1 INTRODUCTION - pa1c/PCE22_503-598_1997.pdf · Contrails forming during times of specific meteorological conditions constitute a visible manifestation of aircraft emissions, although

Pergamon Phys. Chem. Earth, Vol. 22, No. 6, pp. 503-598, 1997

© 1997 Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain

0079-1946/97 $17.00 + 0.00

PII: S0079-1946(97)0018 I-X

1 I N T R O D U C T I O N

The impact of aviation upon the atmosphere has long been considered negligible on account of the fact that aircraft contribute less than 1% of total emission of pollutants worldwide. The present fleet of subsonic aircraft consumes 176Mt of kerosene per year (1990 figures from Schumann [1994]) which is about 5.6 % of the total consumption of petrol. Carbon dioxide (CO2) emitted by aircraft amounts to 554 Mt/yr, about 2.6 % of total the CO2 released from all fossil fuel sources.

The amount of pollutants emitted, however, is not the only criterion for assessing the possible environmental impact. Aircraft fly in the upper troposphere and lower stratosphere, where residence times of exhaust effiuents are considerably longer than for sources at the Earth's surface. Thus pollutants emitted by aircraft and their products can accumulate more efficiently than those from ground sources. Moreover, aircraft are the only in situ emission sources in this regime which is characterized by very low temperatures, minimum background concentrations of water vapor, ozone and carbon monoxide, and high radiative impact of greenhouse gases.

Aviation is one of the fastest growing fields of the world's economy. During the past decade, average growth rates of about 5 %/yr occured, and these are unlikely to shrink during the coming decade. Global jet fuel consumption will grow less rapidly, probably by about 3 %/yr, due to introduction of more fuel-efficient engines and larger aircraft. Nevertheless this growth will increase the fraction aviation has with respect to global CO2 emission, from presently 2.6 % towards higher values. Clearly, when international regulations are sought aiming to reduce the global output of CO~, there is no reason to exclude aircraft emissions, not to mention pollutants other than CO2 and their impact on atmospheric chemistry and climate.

Contrails forming during times of specific meteorological conditions constitute a visible manifestation of aircraft emissions, although the visibility of such an effect does not provide the only information about the environmental impact of aviation. Rather, it is necessary to carry out well coordinated research programs comprising field and laboratory measurements along with modeling studies. Such programs have been established in Europe and North America at the beginning of this decade.

It is the aim of this assessment to review the findings achieved so far, weigh the confirmed effects against uncertainties, and point to important areas where future research needs to be concentrated. We have considered available papers and manuscripts that came to our knowledge up to the beginning of 1997. On the other hand, model evaluation or review of data quality are beyond the scope of this assessment.

503

504 Peter Fabian and Bernd Karcher

This volume is organized as follows. Chapter 2 gives an overview of the basic issues concerning the impact of aviation upon the atmosphere, recalls early studies, and introduces ongoing and recently completed international research programs. Chapter 3 briefly addresses atmospheric dynamics and photochemical and radiative processes as far as the following review chapters refer to them; section 3.4 summarizes in which ways aircraft emissions perturb atmospheric chemistry and climate. Chapter 4 is devoted to a detailed review of observed and confirmed effects and Chapter 5 discusses predicted effects and their uncertainties. At the end of the two latter chapters, brief summaries of basic observations (section 4.4) and predictions (section 5.4) are given. Chapter 6 deals with future aspects of the aviation impact in the changing atmosphere. Chapter 7 points out important areas of research needs and Chapter 8 presents the Executive Summary.

2 I M P A C T OF A I R C R A F T : T H E B A S I C

I S S U E

2.1 Atmospheric composition and structure

Today's conventional subsonic aircraft cruise in the altitude range between about 9 and 13 km, in the upper troposphere and lower stratosphere. This regime is characterized by two important features, minimum temperatures and minimum abundances of various trace gases, such as water vapor (H20), carbon monoxide (CO), and oxides of nitrogen (NOx = NO + NO~) (see Figure 1). This tropopause region marks the pronounced transition from low ozone mixing ratios in the troposphere to high values in the stratospheric ozone layer. It also marks the transition from the moist troposphere towards the very dry stratosphere.

Subsonic airliners cruise at altitudes between 9 and 13km. Planned supersonic aircraft will fly at markedly higher levels, between 18 and 24 kin, depending on the speed of the aircraft. Average tropopause heights decrease from 16 - 18 km in the tropics over 1 0 - 14 km at midlatitudes to about 8km at polar latitudes. Thus, cruising at 12km will be entirely within the troposphere in the tropics, almost entirely within the lower stratosphere, however, at high latitudes. At midlatitudes, the transition between both flight regimes occurs, with flights both at tropospheric and stratospheric levels, depending on the local meteorological conditions.

Due to their thermal structure, troposphere and stratosphere are fundamentally different with respect to vertical mixing and thus residence times of pollutants. In the troposphere, vertical mixing is fairly rapid, in particular in the lower troposphere where turbulent mixing is often enhanced by convection. Pollutants emitted

The Impact of Aviation upon the Atmosphere

temperature [ K ]

170 220 270 320 601 50 NO

4O

"~ 20

10

0 10-4 10-3 10-2 10-1 100 101 10 2 10 3 104 10 s 10 s

volume mixing ratio [ppmV]

CN number density [ cm "3 ]

Figure 1. Schematic presentation of the atmospheric temperature (7") struc-

ture and various trace gas distributions up to 60 km, adapted from Wurzel

[1994]. The temperature profile is representative for the midlatitudes. The

number density profile for the condensation nuclei (CN) with an average ra-

dius of 0.151~m was adapted from Junge [1963]. The shaded region shows the

approximate locations of the tropopause, covering seasonal and latitudinal vari-

ations. Subsonic and supersonic cruise altitudes are 9-13km and 18-~4 km,

respectively.

505

from surface sources, such as nitrogen oxides (NOx= NO + NO2) and sulfur dioxide (SO2), are oxidized and usually washed out as nitrate and sulfate, respectively, within a few days.

The stratosphere acts, due to increasing temperatures with altitude, like a huge inversion layer. Vertical mixing is suppressed and confined to quasi-horizontal motion along sloped isentropic surfaces (see Figure 2). Thus the atmospheric residence time of a pollutant increases with height above the tropopause, reaching values in excess of one year [Fabian, 1974]. Depending on the height and latitude of injection, it increases from about three months at 10km to 15 months at 25 km [Briihl et al.,

1991]. Pollutants emitted in this height regime thus accumulate accordingly.

The composition of the present atmosphere is highly perturbed by long-lived halogenated hydrocarbons from various anthropogenic sources which have accumulated

100 n

a. 300

Peter Fabian and Bernd K~ircher

500

1000 -90 South Pole

-60 -30 30 60 90 North Pole

0 Equator

Latitude

(D

<

506

30

Figure 2. Latitude-altitude cross section for January 1993 showing zonally averaged potential temperature (solid lines) and temperature (dashed lines).

The heavy line denotes the 2-PVU potential vorticity contour, which approx-

imates the tropopause outside the tropics. Shaded areas mark the "lowermost

stratosphere" whose isentropic or potential temperature surfaces intersect the

tropopause. From Holton et al. [1995].

over recent decades. These provide the source of active chlorine and bromine radicals (commonly denoted as C1Ox and BrOx, respectively), which catalyze ozone depletion in the lower stratosphere. Heterogeneous reactions on aerosol and cloud particle surfaces play a dominant role by activating the halogen compounds from rather inert to such species which rapidly destroy ozone [Solomon et al., 1986; Granier and Brasseur, 1992]. The present CIOx level is about 3.8ppbV which is 6 times the abundance of 0.6ppbV from natural methyl chloride emanating from the tropical ocean. The BrOx level has about doubled as a result of anthropogenic emissions, from about 10 pptV to almost 20 pptV [Kriiger et al., 1997].

These C1Ox and BrOx levels will remain high throughout the next decades. Under the present phaseout regulations of the Montreal Protocol it will not be before the year 2060 that pre-ozone "hole" levels of CIOx and BrOx of 2 ppbV and 15 pptV, respectively, will be reached again. Thus any assessment of the impact of aircraft emissions, for the next 10 to 15 years, must take into account the highly perturbed atmospheric halogen levels (see Chapter 6).

The Impact of Aviation upon the Atmosphere 507

Other long-lived trace gases have been increasing in the atmosphere as well, due to anthropogenic emissions. Since the beginning of industrialization, from 1750 onwards, CO2, methane (CH4), and nitrous oxide (N20), have increased in the global atmosphere from 280 ppmV, 0.7 ppmV and 280 ppbV to presently 360 ppmV, 1.7 ppmV, and 310 ppbV, respectively. Together, they contribute to the increasing greenhouse effect and thus affect global climate, with CH4 and N20 also affecting atmospheric chemistry. The growth of CO2 mainly results from burning of fossil fuel, it is continuing at a rate of about 1.5ppmV/yr [Keeling et al., 1995], with aviation contributing almost 3 %.

2 .2 O z o n e t r e n d s

Both ground based and satellite borne instruments reveal continuously progressing global ozone losses in the stratosphere attributable to increased C1Ox and BrOx levels. Compared to the sixties, the ozone layer thickness or total ozone column amount at northern midlatitudes has decreased by more than 5 %. These ozone losses decrease towards lower latitudes and show a pronounced seasonal variation, with largest ozone depletion observed in winter and spring indicating that heterogeneous reactions (see Chapter 4) on particle surfaces play a dominant role [Stolarski et al., 1992; Rusch et al., 1994; Bojkov, 1995; Chandra et al., 1996].

For high sulfate aerosol levels in the lower stratosphere, e.g., at times after major volcanic eruptions, additional ozone depletion has been observed at low [Schoeberl et al., 1993], middle [McGee et al., 1994; Wege and Claude, 1997], and high latitudes [Solomon et aI., 1993].

Heterogeneous reactions of halogen compounds on the surface of polar stratospheric cloud (PSC) particles are the cause of massive springtime ozone depletions observed over polar regions. This effect is most pronounced over south polar latitudes (Antarctic ozone hole), with up to 70 % of total ozone, almost all ozone contained in the lower stratosphere, is temporarily wiped out during early October [Schoeberl et al., 1996; Jiang et al., 1996]. Over Arctic regions where meteorological conditions are less favorable for such ozone "hole" formation, substantial ozone depletions have become common in recent winters [Herman et al., 1993; Menney et al., 1994; Larson et al., 1994; Hansen et al., 1997]. Both the Antarctic ozone hole and corresponding Arctic ozone depletions disappear when, after the breakdown of the wintertime polar vortex, the polar ozone layer is replenished by ozone advected from lower latitudes.


2.3 P o l l u t a n t s e m i t t e d by aircraft

Presently, operating jet engines burn kerosene using atmospheric oxygen as oxidant. Thus the exhaust mainly consists of CO2 and H20 with additional fractions of NOx, carbon monoxide (CO), sulfur (S) in the form of SOx (= SO2 + SO3), unburnt hydrocarbons (CxHy), and soot. Ranges of emission indices varying with engine type and stage of performance are listed in Table 1, along with estimated global emission rates based on 176 Mt/yr of kerosene used in 1990 worldwide.

Table 1. Emission indices and estimated global emissions of exhaust products

of the present fleet of aircraft (reference year 1990, after Schumann [1994], with emission index modifications according to more recent measurements).

Emission

Fuel

CO2

H20

NOx (as NO2)

CO

CxHy

Soot

S

Emission index Emission rate Comparable Comparable emission (g pollutant 1 9 9 0 emissions source per kg fuel) (Mt/yr) (Mt/yr)

1000 176 3140

3150 554 20900

1260 222 45

525000

Total consumption of petrol

Total burning of fossil fuels

CH4 oxidation in the stratosphere

Evaporation from Earth's surface

18 (4-30) 3.2 2.9 4- 1.4 90 ± 35

15 (6-20) 2.6 600 + 300 1490

0.6 (0.2-3.0) 0.1 90

o.1 (o.ool-o.5) o.o18 0.5 (0.01-3) 0.176 0.0625

(as SO2) (as SOs)

134 (as SO2)

Flux from the stratosphere All anthropogenic sources

CH4 oxidation All anthropogenic sources

Antropogenic emissions at the Earth's surface

Rate required to sustain background aerosol in the lower stratosphere Total from fossil fuel combustion

The figure of 176 Mt/yr bases on reported refinery production of destillate labeled as jet fuel. In a recent study, Baughcum [1996] argues that a part of this quantity does not represent jet fuel delivered to airports, thus coming up with a total of only 134 Mr/yr. The average of these independently determined figures is 155 4- 21 Mt/yr


or 155 Mt/yr -t- 14 %. Despite this discrepancy in absolute amounts there is agreement on the relative growth rate of global jet fuel consumption of about 3 %/yr between 1977 and 1989. This growth is expected to accelerate in future years leading to about 304 Mt/yr global fuel consumption in 2015, more than twice the value for 1990 [Baughcum, 1996].

A considerable fraction of jet fuel is burned in the stratosphere. This is an important issue as aircraft are the only direct emission sources in this height regime. Reichow [1990] estimated that Lufthansa consumes 17 - 20 % of the fuel above the tropopause. Ko et al. [1992], using zonally averaged climatologies, estimated that as much as 48 % of the kerosene used 1987 on the northern hemisphere was burnt in the stratosphere. Combining air traffic statistics for flights in the North Atlantic flight corridor with a tropopause surface distribution from analyses of assimilated data, the work of Hoinka et al. [1993] shows an annual (1989-1991) average of about 44 % of today's subsonic fleet cruising in the lowermost stratosphere. These authors also report a pronounced annual variation of the cruising times above the local tropopause, reaching nearly 75 % in February and as low as 25 % in Septem- ber. An annual average of about 40 % of the subsonic emissions taking place above the tropopause has also been confirmed by more recent global model calculations (R. Sausen, personal communication, 1997).

Table 2. Concentration increase due to aviation emissions at 1Okra, globally

distributed (assumed residence time 0.25 yr, emission rates from Table 1)

Emission products

H20 NOx CO S02

Emission Mean concentration Background rate (Mt/yr) increase concentration

at 10 km

222 0.05 ppmV 100 ppmV 3.2 1.9 ppbV 0.1-0.2 ppbV 2.6 0.6 ppbV 10 ppbV

0.176 0.04 ppbV 0.03-0.1 ppbV

Table 2 shows equilibrium concentration increases calculated for global emission rates listed in Table 1 following an approach by Fabian [1990], assuming that no chemical transformations occur. All aircraft operations were assumed to take place at 10km altitude (200hPa), with a residence time of 0.25yr [Briihl et al., 1991], typical for upper tropospheric conditions. The concentration increases for H~O and CO thus obtained are small compared with background concentrations listed in the last column. For NOx, however, an increase by a factor of about 10 results from aircraft operations (see Chapters 4 and 5). For SOs the increase is comparable


to the background. It should be noted, however, that sulfur plays an important role in particle formation making it difficult to assess its environmental impact based on concentrations alone. Moreover, as air traffic is not evenly distributed over the globe, but rather concentrated along several flight corridors, even larger concentration increases may be expected.

2.4 Early studies

2.4.1 Supersonic aircraft

Environmental concern about the impact of aviation upon the atmosphere was stimulated by P. Crutzen's and H. Johnston's historical papers assessing the possible reduction of the Earth's ozone layer by a large fleet of supersonic aircraft (SST) projected at that time for the 1990s [Crutzen, 1971; Johnston, 1971]. John- ston calculated that 500 SSTs operating six hours a day in the stratosphere would emit about two million tons of NOx every year which, ~/a NOx-catalyzed ozone depletion, would cause a global reduction of the ozone layer by about 50 %.

These studies prompted the Climatic Impact Assessment Program (CIAP) initiated and funded by the US Department of Transportation, aiming to assess the impact of a future fleet of SSTs upon the atmosphere and its biological, social, and economic consequences. Through CIAP, an interdisciplinary and international research program (US-based with some contributions from Europe), our knowledge and understanding of atmospheric photochemistry, dynamics, and radiative processes increased considerably. CIAP findings are compiled in a series of mono- graphs, with a final report covering the main results [CIAP, 1975].

The CIAP program had two major consequences. First of all, it clearly demonstrated that important photochemical and dynamical processes were not understood well enough at that time. Thus considerable research efforts were stimulated worldwide, whose results provided a much sounder basis for present assessments. Secondly, CIAP caused a general awareness of anthropogenic effects upon the atmosphere, with emphasis on the middle atmosphere, in particular the ozone layer. CIAP paved the way for the chlorofluorocarbon (CFC) issue not discussed here, and it also provided the motivation for assessing the possible impact of space flight activities.

2.4.2 Rocket and space flight activities

The first studies were carried out during CIAP. The final report [CIAP, 1975] contains information on exhaust composition of small research rockets, such as Arcas,


Aerobee, Black Brant, Nike, and Javelin, whose impact is negligible compared to large space carriers such as Delta, Atlas, Titan, or Saturn. As a significant modi- fication of the atmosphere can be expected for large systems only being launched frequently enough, there was early and ongoing interest in the Space Shuttle. (A review of these early studies in contained in Hindelang et al. [1989].)

Already in the first studies by Cicerone et al. [1973; 1974] it turned out that not H20 emitted by the main liquid fuel engines, but rather emissions of hydrogen chloride (HCI), and aluminium oxide (A12Oj), particles of the solid-state boosters were of concern, due to their impact on chlorine (and ozone) chemistry. The issue of ozone depletion via chlorine atoms released from HC1 in the exhaust of solid- fueled rocket engines has been addressed in various model studies until present. All one-dimensional (1D) [Whitten et al., 1975], two-dimensional (2D) [Prather et al.,

1990] and three-dimensional (3D) assessments [Prather et al., 1990; Hirschberg, 1993; Jackman et al., 1996 and references therein], come up with chlorine increases of less than 1%, with corresponding ozone reductions of a few tenths of a percent at most between 30km and 50km altitude, for realistic launch scenarios. There has been some discussion as to whether or not significant ozone destruction along the flight path of the Shuttle might occur [Aftergood, 1991], but at least based on Total Ozone Mapping Spectrometer (TOMS) ozone data this cannot be confirmed, and no severe local ozone depletion could be detected [McPeters et al., 1991]. Likewise, simulated measurements computed for a Titan IV rocket plume showed that no ozone effect is likely to be detectable by TOMS [Syage and Ross, 1996]. The chlorine input into the atmosphere by solid-fueled rocket engines appears to be negligible concerning its impact on global ozone, at least for present launch frequencies.

The role of aluminium oxide particles emitted from these rockets was recently re- addressed by Jackman et al. [1996]. By means of 2D model calculations including heterogeneous chemistry promoted by A1203 particles they found ozone decreases larger than from HC1 alone. As was pointed out by Robinson et al. [1994], halo- carbons may be decomposed on aluminium oxide particle surfaces thus giving rise to halocarbon depletion in the immediate wake of rocket plumes. Zolensky et al. [1989] found a tenfold increase in the concentration of large solid particles (radius > 1 #m) around 18 km, between 1976 and 1984. They attributed the source of this increase to solid rocket exhaust and ablating spacecraft material. Bekki and Pyle [1992] argue that this rise of very large particles may be mirrored by a large increase of submicrometer particles. Also concerning aviation, there is no doubt that the issue of particles in the atmosphere remains one of the most important ones requiring increased research efforts (see Chapters 4 and 5).


2.5 Ongoing and recently completed research programs

Because of the scientific findings of CIAP and, more importantly, because of eco- nomical reasons, the US SST program was abandoned. Recently there has been renewed interest in the development of faster commercial aircraft. In order to assess environmental aspects related to such a system, NASA started the High Speed Research Program (HSRP) [Watson et al., 1991; Johnston et al., 1991].

Conventional subsonic air traffic, surprisingly, has received little attention until the second half of the eighties, despite rapid growth rates and the clear visibility of contrails forming under suitable meteorological conditions. (A review on earlier observations is given by [Schumann and Wendling, 1990] and [Schumann, 1996]).

Two German studies [Held, 1990; Schumann, 1990] suggested that urgent research needs existed, and the German Science Foundation (DFG) and the Federal Ministry of Education and Research (BMBF, formerly BMFT), funded coordinated research beginning in 1991 and 1992, respectively. While the DFG program focused on the basics of the environmental effects of air and space transport systems, the BMBF- program, in cooperation with industry and other European research proposals, evolved on a much wider scope. In 1994 also in the US research related to sub-

Table 3. Coordinated programs related to assessing the environmental impact

of aviation (see appended list of acronyms).

Program Funding Period Funding Source Approximate Total Funding (US-S)

USA HSRP (incl. S A S S ) 1990-2001 NASA 1.5 Billion SASS 1994-2001 NASA 80 Million

Germany DFG-Focus 1991-1996 DFG 5 Million BMBF-joint program 1992-1997 BMBF 15 Million

E u r op e an U n i o n AERONOX 1992-1994 EU 1.3 Million POLINAT 1994-1995 EU 1.0 Million MOZAIC 1993-1996 EU 1.2 Million STREAM II 1994-1995 EU 1.0 Million LOWNOX 1992-1995 EU 6.7 Million AEROTRACE 1995-1996 EU 0.7 Million


sonic air traffic (the subsonic assessment project, SASS) was funded by NASA, in addition to the HSRP program.

In Table 3 major coordinated programs related to assessing the environmental impact of aviation are listed. Funding volumes are approximate amounts converted to US Dollars, for convenience. A couple of other dedicated EU-projects started in 1996/1997 and are not listed in Table 3.

3 A T M O S P H E R I C PROCESSES

3.1 A t m o s p h e r i c d y n a m i c s

Transport properties in the upper troposphere (UT) and lower stratosphere (LS) are largely determined by the atmosphere's thermal structure and its seasonal variations. The latitudinal structure of vertical exchange reflects the so called Hadley circulation with upward motion in the tropics and downward transport at middle and high latitudes.

Figure 2 shows a latitude-altitude cross section for January 1993 displaying zonally averaged potential temperature (solid) and temperature (dashed) contours [Holton et al., 1995]. The heavy line (cut off at the 380 K isentrope) denotes the 2 PVU potential vorticity (PV) contour (1 PVU = 10 -~ m ~ s -1 K kg-1), which approximates the tropopause outside the tropics. The shaded areas mark the "lowermost stratosphere" (LMS) whose potential temperature surfaces intersect the tropopause. This LMS is dynamically and chemically different from both the stratosphere (above the 380 K isentrope) and the troposphere [e.g., Bregman et al., 1997], as it is heav- ily governed by stratosphere-troposphere exchange (STE). It is obvious that STE needs to be understood for assessing the effects of aircraft exhaust emitted into this height regime.

While STE has traditionally been considered to be related to synoptic and small- scale exchange mechanisms, such as blocking anticyclones, cut-off cyclones and tropopause folds, Holton et al. [1995] present a global scale concept that places STE into the framework of the general circulation. Using ozone, methane, water vapor and nitrous oxide distributions measured aboard the Upper Atmosphere Research Satellite (UARS), Yang and Tung [1996] derived global STE fluxes and a turnover time of 1.6 years for the lower stratosphere with this concept. Similar transport properties and turnover times were derived by Minschwaner et al. [1996] and Volk et al. [1996] from measurements of long-lived tracers, such as N20 and CFCs (see also model studies by Poulida et al. [1996]; Stenchikov et al. [1996]; van Velthoven and Kelder [1996]). Appenzeller et al. [1996], investigating the seasonal

514 Peter Fabian and Bernd K~rcher

variation of STE, found a pronounced annual cycle in the mass of the northern hemisphere LMS, while for the southern hemisphere the corresponding variation is weak.

Global isentropic PV maps reveal strong horizontal PV gradients in the equatorial lower stratosphere. Chen et al. [1995] and Plumb et al. [1996] showed that these regions act as strong barriers to cross-equator mass exchange. The existence of such a permeable barrier was confirmed by observations of the Mt. Pinatubo volcanic aerosols measured by the Stratopheric Aerosol and Gas Experiment (SAGE-II) [Trepte et al., 1993]. A time constant of 3.3 years for this interhemispheric mixing was derived from global fallout data of Strontium-90 [Fabian et al., 1968]. In a recent study Pueschel [1997] showed that soot aerosols, themselves originating from subsonic aircraft, reveal strong interhemispheric gradients in the lower stratosphere, documenting barriers to interhemispheric mixing.

3 .2 P h o t o c h e m i c a l proces se s

3.2.1 Gas phase photochemistry

Ozone is formed in the troposphere through photochemical smog reactions involving NOx, CO, and hydrocarbons, and in the stratosphere by the photodissociation of 02 (at wavelengths below 240 nm) and subsequent recombination of the resulting oxygen atoms with 02 [Crutzen, 1970; 1971]. Ozone abundances are largely controlled by the action of several reaction chains which are catalyzed by the presence of radical species.

In the troposphere, in general, net ozone production takes place via OH-induced oxidation cycles involving CO, CH4, and non-methane hydrocarbons (NMHCs) [Ehhalt et al., 1991]. The hydroxyl radical itself is produced by photolysis of ozone at wavelengths below 320 nm, and the HOx-species (HOx = OH + HO2) are recycled due to very fast CO + OH and NO + HO2 reactions.

Ozone is in a photochemical steady-state, and its mixing ratio is proportional to the abundance ratio of NO2 to NO. Through slow catalytic cycles, the NOx levels are shifted from NO to NO2 due to oxidation by HO2 and peroxy radicals that are generated by the oxidation of CO and hydrocarbons, thereby forming ozone and increasing HOx at the same time. Denoting hydrocarbons by RH, the reaction of which with OH leads to peroxy iadicals (RO2), and aldehydes by RCHO, the reaction of which with OH leads to acylperoxy radicals (RC(O)02), the corresponding generic cycles can be cast into the form


RH+OH °b RO2+H20

RCHO + OH 02> RC(O)O2 + H 2 0

RO2 + NO > NO2 + RO

RC(O)O2 + NO ) NO2 + RC(O)O

HO~+NO ) NO2+OH,

> . . . ) RONOz or H02 + RCC

> - . . ~ RO2

where RCC denotes a carbonyl compound. During their lifetimes, the intermedi- ate alkyl (R) and acyl (RCO) radicals participate in chain propagation reactions and thus can lead to substantial conversion of NO to NO2 and subsequent ozone formation, mediated through RO2 and RC(O)O2. However, in NO-poor regions, ozone is consumed in the presence of CO and related species due to the increased importance of the HO2 + 03 reaction which then dominates over the reaction of HO2 with NO. The transition from ozone loss to ozone production occurs at a ratio NO/O3 of about 2 × 10 -4, or at NOx mixing ratios of 20ppt for typical ozone concentrations of 100 ppb. The nonlinear coupling between NO~, HOx, and net Oa production and its importance for perturbed NOx levels in the upper troposphere has been studied by Ehhalt and Rohrer [1994].

june 12:00 november 12:00 5 . 0 x 1 0 5 . . . . . . . . ~ . . . . . . . . I . . . . . . . . 5 . 0 x 1 0 5 , . . . . . . . I . . . . . . . . I . . . . . . . . I

4 . 0 x 1 0 5

?

3 . 0 x 1 0 5

Z

2 . 0 x 1 0 5

o

• * ' * t . 295 hPa

\ 265 hPa t

• ." %

,: . . . ,

' :dl '% ;: "..:,

z o --.,': -- -,~ -- *'e" ;

- 1 . O x l O 5 , I . . . . . . . . I . . . . . L,

0.01 0 .10 1.00 10 .00 NOx MIXING RATIO [ppb ]

7-

z

4 . 0 x 1 0 5

3 . 0 x 1 0 5

2 . 0 x 1 0 5

1 . 0 x 1 0 5

- 1 . 0 x 1 0 5

0 .001

295 hP~= .%

:., "% .

: :23s hPa'..\ t" : ' , %

• ." *o.~

, , , , l , . l , , , , , , . I , , , , , , , d , , , , , , ,

0.010 0.I00 1,000 I 0 . 0 0 0 NOx MIXING RATIO [ppb]

Figure 3. Net 03 production rates for summer (let) and winter (righ Q condi-

tions at 50 ° N as a function of the background NO= level. The rates are shown

.for 4 labeled altitudes covering the main upper tropospheric flight levels in the

North Atlantic flight corridor. E~rom Perry et aL [1997].

516 Peter Fabian and Bernd K~ircher

Figure 3 shows net ozone production rates, P(O3), calculated for summer and winter conditions at typical flight levels in the North Atlantic flight corridor [Petry et al., 1997]. At the current NOx concentrations, additional release of subsonic aircraft emissions in this region are expected to cause net ozone production. However, Brasseur et al. [1996], and very recently Grooss et al. [1997], have shown that the curves P(O3) versus NOx mixing ratio also strongly depend on assumed background mixing ratios of H20, O3, CO, and NMHCs, and on the solar zenith angle. Hence, the large variability of background levels of the key species renders a detailed analysis more complicated, and aircraft may frequently operate in regions where the NO~ emissions cause a reduction of P(O3), or even a net ozone loss, driven by cycles of the form

X + O 3 ~ X O + Q

X O + O ~ X + Q

O 3 + h u ~ 0 + 0 2

n e t : 2 0 3 + h u > 302,

with the radicals X = OH, NO, Cl.

The OH-cycle dominates 03 removal at altitudes above 45 km. Most ozone is located between 20 and 45 km, where the NO- and Cl-cycles are important, the latter due to the still increasing anthropogenic chlorine levels. Below about 20 km, due to the small abundances of atomic oxygen, HO2 may directly react with ozone, reproducing the OH consumed in the first reaction. In the lower stratosphere, odd- hydrogen and halogen free radical catalysis predominantly determine the rate of removal of ozone [Wennberg et al., 1994] (see Figure 4 left panel), but also cycles involving Br species are important in this regard, among which

C10+BrO > C l + B r + 0 2

O3+C1 > C10+O2

0 3 + B r > B r O + 0 2

n e t : 2 0 3 ) 302,

is most important (see Lary [1996] and Lary et al. [1996] for further details). The main cycle which is responsible for the formation of the ozone "hole" (i.e., under polar stratospheric conditions) involves the C10 dimer molecule [Molina and Molina, 1987]. In the winter polar stratospheres, NOx abundances can become negligible and at the same time available chlorine exists as C10, leading to the observed, severe catalytic ozone depletion.

These and other cycles do not act in isolation and therefore their ozone depletion effects are not simply additive. For instance, there exists a strong coupling between


5O

90

110

• N O : . ? , . . C : O & B r O ,A ..HO x

1 May 1993

o

Rate

T i . . . . . . i , , , . . . . . i

0.1 1 10

e)

03 (3 c3 ~3

0 3 Destruction Rate (%/month) Increasing NO x

Figure 4. Left panel: Photochemical removal rates for ozone in the lower

stratosphere at midlatitudes observed during SPADE in May 1993. Ozone de-

struction was dominated by cycles involving HOx (responsible for more than

~0 ~ of the total loss), ClO and BrO, and NOx. Right panel: Schematic of

various ozone removal rates versus NOx. Similar to tropospheric ozone pro-

duction rates (see Figure 3), the response of stratospheric ozone loss rates to

changes in NOx is highly nonlinear. Adapted from Wennberg et al. [199~].

the NOx- and C10x-cycles involving the reservoir gases ClON02 and HC1 via the reactions C10 + NO2 ~ C10NQ and C1 + CH4 ~ HC1 + CH3. Neither ClON02 nor HCI do react with ozone. As a result, ozone depletion by NOx and especially by C1Ox catalysis is reduced, and increasing levels of NO× will reduce the efficiency of the CIO~ cycle.

High HOx levels lead to the enhanced production of the stable gases HNO3 and HNO4, thereby slowing the NOx depletion cycle. On the other hand, additional HOx will accelerate C10x catalysis by producing active chlorine via HC1 + OH --+ C1 + H20.

At high NOx levels, ozone depletion by the OH cycle is short-circuited, because HO2 reacts with NO to produce NO2 and OH; NO2 in turn is photolyzed to NO and O, the latter immediatley reproducing an ozone molecule, resulting in no net chemical effect. Hence, additional NOx slows HOx catalysis and counteracts the increased importance of the pure NOx depletion cycle. This feedback is important below

518 Peter Fabian and Bernd Khrcher

20km, in the LS and UT. Injection of NOx also produces more HNO3, which is either photolyzed or reacts with OH to form NO3. NO3 in turn is rapidly photolyzed to NO2 and O, thus producing an ozone molecule.

All together, the coupling between the individual cycles lead to the nonlinear response of stratospheric ozone to changes in NOx as shown in Figure 4 (right panel). The preexisting levels of NOx, HOx, C1Ox, and BrOx determine the sign and magnitude of changes when additional NOx from aircraft is added. Clearly, the increasing levels of long-lived reservoir gases CH4 and N20, that are the key sources for water vapor and nitrogen oxides in the stratosphere, as well as chlorine- and bromine- containing gases from various anthropogenic sources, together with increasing aircraft emissions can considerably disturb the subtle balance of the coupled chemical cycles that control the stratospheric ozone levels [Crutzen and Briihl, 1990].

3.2.2 He te rogeneous chemis t ry

Particles and cloud droplets in the troposphere and stratosphere play critical roles in the chemistry of these regions. On the global scale, heterogeneous reactions on aerosols seem to be most important in the industrially affected part of the northern hemisphere, whereas clouds are more important in relatively unperturbed regions [Dentener and Crutzen, 1993]. There is agreement that heterogeneous chemistry must be included in photochemical models used to predict the abundance and temporal variation of stratospheric ozone. The important role aerosols play in anthropogenic ozone depletion at northern midlatitudes and in the polar regions has comprehensively been discussed by Solomon et al. [1996] and Portmann et

al. [1996]. Both reactions on the globally distributed stratospheric sulfate aerosols (SSAs) and on solid nitric acid- and water ice-containing polar stratospheric clouds (PSCs), which require quite low temperatures to form, must be considered. Due to significant efforts that have been directed at particles in the polar regions since the discovery of the ozone "hole" in 1985, the knowledge about the nature of these stratospheric particles has grown tremendously.

The key effect of the known principal heterogeneous reactions is to convert chlorine from the inactive reservoir species ClONO2 and HC1 into active forms such as C10, C1, C12, HOCI, and C1NO2. At the same time, nitrogen is repartitioned from active (NOx) to inactive forms (NOy, mainly HNO3). As a result, more ozone can be catalytically depleted in the LS due to enhanced levels of HOx, C1Ox, and BrOx as the aerosol levels increase. These heterogeneous reactions are:

N205 + H20 > 2HNO~ C1ONO2 + H20 > HOC1 + HNO3


BrONO2 + H20 ~ HOBr + HNOz

C1ONO2 + HCI ~ C12 + HNOz

HOCI + HC1 ~ C12 + H20,

519

N205 is formed in the gas phase by recombination of NO2 and NOa; hence, active NOx is directly converted to NOy v/a the hydrolysis reaction [Fahey et al., 1993]. This process is called denoxification. Calculations show that this reaction pathway, which takes place on the global stratospheric sulfate aerosols, dramatically decreases the importance of the nitrogen-catalyzed 03 depletion cycle in the LS at midlatitudes. At the same time, it renders chlorine- and bromine-catalyzed ozone depletion more important, because less NOx is available to sequester CIOx and BrOx into inactive forms [Rodriguez et al., 1991] (see also Figure 4, right panel). In addition, the reduced loss of HOx into HNO3 and HNO4 also enhances catalytic ozone depletion due to HOx and also due to CIO~, because more HC1 can be converted to active chlorine via the HC1 + OH reaction. Figure 5 illustrates the principal reaction pathways between the nitrogen species, including the hydrolysis of N205 on sulfate aerosols.

Models reach better agreement with observed CIO data and ozone trends when the hydrolysis of N205 is included [Rodriguez et al., 1991; Bekki et al., 1991]. Modeled ozone loss frequencies within 45 ° - 90°N between 14 - 23 km altitude increase by about 20% [Weisenstein e$ al., 1991]. The other heterogeneous reactions also increase the atmospheric NOy budget by forming HNO3. If HNO3 bound in the bulk or at the surface of particles is removed by gravitational sedimentation, the air mass gets permanently denitrified. As does denoxification, denitrification makes ozone more vulnerable to chlorine species. In the UT, heterogeneous destruction of NOx under nighttime conditions via N205 hydrolysis is expected to have a negative

which can take place both on or in liquid and on solid particles. The efficiency of each reaction depends on the phase of the aerosol [Ravishankara and Hanson, 1996]. With regard to reactive uptake coefficients, chlorine activation on/in supercooled ternary H2SO4/HNOs/H20 solutions (often referred to as liquid PSCs) is comparable to that over solid PSCs at equal temperatures. Hence, chlorine and bromine can be rapidly activated even in the absence of solid particles. The N205 hydrolysis is very fast both on liquid aerosol and ice, but less important on nitric acid trihydrate (NAT) and sulfuric acid tetrahydrate (SAT). It is nearly independent of relative humidity, i.e., temperature and partial water vapor pressure. In contrast, the other reactions on/in liquid and solid particles show very pronounced dependences on these parameters, with a marked tendency to become very efficient with increasing water content (i.e., with increasing relative humidity). The reactions of C1ON02 and HOCI with HCI on/in liquid droplets become extremely efficient at very low temperatures due to a steep increase of the HC1 solubility.


.Ic,°N° t . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

NO NO2 / ~ / hv L g ~/~, L

................................. No: ............. t t .............

Sulphalo aorosol 1120 1

Sulphalo N205 I HNO3

hv., OH J T OH

NO V

Figure 5. Schematic of the principal species of the nitrogen (NOy) reser-

voir and reaction pathways. The thickness of the arrows is proportional to the

corresponding conversion rate; hv denotes a photolysis rate. The effect of the

N2 05 and CION02 heterogeneous reactions on sulfate aerosols is to reduce the

steady-state abundance of NOx. From Fahey et al. [1993].

effect on the photochemical formation of ozone and on the OH abundance [Dentener and Crutzen, 1993].

For a high aerosol level in the LS (e.g., after a major volcanic eruption), the chlorine and bromine nitrate hydrolysis reactions too can have a significant effect on ozone concentration by reducing gaseous NOx and producing active C1 and Br. It is known that the N2Os hydrolysis is saturated at background aerosol conditions in the LS (i.e., is limited by the slow formation rate of N20s which decreases quadrat- ically upon NOx-loss) [Fahey et al., 1993]; in contrast, the C1ONO2 hydrolysis is important only at very low temperatures and saturates only at much higher aerosol loadings and is thus the primary cause of much higher ozone loss rates found by models under such conditions [Prather, 1992]. Heterogeneous chemistry involving chlorine compounds can be effective under sunlit conditions even in air where NOx is already depleted and C1ONO2 levels are low. Such a catalytic cycle is initiated by the HOCI + HCI reaction. The reactant C12 molecules are rapidly photolyzed and the two resulting chlorine atoms destroy ozone. The HOC1 molecule is recycled in the gas phase via HO2 Jr- CIO. Bromine reactions provide an additional mechanism for repartitioning nitrogen and increasing HOx under daytime conditions [Randeniya et al. 1996; Tie and Brasseur, 1996].


CATALYTIC OZONE DESTRUCTION CYCLES IN THE STRATOSPHERE (EXAMPLES)

NITROGEN OXIDE CYCLE ( 197 i )

03 + NO ~ 024- NO 2

O + NO 2 ~ 02 + NO

03 + hv ~ O + (12 Dt':,ACT|VATION Net : 203 + hv ~ 3(12

NO 2+CIO. M_ CIONO2

"OZONE IIOLE" CI ILORINE CYCLE (1987)

203 + 2CI ~ 202 + 2(:10

2(:10 ~ C1202

CI202+by ~ 2 C 1 + O 2

N e t : 2 0 3 + h v ~ 3 0 2

ACTIVATION

POLAR STRATOSPHERIC CLOUDS (PSCs)

CIONO 2+ |tCI ~ CI 2+ IINO3/ ¢

521

Figure 6. The two most important catalytic ozone depletion cycles under

polar stratospheric conditions and their coupling via a deactivation reaction.

Re-activation of CION02 and HCl on PSCs drives the ClO-dimer cycle at

temperatures below about ~OO K. From Peter and Crutzen [199,~].

Figure 6 shows two catalytic cycles that are of particular importance under ozone "hole" conditions in the polar stratosphere. Low temperatures (< 200 K) are re-

quired to obtain the necessary high C10 concentrations produced by activation on PSCs between 15 and 25 km altitude, and to prevent rapid thermal decay of the

dimer molecule. There would be no Antarctic ozone "hole" if reactions on polar stratospheric cloud particles did not re-activate the reservoir gases CIONO2 and HC1 through heterogeneous reactions, thereby driving the CIO-dimer cycle.

3.3 Radia t ive processes

3.3.1 G r e e n h o u s e gases

The Earth/atmosphere system receives its energy from the Sun, about 1,354 W m -2 on average (solar constant) within the wavelength range between 0.2 #m (ultravio- let, UV) and 4 #m (near infrared, IR). Averaged over the entire planet (including the night side) this flux amounts to 342 W m -2 about one third of which (105 W m -2) is scattered back to space. The remaining 237 W m -2 are converted to heat within the system, mostly at the Earth's surface, which re-radiates energy back to space, at IR wavelengths between about 4 #m and 100 #m. As satellite measurements show, thermal radiation of 237W m -2 leaves the system (proving that


incoming and outgoing radiation are balanced). This equilibrium would correspond to an average surface temperature of 254 K. The observed surface temperature average, however, is about 288 K, 34 K higher. This is due to the effect of greenhouse gases, H20, COs, 03, N20, and CH4 (in the order of their importance) which absorb in different regions of the Earth's thermal radiation spectrum and re-radiate this energy. The downward flux of this atmospheric thermal radiation,about 301 W m -2, reduces the net radiative energy loss at the Earth's surface, maintaining a considerably higher temperature than without greenhouse gases (greenhouse effect).

The abundances of greenhouse gases have increased as a result of anthropogenic effects. Since the beginning of major industrialization, CO2, CH4, and N20 have increased from 280 ppmV, 0.7 ppmV, and 280 ppbV to 360 ppmV, 1.7 ppmV, and 310ppbV, respectively, at present. Various halogenated hydrocarbons, many of which with strong IR absorption bands, have been emitted and thus enhanced the greenhouse forcing. Stratospheric ozone has been decreasing globally as a result of halocarbon emissions, while in many parts of the globe tropospheric ozone has been increasing during recent decades. Altogether, this anthropogenic greenhouse effect causes an additional radiative forcing of about 3 W m -~, corresponding to about 1% of the natural greenhouse forcing, with COs and the sum of the other constituents contributing about 1.5 W m -2 each.

3.3.2 Ci r rus clouds

High level clouds in the UT are predominantly present as ice clouds due to the low (< 230 K) ambient temperatures. The different types of cirrus form when air is lifted upwards and cools adiabatically, forced by orographic waves or the action of large scale weather systems. As a result, the relative humidity in the air parcel increases beyond the deliquescence points of the CCNs it contains. These solution droplets take up water vapor and freeze, producing cirrus ice crystals. It may be interesting to note that only recently experimental campaigns revealed that the UT contains much more small (size < 20 #m) ice crystals than has been previously thought [Crutzen and Ramanathan, 1996].

Under lower stratospheric conditions, during most of the time the air is too dry to produce significant amounts of visible cloud particles. In the cold polar stratosphere, nitric acid hydrates (especially the trihydrate, NAT, being the most stable form) and possibly sulfuric acid tetrahydrate, SAT, may freeze out, and also water ice particles may nucleate. The formation of the latter is often initiated by leewave activities, as in the UT. Together they constitute the solid type I and II PSCs. The radiative effects of PSCs are negligible.


Thin cirrus clouds are nearly transparent for the incoming solar radiation so that the Earth's surface receives nearly as much shortwave intensity as in clear air. Moreover, the small amount of shortwave radiation reflected back to space by these clouds does practically not depend on the cloud altitude. This tends to cool the system Earth-atmosphere. However, the cirrus ice crystals strongly absorb the outgoing terrestial radiation, because they act as a black body in the infrared wavelength region. As a result, they partly block the longwave radiation that corresponds to a black body spectrum taken at the Earth's surface temperature and replace it by the cloud's own radiation field. Because high level clouds exist at temperatures much colder than those prevailing at the surface, correspondingly less energy is radiated to space. This tends to warm the planet. The net radiative effect of the ice clouds also depends on details of their vertical structure, ice crystal habits, and size distribution and is difficult to predict with models.

3.3.3 Soot and aerosols

The main global radiative impact occurs through the particles in the troposphere including the continental and marine boundary layers. Aerosols in the stratosphere exert a significant impact on the radiation budget only under volcanically perturbed conditions. The stratospheric background aerosol mainly consists of small sulfuric acid particles believed to form from carbonyl sulfide (OCS) emanating from the oceans [Junge et al., 1961; Crutzen, 1976]. They constitute the so called Junge-layer. Hofmann [1990] has shown that major volcanic eruptions inject sulfur dioxide to altitudes of 20 to 25 km, which subsequently forms H~SO4 vapor followed by aerosol nucleation and growth. Thus aerosol mixing ratios and surface areas increase by a factor of 100 after major eruptions, and return to background levels thereafter, with a time constant of about 1.5 years.

Sulfate aerosols are known to exhibit complex cooling patterns as they reflect solar radiation to space (the so-called direct radiative effect). On a global basis, their shortwave albedo outweighs the longwave absorption, which leads to a negative mean forcing of -0.3 to -0 .9W m -2 as an annual mean [Jones et al., 1994]. (For comparison, the forcing due to the increase in CO2 in the last 150 years is about +1.5 W m -2.) Furthermore, cooling is enhanced by an indirect effect: if clouds form in air with increased aerosol concentrations, more clouds form with smaller particles [Twomey, 1977]. As a result, cloud reflectivity increases, causing an additional forcing of -1.3 W m -2 which even exceeds the direct effect. In addition, because mean cloud particle size decreases, polluted clouds are less likely to rain out and thus have longer residence times, producing even more cooling. A question of current debate concerns the fraction of soot that is present as an internally mixed component of tropospheric aerosols. Due to the high absorption cross-section of


black carbon particles, whether or not soot is immersed in an aerosol determines the radiative properties of the mixed aerosol particle.

P~diation-climate model results indicate that carbonaceous particles within the troposphere can significantly modify the clear-sky radiative forcing [Haywood and Shine, 1995]. Soot aerosols strongly absorb solar radiation and therefore act to warm the atmosphere, leading to positive mean global forcings between +0.03 to +0.24W m -2, depending on whether external or internal mixtures of soot with sulfate aerosols are prescribed. This compares to -0.34 W m -2 cooling due to non- soot sulfate aerosols alone according to the simulations of these authors.

3 . 4 A i r c r a f t - i n d u c e d p e r t u r b a t i o n s

Over the past 20 years, scientific progress in air chemistry has shown that the chemical effects of subsonic and supersonic aircraft operation is more complex than previously thought. Our knowledge about the reaction kinetics of and interactions between NOx, HOx, CIOx, and BrOx species has grown substantially; after the discovery of the ozone "hole" in the Antarctic polar stratosphere and the observation of lower stratospheric ozone depletion at northern midlatitudes, the importance of heterogeneous chemistry in the global atmosphere was recognized. It has become clear that clouds and aerosols are important players in the climate system.

Both particulate matter and the gaseous species introduced by the aircraft fleet influence air chemistry via heterogeneous and homogeneous chemical reactions. Aircraft emissions perturb the tropospheric photochemical cycles which are responsible for ozone formation in this altitude region, and may also influence the delicate balance between the various, interwoven catalytic ozone depletion cycles in the lower stratosphere. But also coupling effects can become important, e.g., when the presence of additional particles alter the radiation field and, hence, photolysis rates and therefore subsequent chemical reaction pathways, or when the emission of gaseous aerosol precursor species leads to a greater particle formation probability.

Aircraft can affect cloud and aerosol formation in two ways: either contrails form directly behind the aircraft or the particles emitted by the engines or produced in the plumes act as additional cloud condensation nuclei (CCN) or ice forming nuclei (IN) some time after passage of the aircraft, where otherwise possibly no clouds would form or less aerosols would be present. The resulting perturbations of the coverage and microphysical properties of natural clouds likely causes changes in the radiative forcing and chemical processing by the cloud particles at flight level. Aircraft emissions, on the other hand, also contribute directly or indirectly (via the perturbed chemical balance) to the global warming.


The impact of aircraft emissions upon the atmosphere is strongly influenced by the general circulation, especially by vertical transport processes in the tropopause region and the lowermost stratosphere. But also dynamical barriers to horizontal transport exist that crucially control residence times, and thus the potential chemical and radiative impact, of aircraft emissions. Understanding is required of stratospheric-tropospheric exchange processes, the complex dynamics of the northern hemisphere polar vortex that influences the exchange of air between the polar region and the midlatitudes, as well as the tropical barrier that causes a reduction of transport between the tropics and the midlatitudes. Only then the climate impact of aircraft operations can be addressed adequately.

4 O B S E R V E D A N D C O N F I R M E D E F F E C T S

In this chapter, we focus on observed effects exerted by the present fleet of aircraft in the atmosphere and their experimental confirmation. The hot and highly concentrated exhaust from the aircraft engines is initially introduced into the atmosphere as co-flowing jets, that contain the primary exhaust products. The first section is therefore devoted to a discussion of dynamical effects of plume mixing with the ambient atmosphere and wake dispersion. In aircraft wakes within flight corridors, background conditions remain strongly perturbed up to the timescale of roughly one day. Most of the effects known to date have been observed by following single aircraft at cruise altitude and performing in situ measurements of the constituents (gases and aerosols) in aged aircraft plumes. Experimental and theoretical studies of wake processing are further motivated by the fact that a firm understanding of the near-field issue is a key prerequisite for a reliable assessment of the large-scale impact of aircraft exhaust on chemical and radiative processes in the global atmosphere. Observed gaseous and particulate constituents in aircraft wakes are discussed in sections 4.2 and 4.3, respectively. A brief summary of the key observations is given in section 4.4.

4 .1 W a k e d i s p e r s i o n a n d m i x i n g

The wake regime behind a jet airliner is conveniently subdivided into three distinct regimes. These are the jet, vortex, and dispersion regime governed by different dynamics roughly within plume ages 0-10 s, 10-100 s, and 100-1000 s, respectively [Hoshizaki, 1975]. Several fluid dynamical codes are now available to study the evolution of aircraft exhaust plumes in the atmosphere. Miake-Lye et al. [1993; 1994] use the JANNAF Standard Plume Field Code II (SPF 2) for calculation of


the plume before the individual jets interact with the wingtip vortices. This code uses an axisymmetric mixing layer algorithm to calculate jet expansion, similar to NASA's BOAT code [Dash et al., 1979] employed by Beier and Schreier [1994]. K~rcher [1994] and K~.rcher and Fabian [1994] developed a code for the jet regime that solves the compressible jet mixing problem using a semi-empirical turbulence model. Besides using the BOAT code, Gamier et al. [1997] apply another code that solves the full Navier-Stokes equations together with a k-e-turbulence model. All these numerical models are coupled to finite rate kinetics modules that follow the chemical reactions in the gas phase. To fully capture the jet/vortex interactions, Miake-Lye et al. [1994] use the UNIWAKE aircraft wake model [Quackenbush et al.,

1993; 1996] and incorporated a passive chemistry module. Gerz and Ehret [1997] use both the vortex filament method and large eddy simulations (LES) to follow the attraction and final trapping of the individual exhaust jets by the vortices, and the decay of the vortices due to dynamical instabilities. Schilling et al. [1996] apply fluid dynamical and Monte Carlo models to follow the development and decay of aircraft vortices in a stratified shear flow. Further, Diirbeck and Gerz [1995; 1996] employ LES to study the evolution of emissions in the dispersion regime and considered combined effects of atmospheric stratification, wind shear, and turbulence on wake dispersion.

Calculations of the wake dynamics require huge amounts of computer CPU time and storage. The LES for example needs 50 CPU hours and 1 GByte of memory storage on a Cray J 916 supercomputer to cover plume ages up to 3 minutes (Th. Gerz, personal communication, 1996). Implementing chemical and microphysical processes in multi-dimensional fluid codes starting from the dispersion regime will be even more time consuming, and is probably out of reach in the near future. A computationally efficient, analytical plume model has been presented by Konopka [1995], which can be initialized with arbitrary Gaussian plume shapes and allows for time-dependent anisotropic diffusion and shear. Complemented by turbulent mixing rates extracted from the jet and vortex models, such an approach may be useful to prepare plume entrainment rates driving trajectory box models up to the synoptic scale [Kiircher, 1995]. Whereas individual plumes start losing their identity well after the dispersion regime, local fluctuations of pollutant concentrations in flight corridors can persist for several weeks, as demonstrated by isentropic trajectory calculations [Spading et al., 1995].

Since the exhaust distribution and homogeneous and heterogeneous chemical processing in aircraft wakes are characterized by small-scale spatial inhomogeneities and strong time-dependences, the numerical tools described above are needed for the interpretation of near-field measurements. In general, the results of individual models, or combination of several approaches, have been quite successful in ex- plaining available field data. Anderson et al. [1996] presented an SPF 2/UNIWAKE


analysis of the exhaust plume structure and emissions of the El:t-2 aircraft in the lower stratosphere. Within a few percent over a dilution of more than four orders of magnitude, these calculations agree with mixing rates obtained in the Stratospheric Photochemistry, Aerosol, and Dynamics Expedition (SPADE).

Gerz and K/ircher [1997] presented an analysis of wake dynamics and exhaust distribution behind a B 747 in the upper troposphere, constrained by data taken during the POLINAT mission. Agreement between model and observations is excellent for the plume temperatures and reasonably good for COs mixing ratios. Measured temperature excesses between plume and ambient values at the flight level in the vortex regime result from the fact that the entrained exhaust air is encapsulated in the vortex cores that trail downward in a stably stratified atmosphere. These authors also demonstrated that only a marked reduction of the plume dilution rate (i.e., a reduction of entrainment of ambient air into the plume) during the vortex regime can account for the observed COs mixing ratios. Schumann et al. [1995] concluded from turbulence measurements near the tropopause over the North At- lantic that horizontal diffusivities lie in the range between 5 - 20 m s s -1 and that vertical diffusivities are small (below 0.6mSs-1). Wind shear is found to domi- nate the lateral dispersion of aging plumes after about one hour. Constrained by measured diffusion parameters of exhaust plumes near the tropopanse, the Gans- sian plume model has been successfully applied to interpret single and multiple aircraft emission signatures and to extract plume mixing timescales from the observed NOx data in the North Atlantic flight corridor during POLINAT [Schlager et al., 1997a]. Horizontal and vertical diffusivities from LES without and including a mean-flow shear also agree well with the measured diffusivities in the tropopause region [Diirbeck and Gerz, 1996].

During their lifetime, ice particles in visible contrails can serve as tracers that visualize the plume mixing process. There exist a few observations and numerical simulations of contrails that generally support the above-mentioned turbulence measurements and LES calculations. We will come back to this point later in this chapter.

4 .2 C o n s t i t u e n t s i n t h e w a k e

We will first discuss the major fuel combustion products carbon dioxide and water vapor followed by a description of other observed primary exhaust effluents such as sulfur, nitrogen oxides, carbon monoxide, and hydrocarbons. In a second step, we review abundances and observations of secondary exhaust gases that are formed early in the plume via fast reactions with emitted hydrogen oxide radicals, among which are nitrous and nitric acid, sulfur oxides, and sulfuric acid. Together with


water vapor and involatile combustion aerosols (mainly soot), they are responsible for the buildup and further fate of aerosol particles in the wake of aircraft, which are discussed thereafter. At present, no major differences are known between emissions from subsonic and supersonic jet engines, so the following discussions encompass both types of aircraft.

4.2.1 Primary emissions

By far the most abundant exhaust products from jet fuel combustion are CO2 and H20. For stoichiometric combustion, their respective emission indices, 3,155g (kg fuel) -1 and 1,237g (kg fuel) -1, are determined by the fraction of carbon and hydrogen molecules contained in the fuel (mostly commercial Jet A fuel) and exhibit only very small variations (less than 0.5 %). The total amount of CO2 and H20 dumped into the atmosphere by jet aircraft can therefore be predicted with high accuracy. Since the emitted CO2 is uniquely tied to fuel consumption, emission measurements of any constituent X in the plume made simultaneously with CO2 can be expressed as an emission index for X. In the absence of visible ice contrails, because of the limited capacity of particulate matter to condense the emitted water vapor, this also holds for H20 [Kiircher et al., 1995a; Brown et al., 1996a]. However, when ice contrails form, the emitted H20 fully participates in the growth of the ice particles and cannot be regarded as an inert tracer.

Along with CO2 and H20, various other trace gases are emitted in large amounts into the upper troposphere and lower stratosphere. These include reactive sulfur SOx (= SO2 + SO3) and nitrogen species NOx (= NO + NO2), carbon monoxide (CO), methane (CHa), and non-methane hydrocarbons (NMHCs). As discussed below, the emission indices of these exhaust effluents show considerable spreads for various reasons. Any uncertainty in these emission indices will translate directly into corresponding uncertainties of assessment predictions.

The total number concentrations of emitted sulfur gases are determined by the sulfur (S) levels in the jet fuel which are known to vary from below 0.01% to about 0.2 % by mass. The total kerosene sulfur contents can easily be determined by a chemical analysis of fuel samples. They exhibit a broad range around 0.05 %, or 0.5 g S (kg fuel) -1, depending on the source of the cruide oil and the type of refinery processing [Busen and Schumann, 1995]. The emission indices only rarely exceed the internationally accepted sulfur content of 3gS (kg fuel) -1 and are projected to show a long-term downward trend to an average value of 0.2gS(kg fuel) -1 [Baughcum, 1996]. Whereas the fuel properties determine the total sulfur level, the partitioning of sulfur into partly or fully oxidized species, SOx and H2SOa, depends on details of the combustion process. At high temperatures (> 2000 K) during


combustion, the fuel sulfur is mainly converted to SOs. Limiting SOs formation can only be achieved by reducing the sulfur content. In the subsequent colder turbine flow (temperatures around 1400- 2000 K), conversion of SOs to SOz can proceed efficiently in the presence of excess molecular oxygen. Currently, this conversion is believed to be driven by oxygen atoms (O), the concentrations of which are mainly controlled by the combustion of CO. The key reactions read:

S O s + O .M-> SOa

SOa M> SO2+O

S 0 3 + O > SO2+02

SOs+Os --~ SOa+O.

In the turbine, atomic oxygen concentrations can exceed (chemical) equilibrium levels by up to a factor 1000, allowing equilibrium ratios SO3/SOx of 1 to be reached during cooling. However, kinetic limitations reduce this ratio to considerably smaller values, because the residence time of the exhaust in the turbine dilution zone (typically less than 10 ms) is of the same order as the chemical relaxation time of SOa to reach its local equilibrium concentration. Although the formation of SOa in jet engines has very important consequences for aerosol production in aircraft plumes, it has received attention only very recently and will be discussed in more detail below.

As for SOx, the NOx and CO emission indices are largely determined by the engine design and depend on the combustor geometry and its thermodynamical conditions, i.e., temperature and pressure, fuel-to-air ratio, and fuel flow. The major fraction of nitrogen oxides is emitted in the form of NO that is generated at high temperatures in the combustor (above ,-~ 1200 K) from the oxidation of atmospheric nitrogen (N2), via the Zel'dovich mechanism [Zel'dovich et al., 1985]:

N 2 + O > N O + N

N+O2 > N O + O

N + O H > N O + H ,

involving atomic oxygen, hydrogen (H), and nitrogen (N), and the hydroxyl radical (OH). The Ns + O reaction is the primary NO formation pathway, with additional contributions involving reactions of carbon and cyanide (HCN) radicals. At lower temperatures, the reaction of N20 with atomic oxygen is also important. NO production increases with increasing combustor temperature and pressure. Further reactions of NO, mainly with O and HOs, yields NO2. The conflict of desirable fuel effiency at the expense of higher emissions of oxides of nitrogen due to higher combustion temperatures and pressures is well recognized and considered in new engine design studies. Recent technological development has led to improved eombustors with reduced NO~ emissions.

J ~ 22-6-6


The NOx measurements reported in the literature clearly demonstrate the feasi- bility to determine in-flight emission indices of NOx for commercial jet airliners. Absolute NO and NO2 measurements only 2 km behind a DC-9 aircraft have been reported by Arnold et al. [1992], showing marked enhancements of these species in the plume as compared to background levels. From accidental plume encounters in the upper troposphere and lower stratosphere, Zheng et al. [1994] reported similar enhancements from an analysis of DC-8 data during the second Airborne Arctic Ex- pedition, but without knowledge of the source aircraft. Subsequently, Fahey et al.

[1995a; b] measured NO× emission indices of 4 g (kg fuel)-: (we follow the common convention and give this number in mass equivalents of NO2) in the plume of the subsonic ER-2 research aircraft, and of 23 g (kg fuel)-X for the supersonic Concorde engine, both cruising under lower stratospheric conditions. Recently, Schulte and Schlager [1996] reported NOx emissions indices of medium-sized commercial subsonic jet airliners taken in Southern Germany in the range 6 . 4 - l l .7g (kg fuel) -1 (mostly B 737). During the POLINAT campaign over the eastern North Atlantic, emission indices of larger airliners (B 747, DC-10, A 340) with various jet engines are larger and range between 12 - 30 g (kg fuel) -1 [Schumann et al., 1997]. Detect- ing aircraft emission signatures in dense airtraffic corridors requires a sophisticated sampling strategy and flight logistics, access to back-trajectory calculations and meteorological data, and wake dispersion models to account for the time-dependent lateral and horizontal spread of the plumes and for the addition of signals from individual plumes as well [e.g. Schumann et al., 1995].

Generally, measured NO× emission indices are in good agreement with empirical values scaled from ground-based tests of various subsonic jet engines and thermodynamic engine performance codes [e.g., Fahey et al., 1995b; Lister et aL, 1995; Schulte and Schlager, 1996]. This suggests that global three-dimensional inventories of future NOx aircraft emissions for use in global model calculations can be constructed with a high level of confidence [Gardner et al., 1997]. The difference between existing databases in predictions of the total release of nitrogen due to aircraft operations (1.91 Tg N yr -1, WSL [Mclnnes and Walker, 1992]; 1.92 Tg N yr -1, NASA [Stolarski and Wesoky, 1993a]; 2.78TgN yr-:, ANCAT [Schumann, 1995]) arises from differences in aircraft movement databases and emission calculation methodologies, and will likely be reduced in the near future. This also means that projections of future NOx emissions obtained by combining ground-based measurements with such empirical scaling relations to assess their atmospheric impact are likely to be possible with sufficient accuracy. We note that a similar inventory of emitted fuel sulfur is not available to date. Since sulfur emissions are directly tied to aircraft-related aerosol production, and because these aerosols could prove to be of global importance due to their impact on ozone chemistry and cirrus cloud interaction, such a data base could be required for future assessments.


Whereas the observed total NO~ emissions are in agreement with what has been expected, uncertainties exist concerning the partitioning of NOx into NO and NO~ at emission. Higher NO2 mixing ratios exiting the jet engines lead to an increased formation of HNO3 in the jet plume. It is established that HNO3 can dissolve in aqueous H2SO4 solutions under cold stratospheric conditions [Carslaw et al., 1994; Tabazadeh et al., 1994] or condense on ice particles [e.g., Wofsy eta[., 1990]. Related to aircraft plumes, K~rcher [1996] has shown that gaseous HNO3 can interact with the volatile plume aerosols and condense on contrail ice particles.

A recent model evaluation of the NO2/NOx ratio at the engine exit plane using data for a B 747 with CF6-80C2B1F engines taken during the POLINAT mission in 1994 point to relatively low, if any, emissions of NO2 of the order of 1% of the exhaust NO~ in the plume event at November 13 [K£rcher et al., 1997]. Further evaluation of the POLINAT field data suggests high variability of the NO2-to-NOx ratio between 6 % and more than 20 % on a molecular basis [Schumann et al., 1997]. Lister et al. [1995] reported measured NO2/NOx ratios of less than 5 % behind the RB2111 engine. First non-intrusive measurements of NO2 using a novel Fourier transform infrared (IR) spectrometer [Lindermeir et al., 1997] taken behind a Rolls-Royce M45H engine of the DLR research aircraft Attas that operated under cruising conditions, yielded a ratio of NO2 to NO~ of between 12-22 % [Haschberger and Lindermeir, 1997]. Howard et al. [1996] report ratios grouping around 10 % for similar conditions at the nozzle exit of an engine containing an annular combustor, representative of modern combustor technology, using the IR tunable diode laser technique. Further non-intrusive measurements of NO2 emissions at the exit plane of jet engines [e.g., Sch~ifer et al., 1997] are required to elucidate this issue.

Within a plume age of several minutes under daylight conditions, the emitted NOx species come into photochemical equilibrium with 03 entrained into the aircraft wake. This photochemical equilibrium assumption has been frequently made to interpret the above cited measurements of NOx and 03 in aged plumes. As long as NO values stay very high in the plume (above ~ 100 ppbV) or under nighttime conditions, entrainment of ambient 03 competes with titration by NO, which can lead to local ozone suppressions in the young plume. In summary, model results based on the current understanding of the gas phase chemistry of NOx and 03 are in agreement with the present in situ plume measurements.

Emissions of N20 from aircraft turbine engines are likely to be small. Both measurements on ground-level test stands and in altitude test cells [Wiesen et al., 1994] and values observed in various plume encounters [Zheng et al., 1994; Fahey et al. 1995b] suggest emission indices lower than N 0.1gN20 (kg fuel) -1. Although not being an important exhaust effluent, measurements of the long-lived tracer N20 are useful to determine the origin of air parcels penetrated during aircraft plume

532 Peter Fabian and Bernd Ktircher

crossings. N~O indicates the photochemical and transport histories of the exhaust air within and outside the plumes.

Several in-flight measurements of CO emission indices have been reported that show substantial variations under flight conditions for the same reasons as for NOx. Fahey et al. [1995b] presented values between 18.3 and 20.2 g CO (kg fuel) -1 taken during the ER-2 plume crossings, somewhat lower than estimates from scaling from sea level test data of the ER-2 engine, but in reasonable agreement. Values reported by Zheng et al. [1994] are about a factor 3 lower. In rough agreement, recent IR spectrometric measurements performed under selected cruising conditions for the Attas engine vary between 8 and 14 g C O (kg fuel) -1 [Haschberger and Lindermeir,

1996].

Methane emissions have been found to decrease with increasing engine powers. However, for commercial jet engines, its emission index is small under cruising conditions (less than 0.04g (kg fuel) -1, see Miake-Lye et al. [1993]; Spicer et al. [1994]; Wiesen et al. [1994]) so that no significant increases in CH4 have been detected in plume encounters. Spicer et al. [1994] published a comprehensive investigation of chemical composition and photochemical reactivity of exhaust air from one military and one commercial (CFM-56-3 used on the B 737-300) jet engine burning

JP-5 fuel.

The concentration of organic chemicals (NMHCs from C1 to C17 and polycyclic aromatic hydrocarbons) in the exhaust at idle power was 20 to 50 times greater than at higher power settings. At 80 % power setting, alkanes, dominated by methane, are by far the most significant class of emissions, followed by aldehydes that are dominated by formaldehyde. Many of the organic compounds were below the detection limit of the instruments (< 10 -2 -10 -3 ppmC), pointing to a very limited, if any, role in aerosol formation and chemistry in nascent jet plumes. Among emitted alkenes, ethene is expected to be the most abundant pollutant [Pleijel et al., 1994]. The direct influence of emitted NMHCs on plume chemistry is likely to be small, as suggested by chemical model calculations including gas phase reactions of C1 and C2 species [e.g., Zellner and Weibring, 1994; Pleijel et al., 1994; Hayman and Markiewicz, 1996]. Still, the role of aircraft hydrocarbon emissions in the upper troposphere on large spatial scales has to be studied due to their potential influence on the chemistry of 03, HOx, and NOx and on heterogeneous chemistry in the aqueous phase [Herron and Margitan, 1996]. The recent findings of Singh et al.

[1995] concerning the potential role of oxygenated hydrocarbons in determining the tropospheric HOx, NOx, and O3 cycling emphasize our incomplete understanding of the global tropospheric air chemistry.

Direct emissions of the very important hydrogen oxide radical OH determine the oxidation potential of the nascent jet plumes, where OH levels can exceed the


background mixing ratios (typically pptV) by six orders of magnitude. The chemical lifetime of exhaust OH in the plume is determined by reactions with emitted NO and NO2, and by the OH self-reactions [Miake-Lye et al., 1993; K~ircher et al.,

1996a; Hanisco et al., 1997]:

N O + O H M HNO2

NO2 + OH M) HNO3

O H + O H > H 2 0 + O

O H + O H M) H202.

For NOx emission indices around 1 0 - 15 g (kg fuel) -1, the lifetime of OH varies between 1 ms and 5 ms when the OH exit plane mixing ratio is varied between 0.1ppmV and 100ppmV. The OH self-reactions take over the dominant role for initial OH levels well above 10 ppmV. Whereas higher NOx emissions reduce the lifetime of OH, the oxidation of CO and S02 is catalytic with respect to OH [K~rcher et al., 1996a; Hanisco et al., 1997]. Chemical plume model simulations have also shown that the potential influence of HO2 and other hydrogen or oxygen radicals produced in the jet plume from emitted OH on the buildup of secondary exhaust species appears to be very limited. Direct emissions of these species are likely to be at least one order of magnitude smaller than OH, as conjectured from combustion simulations in jet engines [Miake-Lye et al., 1993]. These simulation results are in general agreement with in situ mass spectrometric measurements of HNO2 in young subsonic jet plumes [Arnold et al., 1992; 1994]. Once the NOx emission index of the source aircraft is known and HNO2 is measured, the lifetime and the amount of emitted OH can be determined by considering the NO + OH reaction, which is the only significant source of HNO2 early in the plume. Interpretation of HNOa data are more difficult due to the a priori unknown NO2 emission index. In general, these in situ observations support OH exit plane mixing ratios around 1 - 10 ppmV, which are likely upper limits because data on how much HNO2 is directly emitted by the engines are not available. With the help of a model, Hanisco et al. [1997] derived an HOx emission index of 0.35 4- 0.15 g OH (kg fuel) -1 from measurements of HOx and HNO2 in the aged plume of the supersonic Concorde [Fa- hey et al., 1995a]. Similar calculations using HO~ data measured in the ER-2 plume yield 0.06gOH (kg fuel) -1. Even higher variabilities between 0 .06- 1.54gOH (kg fuel) -1 have been reported by Schumann et al. [1997] using the field data from various aircraft/engine combinations taken during POLINAT. There are two direct OH measurements published to date. During the CIAP program, McGregor et al. [1972] measured OH mixing ratios exceeding 20 ppmV at the exit plane of a supersonic millitary aircraft engine using UV absorption spectroscopy. Using a similar technique, Howard et al. [1996] reported an upper limit number density of 1012 OH molecules per cm 3 of air.


Internal engine flow chemical simulations for various subsonic and supersonic aircraft engines [Miake-Lye et al., 1993; 1994; Anderson et al., 1996] predict OH exit plane values around 0.5 ppmV to nearly 10ppmV, which generally confirm these observations. However, no work has been published to date that presented both measured and calculated OH values together for the same engine, so that uncertainties about the precision of such complex non-equilibrium calculations remain. The nozzle exit plane of the jet engines is commonly used as a convenient interface between internal engine and external atmospheric flows. Keeping in mind the very short lifetime of OH, the distance past exit where OH is directly measured has to be known precisely and used as input for the simulation, that should also consider the external flow. Ideally, OH should be measured together with exhaust products that result from HOx-induced oxidation of NO~ and SOx at the same location. High amounts of oxidized reactive nitrogen or sulfur species result in low OH levels at a given location, and vice versa.

4.2.2 Secondary exhaust products and aerosol precursor species

Measurements of total reactive nitrogen, NOy (NOx + NO3 + HNO3 + 2 N20~ + C1ONO2 + HNO4 + ...) [Zheng et al., 1994; Fahey et al., 1995a; b], of HNO~ and HNOa [Arnold et al., 1994; Schumann et al., 1997; Schlager et al., 1997] reveal a limited potential for NOx to NOy conversion in the wake. The measurements performed in the immediate near-field yielded conversion efficiencies of the order of a few percent. POLINAT data taken at plume ages imply that less than 10 % of NOx is converted to NOy. The observations at greater plume ages (,,~ 10 min) show that non-NOx species comprise less than 20 % of emitted reactive nitrogen. These findings have been predicted, and are generally supported, by various model calculations [Miake-Lye et al., 1994; Karol and Ozolin, 1994; Danilin et al., 1994; K£rcher, 1995; Hayman and Markiewicz, 1996; Hanisco et al., 1997]. However, observations of NOx and NOy for plume ages of several hours are lacking, and conversion rates larger than 20 % cannot be excluded.

For the OH emission indices around 1 ppmV consistent with many of the observations, K~ircher et al. [1996a] calculated near-field conversion efficiencies of emitted OH to H202, an important tropospheric water-soluble species, of ~ 0.5 %. This conversion rate can reach values up to 5 % if the exhaust OH level approaches 100 ppmV. However, as stated above, most of the OH radicals are transformed into HNO2 in the jet plume. By modeling the measured HNO2, OH, and HO2 data of the Concorde and ER-2 plume crossings, Hanisco et al. [1997] have shown that this HNO2 formed early in the plume was the dominant source of HOx due to photode- composition in the wake at plumes ages of several minutes. The measured OH to HO~ ratios larger than unity was explained in terms of the high NO levels in the

The Impact of Aviation upon the Atmosphcrc 535

plume that shifted the partitioning of HOx towards OH, since species like O3 and CO that convert OH into HO2 do not increase above background values inside the plume.

Mass spectrometric in situ measurements of SO2 in the exhaust plumes of various airliners at plume ages between 0.8 s and 2 min have been reported by Arnold et al.

[1994]. Similar measurements have been made during POLINAT [Schumann et al.,

1997]. The sulfur emission indices vary for different aircraft within the expected range of fuel sulfur levels. This may indicate that most of the kerosene sulfur leaves the jet engines in the form of SOs, and that the potential for chemical processing involving SOs in the exhaust plume is limited, at least for plume ages below ~ 1 hr. The initial step in the formation of H2SO4 in the young jet plume has long been thought to proceed via the following gas phase mechanism [Stockwell and Calvert, 1983]:

SO2 +OH M) HSO3

HSO~ + O2 > HO2 + SO3

S03+n'H20 M> H2SO4"(H20)~_,.

It is currently debated whether the third reaction that produces H2SO4 involves one H20 molecule (n = 1) [Reiner and Arnold, 1993], proceeds via the water dimer (n = 2) or via an SO3 • H20 adduct and subsequent collision with a second H20 molecule [Kolb et al., 1994; Lovejoy et al., 1996]. In any case, all pathways are fast under plume conditions, and the oxidation of SOs by OH is the rate-limiting step. In the absence of SOx emissions other than SO2, this reaction sequence leads to conversion efficiencies of SO2 to H2S04 ranging from 0.5 % to 2 %, depending on the available OH and the NOx emission index [K~ircher et al., 1996a; Brown et al., 1996a]. There is strong experimental evidence that oxidized sulfur species are indeed present in nascent jet plumes. Frenzel and Arnold [1994] observed gaseous HSO~(H2SO4), ion clusters (mostly with n = 1, 2) 20m behind a commercial jet engine by mass spectrometric methods at Lufthansa's ground-level test site in Hamburg.

The conversion efficiency could be enhanced when SO3 that forms within the engine is directly emitted (cf. the discussion above), thereby bypassing the slow SO2 + OH reaction. Brown et al. [1996b] investigated numerically the extent to which fuel sulfur may be directly converted to S03 (and H2SO,) during combustion within subsonic and supersonic jet engines. Their study shows that between 2 % and 10 % of the fuel sulfur could be emitted as SO3, the major fraction of which is promptly converted to H2SO4. Indeed, several recent in situ measurements seem to support H2SO4 production rates much higher than previously predicted by plume chemistry models, and therefore render gas phase oxidation of SO2 according to the above


mentioned mechanism insufficient to explain the high mass and number density of observed volatile aerosols. First direct measurements of H2SO4 by means of passive chemical ionization mass spectrometry in the very young jet plume of an Airbus A310-300 during the German SULFUR4 mission in April 1996 allow for up to 4 % conversion efficiency of fuel sulfur to H2SO4 (F. Arnold, personal communication 1997). The particle measurements of the Concorde [Fahey et al., 1995a] point to even higher values (> 12 %). Observations behind several commercial airliners have been performed during NASA's SUCCESS mission (Subsonic aircraft: Con- trail and cloud effects special study) in April/May 1996. Fuels with different sulfur levels ranging from 70 - 700 ppm by mass have been used, and SOx to H2SOa conversion rates between 6 - 30 % have been inferred within that range [Anderson et al., 1997]. They increase with increasing sulfur level, in contrast to model predictions [Brown et al., 1996b], which either points to an incomplete understanding of sulfur combustion kinetics and modeling, or the influence of yet unknown (homogeneous or heterogeneous) chemical processes inside or outside the jet engines. K~ircher [1997b] has shown that, assuming all sulfur leaving the engines as SO3, the maximum SO~ to H:SO4 conversion efficiency in young jet plumes is limited to 50 - 60 % due to rapid plume mixing during the chemical conversion. Because possible SOa emissions are directly tied to volatile aerosol formation, we will come back to this issue in the next section.

First observations of aircraft NOx signatures, and probably also related chemical Oa changes, at the regional scale of the North Atlantic flight corridor have been reported by Schlager et al. [1997b]. In contrast to measurements inside single aircraft wakes, such regional-scale effects are difficult to identify due to variable meteorological conditions during the measurement periods. The observations were made on subsequent flights in the upper troposphere over 7 days in June 1995 during POLINAT. The probed air masses remained several days in a stagnant anticyclone in the corridor region with a mean traffic load of 770 aircraft per day, allowing aircraft exhaust NOx to accumulate from 60pptV to 140 pptV within 5 0 - 54 ° N, reaching peak values ~ 260 pptV. Calculations using a three-dimensional chemical transport model indicate that this increase cannot be explained whithout aircraft emissions. During the seven days, ozone mixing ratios increased by 70 ppbV. Part of this increase (40 ppbV) can be attributed to subsidence. (A corresponding slight decrease of H20 can be fully explained by the decreasing tropopause height.) The remaining 30 ppbV could be due to ozone production from aircraft NOx and/or due to differences in the origin and photochemical history of the air masses that entered the blocking high.

It should be noted that this is the only observation as yet which is indicative of a possible production of ozone in a flight corridor. Although global chemical models predict a several percent increase of ozone levels in the UT and LMS as


a result of aircraft emissions (see Chapter 5), observations have failed so far to detect any significant increase. Moreover, radiosonde data seem to indicate stagnant or even decreasing ozone levels at this altitude range [Logan I996; 1997]. As to whether or not this is due to CFC-induced ozone depletion in the LS and/or changes in tropopause heights and stratosphere-troposphere exchange at these radiosonde stations remains an open question.

4.2.3 Liquid volat i le aerosols

Hofmann and Rosen [1978] reported the detection of an extensive layer of small particles (radii exceeding 0.01 #m) in association with the passage of a military jet aircraft at 23km altitude. Although these authors estimated that the growing commercial aircraft fleet might increase the equilibrium particle concentrations at the flight levels, no further attention has been given to this subject for more than one decade. One of the reasons for the lack of further research at that time was the missing knowledge of heterogeneous chemical processes in the atmosphere. The discovery of strong seasonal ozone losses in the Antarctic polar stratosphere [Chubachi, 1984; Farman et al., 1985] completely changed this view. Within the upcoming aircraft-related research programs at the beginning of this decade, investigations of the impact of particles on air chemistry and climate were carried out much more rigorously than ever before.

Particle near-field measurements using the NCAR Sabreliner instrumented with various particle counters, hygrometers, and temperature sensors have been reported by Baumgardner and Cooper [1994]. Their data suggest rapid growth of particles into the detectable size range, and an increase in the number of small (sub-micron) particles with increasing distance behind the aircaft. Hagen et al. [1994; 1996] detected partially and fully soluble aerosols in jet exhaust. Together with the in situ

measurement of gaseous sulfur species in nascent jet plumes via chemiions by Fren- zel and Arnold [1994], these observations suggested the formation of new aerosols behind aircraft, likely resulting from binary homogeneous nucleation of H2SO4 and H20, and in addition to the presence of emitted, non-volatile combustion aerosols. Chemiions are charged molecular clusters formed during fuel combustion. The pos- sibility that new particles might form in situ involving oxidized sulfur gases through gas-to-particle conversion, and that these aerosols probably contribute to visible contrail formation, has also been demonstrated by nucleation models [Miake-Lye et al., 1994; K~rcher et al., 1995; Zhao and Turco, 1995].

To investigate the role of sulfur in particle formation, a series of ground tests and in situ plume observations (SULFUR 1-4-missions) was initiated by DLR Oberp- faffenhofen in December 1994, together with groups from Stockholm University,

538 Peter Fabian and Bcrnd Karcher

University of RoUa/Missouri, University of Munich, University of Mainz, MPI for Nuclear Physics (Heidelberg), and NCAR (Boulder). The strategy was to use two aircraft simultaneously, one source aircraft (the DLR Attas and several commercial airliners), and the DLR Falcon that was equipped with standard meteorological in- strumentation and a variety of particle sampling instruments. The fuel sulfur level of the jet engines of the source aircraft were held variable, and ranged from 2 ppm to 5500 ppm by mass. The SULFUR 1-4 missions demonstrated that sulfur emissions at least partly control aerosol formation in jet plumes [Schumann et al., 1996], but have only a limited impact on contrail formation [Busen and Schumann, 1995] and on the optical properties of contrails [Gierens and Schumann, 1996]. The last mission (SULFUR 4) to date, including measurements inside plumes at 80 m minimum distance behind the source aircraft, revealed expected enhancements of plume temperature and water vapor, very low concentrations H2S04 in the gas phase, and that an increase in fuel sulfur causes a shift to more particles [Busen et al., 1997]. Further data analyses and model comparisons are under way (A. Petzold, personal communication, 1997).

The particle measurements made 1994 in the plume of the supersonic Concorde cruising in the lower stratosphere over New Zealand [Fahey et al., 1995a] placed emphasis on the large number of volatile aerosols that have been detected at relatively large plume ages between 15 min and 1 hr. Assuming these aircraft-generated aerosols to consist of H2SOa and H20, which seems to be the most likely explanation due to observed dependences of volatile aerosol formation on the sulfur level, these authors concluded that at least 12 % of the fuel sulfur must have been oxidized and condensed onto these aerosols, in clear contrast to the predictions of the plume chemistry models (see above). Using a detailed chemical-microphysical model of aerosol formation constrained by the observations, Kiircher and Fahey [1997] demonstrated that the Concorde measurements can be fully understood in terms of high emissions of SO3. However, the presence of such high levels of fully oxidized sulfur gases in the nascent Concorde plume is not predicted by current chemical models that deal with sulfur conversion within the jet engine [Brown et al., 1996b; c], although the SOa/SOx ratio at the nozzle exit plane of > 0.2 inferred from the observations is close to what has been measured for gas turbines [Hunter, 1982; Harris, 1990]. Danilin et al. [1997] come to a similar conclusion, but point out that unknown chemical reactions could have been at work to explain the Concorde observation. Kiircher [1997a] presented arguments that at most a few percent of exhaust SO~ could have condensed and increased the sulfur mass of the volatile aerosols due to the low uptake probability (upper bound for the uptake coeffient of the order 10-3).

Figure 7 illustrates the current understanding of volatile aerosol formation processes in aircraft plumes. In combination with elevated SO~ emissions, binary ho-

The Impac t of Av ia t i on upon the A tmosphe re 539

mogeneous nucleation of H2SO4 and H20 is the most likely candidate to explain

the sulfur levels in the observed aerosols. The chemiions detected in young aircraft plumes [Frenzel and Arnold, 1994] could also play an important role in volatile particle formation, but their abundance cannot yet be quantified accurately enough to allow definite conclusions. Soot and sulfur interaction at emission and during wake dispersion will lead to partial or even full liquid coatings on the surfaces of exhaust

soot particles, except for fuel sulfur contents well below average.

Sulfur emissions (SO 2, SO 3)

Soot emissions

0 - 10 ms (plume age) 500 K (plume temperature)

Aerosol formation in aircraft exhaust plumes

Conversion Homogeneous

I --i .2~o. I =i .2so4/n2o l [ I / particles

] Surface chemical activation [ Coated I ' = i soo t J [ particles

10- 100ms 0.1- 1 s 1- 105...106s > 105...106s 400 - 300 K 300 - 250 K 250 K - ambient ambient

Figure 7. Schematic of the temporal evolution of aerosols in an aircraft ex-

haust plume and wake in the absence of a visible ice contrail. Reactive sulfur

gases and nonvolatile combustion aerosols (assumed to be soot) are emitted

from the nozzle exit planes of jet engines at high temperatures. H2S04 in-

creases as a result of gas phase oxidation processes, and the soot particles very

likely become chemically activated by adsorption of S03 and H2S04, leading to

the fomation of a liquid H2SO4/H2 0 coating. Upon further cooling, volatile liq-

uid H2SO4/H20 aerosols are formed by binary homogeneous nucleation. These

aerosols, which may take up HN03 to form ternary mixtures, grow in size and

interact with the coated soot particles by coagulation in the growth stage of the

diluting plume and eventually are scavenged by background aerosols at longer

times. The scavenging timescales are highly variable and depend on exhaust

and background aerosol size distributions and abundances and wake mixing

rates (in the case of scavenging by exhaust soot). Significant perturbations of

the background aerosol size distributions on larger spatial scales can be expected

for low ambient aerosol loadings (< 10 cm-3), where the scavenging loss times

extend from days to weeks. From K~ircher [1997a].


Airborne observations of aircraft volatile particle emissions within NASA's SASS program have been carried out in April/May 1996 [Thompson et al., 1996] during the Subsonic aircraft: contrail and cloud effects special study (SUCCESS). In its goals similar to the German SULFUR missions, but with extended experimental facilities, this campaign aimed to gain a better understanding of volatile particle and contrail formation processes and their impact on radiation. In addition to the studies mentioned above, also an extensive data set was obtained shedding light on cirrus-related open issues, as reported by various SUCCESS investigators at the 1997 NASA AEAP symposium in Virginia Beach. First results reported by Anderson et al. [1997] and Twohy and Gandrud [1997] support many findings reviewed in this section, increasing our understanding of the nature of particle formation by providing a broad data set awaiting to be analyzed by plume aerosol models.

4.2.4 Non-vola t i le combustion aerosols

As a result of incomplete fuel combustion, jet engines generate non-volatile, carbon- rich particulate matter (soot) [Wagner, 1978; Goldberg, 1985]. Soot forms in hot regions in the combustion chamber in patches of low 02, where the aromatic hydrocarbons in the kerosene are not completely broken down to CO2. Such patches exist within the engine because the liquid fuel is sprayed into the heated air and mixes via turbulent diffusion, resulting in spatial inhomogeneities. Eventually, the engines emit 15 - 30 nm-sized (radius) nearly spherical soot particles that may be composed of even smaller spherules. These processes lead to number densities of soot particles at the nozzle exit planes in the range 5 × 105 cm -3 to 5 × 107 cm -3 (typically 10~cm-3), as demonstrated by various exhaust aerosol measurements [Rosen and Greegor, 1974; Hallet et al., 1989; Pitchford et al., 1991; Hagen et al., 1992; 1994; Whitefield and Hagen, 1995; Schumann et al., 1996; Petzold and SchrSder, 1997]. The latter work presents the most detailed characterization of the black carbon particles in jet engine exhaust available to date.

These and other observations demonstrated that jet engine-generated soot aerosols are clearly distinguishible from the ambient background. The recently measured near-field soot abundances and sizes at cruise are consistent with typically 0.05 - 0.1 g soot per kg fuel. Extreme values of emission indices are reported as 0.001 g (kg fuel) -1 [Pitchford et al., 1991] and 0.5 g (kg fuel) -1 [Schumann et al., 1996], probably mainly due to different sampling methods and engine operation conditions. There are indications that some Airbus engines produce less soot particles by mass but more by number than the older Attas engines [Busen et al., 1997]. The available ICAO smoke numbers, derived from optical absorption or filter measurements, allow to derive a crude estimate for the total mass of emitted,


non-volatile particles per mass of fuel burnt at a given power setting. However, microphysical properties for use in nucleation calculations cannot be derived from these data. Since these methods yield no information about what types of particulates contribute to light absorption or filter mass loadings, other particulates than soot (e.g., metal particles, see below) might influence the smoke number, in which case it cannot be simply interpreted as being proportional to the soot emission index. A spherical geometry is usually assumed for the soot particles in the plume models, but deviations from this idealization due to fractal nature of soot tend to increase the specific soot surface area.

Measured black carbon soot abundances at 10 - 11 km altitude are found to covary with commercial air traffic. Aircaft fuel combustion at cruise altitude and vertical transport up to 20km is the principal source for soot in the LS [Pueschel et al., 1992; Blake and Kato, 1995]. The soot concentrations have been found to be highly variable at northern midlatitudes, with mixing ratios ranging from 6 x 10 -a - 3.3 ngm -a. These aircraft-borne measurements using wire impactor collectors do not detect, however, soot that has been already entrained in larger aerosols droplets, so that they likely underestimate the total soot abundance from aircraft emissions. (Scavenging of soot by stratospheric aerosols evolves on the timescales of days to weeks and is much faster than the residence time of dry soot in the same altitude region.) Assuming 20 % of the subsonic aircraft emissions to occur in the lower stratosphere (which is a low estimate, see section 2.3) and a 5 % / yr increase of fuel usage (likely an upper limit) that is translated to the same percentage increase in soot emissions, Bekki [1997] could reproduce some features of the measured soot distribution with the help of two-dimensional model calculations. However, the few soot measurements available to date and the large observed variability of its abundance render a detailed comparison with the calculations difficult and do not allow do draw definite conclusions about the ability of current global models to predict the global (future) distribution of soot.

Unfortunately, very little information exists concerning the chemical reactivity mad surface morphology of soot emitted at cruising altitudes, although this information is essential to evaluate the impact of the combustion aerosols on heterogeneous plume chemistry, cirrus formation, and interactions with the stratospheric aerosol layer. Soot fresh from jet engines is probably hydrophobic, but the particles very likely become activated on their way downstream of the aircraft engines by deposition of water soluble species present in the exhaust. Irregular surface features can also increase the reactivity and amplify nucleation processes. Measurements indicate a large range of C C N / C N ratios of soot varying from 1/1000 [Pitchford et al., 1991] to 1/3 [Whitefield et al., 1993], which probably reflects differences in sampling and analysis methods. However, as we will address below, the ice forming ability of exhaust soot seems to be quite high.


A few measurements shed some light on the interaction between exhaust gases and soot. Wyslouzil et al. [1994] observed hydration of carbon aerosols under water- subsaturated conditions after treatment with gaseous H2SO4. This increase of H20 adsorption after treatment with H2SOa is in qualitative agreement with an analysis of the wetting behavior of graphitic carbon under plume conditions [K~ircher et al.,

1996b]. These authors also performed laboratory experiments to determine the contact angle (64 °) of a H2SO4/H20 droplet (50 wt-% sulfuric acid) on a model soot particle. The contact angle was found to increase by about 10 ° after the graphite surface became more hydrophilic due to exposure to OH radicals, with doses expected to be present in nascent jet plumes. Their experiments also strongly suggest that liquid H2SO4/H20 solutions with immersed carbonaceous substrates can be supercooled to very low temperatures (191 K) without heterogeneous nucleation of H2SOa hydrates, which further stresses that water ice particles comprise the most prominent particle type present in contrails.

Whitefield et al. [1993] reported a correlation between soluble mass fractions found on fresh soot taken a few meters behind jet engines with the fuel sulfur content. The measured soluble mass fractions typically scattered around 8 % per soot particle. This is in agreement with ground-based measurements behind the Attas engine that revealed a decrease of the sulfate mass fraction per soot particle from 70 % at very low (10 %) to 10 % at high (70 %) power settings [Petzold and Schr5der, 1997]. The sticking probability of SOs on amorphous carbon is too small to be relevant [Rogaski et al., 1997], but SO3 and H2SOa easily stick on soot. High emissions of SO3 as suggested by recent observations and combustion models [Brown et al.,

1996b], indeed lead to water-soluble (sulfur) mass fractions in the range between 0.7 - 14 %, depending on jet mixing properties and soot surface parameters, as shown by K~ircher [1997b]. If no SO3 is emitted, the total sulfur mass fraction stays between 0.1 - 1%, contradicting the observations of Whitefield et al. [1993]. Sulfur molecules, however, could already be taken up in the combustor, but this fraction is currently unknown, although it is likely to be small due to the high temperatures within the engines. K~ircher [1997b] has also estimated that only a minor fraction of the emitted SO3 (less than 8 %) and H2SO4 (less than 30 %) can be removed by adsorption on soot prior to binary homogeneous nucleation, indicating that volatile gas-to-particle conversion is not limited by heterogeneous nucleation of H~SO4 and H20 on soot, in agreement with near-field aerosol measurements.

Rogaski et al. [1997] measured the uptake of NO2, HNO3, SO2, and Oa on amorphous carbon at room temperature and found uptake coefficients of 0.11, 0.038, 0.003, and 0.001, respectively. The identified products were NO for the NO~ uptake; NO, NO2, and H20 for the HNOa uptake; and 02 for the uptake of 03. No product was observed during SOs and soot interaction. These measurements suggest that neither of the species can serve as candidates for soot activation in the


nascent jet plume (e.g., prior to contrail formation), making H2SO4 (and possibly also SOn) the most likely exhaust species to activate soot for water condensation. Unless exhaust soot is characterized by very large surface areas, efficient processing of SO2 and 03 on soot within the lifetime of single aircraft plumes (of the order of a day or less) is unlikely according to model estimates [K/ircher, 1997a]. Rogaski et al. [1997] also found an enhanced water uptake on amorphous carbon that has been exposed to HNO3, SO2, and H2SO4, while treatment with NO2 and 03 did not influence the hydration properties.

The formation of a liquid coating which, on the basis of these investigations, likely consists of H2SO4 and H20 and possibly other soluble components, seems to be required to explain the formation of visible contrails. However, as yet it cannot be demonstrated conclusively how soot particles are activated under plume conditions This topic will be discussed in more detail in the following section.

Metals and metal oxides comprise elements like A1, Mn, Cr, Fe, Pb, and V and are estimated to be present at the parts-per-billion level at the nozzle exit planes [Stolarski and Wesoky, 1993b]. Given this molar fraction, the number of emitted metal particles is much less than soot, which limits their role as potential cloud or contrail forming agents. They are also probably negligible compared to background stratospheric metal oxide particles resulting from meteoritic impact and spacecraft debris. However, this abundance and the possible perturbation level of the background abundances is very uncertain. It is based on measurements during the CIAP studies and correspondence to more modern engines is teneous. The metal contents of crude oils would result in lower exhaust molar fractions [CIAP, 1975; Table 2.2]. Potentially more important could be the (time-dependent) release of metal particles due to engine erosion [Stolarski and Wesoky, 1993b]. Based on the various exhaust particle and hydration analyses in the combustors and directly behind the nozzle exit planes in the hot plumes (see above list of references), it is conceivable, however, that by far the greatest contribution by number to the non- volatile particle mass stems from carbon-containing soot that condenses soluble species during and after combustion.

4.3 Contrai l s

Research on contrail formation dates back to 1914 and since then, many expla- nations have been given of how and under which conditions contrails form in the atmosphere. Schumann [1996] gave a comprehensive review of this topic. However, detailed contrail observations and numerical simulations of contrails are still scarce, and only recently contrails have been recognized as an important research topic due


to their potential impact on cirrus nucleation and the tropospheric radiation budget. This adds to the general research on the role of cirrus and other clouds in the climate system [Crutzen and Ramanathan, 1996]. Here, we concentrate on contrails formed behind jet aircraft at typical cruising altitudes.

The formation of contrails is due to isobaric mixing of the hot and humid exhaust with colder and less humid ambient air. For a given aircraft, pressure altitude, and ambient relative humidity, one can, by thermodynamic means, derive a relation- ship that yields the minimum ambient temperature below which a contrail forms (threshold condition). Typically, temperatures below 230K are required at the flight levels 8 - 12 km of subsonic aircraft; for supersonic airliners (high speed civil transport, HSCT) cruising in the LS between 16 - 22 km, temperature thresholds are lower because the air is much dryer. Contrail particles consist of ice crystals, and old, persistent contrails very much resemble natural cirrus clouds. Initially very localized, the cirrus contrails spread horizontally by diffusion processes (much less in the vertical direction) and become distorted by wind shear.

Soot is expected to play an important role in the formation of visible ice contrails under threshold formation conditions. K~ircher et al. [1996b] studied the evolution of optical depth and ice water content of the contrail observed by Busen and Schumann [1995] during the SULFUR 1-mission. A lower bound ,,~ 104 cm -3 for

the number density of ice particles initially present in jet plumes at the onset of freezing has been deduced. In a recent study, Brown et al. [1997] supported this view by making use of their plume nucleation and condensation model applied to the Attas case. This lower bound for the abundance of contrail ice is in quite fair agreement with ice particle number densities measured in young contrails at plume ages from 10s [Baumgardner and Cooper, 1994] up to 2 - 3rain [Strauss and Wendling, 1997]. Together with the estimated mean radius of the ice particles in the range 0.3 - 1 #m, this serves as a visibility criterion for young contrails. The visibility analysis revealed that, under threshold formation conditions, soot must have been involved in the contrail formation process if the visibility criterion is to be fulfilled. Background aerosols very likely play only a minor role in contrail formation. The contrail became visible slightly below liquid water saturation in the plume, although it cannot be ruled out that the plume actually was slightly supersaturated with respect to liquid water due to a possible uncertainty in the measurement of the relative humidity and temperature. Based on a greater number of similar measurements performed during the SUCCESS mission, Jensen et

al. [1997] came to a similar conclusion, namely that the onset of visible contrail formation occurs around liquid water saturation in the plume.

The fact that all contrails observed to date were generated close to the liquid water saturation threshold strongly suggests that the fresh soot particles do not act as


direct ice (deposition) nuclei in the exhaust, i.e., their surfaces are not well suited to initiate the direct gas-to-solid (ice) phase transitions. Rather, ice formation can be explained by freezing of a liquid H2SO4/H20 coating at temperatures below about 230-235 K. The fresh soot particles acquire this coating by interaction with sulfuric acid produced in the plume. It is unclear, although likely, whether the ice nucleating ability of soot, that once participated in contrail formation, would be enhanced compared to that of fresh exhaust soot.

The study of K~cher et al. [1996b] also suggested that under the high saturation levels (i.e., 1.5 - 2.5 with respect to ice saturation) that are reached in cooling jet plumes, many, if not all, of the soot particles are activated and nucleate water ice. The observed contrail behind the Attas aircraft was explained to result from heterogeneous freezing of water ice within a liquid coating of H2SO4 and H20 around the soot particles. Especially, at threshold heterogeneous freezing was found to be fast enough to prevent homogeneously nucleated H2SO4/H20 droplets in the plume from freezing, thereby offering an explanation why the observed contrail formation did not depend on the fuel sulfur content. (Both very low 0.002 g S and average 0.26gS per kg fuel have been used in the experiment.) If homogeneous freezing of (part of) the volatile H2SO4/H20 aerosols would be the dominating mechanism, one would expect a very strong variation of the ice formation with the fuel sulfur content, which was not observed for sulfur levels below average. A subsequent analysis that includes possible emissions of SO3 of the Attas engines showed that, assuming an SO3 to SOx exit plane ratio of 0.1, the soot particles could have acquired a sulfur mass fraction of 2.3 % prior to freezing, likely enough to create a H2SO4/H20 coating.

At very low fuel sulfur contents, the presence of concave surface features or hindered diffusion of molecules in small cavities on the soot surfaces can also be invoked as possible causes for a (partial) coating that lead to the observed ice formation [K~ircher, 1997b]. The present observations do not allow to decide unambigously which mechanism causes freezing.

A follow-up experiment during the SULFUR3 campaign using average and very high S levels (5.5gS per kg fuel) actually showed a visible difference in contrail onset [Schumann etal., 1996], that could be explained by the changing ice particle optical properties due to freezing of more and correspondingly smaller particles [Gierens and Schumann, 1996], presumably soot. Well below threshold formation conditions, at temperatures below 215K, contrail ice particles are predicted to result from both homogeneous and heterogeneous freezing of solution droplets.

The spatial growth of individual contrails between 1 rain and 1 hr after emission has been investigated by Freudenthaler etal. [1995] using a ground-based scanning backscatter lidar. These observations confirmed model calculations of contrail J~l~ 2Z-6-B"

546 Peter Fabian and Bernd Kfircher

spreading, demonstrating a much faster horizontal than vertical growth of the contrail's cross-sectional area. In particular, the impact of shear motion on the evolution of contrails is clearly visible. On the basis of their measurements, Freudenthaler et al. [1995] suggested a general classification of contrail area growth into two categories, namely slow (fast) growth correlated to low (high) vertical shear and low (high) relative humidity. Using the same optical lidar techniques, Freudenthaler et al. [1996] performed depolarization measurements of young contrails at different atmospheric temperatures and humidities. The results suggest that more but smaller ice particles are formed at the lower temperature. The depolarisation of the ice particles has been found to increase with contrail age, and several reasons to explain this are offered by the authors.

Ice crystal sizes and optical depths of thin cirrus clouds and contrails have been retrieved by comparison between remote sensing data using NOAA AVHRR bright- ness temperatures in the infrared and one-dimensional radiative transfer calculations [Betancor-Grothe and Grassl, 1993]. The analysis showed that the measured radiative properties of young contrails are consistent with the model results only if crystal sizes smaller than those given for natural cirrus are adopted for the calculations. The observed continous transition in the optical behavior between cirrus clouds and contrails suggests that persistent contrails likely develop towards cirrus clouds in the course of time.

Contrails are composed of more, but smaller ice crystals than nearby natural cirrus clouds. This is consistent with lidar and infrared radiometer measurements by Gayet et al. [1996] and retrievals of particle size and optical depth in contrails located above cirrus by Duda and Spinhirne [1996]. Analyses of in si tu measurements using an ice replicator (an instrument based on the impactor principle) performed by Strauss and Wendling [1997] and Strauss et al. [1997] revealed that contrail ice particles at a contrail age of about two minutes had typical sizes of 5 - 10#m, with shapes usually only slightly apart from spherical, indicating frozen solution

droplets.

Bakan et al. [1994] evaluated the contrail frequency over Europe from NOAA satellite images. According to these authors, the average contrail coverage exhibits maximum values of almost 2 % along the North Atlantic flight corridor around 50 ° N, and around 0.5 % over western Europe. A significant decrease of contrail cloudiness has been found over continental western Europe and a likewise significant increase over the North Atlantic (between March and July). Although satellite images are well suited to study cloud occurrences because of their regular availability for large areas, the inferred degrees of coverage likely indicate lower limits for the aircraft impact on cloudiness. This is because only linear cloud features are recognized


by the analysis. An unknown fraction of the persistent contrails may develop into diffuse cirrus clouds and are therefore not counted as contrails.

First numerical 2D calculations of persistent contrails, using LES including sim- plified ice microphysics and radiative transfer, have been published by Gierens [1996]. These simulations indicate a limited role of radiative processes within contrails in controlling both, its macroscopic spreading and microscopic ice growth. The turbulence generated by the vortices have been found to be sufficient to explain observed spreading rates, and the lifetime of contrails have been found to be largely controlled by the humidity and temperature of the air surrounding the contrail. A similar conclusion has been reached by Boin and Levkov [1994]. Apply- ing a parameterized cloud model to contrails, these authors found that the local ambient humidity is a controlling factor for crystal size, total ice water content, optical depth, and sedimentation velocity of contrail particles. They also found a (nearly) exponential increase of the contrail lifetime with increasing relative humidity. Sedimentation of ice crystals becomes important only when contrails are formed and persist in strongly supersaturated air, allowing the initially small (i.e., micron-sized) ice particles to grow rapidly by vapor deposition.

4 . 4 S u m m a r y o f b a s i c o b s e r v a t i o n s

From these observations, supported or predicted by models, the following picture of aircraft-induced gas phase chemical processes and particle formation up to the regional scale emerges.

Aircraft exhaust gases are dumped into the atmosphere in hot jets that dissipate by turbulence. The reactive flow is organized in the form of individual jets and vortices within the first few minutes after emission and causes a spatially inhomogeneous wake distribution of exhaust constituents and temperature. Strong dilution during the jet regime and mixing suppression during the vortex regime are important features in this phase. Afterwards, strong wind shear is mainly responsible for the further horizontal spreading and decay of aircraft plumes. The lifetime of single plumes depends on the exhaust species considered and on the respective background conditions, and may range from a few hours up to several days.

Databases for air traffic movements and inventories for fuel use and NO~ emissions have been established and validated. Emissions of nitrogen oxides at altitude can be fairly well predicted by scaling empirical, ground-based values. Oxidation products from NO and SO2 are also emitted, together with HO~ radicals that cause the buildup of acids within the engine or in the nascent jet plume. Within the timescale of a few hours, the oxidation of exhaust NO~ to NOy in flight corridors is limited.

548 Peter Fabian and Bernd K~lrcher

Emissions of oxidized sulfur in the form of SO3 play a central role in volatile aerosol formation and soot activation. Aircraft emission signatures in the form of aerosols, NOx, and other species, as well as in the form of visible, persistent contrails are clearly observable on the regional scale. Ozone production due to aircraft NO~ emissions in the upper troposphere likely occurs on timescales of days. However, such ozone changes have not conclusively been determined as yet.

Volatile sulfuric acid droplets and soot constitute the major fraction of the young exhaust aerosol. They are modified in situ due to condensation and coagulation among themselves and with entrained background aerosols during plume cooling and mixing with ambient air. The soot particles may acquire a liquid coating (presumably composed of sulfate) due to adsorption, heterogeneous nucleation, and scavenging of volatile H2SO4/H20 droplets.

Ice particles in young contrails, resulting from freezing of a subset of these primary aerosol components, are more numerous and smaller than ice crystals in cirrus clouds. A fraction of the contrail ice particles contains soot inclusions, depending on ambient temperature and humidity, and fuel sulfur level. The lifetime and spatial coverage of contrails is highly variable and depends mainly on the evolution of relative humidity and wind shear. Ice supersaturations over large regions in the upper troposphere are required to produce persistent contrails and to transform them into cirrus clouds.

5 P R E D I C T E D E F F E C T S A N D T H E I R

U N C E R T A I N T I E S

The observed and confirmed effects reviewed in the last chapter have largely focused on small- and regional-scale phenomena which were directly accessible to measurements. In contrast, predicted effects arise mainly from a hierarchy of model calculations on all spatial and temporal scales, and partly also from the interpretation of observations. Many of the predicted effects discussed have been investigated with global models, mainly focussing on the impact of NOx emissions from subsonic and supersonic aircraft. Only few global studies to date specifically address the sulfur (or particle formation) issue. In what follows, we will review the results of these studies concerning aircraft-induced chemical and climate perturbations, highlight some of the model uncertainties, and give a brief summary of important model predictions.

5.1


Chemica l effects

549

Complementing the observations of increased NOx levels in the upper troposphere as discussed in the previous chapter, there is strong modeling evidence that the abundance of NOx in the UT has increased significantly in the northern hemisphere as a result of subsonic aircraft operations [Hidalgo and Crutzen, 1977; Derwent, 1982; Beck et al., 1992; Johnson et al., 1992; Kasibhatla, 1993; Fuglestvedt et al., 1993; Hauglustaine et al., 1994; Velders et al., 1994; Ehhalt and Rohrer, 1994; Brasseur et al., 1996; Kraus et al., 1996; KShler et al., 1997; van Velthoven et aL, 1997; Wauben et al., 1997; Stevenson et al., 1997]. In general, nitrogen oxides are found to be almost 10 times more abundant in the northern than in the southern hemisphere, pointing to a strong anthropogenic contribution. These results, obtained from a variety of global transport models of different degrees of complexity and self-consistent description of chemical and dynamical processes, indicate a seasonally dependent increase of the NOx abundance by up to ~ 20 - 80 % in this region due to subsonic aviation. This increase in NOx results in modeled ozone increases between ~ 2 - 12 %, the higher values reached during summer due to enhanced photochemical activity.

On the basis of current subsonic airtraffic growth scenarios, NOn and 03 levels will increase even more in the future. Brasseur et al. [1996] report that in the year 2050, NOx emissions (at 10 km altitude) could cause a doubling of the present ozone levels. However, the exact magnitude of this increase (besides its well-known seasonal and latitudinal components) remains uncertain and depends on the level of background NOx and the balance between its poorly quantified sinks and sources [WMO, 1995; Lamarque et al., 1996]. Especially, the relative contribution of lightning to the total upper tropospheric NOx budget must be better quantified [Levy II et al., 1996; Penner et al., 1997], more knowledge about rapid convective transport of polluted surface air to cruising levels is required [Flat0y and Hov, 1996; Strand and Hov, 1996; Berntsen and Isaksen, 1996], and cross-tropopause exchange processes must be better represented in the calculations [Velders et al., 1994].

Applying a mesoscale chemical transport model, Lippert [1996] and MSllhoff [1996] have demonstrated that the chemical conversion of NOx emitted in the tropopause region depends sensitively on the emission time (photochemical activity) and altitude (relative location of the tropopause), the abundances of background trace gases (efficiency of ozone production mainly through the NOx levels), and the general synoptical situation (transport processes into chemically different regions). The latter point indicates that atmospheric models must consider both tropospheric (hy- drocaxbon) and stratospheric (halogen) chemistry to assess the impact of aircraft at these altitudes properly. To date, the impact of emitted NMHCs resulting from incomplete jet fuel combustion on ozone production in the airlanes has not been


satisfactorily addressed, but is believed to be small primarily due to the comparably low emission indices ITie, 1994], see section 4.2.

Current 3D global chemical models for the troposphere are of high complexity and contain a wealth of individual processes, but concerning chemical and microphysical processes they are still far from the comprehensiveness that has been reached with 1D or 2D models due to exceedingly large computational demands. Although various parameterizations for dry and wet deposition processes are used in models of the global troposphere that are employed to assess the chemical impact of subsonic aircraft, none of them can make use of fully consistent background aerosol distributions for calculating heterogeneous chemical reaction probabilities (of e.g., N205 and NO3 on sulfate particles). This is mainly due to the almost complete lack of a consistent tropospheric aerosol climatology and our incomplete knowledge about the role of small particles in the UT. In addition to the relative importance of the (not well quantified) various tropospheric NOx sources, the pronounced natural, interannual variability of upper tropospheric and lower stratospheric O3 [Logan, 1994] and the poorly quantified, localized convective activity [Arnold et al., 1997] further render extraction of the pure aircraft effect on ozone more complicated. The results from current models attempting to include heterogeneous tropospheric processes [Dentener and Crutzen, 1993] (mainly the N205 hydrolysis) indicate a substantial influence of heterogeneous reactions involving NO3 and N20 on concentrations of NOx, 03, and OH. These authors find ozone reduction by about 25 % in the northern hemisphere subtropics and at midlatitudes in a simulation including heterogeneous chemistry compared to a simulation neclegting it.

Recently Dameris et al. [1997] reported results on the impact of NOx emissions on atmospheric ozone, using the global 3D climate model ECHAM 3 coupled to a detailed chemistry module. The chemistry is treated off-line in these calculations, i.e., there exists no feedback of chemical species on radiation and dynamics. The investigation suggests that present day subsonic aircraft have caused an increase of tropospheric ozone by 3-4 % in the northern hemisphere, and no significant increase in the LS. An ozone increase near the cruising levels of future (2015) subsonic air traffic of ~ 15% is predicted, accompanied by an ozone reduction in the range 3-4 % in the LS due to supersonic aircraft operations. The ozone decrease is even stronger in polar winter due to the impact of heterogeneous chemistry related to polar stratospheric clouds.

The impact of HSCTs on stratospheric ozone has been investigated with models since the 1970s, and the model predictions changed several times according to improved understanding of atmospheric processes, ranging from severe global ozone depletion up to the year 1975 and between 1982 to 1988 to slight ozone production around 1978 [Wuebbles and Kinnison, 1990; K~ircher and Peter, 1995]. When only


gas phase reactions are considered, supersonic aircraft scenarios currently predict around 10 % decreases of the ozone column densities due to the increased importance of the NOx and HOx catalytic cycles [Johnston et al., 1989]. It should be noted, that the location of injection mainly determines the response of global models to aircraft perturbations [Considine et al., 1995; Plumb et al., 1995]. The high sensitivity of the ozone response to emission locations suggest that, in general, the model transport processes might be a source of considerable uncertainty for predictions of aircraft-induced ozone depletions. The calculated O3 losses are larger for higher NOx emission indices and for higher cruising altitudes because more emitted material is deposited in regions with longer residence times.

The inclusion of the heterogeneous N205 hydrolysis on stratospheric background aerosols in global models (see section 3.2.2 and Figure 5) has dramatically changed the ozone response to NOx emissions from HSCTs [Weisenstein et al., 1991]. Taking this reaction into account results in a less dramatic reduction of the ozone column of less than 1% [e.g., Bekki et al., 1991; Weisenstein et al., 1991; 1993; Considine et al., 1995].

As for the background atmosphere, the inclusion of the heterogeneous N205 hydrolysis reaction on the global sulfate aerosol layer removes most of the capacity of the emitted nitrogen oxides to destroy ozone and therefore decreases the sensitivity of O3 to the NOx injections. Decreasing aerosol surface area densities and background chlorine levels increases the sensitivity of O3 to the NOx perturbations. With the N205 reaction pathway included in the models, supersonic aircraft scenarios predicted only minor local ozone losses and even a slight increase in ozone column (including the ozone increases due to growing subsonic alrtraffic). This is because, although additional aircraft NOx still increases the ozone-depleting efficiency of the NOx-reactions, this effect is overcompensated by the concomitant decrease of the other (especially the dominating HOx) cycles.

With additional aircraft NOx injections in the LS, decreases in the chlorine (through NOx/C1Ox-coupling reactions like C10 + NO) and hydrogen (through NOx/HOx- coupling reactions like HO2 + NO, followed by NOx to HNO3 conversion) radical budgets tend to compensate for increases in the ozone removal rate due to additional aircraft NOx. The change in HOx from the emission of H20 is complicated and depends on the amount of NOx deposited by the fleet and on the fate of odd oxygen (i.e., 03 and O). Models predict either a slight decrease or increase of lower stratospheric HO~, whereas in the upper stratosphere, HSCTs induce a net HOx increase from H20 emissions.

All models show consistently that CO emissions have no significant effects on global ozone and that more ozone is depleted with increasing height of injection. However, there still exist considerable difficulties in calculating ozone depletion in the LS, as


pointed out by Considine et al. [1992] and Jones et al. [1993], due to uncertainties in kinetic reaction and photolysis rates of relevant chemical reactions. Simultanenous in situ measurements of key radical species, along with modeling, may provide an important tool for resolving at least part of these problems [Wennberg et al., 1994; Jucks et al., 1996; Wennberg et al., 1997].

The partitioning of the odd nitrogen species in the LS, and hence also the sensitivity of Os to NOx emissions by aircraft, changes when the surface area density of the background sulfate aerosols changes [Weisenstein et aL, 1993]. The background aerosol layer is sustained by oxidation or photolysis of SO2, eventually leading to H2SO4 and subsequent incorporation into sulfate aerosol particles and nucleation. In earlier modeling studies, that work addressed the impact of the aircraft sulfur emissions on a global scale. Gaseous SOs emitted by the jet engines was instanta- neously mixed into a model grid box, just as this is usually done for NOx and H20 emissions. Global models taking into account emissions of SO2 and stratospheric sulfur chemistry suggest that both the current subsonic and the planned supersonic fleet represent substantial sources for aerosols (increases between 50 % and 100% of their surface area density) in the northern hemisphere LS, which in turn lead to an enhanced ozone loss due to resulting higher levels of active BrOx, CIO~ and HOx [Bekki et al., 1992; Bekki and Pyle, 1993; Pitari et al., 1993; Tie et al., 1994].

First attempts to include in situ H2SO4/H20 particle production ("plume/wake processing") in a 2D assessment model [Weisenstein et al., 1996] revealed the new aspect that these additionally enhanced aerosol surface area densities are potentially more significant to Os than the concomitant NOx emissions, especially when nitrogen oxide levels are reduced in future jet engines. The model predicts background aerosol surface area density increases by a factor 2 - 3 if significant (> 10 %) amounts of the emitted SOx molecules are converted to H2S04 nucleating homogeneously in the plume. The resulting higher aerosol loading leads to a pronounced decrease in ozone column densities as compared to earlier assessments due to the greater relative importance of the catalytic C1Ox depletion cycles. E.g., in the winter northern hemisphere at midlatitudes, the ozone column reduction is 1.6 % when all sulfur emissions are introduced as particles compared to a reduction of only 0.2 % when sulfur emissions are completely neglected. For a fixed chlorine background level, the greatest impact from the sulfur emissions is found when NOx emissions are small. The possible future volcanic activities would make the impact of aircraft emissions variable as well. In a follow-on study, using the Concorde particle measurements to place constraints on the conversion of fuel sulfur to volatile aerosols, Danilin et al. [1997] calculate 0.75 - 1.1% as the range of the annually averaged 03 column depletion at 40 - 50 ° N. The global ozone response is found to be more sensitive to the SOa/SO× partitioning at emission than to the wake dilution rate.


An issue which has not been addressed adequately by models is the possible chemical impact of the emitted soot particles. It may be of considerable importance that heterogeneous reactions on soot surfaces may occur through synergistic chemical effects among various trace gases emitted or built up in aircraft exhaust plumes. Once entrained in the background aerosols, it is conceivable that the soot cores could induce heterogeneous freezing of nitrogen- or sulfur-containing hydrates or pure water ice in the surrounding solution much more efficiently than via homogeneous freezing nucleation. This will certainly depend on the detailed temperature history of the air parcels. A first at tempt to investigate the possible role of aircraft- generated soot in the midlatitude ozone depletion has been made by Bekki [1997] using a stratospheric 2D global model with explicit sulfur chemistry and particle microphysics. Two heterogeneous reactions on soot have been considered, a direct ozone destruction reaction and an HNO3 to NOx reduction reaction, both working also in darkness. Although the model calculations based on this ozone-depleting mechanism have the potential to explain the observed ozone depletion in the LS at northern latitudes, unspecified reaction products of the soot-induced reactions, unknown chemical reactivity of aircraft soot, and uncertainties in soot emission levels and surface area densities render the model predictions uncertain.

Aircraft-induced perturbations of polar chemistry at low (< 200 K) temperatures deserve special attention. Heterogeneous reactions involving C1ONO2 and HC1 appear to exert only a small influence on ozone on an annual average basis, but this might not hold when also the influence of PSCs and in situ produced aerosols are included. The considerations discussed above do not take into account the possible effects of chemical processing on PSCs. HSCT emissions (gaseous H20 and NOx, which eventually form HNO3) might raise the threshold temperature for the nucleation of type I PSC by 1 - 2 K over current conditions, which would double zonally averaged type I PSC frequencies at 70°N at 30 hPa and 50 hPa levels and increase the corresponding type II PSC frequencies even more dramatically [Peter et al., 1991]. More frequent occurence of NAT particles could exacerbate the ozone depletion by additional chlorine activation, especially for high-flying (22 - 24 km) aircraft. On the other hand, excess CIO due to enhanced PSC processing could combine primarily with added aircraft NOx to form C1ONO2, which could atten- uate ozone depletion when the occurence of PSCs cannot be sustained sufficiently long. In this regard, PSCs would act similarly to sulfate aerosols.

Presently, the PSC parameterizations used in the global models appear to be very crude and have to rely on external temperature climatologies to estimate temperature deviations and consistent cooling rates around the zonal means in order to account for the extremely strong dependence of the rates for polar heterogeneous processes and PSC formation on these factors. In addition, even if these models would be further developed and an HSCT fleet would be in service, the very limited


set of observational data about large-scale CIO and C1ONO2 distributions would not allow to fully validate the predicted chemical processing of PSC-containing air. Including the effect of PSCs (but not the impact of fuel sulfur), the mid-winter atmosphere at northern high latitudes responds with a 1% O3 column decrease, whereas in late spring an increase of column ozone is found [Grooss et al., 1994]. Global depletion is reduced slightly with PSCs included, but the emissions cause enhanced 03 reduction in the southern hemisphere in winter [Considine, 1994]. Finally, whereas Tie et al. [1994] observed a substantial increase in PSC surface area densities and therefore a great ozone depletion potential in the late winter northern hemisphere, Pitari et al. [1993] predict much less impact and even ozone production due to enhanced growth and sedimentation of HSCT-influenced PSCs. This range of different predictions clearly demonstrates the current inability of the global models to resolve this issue.

The detailed heterogeneous chemistry of liquid aerosols, ice crystals, and soot particles in expanding aircraft wakes and contrails is virtually unexplored. It is crucial to learn more about how subgrid plume processing modifies the aerosol particle spectrum, concerning number density, radial distribution, and chemical composition. There is modeling evidence that in very young aircraft plumes HNO3 can dissolve in newly generated H2SO4/H20 droplets to form ternary solution droplets [K~ircher, 1996]. Hence, it will also be important to find out if and which heterogeneous reaction rates will be altered by the presence of dissolved HNO3. New heterogeneous reactions that appear to be of global importance might also have to be considered in aircraft-related research. For instance, uptake of HOx on SSAs, with or without soot inclusions, could lead to an enhanced loss of HOx and therefore impacts the balance between the coupled HOx/NOx/C1Ox-catalytic cycles [Hanson et al., 1992]. Nitrosyl sulfuric acid (NSA) has been observed in stratospheric sulfate aerosol [Farlow et al., 1978] and can be present in strongly concentrated, supercooled H~SO4/H20 solutions (> 60 wt-% H2SO4) as an ionic solid or as H2ONO + and HSO~ ions in solution [Burley and Johnston, 1992; Kinnison and Wuebbles, 1994]. Both nitrous acid and H2SO4/H20 droplets are present in high concentrations in young aircraft plumes. Recent laboratory studies showed that HNO2 is highly soluble in concentrated sulfuric acid solutions to form NSA [Zhang et al., 1996; Fenter and Rossi, 1996]. K~ircher [1997a] demonstrated by model calculations that HNO2 indeed could have been taken up by the aerosols present in the Con- corde plume [Fahey et al., 1995a]. It is known that HC1 reacts with NSA to form C1NO, which might lead to acid-catalyzed conversion of HCI to Cl, thereby affecting chlorine partitioning. Further, there is evidence that HNO4 reacts heterogeneously on ice surfaces [Li et al., 1996].

In the absence of detailed knowledge of the importance of various heterogeneous reaction pathways in aircraft plumes, in s i tu observations are required to shed more


light on this issue. Using information about aircraft-generated particle abundances and size distributions from recent near-field measurements, K~cher [1997a] inferred lower bounds 0.003 - 0.007 for uptake coefficients for heterogeneous reactions in order to proceed efficiently on the scale of the wake. The given range of uptake coefficients depends on the available surface area of exhaust soot. Rapid coagulation and dilution were found to be responsible for this constraint. Further, uptake coefficients lower than ~ 0.1 are not expected to be sufficient for processing on rapidly evaporating contrail ice particles, in general agreement with previous numerical simulations [Danilin et al., 1994; Karol and Ozolin, 1994]. However, this limit will be relaxed for persistent contrails developing into cirrus clouds in proportion to their lifetime.

5.2 Climatic effects

5.2.1 Radiative effects of emitted t r ace gases

Since CO2 is a well-mixed gas, its radiative impact can easily be estimated, independently of the source. Taking a radiative forcing of 1.5 W m -2 exerted by all CO2 emitted from anthropogenic sources (section 3.3.1) the thermal effect of CO2 from aircraft alone can be calculated in proportion to its fraction of the total. Based on 155 Mt/yr-l-14 % kerosene burnt in 1990 (section 2.3) yielding 489 Mt/yr+14 % CO2, a fraction of 2.24 % results for 1990, taking into account a recent figure for the 1990 world total CO2 emission from all fossil fuel burning, of 21,791 Mt/yr [Kondratyev, 1997]. In order to relate aircraft CO2 to thermal forcing, accumulated emissions need to be known. For the 1980-1990 period, accumulated CO2 from aicraft amounts to 4,587 Mt based on the 1990 figure and assuming a 4 %/yr growth during that time. The corresponding total CO2 globally emitted from all fossil fuel sources based on Kondratyev's table is 216,971 Mt. Thus aircraft contribute 2.1% to CO2 accumulated during the last decade. In view of the fact that CO2 emissions during recent years were highest and thus most effective, 2 % appears to be a reasonable value to that from all anthropogenic sources. Along with ±14 % uncertainty for the 1990 kerosene usage, a total uncertainty of +25 % appears appropriate yielding a total thermal forcing exerted by CO2 from aviation of 0.03 W m-2±25 %. Although this is a small number at present, its impact should be considered in view of the expected large growth (and, hence, CO2 emission) rates of commercial air traffic.

The direct radiative effect of water vapor emitted by aircraft is likely to be ne- glegible. Ponater et al. [1996] using the ECHAM model for simulating the global climatic impact of H20 released from the present fleet of aircraft found no discernable effect, even when present emissions were increased by a factor 100. The


picture changed for a factor-1000 experiment, but the enforced water vapor increase is still much smaller than background concentrations. Similarly minor effects were obtained by Rind et al. [1996] using the Goddard Institute for Space Studies (GISS) global climate model. With emissions some 15 times higher than 2015 projections corresponding to at least 30 times the present emissions, a small impact of a few tenths K is observed globally and locally. With H20 emissions 300 times the 2015 values, a global warming of 1 K results. However, they also showed that for such high emissions only about 5% of the water actually resides in the atmosphere long enough to be thermally important.

It should be noted, however, that the direct radiative impacts of aircraft-relevant greenhouse gases, such as H20 and O3 (affected mainly through the NOx emissions), as well as cloud particles and aerosols, cannot be simply separated as done in these studies, because their abundances are altered simultaneously by the large-scale operation of aircraft, and because of various feedback processes with clouds and between the troposphere and stratosphere [Rind and Lonergan, 1995; Fortuin et

al., 1995]. Especially ozone acts as an efficient absorber of both short- and longwave radiation; its impact on changes of the radiation balance depends on the changes of its vertical distribution which in turn is partly influenced by dynamical processes. Ozone increases due to the operation of subsonic aircraft are likely to produce a small positive forcing [Johnson et al., 1992; Hauglustaine et al., 1994; Brasseur et

al., 1996].

The effect of aircraft-induced ozone changes on the global climate has been studied by Sausen et al. [1997] using the general circulation model ECHAM 4. By adding perturbations to the operational ozone distribution of the model due to aircraft emissions, the zonal mean temperature response patterns were found to be statis- tically significant, with warming in the troposphere and cooling in the stratosphere in the range ±0.2 K for present day aircraft-induced forcing conditions. The magnitude of the temperature signal was found to depend nonlinearly on the magnitude of the assumed ozone perturbation.

It is difficult to quantify the radiative forcing exerted by aircraft induced ozone changes, in particular since these changes are uncertain to a large degree and not confirmed by measurements as yet. Mohnen et al. [1993] by means of a rough estimate come up with a positive radiative forcing of 0.04-0.07W m -2 for a prescribed 4-7 % ozone increase between 8 and 12 km and the latitude range 30-50 °N. By means of a coupled chemical dynamical radiative 2D high resolution model, Hauglustaine et al. [1994] obtain an aircraft induced ozone increase of up to 7 % during summer in the upper north-midlatitude troposphere. The resulting radiative forcing amounts to 0.015 W m -~ as a global average, but up to 0.08 W m -2 for the highest ozone increases in summer in the northern hemisphere. Brasseur


et al. [1996] by means of their 3D IMAGES model come up with similar values of 0.01-0.02 W m -2 as a global average and up to 0.04 W m -2 for midlatitude summer. Thus if NOx emitted by aircraft would really cause ozone increases in the UT, a positive radiative forcing is likely to result, which is of the same order of magnitude as that exerted by aircraft CO2 [Brasseur et al., 1995].

5.2.2 Effects d u e t o contra i l s and s o o t

Just as for high altitude cirrus clouds, the net radiation effect of contrails is believed to enhance warming. A possible contribution by contrails to climate change has been noted, but it is not clear as to whether their net effect would by warming or cooling. As stated in the foregoing chapter, the observations show that young contrails, in general, are composed of much more and smaller ice crystals than natural cirrus clouds, the latter also showing a large variety with respect to microphysical properties, i.e., crystal habits, ice water content, and size distribution [Dowling and Radke, 1990]. Further, this first straight-forward conclusion ignores possible modifications of existing cirrus clouds by interaction with contrails. In any case, a significant radiative impact can only be expected from long-lived contrails that develop in regions where the atmosphere is ice supersaturated. An at tempt to estimate the climatic impact of (direct) cirrus cloud cover due to aircraft contrails using a climate model showed that a distinct signal (significant warming of the lower troposphere) can be expected for additional 5 % or more contrail coverages (compared to estimated present levels) [Ponater et al., 1996].

The radiative effect of non-contrail clouds triggered by the aircraft exhaust is much harder to estimate because the occurrence of such clouds depends on the synoptical conditions, i.e., on the trajectories on which the polluted air parcels become trans- ported, and is therefore highly variable and essentially impossible to quantify to date. If an air parcel, immediately after passage of the aircraft, is forced to ascent adiabatically, cirrus can form, but with different spatial structure and extent than contrails. If cirrus form at lower altitudes, their warming effect is smaller; low, thick cirrus might even cause cooling.

The thermal impact of contrail induced cirrus clouds was studied by Strauss et al.

[1997] by means of a 1D radiative convective model. A 10 % increase in ice cloud cover leads to a surface temperature increase of 1.2 - 1.4 K depending on season. According to Ponater et al. [1996] a value of 2 % forms an upper limit for the average contrail coverage over the regions associated with the main flight routes. This would correspond to a positive radiative forcing of about 0.5 W m -2 for these areas and times of maximum contrail coverage [Toon, 1996]. Thus taking a mean for the area fraction of persistent contrails for the northern extratropics of 18 %


[Ponater et al., 1996] an average positive forcing of 0.045 W m -2 would result for the entire northern hemisphere.

Since on average 40 % of the present subsonic air traffic across the North Atlantic takes place above the tropopause in the LS, the fleet presents a constant source of new aerosols, with typical residence times of several months up to 1 yr. Locally, new particles size modes, possibly of different chemical composition, are added to

the background spectrum which represent a significant perturbation. The projected HSCT fleet would act into the same direction, but much stronger. Model simulations suggest that the current air traffic is a substantial source of sulfur below 20 km altitude and that the planned fleet of supersonic aircraft might even double the stratospheric aerosol surface area in northern midlatitudes. Liquid aerosols that are generated and injected in the UT may become mixed upwards through tropopause foldings, but the majority of the droplets can be chemically and ra- diatively active only within the typical residence time of upper tropospheric constituents, i.e., 1-2 months. When emitted just above the tropopause, these aerosols could form a potential source of (subvisible) cirrus clouds, an issue which needs to be investigated in more detail in the future.

The aircraft fleet is a significant direct source of soot in the UT and LS. As outlined in the previous chapter, exhaust soot very probably experiences modifications of surface morphology and chemical reactivity to a degree which is sufficient to initiate the formation of water ice contrails in jet plumes. Upon evaporation of the ice crystals, the soot cores become released and could host heterogeneous reactions. Soot particles that never went through a contrail cycle can directly affect the incoming solar (shortwave) radiation, but also when they become incorporated in contrail ice crystals or in the bulk of liquid aerosol due to coagulation, they might affect optical properties and could facilitate freezing nucleation. A model study pointing towards the potential importance of soot emissions for cirrus nucleation has been published by Jensen and Toon [1997]. The formation of high-level clouds due to the presence of aircraft soot is probably an important indirect consequence of aircraft pollution. Much more research is needed to understand this anthropogenic impact on cloud formation. The number of long-lived (when emitted in the LS) sulfuric acid aerosols internally mixed with soot cores is increased by the operation of jet aircraft, and thus may contribute to the positive radiative forcing.

5.3 U n c e r t a i n t i e s in g loba l m o d e l p r e d i c t i o n s

Model predictions of ozone production due to aircraft NOx emissions remain uncertain because of our limited knowledge of the budget and distribution of NOx


and other species such as H20, O3, and CIO in the UT and LS. Further difficulties are introduced due to our poor understanding of stratospheric-tropospheric exchange processes and mixing properties of the so-called lowermost stratosphere [Holton et al., 1995], NOx released in lightning discharges, and the missing climatology of convective events that are responsible for the transport of polluted air from the planetary boundary layer to the free troposphere. Large-scale models treat such processes in a highly parameterized manner and reveal quite substantial sensitivities of the NOx abundance and its spatial and temporal variability to variations of these parameterization. A clear outcome of these investigations is that a proper assessment of subsonic aircraft emissions requires 3D global model studies to resolve the small-scale transport of pollutants across the tropopause [see also Rodriguez, in: Thompson et al., 1996; pp.123]. For aircraft emissions in the stratosphere, the existence of a barrier of transport between the tropics and midlatitudes is of central importance [Plumb, 1996]. Current 2D models that take into account reduced transport between these regions come into much better agreement with observations (e.g., of the ratio NOy/O3) [Fahey et al., 1996; Volk et al., 1996]. Three-dimensional studies of the impact of high-flying (supersonic) aircraft are regarded less crucial for assessments as compared to the subsonic issue given the somewhat simpler dynamical situation in the stratosphere, but have still to be improved.

If we are to predict aircraft-induced perturbations of tropospheric chemistry, climatologies of aerosol abundances are certainly required. Unfortunately, we have no systematic data on the nature (i.e., phase, composition, and morphology) of aerosols in the troposphere, especially in remote regions not directly affected by anthropogenic activities. Further, the importance of cirrus clouds and other sub- micron aerosols in the photochemistry of the UT needs to be evaluated, as indicated by lidar observations of unexpectedly low ozone levels in cirrus clouds [Reichardt et al., 1996] and by the potential importance of chlorine chemistry in upper tropospheric ice clouds pointed out by Borrmann et al. [1996]. Clearly, more laboratory, field, and modeling research is needed to bridge this gap [Heintzenberg et al., 1996], and to allow for a detailed incorporation of related processes in chemical transport and climate models.

One should keep in mind that our knowledge about the oxidation rates of nitrogen is incomplete [Chatfield, 1994]. Heterogeneous reduction reactions of HNOa into NO on carbonaceous aerosols bring modeled and observed HNOs/NOx ratios into better agreement [Hauglustaine et al., 1996; Lary et al., 1997]. This discrepancy between observations and current model calculations directly influences aircraft assessment predictions. Keim et al. [1996] presented in situ observations of large NO/NOy ratios near the midlatitude tropopause. Because large changes of C10 and HOx together with enhanced aerosol levels and H20 were observed simultane-


ously, heterogeneous chemical reactions of C1ONO2 with HC1 and H20 on sulfate aerosols explain the associated conversion of NOx to HNO3 due to sufficiently low temperatures.

The interplay between ozone chemistry, transport, and radiative processes in the LS, the involvement of temporary reservoir species that cause strong coupling between the catalytic ozone depletion cycles, and natural perturbations due to volcanic eruptions renders the assessment of anthropogenic perturbations with the help of models difficult in this region of the atmosphere. This especially holds concerning the impact of current and future subsonic aircraft operations in the lower(most) stratosphere. Many important questions remain unanswered, among which are the problems of how to predict microphysical properties of stratospheric particles from first principles, under which conditions do PSC particles nucleate and what is their exact composition and lifetime, and how to calculate the rate of particle injection into and removal from the stratosphere.

Denitrification has been found to be very important in polar ozone destruction. Enhanced water vapor and nitric acid from aircraft could alter the chemical composition of existing PSCs and heterogeneous reaction rates [DelNegro et al., 1997], and could enhance their growth and accelerate sedimentation [Peter et al., 1991]. On the other hand, Patten and Wuebbles [1997] have shown that emissions of both subsonic and supersonic aircraft can cause additional PSCs to form. In such a situation, due to the limited amount of available vapor, the mean size of PSC particles could decrease, accompanied by a decrease in the sedimentation losses, which enhances the average residence time of the clouds.

It is very difficult to estimate the relative importance of the PSC-induced chlorine activation pathways until we get a better understanding of the underlying microphysics of PSC formation and subsequent incorporation of these findings into assessment models. In general, more research work has to be done related to questions concerning chemical processing in the plume and wake regimes of aircraft, from the emission up to the global scale. The potential importance of aircraft- related aerosol and contrail formation to chemistry and climate parameters cannot be properly addressed without a detailed understanding of the strongly interde- pendent microphysical, chemical, and dynamic processes that characterizes the gas-aerosol system in aircraft wakes.

The capability of current global models to cope with highly nonlinear feedback mechanisms (e.g., coupling between chemical changes and radiative forcing) is very limited. Especially, current results from climate models concerning the impact of aircraft emissions cannot be regarded as quantitative assessments, they merely point towards possible mechanisms by which aircraft might affect climate.


5.4 Summary of basic predictions

561

The global distribution of NOx and ozone, as well as their seasonal and latitudinal variations predicted by current two- and three dimensional chemical transport models is in qualitative agreement with observations, but a full explanation of observed trends is not yet possible. The main reason for this deficiency of models is the poor understanding of sinks and sources of NO×, of convective activity and lightning, of stratospheric-tropospheric exchange processes, and the lack of observational data (allowing sufficient global and annual coverage) with regard to the key chemical species and abundance and composition of tropospheric aerosols. On the basis of

these uncertainties, aircraft emissions are predicted to be largely reponsible for the NOx at cruising altitudes and the related photochemical production of ozone, with growing tendency in the future. The planned fleet of supersonic aircraft is predicted to reduce the lower stratospheric ozone column.

While observed NOx distributions are, considering the large uncertainties of NOx sources other than aircraft exhaust, consistent with model predictions, no ozone changes resulting from aircraft NOx have unambiguously been detected by observations.

The direct radiative forcing due to CO2 emitted by aircraft amounts to about 0.03W m -2 as a global average. The direct forcing of H20 is negligible, while contrails may cause about 0.04W m -2 on average for the northen extratropics. Another 0.02W m -2 may be exerted by ozone increases in the UT, which may amount a few percent on the northern hemisphere. While the figure for the additional CO2 forcing is quite certain, both the contrail and the ozone contributions to the thermal budget are uncertain. If confirmed, total radiative forcing as a result of aircraft effluents may be about 3 times that exerted by CO2 alone, at least for the northern hemisphere. Possible other radiative effects due to soot or sulfate aerosols are unknown.

6 F U T U R E A S P E C T S

6.1 The changing atmosphere

The present atmosphere is perturbed due to anthropogenic emissions from various sources (see Chapter 2). Continuous global ozone depletion in the stratosphere, superimposed by deep polar ozone depressions during the spring seasons are the consequence of increased levels of chlorine and bromine species in the stratosphere resulting from the emission of various halogenated hydrocarbons. Regional increases


of tropospheric ozone resulting from photochemical reactions of NOx and volatile organic compounds show the impact of combustion processes.

Both stratospheric ozone depletion and tropospheric ozone increase are intimately related to the climate system. They enhance the thermal effects exerted by the increasing anthropogenic greenhouse effect, i.e. warming the Earth's surface and lower atmosphere and cooling the stratosphere above about 20 km. C F C s and halons leading to stratospheric ozone depletion contribute to the increasing greenhouse effect as well, along with COs, N20, and CH4 (see Chapter 3).

While middle atmosphere cooling trends have unambiguously been determined from measured data [Angell, 1988; Taubenheim et al., 1990], there still is discussion as to whether or not observed anomalies of tropospheric temperatures, precipitation, ocean temperatures, extension of the cryosphere etc. are indicating climate changes related to the increasing greenhouse effect. Recent studies show, however, the observed spatial patterns of temperature change in the free atmosphere very similar to those predicted by state-of-the-art climate models [Santer et al., 1996; Hegerl et al., 1996]. By means of an optimal fingerprint method, Hegerl and Cubasch [1996] and Hegerl et al. [1996] could even show that, with a probability of more than 95%, observed near surface temperature trends are man-made, reflecting the increasing anthropogenic greenhouse effect. "The balance of evidence suggests a discernible human influence on global climate" [IPCC, 1996].

During the next 20 to 30 years, atmospheric perturbations are unlikely to disminish. Chlorine and bromine levels will increase further, although more slowly as a result of the Montreal Protocol phasing out production and usage of CFCs and halons (see Chapter 2). High stratospheric C10× and BrOx levels of not much less than 4 ppbV and 20 pptV, respectively, have to be envisaged for this time span, with corresponding negative ozone responses. Stratospheric temperatures will continue to decrease. Thus additional sulfate particles and PSCs may be formed possibly leading to more severe ozone depletion (to the best of present knowledge).

The replacement of CFCs and halons by HCFCs and HFCs mostly decomposed by OH at tropospheric levels will give rise to decomposition products whose further photochemical pathways are largely unknown.

The growth of atmospheric CO2 with further increase of the greenhouse effect will continue as no strategies for a significant reduction of global emissions are at sight.

6.2 F u t u r e a i r c r a f t emis s ions

Global jet fuel consumption is likely to grow about 3%/yr or even more during the next decade. The same growth rate applies to the exhaust products unless


major jet engine modifications will be introduced, which is unlikely within this short time span. The largest growth rates are expected for the East Pacific region thus shifting the bulk of effluents presently emitted at midlatitudes towards lower latitudes. Cruising altitudes will not decrease but rather tend to increase. Newly designed engines can operate up to about 14 kin. The projected fleet of HSCTs, if built and put into service at all, would be flown around 20 km altitude. Figure 8 depicts current (1991) and estimated (2000) air traffic volume along the main flight corridors. Note that the Far East and Transpacific routes are expected to grow fastest.

Year

:+90%

• ~ North Atlantic Europe- -Far -Eas t Transpacif ic

Figure 8. International air tragic in 1991 (in billion passenger kilometers)

and expected figures in 2000. The schematic indicates the main flight routes.

From Lecht et al. [1994].

Exhaust products will be dumped into a continously changing atmospheric environment briefly described above. Based on the findings reviewed in Chapters 4 and 5 the following effects have to be envisaged related to atmospheric composition and climate. Growing emissions of H20 and NOx will cause increasing contrail coverage and likely ozone formation in the UT. The relative contribution of aircraft CO2 to total global radiative forcing by CO2 from all anthropogenic sources will become larger, as the growth rate of 3%/yr predicted for global jet fuel usage is larger than that of total fossil fuel. Furthermore, when radiative forcing of contrails and NOx-induced O3, both of the same order as that of aircraft CO2 alone, are added, the relative contribution of aviation towards the anthropogenic greenhouse effect is bound to grow even faster.

Growing emissions of SOx are likely to produce more stratospheric sulfate particles, in particular when more SOx is emitted in the stratosphere due to higher cruising altitudes. This also might occur when flight activities are intensified in the tropical upper troposphere leading more SOx to be injected into the stratosphere via tropical


upwelling. More sulfate aerosol particles will lead to additional ozone depletion through heterogeneous reactions on the particle surfaces. Further, due to the lower temperatures, the contrail formation frequency will be increased, probably leading to enhanced chemical processing and greater perturbations of the radiative balance as compared to midlatitudes. If a fleet of future HSCTs will operate at about 20 km, entirely within the stratosphere, the problem of sulfate aerosol production and related ozone depletion may become even larger. Emissions of both subsonic and supersonic aircraft can cause growth of existing or even formation of additional polar stratospheric clouds, giving rise to further enhanced ozone depletion.

6 .3 T h e i s s u e o f c a r b o n t a x e s

Environmental taxes can be effective instruments for the internalization of external costs, i.e., the incorporation of costs of environmental services and damages (and their repairs) directly into the prices of the goods, services, and activities causing them. They thereby satisfy criteria of the Polluter Pays Principle #16 of the Rio Declaration and promote the integration of economic and environmental policies.

Environmental taxes can further provide incentives for both consumers and producers to change their behavior towards a more "eco-efficient" use of resources, to stimulate innovation and structural changes, and to reinforce compliance with regulations.

Environmental taxes can be classified into three main categories, (1) cost-covering charges designed to cover the costs of environmental services and abatement mea- sures, (2) incentive taxes designed to change the behavior of consumers and/or producers and (3) fiscal environmental taxes primarily to raise revenues. Often a mixure of these functions is executed.

According to the Chicago Convention kerosene used for international aircraft operations is exempt from taxation. Thus the average price of jet fuel in Europe is about US $ 0.20/1 only, whereas Diesel fuel prices range between US $ 0.60/1 and 0.90/1.

Taking CO2 emissions as a basis, taxation of kerosene for aviation should be the same as for Diesel fuel or gasoline. Taking into account, however, that the climatic impact of aircraft emissions may be about 2 to 3 times larger than the CO2 effect alone (see Chapter 5), taxation of aircraft fuel should be accordingly higher. There is no reason to exempt kerosene from taxation. In view of the critical development of global climate due to the increasing greenhouse effect the Chicago Convention should therefore be revised. Instead, international laws need to be worked out to provide a basis for international taxation of jet fuel.


So far, except for a few examples of national taxes levied in some north European countries, no international regulation for environmental taxes exists. Finland col- lects a carbon tax of about US $ 0.015/1 kerosene used for domestic flights which is close to a figure discussed within the EU. There is little progress, however, in implementing such environmental taxes at EU level which would correspond to US $ 6.50/t CO2 at present planned to gradually increase to about US $ 20/t CO2 within the next years. It will be even more difficult to achieve such a tax regulation on a worldwide basis, at least among the industrialized nations, although the global climate urgently requires it.

Total revenues of such a taxation would amount to 3.7 billion US $ worldwide at this initial stage which could gradually be increased with time. It is beyond the scope of this review to contemplate on how these revenues of such taxation should be spent most efficiently for the global environment or how international laws and regulations can be created that are necessary to enable such taxation.

In view of the increasing global greenhouse effect and related climate changes, environmental taxes will be introduced sooner or later, with aviation being included. As competitive imbalances as a result of possible national carbon tax schemes are highly undesirerable, a worldwide scheme encompassiiLg all major nations needs to be developed and enforced for international aviation. Thus it is highly recommended that international laws be prepared now to provide a frame for such international regulations.

7 I M P O R T A N T A R E A S OF R E S E A R C H

N E E D S

Building on today's knowledge, it seems impossible to conclusively quantify the impact of present and future aviation on global chemistry and climate, although many open questions have been solved during the past decade. Recalling the chapters of this evaluation, most confirmed effects have been observed on a local scale, where aircraft-induced perturbations are largest and the impact of other pollution sources is minimal. On the other hand, we find most uncertainties and open questions related to regional and global scale predictions, where experimental ver- ification is difficult and possible aircraft effects are likely masked by a multitude of other processes.

Guided by our evaluation of past research, we propose intensified and more focused investigations in the following areas:


Characterization of emissions. This concerns both, the future development and trends of traffic scenarios, fuel consumption, and related emission indices (including soot), and the understanding of how aerosol precursor gases and particulates are formed and interact at emission.

Plume/wake processing. This concerns studies of the size distribution (below 10nm) and chemical composition of new aerosols at emission and how they evolve up to the global scale. But also the role of aircraft-generated aerosols, soot, and ice particles on heterogeneous chemical processes in the wake needs to be investigated to obtain proper input data for use in global model calculations.

Aircraft exhaust/cirrus interaction. This concerns the question to which degree aircraft exhaust particles, especially soot, trigger the formation of cirrus clouds and modify existing cirrus cloud properties in order to gain more insight into changes of radiative forcing due to the presence of contrails and cirrus.

Characterization of the background atmosphere. Our present knowledge about transport and chemistry near the tropopause, in terms of residence times, dynamical barriers, oxidation capacity, reconversion of chemically active species from reservoir gases, aerosol climatologies, natural and other anthropogenic sources and sinks of key species is still incomplete, being a major obstacle to properly assess the aviation impact on the climate system.

Global chemical and climatic impact. Many important processes in current global chemical models controlling air chemistry need to be better parameterized and validated by comparison to observations. Introduction of interactive air chemistry modules, the radiative impacts of aerosols and clouds, and consideration of feedback mechanisms in climate models is in a very preliminary state and needs to be put on a much sounder basis.

Clearly, to tackle these issues, intensive cooperation between atmospheric scientists and engineers is required, and future programs should consider a combination of field, remote sensing, and laboratory studies as well as continued instrument and model development.

8


E X E C U T I V E S U M M A R Y

567

8.1 T h e i s s u e

Today's conventional subsonic aircraft operate in the altitude range between about 9 and 13km, in the upper troposphere and lowermost stratosphere. This height regime is characterized by a temperature minimum and a minimum background concentration of particles and of various trace gases, such as water vapor (H20), carbon monoxide (CO), and oxides of nitrogen (NOx = NO + NO2). This region marks the pronounced transition from low troposphere ozone (Oa) mixing ratios to high values in the stratospheric ozone layer. It also marks the transition from the moist troposphere towards the very dry stratosphere, with stratosphere-troposphere exchange processes playing an important role. Visible and subvisible cirrus (ice) clouds form at tropopause altitudes.

Jet engines burn kerosene using atmospheric oxygen as oxidant. Thus the exhaust mainly consists of CO2 and H20, with additional fractions of NOx, CO, SOx (= SO2 + SO3), unburned hydrocarbons (CxHy), and soot. These effluents are dumped along the flight corridors mainly concentrated at northern midlatitudes at present. Aircraft are the only direct polluters of the upper troposphere/lower stratosphere. Both gaseous species and particulate matter introduced by the aircraft fleet interact with solar and thermal radiation and influence air chemistry via homogeneous and heterogeneous chemical reactions, thereby giving rise to production of secondary species such as ozone, sulfate aerosols, or ice crystals often visible in contrails.

Effluents from aircraft are emitted into an atmosphere whose composition is changing due to man's activities: Increasing abundances of greenhouse gases such as CO2, CHa, N20, and various halogenated hydrocarbons (also causing stratospheric ozone depletion), increasing levels of stratospheric chlorine and bromine radicals resulting in a continuously shrinking ozone layer, with particulary large ozone losses at high latitudes, and photochemical ozone production in wide areas of the troposphere due to emissions of NOx, CO, and hydrocarbons from fossil fuel and biomass burning, just to mention some important issues of global change. The climatic impact of these effects, i.e., increasing global temperatures in the troposphere and decreasing temperatures in the stratosphere, is discernable from worldwide climate data.

Aviation is one of the fastest growing fields of the world's economy. Past growth rates of passenger and freight miles by about 5 %/yr are likely to persist during the coming decade, with global jet fuel consumption growing less rapidly, probably by about 3 %/yr. Thus the emission of effluents from aircraft exhaust will grow accordingly. Contrails forming under specific meteorological conditions clearly indi-


cate that present aircraft have an impact upon the atmosphere. In view of growing emissions the atmospheric impact of aviation is bound to grow in future years.

8.2 Aircraft emiss ion data base

Total jet fuel usage amounts to 155 Mt/yr+14 % (1990 figure) increasing by 3 %/yr. Air traffic movement databases and inventories for fuel use global distribution have been established and validated. Thus using stoichiometric relations, the global source distributions of CO2 and H20 emitted from aircraft are quantified with an uncertainty range of about ± 20 %.

The uncertainty ranges of NOx, CO, and hydrocarbons sources from aircraft effluents are larger, of the order of ± 50 %, as emission indices are dependent on engine type and age, power setting etc.

The emission of sulfur compounds is determined by the sulfur content in jet fuel which is known to vary from below 0.01% to about 0.2 % by mass, with a broad maximum around 0.05 %. Partitioning of fuel sulfur into oxidation products (leading to particle formation) is uncertain, and possibly highly variable. Due to their possible role in particle formation, more information about chemiion abundances at emission are required.

Emission indices of soot range typically between 5 x 10 x4 - 10 xs particles per kg fuel, corresponding to 0.05 - 0.1 g soot per kg fuel. In contrast to abundances and approximate size distributions, the surface morphology, chemical activity, and nucleation ability (with respect to acidic liquids and ice crystals) of soot is only poorly characterized.

8.3 Observed and conf irmed effects

In situ measurements of constituents (gases and aerosols) in aged aircraft plumes, interpreted by theoretical model studies, have yielded the following results:

Emission indices of gaseous constituents determined directly under real flight conditions agree fairly well with predicted ones obtained by scaling empirical, ground-based values used for earlier assessments. This especially holds for emissions of NOx.

Aircraft produce volatile aerosol particles likely resulting from binary homogeneous nucleation of sulfuric acid (H2SO4) and H20, in addition to emitted non-volatile combustion aerosols (mainly soot). Although the conversion of


exhaust SO2 to H2804 is not fully understood as yet, the existence of aviation- induced particles, which are likely to contribute to visible contrail formation,

has clearly been shown.

• Young contrails are composed of more, but smaller ice crystals than older ones and nearby natural cirrus clouds. The observed continuous transition in the optical and microphysical behavior suggests that persistent contrails develop towards cirrus clouds in the course of time.

• Over regions associated with the main flight routes contrails cause additional coverage by high clouds of up to 2 %. Contrails clearly contribute to growing cirrus cloudiness.

• Aircraft emission signatures in the form of volatile aerosols, soot, NOx and other species are clearly observable on the regional scale. The ratio of volatile aerosol to soot emission indices typically exceeds a factor 10.

8 . 4 P r e d i c t e d e f f e c t s a n d t h e i r u n c e r t a i n t i e s

8.4.1 Chemical effects

• Although the data base for the global distribution of NOx, CO, and hydrocarbon emissions from aircraft is fairly well (~-, • 50 %) established, large uncertainties (more than a factor 10) still exist with respect to natural and other anthropogenic sources of these species.

• Models of different degrees of complexity predict an increase of the NOx abundance between 20 % and 80 % over the northern hemisphere, depending on the season. Although regional NOx increases measured in the main flight corridors are found consistent with model results, global effects must be regarded with care considering the large uncertainties related to sources other than aircraft.

• Based on elevated NOx, models predict ozone increases at flight altitudes, between 2 % and 12 % depending on season, with higher values during summer. However, with the exception of one measurement over 7 days in a stagnant anticyclone over the North Atlantic corridor, with 770 aircraft passing per day


allowing aircraft exhaust to accumulate, when ozone was found to increase

by 70 ppbV most of which was attributed to subsidence, no ozone effect due to aircraft is documented so far. Given the large natural variability, global

ozone changes near the tropopause caused by aircraft emissions are unlikely to be discernable within the near future.

No observations exist about the size distribution and chemical composition

of volatile plume aerosols in the 1 - 10am size range, leaving model pre-

dictions concerning the role of chemiions and uptake of nitric acid during

formation and growth of these aerosols unsupported. Similarly, the role of

volatile aerosols in contrail formation at very low temperatures and the role

of sulfur-induced activation of soot particles in contrails generated at very

low fuel sulfur contents remains to be investigated.

The chemical impact of aerosols from aviation, both at midlatitudes and in

the polar regions, is largely unknown. Global model simulations of the aerosol

impact from the supersonic HSCT fleet on stratospheric ozone suggest signifi-

cant ozone depletion due to increases in total aerosol surface area. Simulations

also indicate that subsonic aircraft soot could potentially contribute to the

observed ozone decline at midlatitudes in the northern hemisphere, but re-

sults are sensitive to the assumed ozone loss rates involving soot. The detailed

heterogeneous chemistry and microphysical evolution of liquid aerosols, soot

particles, and ice crystals in aircraft wakes and contrails is virtually unex-

plored.

8.4.2 Cl imatic effects

• CO2 emitted from aircraft amounts to 2 % of total CO2 emissions from fossil

fuel burning. Thus aircraft CO2 contributes 1.5 % to anthropogenic greenhouse warming corresponding to a global thermal forcing of 0.03 W m -2.

The direct radiative effect of H20 emitted from aircraft is negligible. Con- trails, however, may exert an additional positive thermal forcing of about

0 .04Wm -2 for the northern hemisphere. Contrails may have an important

climatic effect, at least on a regional scale, but uncertainties in predicting

detailed forcing patterns are still large. Especially, how often contrails are transformed into cirrus clouds, or cause the formation of cirrus, is an unre-

solved problem.


• If aircraft NOx would really give rise to ozone production as predicted by models, another positive thermal forcing of about 0.02 W m -2 as a global average, with up to 0.08Win -2 for the highest ozone increases in summer, will result.

8.5 Final remarks

Effluents emitted by aircraft give rise to changes in atmospheric composition with impact on air chemistry and the radiation field and thus climate. Aircraft emission signatures in the form of aerosols, contrails, NOx and other species are clearly observable on the regional scale. While particle emission and formation as well as NOx emission with subsequent ozone production are likely to exert the most important chemical effects, CO2, contrails and ozone (if confirmed to be affected by aircraft NOx) affect radiative transfer and thus climate. It should be noted that thermal forcing exerted by aircraft ei~uents and their products is about 3 times higher than that of aircraft CO2 alone, at least for the northern hemisphere. Thus, in view of this triple effect, incentives aimed to reduce global emissions of CO2 should not exclude air traffic if environmental taxes are introduced, in particular as aviation is growing faster than most other fields of the world's economy.

9 A C K N O W L E D G E M E N T S

This study has been funded in part by Umweltbundesamt (German Federal Au- thority of the Environment) in Berlin, whose financial support is gratefully ac- knowledged. Sincere thanks are due to Thomas Peter for his critical review of the first version of this paper. The authors wish to particularly thank Michaela Hirschberg for her indefatigable efforts in bringing the many different and subtle pieces together.

572

10

Peter Fabian and Bernd K~lrcher

LIST OF A C R O N Y M S

AEAP

AERONOX

AEROTRACE

AVHRR

BMBF

BMFT

CCN

CFC

CIAP

DFG

DLR

ECHAM

ECMWF

EI

HSCT

HSRP

IN

IPCC

LES

LMS

LOWNOX

LS

Atmospheric Effects of Aviation Program

The Impact of NOx Emissions from Aircraft at Flight Altitudes 8-15 km

Measurement of Trace Species in the Exhaust from Aero-Engines

Advanced Very High Resolution Radiometer

German Federal Ministry of Education, Science, Research and Technology (Bundesministerium ffir Bildung, Wissenschaft, Forschung und Technologie)

German Federal Ministry of Research and Technology (Bundesmin- isterium ffir Forschung und Technologie, now BMBF)

Cloud condensation nuclei

Chlorofluorocarbon

Climatic Impact Assessment Program

German National Science Foundation (Deutsche Forschungsgemein- schaft)

German Aerospace Research Establishment (Deutsche Forschungs- anstalt ffir Luft- und Raumfahrt)

ECMWF model, Hamburg version

European Center for Medium Range Weather Forecast

Emission index

High-Speed Civil Transport

NASA High Speed Research Program

Ice forming nuclei

Intergovernmental Panel of Climate Change

Large eddy simulation

Lowermost stratosphere

Low Emission Combuster Technology Pilot Phase

Lower stratosphere

MOZAIC

MPI

NASA

NAT

NCAR

NMHC

NOAA

POLINAT

PSC

PV

SAGE

SASS

SAT

SPADE

SSA

SST

STE

STREAM

SUCCESS

TOMS

UARS

UT

WMO


Measurement of Ozone on Airbus In-service Aircraft

Max-Planck-Institute

National Aeronautic and Space Agency

Nitric acid trihydrate

National Center for Atmospheric Research

Non methane hydrocarbons

National Oceanic and Atmospheric Administration

Pollution from Aircraft Emissions in the North Atlantic Flight Corridor

Polar stratospheric cloud

Potential vorticity

Stratospheric Aerosol and Gas Experiment

Subsonic Assessment Project

Sulfuric acid tetrahydrate

Stratospheric Photochemistry, Aerosol, and Dynamics Expedition

Stratospheric sulfate aerosol

Supersonic transport

Stratosphere-Troposphere-Exchange

Stratosphere-Troposphere Experiments by Aircraft Measurements

Subsonic Aircraft: Contrail and Cloud Effects Special Study

Total Ozone Mapping Spectrometer

Upper Atmosphere Research Satellite

Upper troposphere

World Meteorological Organization

573

574

11

Peter Fabian and Bernd K~ircher

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Documents

1 INTRODUCTION - pa1c/PCE22_503-598_1997.pdf · Contrails forming during times of specific meteorological conditions constitute a visible manifestation of aircraft emissions, although