Atmospheric Environment 35 (2001) 1673}1676

Water adsorption on aircraft-combustor soot underyoung plume conditions

O.B. Popovitcheva*, M.E. Trukhin, N.M. Persiantseva, N.K. Shonija

Moscow State University, 119 899, Moscow, Russia

Received 28 April 2000; accepted 21 August 2000

Abstract

Laboratory studies of water adsorption and initial water uptake on aircraft-combustor soot, produced by combustionof sulphur-free hydrocarbon fuel, are presented. Due to the speci"c microporous structure and surface heterogeneity, thesoot particles can acquire a substantial fraction of water monolayer, even in the youngest plume. This sulphur-independent heterogeneous mode, driven by presence of water on soot surface, may be the cause for a visible contrailformation at low sulphur content in the jet fuel. ( 2001 Published by Elsevier Science Ltd.

Keywords: Aircraft exhaust; Soot; Water uptake; Contrail formation

1. Introduction

Carbonaceous particles emitted from aircraft enginesare suggested to be responsible for visible contrail forma-tion (Karcher et al., 1996). However, it was hypothesized(Brown et al., 1996; Schumann et al., 1996; Gleitsmannand Zellner, 1998a) that fresh exhaust soot particles,which are simulated in the laboratory studies either bysynthetic carbon (Wyslouzil et al., 1994) or by spark-discharge soot (Niessner et al., 1990), are hydrophobic. Itmeans there is a high contact angle of water embryos onthe soot surface and small rate of heterogeneous nuclea-tion at the supersaturations predicted in contrail } form-ing conditions. Yet to ful"ll the visibility criterion within25}30m, a large fraction of soot particles must possessa wetted surface, which allow the droplets to grow largeenough in order to overcome the Kelvin barrier (Karcheret al., 1996). To resolve this problem, models of `sootactivationa were suggested (Karcher et al., 1996; Brownet al., 1996; Gleitsmann and Zellner, 1998a) which arebased on the statement that exhaust soot particles wouldbe poor substrates for nucleation until they have under-

*Corresponding author. Tel.: #7-95-939-4954; fax: #7-95-939-3956.

E-mail address: olga@mics.msu.su (O.B. Popovitcheva).

gone interaction with the gaseous species. To describe thesulphur-induced activation, the accomodation coe$cientfor sulphuric acid was chosen extremely high, but forwater vapour it was taken equal to zero (Brown et al.,1996). However, for low sulphur content of the jet fuel thesulphur activation alone cannot explain the buildup ofvisible contrail (Karcher et al., 1996; Gleitsmann andZellner, 1998a).

Hydroscopicity of soot particles is strongly in#uencedby chemical and morphological characteristics thatchange markedly with conditions of soot formation. Un-der laboratory conditions n-hexane and commercial car-bon black have been used to study water adsorption andinitial uptake (Chughtai et al., 1996; Rogaski et al., 1997).However, lack of experimental data exists concerning air-craft-generated soot particles. That is why the laboratorycombustion technique operating on typical combustor ofthe gas turbine engine was used for soot sampling in ourprevious work (Popovitcheva et al., 2000). Speci"c hydro-scopic properties of the combustion soot should be deter-mined to clarify the role of water adsorption in the youngplume and the possibility of wetting of the soot surface.The uptake e$ciency of water vapour will allow estima-tion of the timescale for achievement of the quasi-equilib-rium adsorption during nonequlibrium plume conditions.

This paper presents the "rst results of a laboratorystudy of water adsorption and initial water uptake on

1352-2310/01/$ - see front matter ( 2001 Published by Elsevier Science Ltd.PII: S 1 3 5 2 - 2 3 1 0 ( 0 0 ) 0 0 4 4 7 - 7

Fig. 1. Adsorption isotherms of water vapour on combustorsoot at temperature 295, 282, 261 and 253K. Filled rhombicsymbols indicate the conditions at the plume axis.

aircraft-combustion soot for temperatures in the range295}253K. The amount of water adsorbed on exhaustsoot particles is estimated in correspondence to theevolution of both temperature and relative humidity inexpanding plume. Unusual behaviour of water adsorp-tion with decreasing temperature provides a noticeableamount of water on soot particles in the earliest stages ofthe plume evolution.

2. Experimental

The combustor of a gas turbine engine has been usedfor soot production under typical cruise combustion con-ditions at an average air/fuel ratio 4, #ame temperature1500}2000K, and pressure up to 0.4MPa. HydrocarbonC

3H

8/n-C

4H

10mixture was choosen to simulate the

sulphur-free fuel. Soot was sampled on a stainless-steelprobe cooled by air which was situated just behind thecombustor exit (at the distance of 12 cm) to minimize thetime of soot interaction with the hot exhaust gases. Sam-ples were removed, dried at 1303C for 2 h, outgassed andstored in the fridge.

A gravimetrical experimental set-up, equipped byMcBain scales, has been applied primarily to determinethe surface area and porosity of soot using the procedureof C

6H

6adsorption/desorption. The chemical nature

(acidity) of surface functional groups has been detectedby acid}base titration.

The volumetric method was used to measure the wateradsorption in a wide range of temperature 295}253K.Soot was suspended on the bottom of a sample tubeimmersed in an ethanol bath. To minimize the wateradsorption on the cooled tube walls, they were sur-rounded by a heating wire.

Water vapour uptake by combustor soot was mea-sured in a low-pressure #ow Knudsen reactor whichconsists of two chambers separated by a valve (Persiant-seva et al., 1998). The upper chamber was equipped witha coil in order to provide the additional heating of thechamber walls up to 350K. The water vapour pressuresstudied were in a range 10~4}10~5 Torr. The lowersample chamber contains the soot sample, placed on thetop of a Te#on rod, cooled by ethanol bath. The uptakee$ciency is obtained using the elementary gas}kinetictheory similar to Rogaski et al. (1997). Sample pretreat-ment consisted of 10 h heating at 1803C and outgassingat 10~5Torr.

3. Results and discussion

Important features, such as the irreversible adsorptionand wide hysteresis loop of nearly horizontal shape, areobserved upon the C

6H

6adsorption/desorption. It indi-

cates the existence of micropores of slit-like nature, which

play a prominent role in the adsorption properties. Thespeci"c surface area, S, calculated by applying the BETequation gives 47m2g~1.

Isotherms of water adsorption on the combustor sootare shown in Fig. 1. Compared with a reference hydro-phobic graphitized carbon black (Graphon) (Zettlemoyerand Mc Ca!erty, 1973) combustor soot is not hydropho-bic in character. In the initial relative humidity region,RH+20%, the amount of water molecules adsorbed is250 times higher than for Graphon. These results arerelated to the adsorption on the surface polar heterogen-eities, which are assumed to be related to the activeoxygen-containing groups (Gregg and Sing, 1982).Formation of water associated clusters on the previouslyadsorbed water molecules may be a possible mechanismof water adsorption as the relative humidity, RH, isincreased. The steep rise at RH*60% is correlated withbehaviour of water isotherms on active carbons (Greggand Sing, 1982), that indicates the cooperative e!ect ofrapid "lling, preferably of the micropores. Such a speci"cmechanism cannot be described by the classical Lan-gmuir theory of layer-by-layer "lling, as it was attemptedin Karcher et al. (1996).

Fig. 1 shows that from 295 to 253K the amount ofwater adsorbed at a given relative pressure decreaseswith temperature. The explanation for such unusual be-haviour lies in the relative value of the isosteric heat ofwater adsorption, q

45, and the heat of water condensa-

tion, q7, given by the equation

q45!q

7R¹2

"Ad ln(RH)

d¹ Ba

, (1)

1674 O.B. Popovitcheva et al. / Atmospheric Environment 35 (2001) 1673}1676

Fig. 2. Typical water uptake cycles measured from Knudsen cellat 293 and 250K. Times when the valve is open to expose thesoot surface or is closed are indicated as `uptake ona or `uptakeo!a.

and is determined from the isotherms of water adsorp-tion at a given amount of water adsorbed (Gregg andSing, 1982). If q

45(q

7, the isotherms should move to-

wards lower values of RH, as the temperature increasesthat is observed in Fig. 1. Such behaviour has beenobserved on certain nonoxidized carbon blacks, includ-ing Graphon (Zettlemoyer and Mc Ca!erty, 1973).

The impact of the phenomena of thermally activateddi!usion of water molecules into the micropores is likelyto appear at the temperatures above the bulk triple point,when the di!usion coe$cient of water molecules into themicropores is high enough.

The result of a typical water uptake cycle measuredin Knudsen cell experiments are shown in Fig. 2. Highuptake e$ciency c+(4$2)]10~3 is produced at¹+293K. At 250K there is no measured uptake of watervapour that is in good correlation with the small amountof water adsorbed on soot at low temperatures.

3.1. Quasi-equilibrium water adsorption in the plume

Time to reach the equilibrium of water adsorption, a,may be estimated as

t%"

4aNA

cnwS, (2)

where n is the number density of water vapour, w is thethermal velocity, N

Ais the Avogadro constant. Using the

results of a numerical model of the young plume ofa B-747 aircraft at the ambient H

2O content of 0.02mbar

(Gleitsmann and Zellner, 1998b), the RH is obtained

+0.28 at plume axis at ¹+295K. Then t%

is estimatedfrom Eq. (2) as 9]10~5 s. The plume age at the watersaturation is near 0.6 s. Obviously, this is much morethan t

%.. As a consequence, the water adsorption is in

a quasi-equilibrium state in the earliest stage of the plumeevolution and we can take for estimation the experi-mental adsorption data at the plume age more than t

%.

Filled rhombic symbols in Fig. 1 indicate the amountof water adsorbed on exhaust soot particles in corre-spondence with the relative humidity and temperaturedata at the plume axial distance 55, 68, 100, and 120mplume, as was obtained by Gleitsmann and Zellner(1998b). We see a noticeable amount of water on sootparticles already in the youngest plume. At ¹+295K air-craft-combustor soot adsorbs about +10~4 g H

2Om~2

on the soot surface corresponding approximately 0.3 ofa equivalent of surface monolayer. As the plume temper-ature relaxes, there is a competition between increasingthe amount of water adsorbed with RH and decreasingwith temperature. Thus the formation of wetted sootsurface in the expanding plume precedes the nucleationstep. Then it leads to a decrease in the contact angle, thuslowering the free energy barrier to nucleation, therebylowering the supersaturation required for nucleation.This e!ect has most likely been observed on diesel soot ina #ow-cloud chamber (Chen et al., 1993).

4. Conclusions

Insoluble fresh exhaust particles may act as the hetero-geneous ice nuclei in an expanding plume if there is thee!ective mechanism of soot wetting. Speci"c microstruc-ture of combustor soot and high water uptake providethe conditions for signi"cant quasi-equilibrium wateradsorption on soot particles in the young plume. Thus,there is a mechanism of reduction of the supersaturationrequired for heterogeneous water nucleation on exhaustsoot. As a result, the sulphur-independent heterogeneousnucleation mode driven by presence of water on sootsurface is suggested.

Finally, it is important to note that during the combus-tion of real aviation fuel containing sulphur impurities,the soot particles might acquire a certain sulphur massfraction, which may in#uence the ice nucleating proper-ties. Therefore the results presented here correspond tothe lower limit for water adsorption on aircraft soot. Theresults may be important for explaining visible contrailformation from sulphur-free kerosene, or low sulphurcontent in the jet fuel.

Acknowledgements

Financial support partially from EC Environmentaland Climate project, contract N CT97/0620 and Russian

O.B. Popovitcheva et al. / Atmospheric Environment 35 (2001) 1673}1676 1675

Fund of Basic Research, contract N00 15 96554 aregratefully acknowledged. We should be grateful to theCIAM group, under Prof. A.M. Starik for supervisionof soot production and support. Also the authors thankG. Gleitsmann for numerical calculations and great in-terest.

References

Brown, R.C., Miake-Lye, R.C., et al., 1996. Aerosol dynamics innear-"eld aircraft plume. Journal of Geophysical Research101, 22939.

Chen, C.-C., Hung, L.-C., Hsu, H.-K., 1993. Heterogeneousnucleation of water vapor on particles of SiO

2, Al

2O

3, TiO

2,

and carbon black. Journal of Colloid Interface Science 157,465.

Chughtai, A.R., Brooks, M.E., Smith, D.M., 1996. Hydration ofblack carbon. Journal of Geophysical Research 101, 19505.

Gleitsmann, G., Zellner, R., 1998b. The e!ect of ambient temper-ature and relative humidity on particle formation in the jetregime of commercial aircrafts: a modelling study. Atmo-spheric Environment 32, 3079.

Gleitsmann, G., Zellner, R., 1998a. A modelling study of theformation of cloud condensation nuclei in the jet regime ofaircraft plumes. Journal of Geophysical Research 103, 19543.

Gregg, S.J., Sing, K.S.W., 1982. Adsorption, Surface Area andPorosity, 2nd Edition. Academic Press, New York.

Karcher, B., Peter, Th., Biermann, U.M., Schumann, U., 1996.The initial composition of jet condensation trails. Journal ofAtmospheric Science 53, 3066.

Niessner, R., Daeumer, B., Klockow, D., 1990. Investigation ofsurface properties of ultra"ne particles by application ofa multistep condensation nucleus counter. Aerosol Scienceand Technology 12, 953.

Persiantseva, N.M., Popovitcheva, O.B., et al., 1998. Mass spec-trometric studies of the e$ciency of absorption of HClmolecules by ice under atmospheric conditions. ChemicalPhysics Reports 17, 543.

Popovitcheva, O.B., Perseantseva, N.M., et al., 2000. Experi-mental characterization of aircraft combustion soot: micro-structure, surface area, porosity and water adsorption. Phys-ical Chemistry Chemical Physics 2, 4421.

Rogaski, C.A., Golden, D.M., Williams, L.R., 1997. Reactiveuptake and hydration experiments on amorphous carbontreated with NO

2, SO

2, O

3, and H

2SO

4. Geophysical Re-

search Letters 24, 381.Schumann, U., Strom, J., et al., 1996. In situ observation of

particles in jet aircraft exhaust and contrails for di!erentsulfur containing fuels. Journal of Geophysical Research 101,6853.

Wyslouzil, B.E., Carleton, K.L., et al., 1994. Observation ofhydration of single, modi"ed carbon aerosols. GeophysicalResearch Letters 21, 2107.

Zettlemoyer, A.C., Mc Ca!erty, E., 1973. Water on oxide surfa-ces. Croatica Chemica Acta 45, 173.

1676 O.B. Popovitcheva et al. / Atmospheric Environment 35 (2001) 1673}1676