Non-methane hydrocarbons at a high-altitude rural site in the Mediterranean (Greece)

Atmospheric Environment 34 (2000) 973}984

Non-methane hydrocarbons at a high-altitude rural sitein the Mediterranean (Greece)

Nektarios Moschonas, Sotirios Glavas*

Department of Chemistry, University of Patras, EL-26500 Patras, Hellas, Greece

Received 9 December 1998; received in revised form 31 March 1999; accepted 19 April 1999

Abstract

Acetylene, C3}C

10para$ns, ole"ns, aromatic hydrocarbons and the biogenic isoprene, were determined in six sam-

pling periods from May to October in 1996}1997. The concentrations of the determined species were generally lowerthan those measured in southeastern US and western France, but higher than those measured in more remote regions.Acetylene and C

3}C

5hydrocarbons exhibited seasonal variation with higher concentrations in the late October sampling

period. The aromatic hydrocarbons, except benzene, exhibited maximum concentrations in the summer. Isoprene hadmaximum concentrations in the summer, with its concentration linearly increasing as a function of the logarithm oftemperature, in agreement with past studies. Its emission #ux was calculated to be on the lower end of similar studies inUS. The main destruction route of all hydrocarbons was determined to be due to their photochemical reactions. Thephotochemical reactivity, calculated in terms of propylene equivalent concentration, was in the summer dominated byisoprene which accounts for 69% of the total, aromatics with 16%, ole"ns with 11% and para$ns with 4%. In the Mayand October periods, the aromatics contributed the most (&37%) to the photochemical reactivity. Air mass backtrajectories indicated higher concentrations when air masses arrived from northwestern and northeastern, compared tosouthern, directions. ( 2000 Elsevier Science Ltd. All rights reserved.

Keywords: Para$ns; Ole"ns; Aromatics; Isoprene; Isoprene emissions; Photochemical activity; Propylene-equivalent concentrations;Air mass back trajectories; High elevation rural; Mediterranean

1. Introduction

It is well established that the hydroxyl radicals are thekey species in the photochemistry of the troposphere.OH radicals control the formation of ozone and otheroxidants, as well as the acidity of the atmosphere. There-fore the sources and sinks of these radicals are of para-mount importance. In the troposphere OH radicalswould react mainly with CO and CH

4to form ozone

(Fishman and Crutzen, 1978). However, the hydroxylradicals react very fast with non-methane hydrocarbons(NMHC) both of anthropogenic origin and natural sour-ces; therefore these hydrocarbons will also contribute to

*Corresponding author. Tel.: #30-61-997-134; fax: #30-61-997-134.

E-mail address: [email protected] (S. Glavas)

the regulation of the hydroxyl radicals. The main sourcesof NMHC are fossil fuel combustion, mostly mobile sour-ces, solvent evaporation, re"neries, etc. (Field et al., 1992;Sche! and Wadden, 1993). All these sources are situatedin or near urban areas.

The emitted anthropogenic NMHC consist of paraf-"ns, ole"ns and aromatics from two to more than 10carbon atoms. The lifetimes of para$ns and aromaticsare controlled by their reaction with hydroxyl radicals.They vary from several days (43 days for ethane) to a fewhours (4.8 h for 1,3,5 trimethyl benzene), assuming a hy-droxyl radical concentration of 1]106 molecules cm~3.Thus considerable concentrations of para$ns and aro-matics may be found in rural regions due to longrange transport. Ole"ns are more reactive with hydroxylradicals and in addition they react with ozone. Underusual hydroxyl radical and ozone concentrations theirlifetimes do not exceed a few hours. Studies in remote

1352-2310/00/$ - see front matter ( 2000 Elsevier Science Ltd. All rights reserved.PII: S 1 3 5 2 - 2 3 1 0 ( 9 9 ) 0 0 2 0 5 - 8

sites indicate the presence of anthropogenic origin hydro-carbons (Jobson et al., 1994; Solberg et al., 1996; Laurilaand Hakola, 1996). Studies in rural environments aremore frequent, but they are limited to north America andnorth Europe (Bottenheim and Shepherd, 1995; Hager-man et al., 1997; Boudries et al., 1994; Hov et al., 1991;Laurila and Hakola, 1996). In all of these studies C

2}C

5hydrocarbons exhibited seasonal variation with higherconcentrations in the winter, because of the reducedphotochemical activity in the winter (smaller hydroxylradicals concentration) and because of the generally lar-ger mixing heights in the summer (Jobson et al., 1994).The suggestion by Tille et al. (1985) on larger emissions inthe winter is disputed by Rotty (1987), who reportedsigni"cant seasonal variation only for gaseous fuel, whichwould not be expected to a!ect all the measured specieshere. This seasonal variation does not seem to be fol-lowed by the aromatic hydrocarbons in the two existingstudies that report more than one aromatic hydrocar-bons (Boudries et al., 1994; Hagerman et al., 1997). Thus,Boudries et al. (1994) reported higher benzene and tol-uene concentrations in winter and early spring for west-ern France, but xylenes did not exhibit a distinct seasonalvariation. Hagerman et al. (1997) on the other hand,larger aromatic hydrocarbon concentrations in the sum-mer reported for south east US.

The naturally emitted NMHC consist mainly of iso-prene, pinenes and limonene. Isoprene is emitted mainlyby deciduous trees and it thus exhibits a seasonal vari-ation with larger concentrations in the summer (Botten-heim and Shepherd, 1995; Hagerman et al., 1997; Bou-dries et al., 1994; Laurila and Hakola, 1996). Because ofits high reactivity with hydroxyl radicals and ozone,isoprene has a short lifetime, circa 2.5 h. In addition,isoprene has been shown to increase with photoreactiveradiation (Pier, 1995; Fuentes et al., 1995; Guenther et al.,1991; Kempf et al., 1996). Thus, isoprene would exhibitdiurnal variation.

The lifetimes of NMHC, at least of the more reactiveones, as well as the spread of the strengths of theiremissions indicate that these hydrocarbons would exhi-bit geographical variability. As far as we know no otherstudy exists of these hydrocarbons in the rural Mediter-ranean, with the exception of an airborne study of someof the here reported hydrocarbons in western Mediterra-nean (Tille et al., 1985).

In this work we present acetylene, C3}C

10NMHC and

isoprene measurements carried out in the spring, sum-mer and fall of 1996}1997 at a high elevation rural site inPeloponnisos, Greece. In addition to the seasonal vari-ation of the NMHC we determined the diurnal variationof isoprene concentrations and derived isoprene emission#ux of 1300 lg m~2 h~1. The photochemical reactivityof each category of hydrocarbons alkanes}ole"ns}aro-matics and isoprene is evaluated in terms of their propy-lene equivalent concentration. In addition, the e!ect of

the origin of the air masses arriving at the sampling sitewas evaluated by air mass back trajectories.

2. Experimental

All northern Peloponnisos is mountainous with sev-eral peaks exceeding 2000 m. The monitoring site is lo-cated at the top of a hill, at altitude 1070 m, on the slopesof the volume of Aroania mountains (peak at 2345 m)near village Messorougion. Most of the major peaks areshown in Fig. 1. The lines indicate the contours at 1000and 2000 m. The height of the major peaks at the triangleis also indicated. Between these mountains and the coastextends a #at corridor with width of a few hundredmeters just before Patras (pop. 170 000) and widening toa few kilometers near Korinthos (pop. 30 000). Severalsmall villages with about 10}20 inhabitants are locatedwithin a radius of 10 km from the site. The nearest townis Akrata, AK (pop. 2000) 35 km in the NNW. A numberof more populous towns (3000}10 000 inhabitants) arefound along the coast line of Korinthian gulf. The majornational road runs along the coast.

The sampling site is adjacent to the forest of Zarou-chla-Messorougion. This forest extends from 1000}1650 m and is populated mainly by coniferous spruce(Abies cephalonica) and pine trees (Pinus nigra) intersper-sed with deciduous trees like chestnut trees (Castaneasativa) and platanus (Platanus orientalis), as well as thenon-deciduous Quercus coccifera.

Ambient air was pressurized to 30 psig in Summapassivated canisters using a stainless steel pump withte#on diaphragm, all obtained from BRC-Rasmussen;samples were collected every day at 13}14 hours (localtime, UTC#2 h) 4 m above ground, in the following sixsampling periods: 29 April}4 May 1996, henceforthcalled April 96, 25}30 June, henceforth called June 96,2}10 September, henceforth called September 96, 28 Oc-tober}4 November 1996, henceforth called October 96and 5}22 May 1997, henceforth called May 97. In twoperiods of September and October the diurnal pro"le ofthe hydrocarbons was determined by collecting four sam-ples every six hours within 24 h. No winter data werecollected because access to the site was not secured.

The canisters were analyzed right after the end of thesampling period. Preliminary experiments had shownthat para$ns and aromatic hydrocarbons stayed un-changed in the canisters for more than a month. Firstan aliquot of 1500 ml passed through a Na"on dryer toremove the humidity and was then cryoconcentrated ona glassbeads trap cooled at !1803C of a Tekmar 5010GT Automatic Thermal Desorber. Subsequently, the sam-ple was cryofocused on a small portion of the analyticalcolumn and then #ash injected into the analytical col-umn. The analytical column was a 50 m long, 0.32 mmi.d., HP-1 non-polar column, coated with 1.05 lm "lm

974 N. Moschonas, S. Glavas / Atmospheric Environment 34 (2000) 973}984

Fig. 1. Map of the area of sampling site indicated by a cross. Lines are contours at 1000 and 2000 m. Numbers are peak heights at themajor peaks, indicated by solid triangles. Solid circles indicate small towns.

thickness 100% cross linked methyl silicone gum. Thiscolumn was housed in a HP-5890 Series II gas chromato-graph and could be connected either to a #ame ioniza-tion detector, FID, or to a HP-5971A mass detector,MSD. The GC oven program used was: !403C for1 min rising at 103C/min to 1503C where it remains for7 min and then rises at 103C/min up to 2003C. Thisprogram allowed the resolution of 50 hydrocarbons,given in Table 1, from acetylene up to C

10, including

isoprene. The pinenes were not detected. They were mostprobably destroyed by the Na"on dryer, as was shown ina past study (Burns et al., 1983). All the results reportedhere were obtained with the FID. The MSD was used forthe identi"cation of the peaks. This procedure involvedthe analysis of an ambient air sample in Athens in thescan mode of the MSD. Thus all peaks were identi"ed

through their mass spectra and the pro"le of the chro-matogram was established. This procedure could not beapplied to a sample from the rural site because thesensitivity of the MSD was not su$cient. The pro"le ofa chromatogram from the same sample, however, is thesame irrespective of the used detector. All the non-meth-ane hydrocarbons expected and found at rural sites arethe same ones found in urban areas. Indeed comparisonof the species of Table 1 with those of Table 1 from ourwork in Athens (Moschonas and Glavas, 1996) indicatesthat almost all hydrocarbons observed in Athens werealso determined at Messorougion. Thus the pro"le of thechromatogram obtained with the FID at the rural sitewas known. Injection and analysis of a single knownhydrocarbon allowed the veri"cation of its locationin the chromatogram. All obtained peaks were then

N. Moschonas, S. Glavas / Atmospheric Environment 34 (2000) 973}984 975

Tab

le1

Mea

nva

lue,

stan

dard

dev

iation

,m

edia

nan

dra

nge

ofth

eco

nce

ntra

tion

ofN

MH

Cfo

rth

esa

mplin

gsat

Mes

soro

ugio

n

Hyd

roca

rbon

May'9

6an

dM

ay19

97Ju

ne

and

Sep

tem

ber

1996

Oct

ober

1996

Mea

nSd

Med

Min

Max

Mea

nSd

Med

Min

Max

Mea

nSd

Med

Min

max

(15)

(15)

(15)

(13)

(13)

(13)

(6)

(6)

(6)

ppb

Ace

tyle

ne

0.13

0.14

0.08

0.03

0.55

0.40

0.32

0.31

0.08

1.03

0.96

0.49

0.70

0.53

1.70

Pro

pyl

ene

0.02

0.01

0.01

0.00

0.04

0.06

0.04

0.05

0.02

0.13

0.06

0.04

0.05

0.04

0.14

Pro

pan

e0.

410.

240.

390.

180.

930.

640.

330.

680.

171.

390.

950.

420.

830.

591.

67Is

obut

ane

0.04

0.05

0.01

0.01

0.19

0.04

0.02

0.04

0.02

0.07

0.07

0.03

0.06

0.05

0.13

Isob

ute

ne#

1-bu

tene

0.02

0.02

0.02

0.00

0.06

0.07

0.06

0.06

0.00

0.20

0.07

0.03

0.07

0.05

0.11

n-B

utan

e0.

280.

410.

020.

011.

220.

110.

070.

080.

030.

250.

320.

180.

270.

180.

68tr

ans-

2-but

ene

0.01

0.01

0.01

0.00

0.04

0.02

0.01

0.03

0.01

0.05

0.01

0.00

0.01

0.01

0.02

cis-

2-but

ene

0.03

0.02

0.03

0.01

0.09

0.04

0.02

0.05

0.01

0.07

0.02

0.01

0.01

0.01

0.03

Isop

enta

ne

0.04

0.06

0.02

0.01

0.23

0.09

0.08

0.08

0.01

0.27

0.11

0.06

0.10

0.05

0.19

1-P

ente

ne

0.00

0.00

0.00

0.00

0.01

0.01

0.01

0.01

0.00

0.03

0.01

0.00

0.01

0.00

0.02

n-P

enta

ne

0.02

0.02

0.01

0.00

0.07

0.04

0.03

0.03

0.00

0.11

0.07

0.05

0.06

0.03

0.15

Isop

rene

0.04

0.02

0.03

0.02

0.10

0.78

0.79

0.34

0.15

2.35

0.04

0.02

0.04

0.02

0.08

1,1-

Dim

ethyl

-cyc

lopr

opan

e0.

020.

020.

020.

000.

050.

010.

010.

010.

000.

020.

000.

000.

000.

000.

012-

met

hyl-1-

but

ene

0.01

0.01

0.01

0.00

0.03

0.01

0.01

001

000

0.02

0.01

0.00

0.01

0.00

0.01

2-M

ethyl

-2-b

ute

ne

0.01

0.01

0.00

0.00

0.03

0.03

0.03

0.02

0.00

0.08

0.01

0.00

0.01

0.00

0.01

2,2-

Dim

ethyl

-buta

ne

0.01

0.01

0.01

0.00

0.03

0.02

0.01

0.02

0.00

0.04

0.01

0.00

0.01

0.00

0.02

Cyc

lopen

tene

0.01

0.01

0.01

0.00

0.04

0.03

0.02

0.03

0.00

0.06

0.01

0.00

0.01

0.01

0.01

2,3-

Dim

ethyl

-buta

ne

0.02

0.02

0.01

0.01

0.07

0.08

0.06

0.08

0.01

0.24

0.06

0.03

0.05

0.03

0.09

2-M

ethyl

-pen

tane

0.02

0.02

0.02

0.00

0.07

0.06

0.03

0.07

0.01

0.12

0.04

0.01

0.04

0.03

0.05

3-M

ethyl

-pen

tane

0.05

0.04

0.04

0.02

0.17

0.14

0.10

0.10

0.01

0.29

0.04

0.02

0.04

0.02

0.07

1-H

exen

e0.

000.

000.

000.

000.

010.

010.

010.

010.

000.

020.

010.

000.

000.

000.

01n-

Hex

ane

0.01

0.01

0.01

0.00

0.03

0.04

0.04

0.04

0.01

0.15

0.05

0.02

0.05

0.03

0.08

tran

s-2-

Hex

ene

0.01

0.01

0.00

0.00

0.02

0.01

0.01

0.01

0.00

0.02

0.00

0.00

0.00

0.00

0.01

2-M

ethyl

-2-p

ente

ne0.

000.

000.

000.

000.

010.

010.

000.

010.

000.

010.

000.

000.

000.

000.

00M

ethyl

-cyc

lope

ntan

e0.

010.

010.

010.

000.

020.

030.

040.

020.

010.

150.

020.

010.

020.

010.

04Ben

zene

0.03

0.02

0.02

0.01

0.09

0.08

0.06

0.07

0.02

0.21

0.17

0.10

0.12

0.09

0.33

Cyc

lohex

ane

0.01

0.00

0.00

0.00

0.01

0.01

0.01

0.01

0.00

0.03

0.01

0.01

0.01

0.00

0.02

2-M

ethyl

-hex

ane

0.01

0.00

0.01

0.00

0.02

0.02

0.02

0.02

0.00

0.06

0.01

0.01

0.01

0.00

0.02

3-M

ethyl

-hex

ane

0.02

0.02

0.02

0.00

0.06

0.07

0.05

0.06

0.01

0.16

0.04

0.01

0.04

0.04

0.05

tran

s-1,

3-D

imet

hyl-cy

clop

enta

ne

0.01

0.01

0.01

0.00

0.02

0.03

0.02

0.02

0.01

0.07

0.01

0.01

0.01

0.00

0.03

n-H

epta

ne

0.01

0.00

0.01

0.00

0.02

0.02

0.01

0.02

0.00

0.04

0.01

0.01

0.01

0.00

0.04

3-M

ethyl

-2-e

thyl

-1-b

ute

ne

0.01

0.00

0.01

0.00

0.01

0.01

0.00

0.01

0.00

0.02

0.00

0.00

0.00

0.00

0.01

Met

hyl-cy

clohe

xane

0.01

0.00

0.01

0.00

0.01

0.01

0.01

0.01

0.01

0.03

0.00

0.00

0.00

0.00

0.01

Tolu

ene

0.04

0.03

0.03

0.01

0.09

0.16

0.13

0.15

0.03

0.48

0.13

0.07

0.13

0.06

0.22

2-M

ethyl

-hep

tane

0.01

0.01

0.01

0.00

0.03

0.01

0.01

0.01

0.00

0.04

0.00

0.00

0.00

0.00

0.01


3-M

ethyl

-hep

tane

0.03

0.02

0.02

0.00

0.07

0.07

0.05

0.05

0.02

0.18

0.04

0.01

0.04

0.02

0.06

1,4-

Dim

ethy

l-cy

clohe

xane

0.01

0.02

0.01

0.00

0.07

0.01

0.01

0.01

0.00

0.03

0.01

0.01

0.00

0.00

0.03

n-O

ctan

e0.

020.

010.

020.

000.

060.

010.

010.

010.

000.

020.

000.

000.

000.

000.

01Eth

yl-b

enze

ne

0.01

0.01

0.01

0.00

0.02

0.05

0.04

0.05

0.00

0.13

0.03

0.02

0.03

0.01

0.05

m#

p-X

ylen

es0.

020.

020.

010.

000.

060.

180.

160.

190.

010.

540.

090.

040.

090.

040.

143-

Met

hyl

-oct

ane

0.00

0.00

0.00

0.00

0.01

0.01

0.01

0.01

0.00

0.03

0.01

0.00

0.00

0.00

0.01

Styr

ene

0.03

0.03

0.02

0.01

0.10

0.12

0.09

0.09

0.02

0.31

0.08

0.02

0.07

0.06

0.11

o-X

ylen

e0.

010.

010.

010.

000.

040.

060.

050.

060.

010.

140.

030.

010.

030.

010.

053,

5-D

imet

hyl-oct

ane

0.01

0.03

0.00

0.00

0.10

0.01

0.01

0.01

0.00

0.03

0.01

0.01

0.01

0.01

0.02

1,2,

3-Trim

ethy

l-be

nzen

e0.

020.

020.

010.

000.

090.

030.

030.

020.

000.

100.

030.

010.

030.

010.

051,

3,5-

Trim

ethy

l-be

nzen

e0.

040.

070.

020.

000.

240.

090.

160.

010.

000.

421,

2,4-

Trim

ethy

l-be

nzen

e0.

010.

040.

000.

000.

140.

040.

040.

020.

000.

130.

020.

010.

020.

010.

041,

2-D

ieth

yl-b

enze

ne0.

010.

020.

010.

000.

040.

030.

030.

020.

000.

070.

080.

100.

040.

030.

281-

Met

hyl

-2-p

ropyl

-ben

zene

0.05

0.09

0.00

0.00

0.23

0.01

0.01

0.00

0.00

0.03

0.02

0.01

0.01

0.01

0.04

2-E

thyl

-1,4

-dim

ethyl

-ben

zene

0.12

0.21

0.01

0.00

0.70

0.02

0.04

0.01

0.00

0.13

0.01

0.00

0.01

0.00

0.01

assigned through their relative retention. In addition,injections of the following authentic hydrocarbons, afterstatic dilution in the gas phase con"rmed the GC-MSresults: propane, n-butane, isobutane, trans-2-butene,cis-2-butene, obtained from Messer Griesheim; 2-3-methyl pentane, hexene-1, n-hexane, toluene, xylenes,ethyl benzene and trimethyl benzenes, obtained fromAldrich. The quantitative determination was performedusing a benzene primary standard, obtained from NBS.This standard was dynamically diluted to a concentra-tion around 2}10 ppbv and analyzed following the analy-sis procedure of the samples. The detector's response forall the hydrocarbons of Table 2, was derived relative tothat of benzene. For this purpose were prepared gaseousmixtures of all the compounds of Table 2. In these mix-tures benzene was always present. The obtained, versusbenzene, detector's response was further corrected versusthe benzene response obtained by the NBS primary ben-zene standard. The remaining hydrocarbons of Table 1,were quanti"ed using as detector's response that of thenearest hydrocarbon (same homologous series, same car-bon atoms) whose response was determined in the pre-vious stage.

Ozone was measured with a Dasibi Model 1008RSUV absorption instrument with stated detection limit1 ppbv. NO

xwas measured with a Thermo Environ-

mental Model 42S high sensitivity chemiluminescenceinstrument with stated detection limit 50 pptv. Spanchecks were performed for the ozone analyzer every day.Both instruments were multipoint calibrated before andafter each sampling period. More details can be foundelsewhere (Glavas, 1999). Temperature and relative hu-midity were continuously measured in 1997 along withwind speed and wind direction using Rotronic and Vec-tor Instruments sensors for the temperature/relative hu-midity and wind measurements respectively. In the 1996sampling periods temperature and relative humidity weremeasured, two-three times during the hour of "lling ofthe canisters, using a Vaisala HMI 31 humidity andtemperature indicator.

3. Results and discussion

The mean, standard deviation of the mean, the medianand range of concentrations of the measured species inspring (early May 1996 and in May 1997), summer (Juneand early September 1996) and fall (late October 1996)are presented in Table 1. The observed ambient airNMHC concentrations depend on the strength of thesources (emissions) and destruction. The sources dependon the activities of the region. The main sources ofNMHC are the same all over the world. Mobile sourcescontribute the most, followed by solvent use, fuel produc-tion, etc. These sources emit the same species, irrespectiveof geographical location, as is inferred by the NMHC


Table 2Mean value, standard deviation, median and range of the ratio HC/acetylene at Messorougion and in Athens for summer period

Hydrocarbon Messorougion Athens

June 1996 and September 1996 13 June 1997

HC/acetylene, ppb/ppb

mean S.D. Median Min Max Mean S.D. Median Min Max(13) (13) (13) (8) (8) (8)

Propylene 0.18 0.16 0.14 0.08 0.69 0.27 0.05 0.28 0.17 0.32Propane 2.28 1.87 1.62 0.82 8.07 0.22 0.10 0.17 0.13 0.41Isobutane 0.14 0.10 0.10 0.05 0.41 0.24 0.14 0.21 0.10 0.54N-Butane 0.46 0.67 0.24 0.18 2.63 0.62 0.29 0.56 0.26 1.24Isopentane 0.23 0.21 0.17 0.06 0.87 0.60 0.17 0.59 0.36 0.83n-Pentane 0.11 0.08 0.10 0.01 0.31 0.26 0.07 0.36 0.15 0.40Isoprene 5.03 8.93 1.62 0.22 26.06 0.07 0.04 0.06 0.03 0.132-Methyl-pentane 0.28 0.39 0.12 0.06 1.34 0.04 0.03 0.03 0.01 0.103-Methyl-pentane 0.53 0.63 0.35 0.06 2.50 0.15 0.04 0.15 0.10 0.22N-Hexane 0.15 0.10 0.12 0.03 0.33 0.21 0.08 0.20 0.13 0.34Benzene 0.23 0.12 0.17 0.12 0.45 0.20 0.02 0.21 0.16 0.22Toluene 0.51 0.46 0.43 0.15 1.89 0.71 1.26 0.72 0.41 1.09Ethyl-benzene 0.16 0.24 0.09 0.01 0.93 0.13 0.06 0.12 0.08 0.25m#p-Xylenes 0.55 0.61 0.36 0.08 2.35 0.41 0.18 0.39 0.22 0.81o-Xylene 0.24 0.31 0.14 0.04 1.18 0.13 0.03 0.13 0.08 0.17

speciation determined in urban (Moschonas and Glavas,1996) and rural areas (this work, Hagerman et al., 1997;Boudries et al., 1994; Bottenheim and Shepherd, 1995)with similar relative quantities. It is the quantity of emis-sions that varies with region. The destruction dependsmainly on the concentration of the hydroxyl radicals,which may also vary considerably with the region. Thus,comparison of the species and their ambient air concen-trations in di!erent locations is meaningful. Indeed therange of concentrations of the measured C

2}C

5hydro-

carbons, Table 1, at our site is comparable to the range ofconcentrations observed in other studies in rural north-ern Europe (Hov et al., 1991; Laurila and Hakola, 1996)but lower than concentrations given in western France(Boudries et al., 1994) and Canada (Bottenheim and Shep-herd, 1995). The concentrations of benzene at our sitewere similar to those in south Finland (Laurila andHakola, 1996) while those at Porspoder western Francewere slightly higher (Boudries et al., 1994). Toluene andethyl benzene were similar at our site and westernFrance, but m#p-xylenes were higher at our site. Allmeasured hydrocarbons had lower concentrations thanthose reported for south east US; in fact our summerconcentrations are one order of magnitude smaller(Hagerman et al., 1997). In more remote sites in northCanada (Jobson et al., 1994) and north Finland (Laurilaand Hakola, 1996) the concentrations were generallysigni"cantly lower.

In terms of the seasonal variation, Table 1 presents twodi!erent pro"les. First, hydrocarbons with up to "ve car-bon atoms exhibited maximum concentrations in the fallperiod and minimum in the spring}summer. Second, allaromatic hydrocarbons except benzene exhibited max-imum concentrations in the summer. The seasonal vari-ation was particularly distinctive for isoprene, with thelargest concentrations observed in the summer. Similarseasonal variations have been observed in all studies ofC

2}C

5hydrocarbons and isoprene. The explanation of-

fered by Tille et al. (1985) for the lower concentrations ofthe C

2}C

5hydrocarbons in the summer and higher in the

winter, is the greater emissions in the winter. This pointis, however, discounted by Rotty (1987). In addition,higher destruction rates are operating in the summer dueto photochemical activity (Penkett et al., 1993; Tille et al.,1985), as well as to the generally higher mixing heights inthe summer (Jobson et al., 1994). The higher destructionrates in the summer are due to the increased concentra-tion of the hydroxyl radicals and in the case of ole"nsalso of ozone.

In the case of the aromatic hydrocarbons, all exceptbenzene exhibited larger concentrations in the summer.Similarly, in the study of Hagerman et al. (1997) insouth-eastern US, the maximum concentrations of thearomatic hydrocarbons were observed in the summer.No explanation was o!ered. The picture is less clearfor the only other existing work with measurements of


several aromatic hydrocarbons in western France (Bou-dries et al., 1994). In their work benzene and toluenepeaked in February and March with minimum concen-trations in October. For the other aromatic hydrocar-bons ethylbenzene, m#p- and o-xylene, minimum andmaximum concentrations were scattered throughout theyear, although the larger values were observed in March.The explanation we o!er for the higher aromatic hydro-carbons in the summer is based on the higher sources ofsome of these hydrocarbons in the summer. C

2}C

5al-

kanes, ole"ns and benzene are mainly emitted into theatmosphere by the exhaust of mobile sources and evapor-ation of their fuels. Larger aromatics, however, like tol-uene, xylenes and trimethyl benzenes, in addition to autorelated sources, may be emitted from the evaporation ofsolvents, of which are major components. These evapor-ation sources would become more important at the high-er summer temperatures and at the warmer regions, suchas ours. In order for the larger aromatics to exhibit themaximum yearly concentrations in the summer, as isobserved, the solvent evaporation emissions must exceedtheir destruction due to the enhanced in the summerphotochemical activity. Evidence for this hypothesis maybe obtained from the work of Derwent et al. (1995). Theymonitored for 12 months the C

2}C

9hydrocarbons on the

kerbside of a London street. From their data we cal-culated the hydrocarbon/acetylene concentration ratiosfor each of the given quarters of the year. The aromatichydrocarbons vs. acetylene ratios were higher in thewarmer July}August}September quarter, followed by thevalues of the quarter April}May}June. The October}No-vember}December and January}February}March quar-ters had the lower ratios. It must be pointed out thatthese seasonal di!erences existed even though the De-rwent et al. (1995) study was conducted on the kerbside ofa street and thus automobile exhaust and fuel evapor-ation would be the main factors that contribute to themeasured hydrocarbon concentrations. In the case ofAthens we have shown that in addition to mobile sourcessolvent evaporation could contribute to the observedconcentrations of larger than benzene aromatic hydro-carbons (Moschonas and Glavas, 1996) and a similaremission pattern would be observed in the greater region.

As an evidence of photochemical destruction of allmeasured hydrocarbons we present the hydrocarbon/acetylene ratios at Messorougion and in Athens for se-lected hydrocarbons with the larger concentrations, asshown in Table 2. Summer measurements are used fromboth sites. Acetylene was selected because it has beenshown to be emitted exclusively in auto exhaust (Whitbyand Altwicker, 1978). Like alkanes and aromatics acety-lene reacts exclusively with hydroxyl radicals. In factacetylene has the lowest rate constant with hydroxylradicals after ethane. Examination of the hydrocarbonconcentration ratios eliminates the dispersion factor anddepends only on their chemical degradation (Roberts et

al., 1985; Parrish et al., 1992; Jobson et al., 1994). There-fore, as result of photochemical activity the hydrocar-bon/acetylene ratio at Messorougion could not exceedthat in Athens. This ratio would be maximum in Athenswhere occur the emissions and it would decrease atMessorougion where no emissions exist, as result of thegreater reactivity towards the hydroxyl radicals of thehydrocarbon in the numerator. As expected most hydro-carbon/acetylene ratios were higher in Athens exceptpropane and isoprene. Apparently the isoprene/acetyleneratio is larger at Messorougion compared to Athens be-cause of the largest tree density at the rural site. In thecase of propane an explanation for higher sources at therural site is di$cult, even though propane has been pro-posed to be emitted by biomass burning and microbialproduction (Ehhalt et al., 1986), sources associated withrural regions. The ratios of 2-methyl- and 3-methyl-pentane to acetylene are signi"cantly larger at Mes-sorougion than in Athens. No explanation is available forthis observation. The remaining benzene, ethyl benzeneand xylenes to acetylene ratios were slightly larger atMessorougion compared to Athens, probably as result ofthe experimental uncertainty.

A quantitative veri"cation of the photochemical de-struction of the measured hydrocarbons can be obtainedfrom the ln}ln plots of hydrocarbon ratios, as was shownby Roberts et al. (1984). A plot of ln[HC

1]/[HC

3] vs.

ln[HC2]/[HC

3] is independent of dilution and of the

concentration of the hydroxyl radicals and it should yielda straight line with slope equal to (k

1!k

3)/(k

2!k

3).

[HCi] are the concentrations of the measured hydrocar-

bons and kitheir respective rate constants of their reac-

tions with hydroxyl radicals. These reactions are as-sumed to be pseudo-"rst order with respect to hydroxylradicals. Two of these plots are shown in Fig. 2a and b fortwo di!erent sets of hydrocarbon ratios. In the "rst plotof ln(3-methyl-pentane/n-butane) vs. ln(n-pentane/n-bu-tane) the reactivities of alkanes HC

1and HC

2towards

hydroxyl radicals, are very di!erent. In the second plot ofln(3-methyl pentane/n-butane) vs. ln (n-hexane/n-butane)the reactivities of alkanes are similar. Ratios of concen-trations of aromatic hydrocarbons of similar reactivitytowards OH radicals, are shown in Fig. 3a and b. Thebest agreement between the theoretical ratio of rate con-stants, calculated from rate constants tabulated by At-kinson (1994), and the calculated slope from Fig. 2a andb is obtained in Fig. 2b, when reactivities of examinedalkanes are similar, as was observed in past studies (Job-son et al., 1994; Parrish et al., 1992). In the aromatichydrocarbon ratio, shown in Fig. 3a and b, even thoughthe examined HC

1and HC

2hydrocarbons have similar

reactivity the deviation is signi"cant. The deviation wassigni"cantly improved, when only benzene concentra-tions larger than 0.09 ppb were used, as shown in Fig. 3b.The reasons for disagreement between experimental andtheoretical slopes in Fig. 3a may be the di!erent sources


Fig. 2. (a) ln(3-methyl-pentane/n-butane) vs. ln(n-pentane/n-butane) plot and (b) ln(3-methyl-pentane/n-butane) vs. ln(n-hexane/n-butane) plot.

Fig. 3. (a) ln(Toluene/benzene) vs. ln(ethylbenzene/benzene) plot. (b) ln(Toluene/benzene) vs. ln(ethylbenzene/benzene) plot. Only pointswith benzene concentration larger than 0.09 ppb.

of emissions which may imply di!erent dilution, as wasproposed by Roberts et al. (1984) and Parrish et al.(1992).

3.1. Biogenic hydrocarbons

From these hydrocarbons for the reasons given in theexperimental section only isoprene was measured. Asshown in Table 1, isoprene exhibited seasonal variationwith maximum concentrations in the summer. This is inagreement with all other studies (Boudries et al., 1994;Bottenheim and Shepherd, 1995; Jobson et al., 1994;Hagerman et al., 1997) who observed log[isoprene] toincrease linearly with the temperature. As shown in Fig. 4,the same relation applies to our data. The small lifetimeof isoprene, due to its reaction with hydroxyl radicals andozone, about 2.4 h, and the greater emissions when photo

active radiation is maximum, around midday, indicatethat isoprene would exhibit a diurnal variation. Thisvariation is clearly shown in Fig. 5, for data collected in4}5 September and 30}31 October 1996. The diurnalvariation is more pronounced in September as expected.Our summer isoprene concentrations are approximatelyhalf of the respective values reported by Hagerman et al.(1997), but are much larger than those reported by Bou-dries et al. (1994).

Model calculations predict summer hydroxyl radicalconcentrations for our latitude of the order of 1]106

molecules/cm3 (Spivakovsky et al., 1990). The mean mid-day ozone concentrations were measured at our site inthe summer to be around 50 ppb. Under these conditionsand without any other destruction route to isoprene, itslifetime would be q"(k

OH[OH]#k

O3[O

3])~1"

8547 s"2.4 h. The emission strength of isoprene in the


Fig. 4. Dependence of log(isoprene) on the ambient temperatureduring sampling.

Fig. 5. Diurnal variation of isoprene in two sampling periods inSeptember and October 1996.

summer can be calculated using the mean isoprene con-centration from the June and September sampling peri-ods of 0.78 ppb and the above lifetime. The emissionstrength is [isoprene]/q"0.78 ppb/8547 s"0.87 lgisoprene m~3 h~1 at 297 K and 0.86 atm. For an as-sumed 1.5 km mixing height, this would result in a isop-rene #ux of 1300 lg m~2 h~1 (at 243C, the average at oursite temperature during the sampling periods of June andSeptember). This value lies at the lower end of the #uxdeterminations reported by Lamb et al. (1985).

3.2. Origin of air masses

The origin of the air mass that a!ects the monitoringsite is of great importance to the concentration of thestudied hydrocarbons at our sampling site. During theday, most of the time, judging from the wind direction,the air masses arriving at our site were from west-north-

western or east-northeastern directions. On some occa-sions, however, winds from southern direction prevailed.In general one would expect southern wind directions toadvect cleaner air masses than the other directions. Inorder to quantify the origin of the air masses we obtainedair mass back trajectories from Meteo France for theperiod of May 1997. This period was selected because inaddition to the wind direction which varied from south-ern to western to eastern direction, clear variations in thetemperature and relative humidity were also observed. Atthe start of monitoring on 06 May 1997 wind directionwas from the south with temperature ranging from 17}223C and relative humidity 20}30%. After 09 May 1997and until 11 May wind direction changed to westerndirections, temperature dropped to 8}173C and relativehumidity increased to the usual 30}70%. Similar temper-ature and relative humidity ranges were observed from11}15 May when wind was from northeastern directions.These observations clearly indicate the advection to oursite of di!erent air masses. The obtained air mass trajec-tories, shown in Fig. 6, con"rm our indications. From06}09 May air masses were originating, 4 days before, innorthern Africa. After 09 May and until 11 May, the airmasses were originating in western Mediterranean(North Spain}South France). From 12}15 May the airmasses were originating in north-eastern directions(Black Sea}South Russia}Ukraine). It is apparent thatfrom these three directions the cleaner air masses wouldbe expected from northern Africa. The other two direc-tions may have similar emission sources, although thespeci"c path of a trajectory will ultimately determinethe quality of advected air masses. The average of themajor 15 hydrocarbons of Table 2 was calculated for theabove three directions and is shown in Table 3. FromTable 3 it may be inferred that western directions may

Fig. 6. 4-Day air mass back trajectories at 850 mBar. A. Traject-ory arriving at 13:00 h, local time, of 07 May 1997 B. Trajectoryarriving at 13:00 h of 10 May 1997 and C: Trajectory arriving at13:00 hours of 13 May 1997.


Table 3Mean values of concentrations of selected hydrocarbons, ozoneand NO@

xfor the samplings of May 1997 distributed in groups

according to the origin of the air mass

Hydrocarbon 6}8/5/97 9}10/5/97 11}15/5/97

Mean value, ppb

Acetylene 0.11 0.06 0.06Propylene 0.01 0.01 0.01Propane 0.29 0.58 0.36Isobutane 0.01 0.03 0.01n-Butane 0.02 0.05 0.02Isopentane 0.01 0.04 0.02n-pentane 0.01 0.02 0.01Isoprene 0.06 0.03 0.022-Methyl-pentane 0.03 0.02 0.013-Methyl-pentane 0.03 0.04 0.03n-Hexane 0.01 0.02 0.01Benzene 0.03 0.04 0.02Toluene 0.04 0.05 0.02Ethyl-benzene 0.01 0.01 0.00m#p-Xylenes 0.02 0.03 0.01o-Xylene 0.01 0.03 0.00NO@

x0.37 1.07 0.75

O3

51 52 55

carry slightly more polluted air masses. Northeasterndirections, which would be expected to carry polluted airmasses, do not carry signi"cant amounts of hydrocar-bons. It is probable that high amounts of hydrocarbonsare not emitted in these regions, unlike their high sulfuremissions (Danalatos and Glavas, 1999). An additionalexplanation may be obtained from Fig. 6, indicating thatparticular trajectories cross mostly marine areas withlittle contact with direct emissions.

It must be pointed out here that isoprene had doubleconcentration in the period from 6}8 May compared tothe remaining sampling days in May 1997. The reason isthat in 6}8 May the average temperature was signi"-cantly larger than the following days and as was discus-sed earlier temperature, as result of high photoreactiveradiation, is the major factor that determines the isop-rene concentrations.

3.3. Propylene equivalent concentrations

In order to estimate the contribution of each hydrocar-bon to the formation of ozone, various reactivity scaleshave been proposed. One of them is the propylene equiv-alent concentration proposed by Chameides et al. (1992).This scale takes into account both the concentration andthe rate constant of a hydrocarbon's reaction with thehydroxyl radicals; the second factor accounts for thewell established signi"cance of the reaction of hydroxyl

Fig. 7. Seasonal variation of propylene-equivalent concentra-tions for the sum of concentrations of measured hydrocarbons(TNMHC), isoprene, sum of para$ns, sum of ole"ns and sum ofaromatic hydrocarbons.

radicals with all hydrocarbons to the formation ofozone.

In Fig. 7 are shown the propylene equivalent con-centrations of the total non-methane hydrocarbons(TNMHC) as well as the groups of which the TNMHCconsist, namely para$ns, ole"ns and aromatics. Thebiogenic isoprene is given separately from the ole"ns,because of its great importance. Maximum reactivity isclearly exhibited in the summer, as shown in Fig. 7, asresult of the increased contribution of isoprene. Thisin turn is due to the greater isoprene emissions in thewarmer period, as was discussed earlier. In the summerisoprene dominates the hydrocarbon reactivityaccounting for 70% of the total. The second most im-portant group of hydrocarbons in terms of propyleneequivalent concentrations is the aromatics followed byole"ns, while para$ns are last. Outside the warm monthsaromatics and ole"ns, with similar contribution, accountfor about 65% of the propylene equivalent concentration.

4. Conclusions

The larger measured hydrocarbons in decreasing orderof concentration, in ppbv, are: propane, acetylene, n-bu-tane, toluene, m#p-xylene, sum of trimethyl-benzenes,styrene, 3-methylpentane, isopentane, benzene, iso-butane, isobutene#butene-1. Similar order of concen-trations is observed in all other rural studies in south eastUS, east Canada, western France, as well as Scandinaviaand north Canada. It is interesting to note that aro-matic hydrocarbons are high in the order of concen-trations. This series may somewhat vary because paraf-"ns and ole"ns exhibit their minima in the summer.Aromatics have their maxima in the summer, indicating


higher emissions in the summer enough to overcome thelarger photochemical destruction and generally highermixing heights.

The levels of the concentrations of hydrocarbons ofanthropogenic origin are lower at our site compared torespective rural sites in eastern US and Canada andwestern Europe. However, the concentrations deter-mined here are larger than the ones measured in moreremote regions. These "ndings indicate that the anthro-pogenic emissions in our region are lower than those innorthwestern Europe and eastern north America. Thebiogenic isoprene exhibited lower concentrations herethan in the rural east US, but higher than those inwestern Europe. The here determined isoprene #ux of1300 lg m~2 h~1 depends critically on the assumed mix-ing height of 1500 m.

Degradation of the hydrocarbons determined here oc-curs mainly by the hydroxyl radicals, as is also reportedelsewhere (Roberts et al., 1984; Jobson et al., 1994) indi-cating their importance in the photochemical activity.Quantifying their reactivity, inferred from the calculationof propylene-equivalent concentrations, indicates thatthe biogenic hydrocarbon isoprene dominates in thesummer, followed by the aromatics, ole"ns and para$ns.In the two sampling periods of late October and May,representing fall and spring the aromatic hydrocarbonsare most important followed by the ole"ns and para$nsthat were measured.

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Documents

Non-methane hydrocarbons at a high-altitude rural site in the Mediterranean (Greece)