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Atmospheric Environment

Atmospheric Environment 42 (2008) 6000– 6010

1352-23

doi:10.1

� Cor

E-m

journal homepage: www.elsevier.com/locate/atmosenv

Estimate of initial isoprene contribution to ozone formation potentialin Beijing, China

Xin Xie a, Min Shao a,�, Ying Liu a, Sihua Lu a, Chih-Chung Chang b, Zhong-Ming Chen a

a State Joint Key Laboratory of Environmental Simulation and Pollution Control, College of Environmental Sciences and Engineering, Peking University,

Beijing 100871, Chinab Research Center of Environmental Changes, Academia Sinica, Nankang, Taipei 115, Taiwan

a r t i c l e i n f o

Article history:

Received 11 December 2007

Received in revised form

21 March 2008

Accepted 25 March 2008

Keywords:

Initial isoprene

Chemical loss

MIR

VOCs

10/$ - see front matter & 2008 Elsevier Ltd.

016/j.atmosenv.2008.03.035

responding author. Tel.: +86 10 6275 7973.

ail address: mshao@pku.edu.cn (M. Shao).

a b s t r a c t

Volatile organic compounds (VOCs) are important precursors of tropospheric ozone

formation. Isoprene contributions to ozone formation by using ambient mixing ratios are

generally underestimated because of rapid chemical losses. In this study, ambient mixing

ratios of major VOC species were continuously measured at Peking university (PKU) and

YUFA, urban and sub-urban sites in Beijing, the city that will host 2008 Olympic Games.

The observed mixing ratios of methyl vinyl ketone (MVK), methacrolein (MACR) and

isoprene were used to derive the mixing ratios of initial isoprene, which means the

ambient isoprene level before it undergoes any photochemical reaction with OH radicals.

The average mixing ratios of initial isoprene were 3.371.6 and 2.971.5 ppbv at PKU and

YUFA sites, respectively. The percentages of initial isoprene in total initial VOCs were

10.8% at PKU site and 11.4% at YUFA site, in reasonable agreement with the isoprene

contribution in total VOC emissions as derived from source inventories. Maximum

increment reactivity (MIR) was used to evaluate the ozone formation potential (OFP) for

major VOC species. The OFP for initial isoprene accounted for 23% of the total OFPs for all

measured species, compared to 11% using ambient mixing ratios of isoprene at PKU site.

Similarly, at YUFA site, the ambient measured isoprene and initial isoprene contributed

10% and 22%, respectively, to the OFPs for total measured VOCs. It seems that isoprene has

similar contribution to ozone formation at both sites in Beijing city.

& 2008 Elsevier Ltd. All rights reserved.

1. Introduction

The annual global isoprene emission estimated withMEGAN ranges from about 500 to 750 Tg isoprene(440–660 Tg carbon) depending on the driving variables,which include temperature, solar radiation, leaf areaindex, and plant functional type (Guenther et al., 2006).In more remote forested regions at low ambient NOx

mixing ratios, ozone formation is limited despite theabundance of biogenic volatile organic compounds (VOCs)such as isoprene. However, in rural, suburban, or urban

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areas that are impacted by anthropogenic emissions ofNOx, isoprene has potentially a very large impact on ozoneformation (Chameides et al., 1997; Starn et al., 1998;Stroud et al., 2001).

Chameides et al. (1988) used a model to simulate theformation of ozone in Atlanta. Model calculation indicatedthat natural emissions could significantly affect urbanozone levels. Ryerson et al. (2001) used data taken fromaircraft transects of emission plumes from US rural coal-fired power plants to confirm and quantify the nonlineardependence of troposphere ozone formation on plumeNOx mixing ratios. The ambient reactive VOCs, principallybiogenic isoprene, were also found to modulate ozoneproduction rates and yields in these rural plumes. Dreyfuset al. (2002) made observations of isoprene, methyl vinyl

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X. Xie et al. / Atmospheric Environment 42 (2008) 6000–6010 6001

ketone (MVK) and methacrolein (MACR) along thewestern slope of the Sierra Nevada Mountains, and foundthat the daytime mixing ratios of isoprene’s oxidationproducts and ozone were highly correlated. On average,over 40% of the observed midday ozone formation in thisregion was attributed to isoprene oxidation. On ozoneepisode days (maximum [O3]490 ppbv), the mean iso-prene contribution was over 70%. Stroud et al. (2001)compared the observation of MVK and MACR to theresults from two chemical models: a simple sequentialreaction scheme and a one-dimensional (1-D) numericalbox model. Good agreements suggested that the first fewstages of isoprene oxidation in an urban forested environ-ment were well understood. Barket et al. (2004) examineda large dataset of mixing ratios of isoprene and itsoxidation products at four North American sites to assessthe NOx dependence of the daytime oxidation of isoprene.Isoprene oxidation resulted in the largest ozone produc-tion rates with NOx concentrations in the range of1–10 ppb. This range of NOx mixing ratio was representa-tive for rural areas with human activities’ influence. Theresults clearly showed the large impact that urbanemissions and power plant plumes might have oninitiating isoprene chemistry in downwind forest envir-onments.

It is well documented that ambient isoprene willexperience large chemical loss due to its chemicalreactivity, especially via reaction with OH radicals in thedaytime. Therefore, it is very likely that the ozoneformation potential (OFP) for isoprene is underestimatedby using solely observed isoprene mixing ratios. To correctfor this effect, an approach to extrapolate observedisoprene mixing ratios back to the mixing ratios of initialisoprene, which means the ambient isoprene level beforeit undergoes any photochemical reaction with OH radicals,was developed by using the measurements of isoprenephotoproducts, MVK and MACR. The approach wasemployed in several studies. Stroud et al. (2001) madeuse of correlation between the MVK/ISOP and the MACR/ISOP ratio for the daytime measurements (0900–1600LST) during SOS-1990 and SOS-1999 campaigns in USA.The conclusion was that air masses with more photo-chemically aged isoprene were observed during SOS 1999at Cornelia Fort (0.3–1.6 h) compared to the SOS 1990canopy study at Kinterbish (0.1–0.6 h). Apel et al. (2002)did similar work and calculated isoprene photochemicalages ranging from 0.06 to 0.3 h in a rural forested site, andthe location of the sampling manifold was shown to affectthe measured ratios of MACR and MVK to isoprene. Thephotochemical lifetime of measured isoprene was sig-nificantly less than the photochemical lifetime of isoprene(45 min at [OH] ¼ 3.35�106 molecules cm�3). Thus alarge portion of the isoprene that reached the manifoldhad no time to react completely with OH, yielding lower-than-expected ratios. de Gouw et al. (2005) calculated theinitial mixing ratios of isoprene to be in the range fromvery low levels to about 14 ppbv and found that thesewere well correlated with the mixing ratios ofMVK+MACR.

Beijing city is busy preparing for 2008 Olympic Games.As the ambient air quality standard for ozone is frequently

exceeded in Beijing, China (Wang et al., 2006), ground-level ozone abatement has become an urgent need for thecity. Therefore it is important to have reliable estimates ofVOC sources. Song et al. (2007) used a positive matrixfactorization (PMF) model to perform source apportion-ment of VOC in Beijing, and concluded that biogenicsources were one of the major contributors to total VOCOFP. The Beijing municipal government is hastening toincrease vegetation coverage by implementing 9 vegeta-tion coverage projects, to achieve over 45% of green area inBeijing (Beijing Olympic Games Action Plan(3)). This planwill lead to increased biogenic emissions of isoprene, from21,000 to 34,900 t (Xie, 2007), then contributing to theformation of ground-level ozone. In this study, the mixingratios of ambient VOCs at the campus of PekingUniversity, a downtown site, and YUFA, a sub-urban sitein south Beijing, were measured continuously from15 August to 10 September, 2006. These two sites havequite different vegetation coverage, and are used as twocases for study of isoprene photochemistry to ozoneformation. The role of biogenic isoprene in ozone produc-tion was evaluated by estimating initial isoprene mixingratios and their contributions to OFP.

2. Experimental method

2.1. Site description

The measurements were performed between 15 Au-gust and 10 September, 2006, at Peking University (PKU)and YUFA sites in Beijing, shown in Fig. 1. Other airpollutants, including NO, NO2, and O3, were also measuredat both sites. PKU site is an urban site located in northwestBeijing, about 14 km from downtown. Directly east of thePKU site is a very busy north–south connecting road;directly south of the site is the 4th ring road situated, withvery heavy traffic. This site is surrounded by residentialapartments and shops (predominantly of electronicequipment). The instruments are located on the roof of asix story building (about 20 m above ground level) in thecampus. YUFA site is situated in the southern part ofBeijing. It is a rural site around 65 km from the city center.No strong local emission sources are present in thevicinity of the site. Ambient air was drawn through asample inlet at a height of 2 m above the roof of a fourstory building (about 12 m above ground level) in thecampus of Huangpu University.

During the whole campaign, the weather was char-acterized by sunshine with very low frequency of rainevents, the ambient temperatures ranged from 10 to 35 1C,and the prevailing winds came from south or southeastwith an average wind speed of 1.671.4 m s�1.

2.2. VOCs and OVOCs analysis

At PKU site, continuous measurements of 47 VOCspecies were performed every hour using an automatedonline GC-FID system from the Research Center forEnvironmental Changes (RCEC), Academia Sinica ofTaiwan. The detection limit of VOC species ranged from

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Fig. 1. Map of the sampling sites and wind speed and wind direction during field measurements.

X. Xie et al. / Atmospheric Environment 42 (2008) 6000–60106002

0.002 to 0.098 ppb v/v. The calibration of the instrumentwas performed before and after the campaign by use of astandard gas prepared by the gravimetric method (Spectragases, Branchburg, NJ, USA), containing 50 target specieswith mixing ratios in the range of 3 and 15 ppbv; theresults indicated that the variation of sensitivities of allthe measured compounds were o2%. More details of theGC-FID systems are described elsewhere (Wang et al.,2000, 2002, 2004).

At YUFA site, continuous measurements of 39 VOCspecies were automatically performed with a timeresolution of a half hour by using two online GC-FID/PIDsystems (Syntech Spectra GC-FID/PID GC955 series 600/800 VOC analyzer): one, for the C2–C5 VOCs, uses a gaschromatograph with pre-concentration on CarbosievesSIII at 5 1C, followed by thermal desorption and separationon a combination of two columns, a capillary film columnand a capillary PLOT column. Photo ionization detector(PID) and a flame ionization detector (FID) are used forquantification. The other system is for C6–C10 VOCsanalysis; air samples are pre-concentrated on Tenax GRat normal temperature, thermal desorbed, separated on anATTM-1 column, and detected by a PID. For each analysis,an air sample with a volume of 250 mL was sampled.Calibration was also performed before and after thecampaign by using a gas standard containing 57 targetspecies with mixing ratios of 1 ppm in nitrogen, preparedby the gravimetric method (Spectra gases, Restek Cor-poration, USA). The sensitivity changes of the measuredVOCs compounds were within 10%. The detection limit ofVOCs ranged from 0.001 to 0.09 ppbv.

Ambient oxygentated VOCs (OVOCs) including MVKand MACR were sampled by dinitrophenylhydrazine(DNPH)-coated C18 cartridges (Waters and Associates).At PKU site samples were taken every 3 h from 8:30 to23:30 from 24 August to 29 August and 7–8 September. At

YUFA site, the OVOCs were sampled every 3 h from 8:50 to20:50 from 2 September to 11 September except for9 September. The ambient air was collected at a flow rateof 100 L min�1 through a KI ozone scrubber as describedby Gaffney et al. (1997) during a 3 h sampling period. Thesample cartridges and field blanks were sent back tolaboratory for analysis according to the TO-11 recom-mended by US EPA. Linearity (R2) of calibration curves ofHPLC for all the measured species was higher than 0.9994.The detection limits of OVOCs are below 10�8 mol L�1.Daily calibration was done before samples’ analysis. Theprecisions of HPLC for all OVOC species have been checkedby duplicated analysis, and the RSD (relative standarddeviation) values are measured to be 0.32% for MACR and0.70% for MVK.

2.3. Methodology for estimating initial isoprene

mixing ratios

Isoprene is a reactive compound, leading to significantchemical losses during transport from sources to themeasurement site; so ambient levels do not correspond toemissions. It is well known that based on analyses of MVK,MACR and isoprene estimates can be derived of time scaleson which isoprene photochemistry has taken place (Dreyfuset al., 2002; de Gouw et al., 2005; Roberts et al., 2006).Makar et al. (1999) concluded that isoprene emissionswould be underestimated by up to 40% if chemical losseswere neglected. As isoprene and its oxidation products MVKand MACR are simultaneously measured, the initial isoprenemixing ratio can be derived from the sequential reactionmodel described by Stroud et al. (2001):

Isopreneþ OH! 0:23MACR þ 0:32MVK;

k1 ¼ 1:00� 10�10 cm3 molecules�1 s�1 (1)

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X. Xie et al. / Atmospheric Environment 42 (2008) 6000–6010 6003

MVKþ OH! products; k2 ¼ 2:0� 10�11 cm3 molecules�1 s�1�MACR

þOH! products; k3 ¼ 2:9� 10�11 cm3 molecules�1 s�1

(2)

Therefore,

½MVK�t½ISOP�t

¼0:32k1

k2 � k1ð1� eðk1�k2ÞOHavgtÞ (3)

½MACR�t½ISOP�t

¼0:23k1

k3 � k1ð1� eðk1�k3ÞOHavgtÞ (4)

ISOP�0 ¼ ½ISOP�t � ek1�OHavgt (5)

where k1, k2, k3 are the rate constants of isoprene, MVK andMACR reacting with OH (Atkinson et al., 2006). [ISOP]0

represents initial isoprene mixing ratio. [MVK]t, [MACR]t,[ISOP]t represent observed mixing ratios at time t, respec-tively, and OHavg is the average OH level in the air mass.

From the measured ratios of ambient MVK, MACR andisoprene, the chemical age of air mass from emissionsource to measurement site is determined by Eq. (3) or(4), and the initial isoprene mixing ratio can be calculatedfrom measured isoprene by Eq. (5). Ratios of MVK andMACR to isoprene are used to compensate for the effect ofdilution during transport. As there is no significantdifference by using either Eq. (3) or (4), the OHavgt canbe calculated as the average of the results from Eqs. (3)and (4). By using Eq. (5), the value of OHavgt is used as aninput for [ISOP]0 calculation. It is noteworthy that, in thisapproach, Eqs. (1), (2), Eqs. (3)–(5) are only valid in

Fig. 2. Time series of hourly average mixing ratios of O3, O3+NO2, NO and TVOC

daytime. Therefore only daytime measured data (from9:00 AM to 17:00 PM) were used for calculations in thiswork.

3. Results and discussion

3.1. Variations of O3, O3+NO2, NO and VOCs

Fig. 2 presents the hourly time series of the mixingratios of ground-level total oxidant (O3+NO2), NO, andtotal measured VOC species (TVOCs) at PKU and YUFAsites (Levy II et al., 1985; Liu, 1977). The average dailymaximum of 1 h mixing ratio of ozone was 69736 ppbv atPKU site. There were 7 days (28% of overall measurementdays) when ozone mixing ratios were over 100 ppbv(Grade II, China State Environmental Protection Adminis-tration, revised at 2001). At YUFA site, the ozone levelswere generally higher than at the PKU site; the averagedaily maximum of 1 h ozone mixing ratio was81722 ppbv. There were 4 days (16% of overall measure-ment days) when ozone mixing ratios were over 100 ppbv(Grade II). Ozone mixing ratio had higher average level atYUFA site, but had lower variability compared to PKU site.

The average daily maximum of 1 h total oxidant(O3+NO2) at the two sites was 98742 ppbv at PKU, withhigher mixing ratio and larger variability compared to theYUFA site, where it was 88723 ppbv. As Fig. 2 indicates,the day-to-day diurnal variations of Ox were similar at thetwo sites.

at PKU and YUFA sites in Beijing. (Gridlines indicate zero hour of a day.)

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alkene24%

aromatic21%

alkane53%

isoprene2%

alkane alkene aromatic isoprene

alkene9%

aromatic32%

alkane57%

isoprene2%

alkane alkene aromatic isoprene

Fig. 3. Percentages of mixing ratios of different VOCs groups in total VOCs at PKU and YUFA sites in Beijing.

X. Xie et al. / Atmospheric Environment 42 (2008) 6000–60106004

NO mixing ratio was generally higher at PKU site thanat YUFA site. The average mixing ratios of NO were 7.5 and3.7 ppbv, and its maximum concentrations were 66.4 and57.1 ppbv at PKU and YUFA site, respectively.

TVOC mixing ratios were at similar levels at both sites,with average mixing ratios of 29.1717.2 ppbv at PKU siteand 28.2715.3 ppbv at YUFA site. The average composi-tions of VOCs (based on mixing ratios, ppbv) are given inFig. 3. At both sites, anthropogenic hydrocarbons aredominant, compared to the small contribution of isoprene(2%). Alkanes contribute most (450%), followed byaromatics (21–32%) and alkenes (9–24%).

The VOC speciation results at both sites are shown inTable 1. Because the method to estimate initial isoprenemixing ratios is only applicable in the daytime, the datashown in the table are sampled from 9:00 AM to 17:00 PM(Beijing time). Most species at PKU site had higher mixingratios and larger mixing ratios variability than at YUFAsite, for instance, BTEX and isoprene.

As shown in Table 1, during the measurement period,toluene and i-pentane were the most abundant VOCspecies at PKU site; the next abundant species werebenzene, butane, pentane, hexane, m,p-xylene and iso-prene. Comparing to PKU site, the top 10 abundant VOCspecies at YUFA site were (in order of mixing ratio) ethane,ethene, propane, butane, i-pentane, cyclohexane, benzene,toluene and propylene.

Winer et al. (1992) proposed that isoprene andterpenes were major VOCs emitted from vegetation. Butvehicular emission also contributes to ambient isoprenemixing ratios (Reimann et al., 2000; Borbon et al., 2001).Isoprene mixing ratio and variability are lower at YUFAsite than at PKU site. Average isoprene mixing ratios atboth sites are comparable to values of 0.65 ppbv observedin Hong Kong (So and Wang, 2004), 0.8 ppbv in Karachi(urban area) in the southeast of Pakistan (Barletta et al.,2002) and 0.5 ppbv in Santiago (Chen et al., 2001).

The oxidized products of isoprene, MVK and MACRwere also measured. The average mixing ratios weresimilar at PKU and YUFA site, MVK average mixing ratiowas 0.3470.17 ppbv at PKU site and 0.3770.24 ppbv at

YUFA site; MACR average mixing ratio was 0.7070.42 and0.5570.31 ppbv, respectively.

3.2. Initial mixing ratios of isoprene

The method described in Section 2 is used to calculatethe initial mixing ratios of isoprene in the daytime. Theresult is that the initial isoprene mixing ratios are3.2571.63 ppbv at PKU site and 2.8671.47 ppbv at YUFAsite, much higher than the observed ambient mixingratios: 0.8970.55 and 0.6470.44ppbv, respectively. Thepercentages of initial isoprene in total initial VOCs, whichwere calculated in Section 3.3, increased to 10.8% at PKUsite and 11.4% at YUFA site.

To compare with the estimation of initial isoprenelevels in the air, the previous studies on source inventoryof isoprene are summarized in Table 2. Estimates of thecontributions of initial isoprene to total initial VOC at thetwo sites are comparable to the biogenic isoprenecontribution obtained from the source inventory studiesfor Beijing city, which was 12.9% as shown in Table 2. Itdiffers greatly from the observed ambient isoprenecontributions to total VOC, 4.5% and 4.4% for PKU andYUFA site, respectively. However, they are comparable tothe percentage of initial isoprene in the total initial mixingratios of VOCs at both sites. It can also be seen from thetable that the discrepancies of the source inventories arelarge, mainly due to the differences in the estimates onabundance of plant species and the forest area of thesespecies (Guenther et al., 2000). We consider that thebiogenic isoprene in Beijing comes from two parts: one isthe emissions from forests. We estimate this part ofisoprene as the average of all the previous studies onforest sources in Beijing; the second part is from urbangreen-lands developed under the 2008 Green Olympicprojects (Xie et al., 2007). It is noteworthy that theisoprene emissions by this approach may be overesti-mated due to possible overlapping in the emissions fromforests and green-lands in urban areas.

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Table 1Daytime mixing ratios and maximum incremental reactivity (MIR) of measured VOC species in Beijing, units: ppbv (average7standard deviation) (Some

compounds are not measured at each site because of different measure systems.)

Group Species Urban (ppbv) PKU site Rural (ppbv) YUFA site MIRa (g ozone g VOC)�1

Alkanes Ethane – 4.6770.24 0.25

Propane 1.4070.92 2.0672.47 0.48

Isobutane 1.5771.18 1.3871.28 1.21

n-Butane 1.7471.43 1.5271.53 1.02

Cyclopentane 0.1370.09 – 2.4

Isopentane 2.4371.78 0.9370.78 1.38

n-Pentane 1.1570.93 0.6170.54 1.04

2,2-Dimethylbutane 0.2470.17 – 0.82

Methylcyclopentane 0.1370.10 – 2.8

2,3-Dimethylbutane 0.1970.11 – 1.07

2-Methylpentane 0.5970.42 0.3570.26 1.5

3-Methylpentane 0.5370.33 0.3170.16 1.5

2,4-Dimethylpentane 0.2270.19 0.4470.46 1.5

n-Hexane 1.1170.80 0.2070.11 0.98

Cyclohexane 0.2270.17 0.7570.91 1.28

2-Methylhexane 0.3770.26 0.3970.19 1.08

2,3-Dimethylpentane 0.0870.05 – 1.31

3-Methylhexane 0.4970.34 0.2470.13 1.4

2,2,4-Trimethylpentane 0.0470.03 0.1770.07 0.93

n-Heptane 0.3270.23 0.2670.12 0.81

Methylcyclohexane 0.2370.17 0.1270.03 1.8

2-Methylheptane 0.1270.08 0.0670.05 0.96

3-Methylheptane 0.1470.10 – 0.99

n-Octane 0.1770.12 0.3570.06 0.6

n-Nonane 0.1670.19 – 0.54

n-Decane 0.1970.24 – 0.46

n-Undecane 0.1270.17 – 0.42

n-Dodecane 0.0670.04 – 0.38

Alkenes Ethene – 3.6770.82 7.4

Propylene 0.4070.23 0.9070.45 9.4

trans-2-Butene 0.1970.17 0.1570.03 10

iso-Butene – 0.3070.11 5.3

1-Butene 0.4170.27 0.3170.23 8.9

cis-2-Butene 0.2070.14 0.1570.05 10

trans-2-Pentene 0.1370.09 0.11270.01 8.8

1-Pentene 0.1370.08 0.1370.03 6.2

cis-2-Pentene 0.1070.05 0.1170.01 8.8

Isoprene 0.8970.55 0.6470.44 9.1

1,3-Butadiene – 0.1670.07 10.9

3-methylbutene – 0.1670.13 6.2

Aromatics Benzene 2.1572.09 1.4571.01 0.42

Toluene 2.4971.76 1.2770.82 2.7

Ethylbenzene 0.7570.55 0.4070.27 2.7

m,p-Xylene 1.0870.78 0.5070.40 7.4

Styrene 0.2270.17 0.1370.02 2.2

o-Xylene 0.4470.31 0.2670.17 6.5

Isopropylbenzne 0.0570.03 – 2.2

n-Propylbenzene 0.0970.06 – 2.1

1,3,5-Trimethylbenzene 0.1170.10 – 10.1

1,2,4-Trimethylbenzene 0.3070.25 – 8.8

1,2,3-Trimethylbenzene 0.1270.08 – 8.9

OVOCs MVK 0.3470.17 0.3770.24

MACR 0.7070.42 0.5570.31

a MIR denotes maximum incremental reactivity (g O3/g VOC), Carter, 1994.

X. Xie et al. / Atmospheric Environment 42 (2008) 6000–6010 6005

The correlation between the [MACR]/[ISOP] andthe [MVK]/[ISOP] ratios for daytime measurement iso-prene is an indicator for photochemical ages in airmasses (Stroud et al., 2001; Apel et al., 2002). Fig. 4shows this correlation at PKU (triangles) and YUFA(circles) site, indicating that the air masses at YUFA have

undergone more photochemical conversion, compared tothe PKU site.

Scatter plots of initial isoprene versus MVK+MACR atboth sites are given in Fig. 5. It is seen that the initialisoprene is well correlated with MVK+MACR at both urbanand sub-urban sites, and that the slopes are higher than

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Table 2Comparison of the biogenic emissions from source inventories and ambient measurement in Beijing

Biogenic source estimation Isoprene emissions, (Gg) Reference

Forest 7.0 Wang et al. (2003)

24.5 Klinger et al. (2002)

12.3 Guenther et al. (1995)

Forest average 14.6

Greenland vegetation 27.3 Xie, et al. (2007)

Anthropogenic source 323 Zhao (2004)

Biogenic isoprene emissions in total VOCs sources, % 12.9

Measured Isoprene contribution to total VOCs levelsa PKU 4.5% This work

YUFA 4.4%

Estimated Initial isoprene contribution to total VOCs levelsa PKU 10.8% This work

YUFA 11.4%

a Calculated based on mass concentrations of VOCs species.

0.01

0.10

1.00

10.00

0.10 1.00 10.00

[MACR]/[ISOP]

[MV

K]/

[ISO

P]

PKUYUFA

Fig. 4. The correlation between the [MACR]/[ISOP] and the [MVK]/[ISOP]

ratio from the daytime measurement (0900–1700 LST) at PKU (triangles)

and YUFA (circles) sites.

X. Xie et al. / Atmospheric Environment 42 (2008) 6000–60106006

the yield of MVK+MACR from the reaction between OHand isoprene (54%). The reason is that reactions with OHalso cause some losses of MVK and MACR in the daytime.So the result is that the calculated initial isoprene is stillwell correlated with MVK+MACR, but the slope is higherthan (MVK+MACR).

The slope at YUFA site is similar to that at PKU site asshown in Fig. 5. But, the intercept is larger at PKU site. Thereason might be that at PKU site there is isopreneaccumulating in the nighttime from traffic emissions. 1-Butene is usually regarded as an indicator of trafficemissions. Initial isoprene shows a correlation with 1-butene (R2

¼ 0.30, n ¼ 22) at PKU site, but not at YUFA(R2¼ 0.06, n ¼ 22). In addition, no correlation is found

between isoprene and other species from anthropogenicsources such as m,p-xylene and toluene with R2

¼ 2.1E�7and R2

¼ 0.16, respectively; so isoprene is not emitted bythese sources. The relative small intercept indicates thatthe contribution of traffic is limited.

3.3. OFPs for ambient and initial VOCs

To evaluate the relative importance of VOC species inthe production of ground-level ozone, the concept of OFPs,

which is the product of mixing ratio of VOC species andthe maximum incremental reactivity (MIR) as developedby Carter (1994), is employed in numerous studies (Changet al., 2005; Chiang et al., 2007; Hsieh and Tsai, 2003; Yuet al., 2000).

The OFPs for individual VOC species at PKU and YUFAsite were calculated. Fig. 6 shows the top ten VOC specieswith the highest OFP values. These ten species areobviously different from the top ten abundance VOCspecies. At PKU site, the top ten OFP VOC species accountfor 68% in the total OFP of all measured species. Aromaticshave significant contributions to the OFPs at PKU site. Sixaromatic species, namely m,p-xylene, toluene, o-xylene,1,2,4-trimethylbenzene, ethylbenzene, and 1,3,5-tri-methylbenzene, account for 46% in the total OFPs at PKUsite. Isoprene only contributes 11% to the total OFPs, lowerthan m,p-xylene and toluene. YUFA site is different, thetop ten OFP VOC species accounting for 68% of the totalOFPs. The OFP for ethene was almost twice as much asother reactive species. The OFP for isoprene, similar topropylene, m,p-xylene, toluene, and accounts for 10%, nextto ethene.

The mixing ratios of initial isoprene must be consid-ered if measures are considered for ozone abatement. Butother species also have chemical losses before reachingthe receptor site. Wiedinmeyer et al. (2001) developed amethod to derive VOCs losses from isoprene conversion.Consumed isoprene, which is actually the differencebetween initial and measured level of isoprene in the airmass, is calculated. Then, the change in mixing ratios ofVOCi due to losses (or the photochemical loss, DVOCi) canbe estimated from the isoprene conversion

DVOCi ¼ Disoprene�kVOCi

kisoprene�½VOCi�

½isoprene�(6)

where [isoprene] and [VOCi] are the measured mixingratios of isoprene and VOCi, respectively, and kisoprene andkVOCi

are the rate constants for the reaction of isopreneand VOCi with the hydroxyl radical, respectively. Eq. (6)assumes that the relative source strengths of VOCs areconstant in the immediate area surrounding the site andthat atmospheric transport and dispersion are non-limit-ing factors compared with chemistry. For this purpose, the

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slope=1/0.54

y = 2.96x + 0.02R2 = 0.99

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

MVK+MACR (ppbv)

Initi

al is

opre

ne (

ppbv

)

slope=1/0.54

y = 2.89x + 0.25R2 = 0.96

0.00.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

MVK+MACR (ppbv)

Initi

al is

opre

ne (

ppbv

)

0.4 0.8 1.2 1.6 2.0 0.0 0.4 0.8 1.2 1.6 2.0

Fig. 5. Scatter plot of initial isoprene versus MVK+MACR at (a) PKU and (b) YUFA sites in Beijing.

Fig. 6. The top 10 VOC species in terms of OFP at PKU and YUFA sites in Beijing.

X. Xie et al. / Atmospheric Environment 42 (2008) 6000–6010 6007

isoprene chemistry is employed in this work to estimateDVOCi, because it is the fastest comparing with thechemical processes of other VOC species, e.g. aromatics.It would be more reasonable to assume constant emis-sions in shorter time period. And secondly, the ratio of[VOCi] and [isoprene] is used in Eq. (6); if they follow asimilar transportation path, the effects of dilution ordispersion could then be cancelled out.

As seen in Figs. 6 and 7, at PKU site, the OFPs for totalVOCs increase by 70% after correction for chemicalconversion. The main reason is that the OFP for isopreneincreases greatly from 12 to 42 ppbv. As a result, the OFPfor isoprene accounts for 23% of the total OFPs of allmeasured species, and its OFP value is 56% higher thanthat of m,p-xylene and nearly two times higher than that

of toluene. At YUFA site, the OFPs for total VOCs increase112%. The reason is similar: the OFP for initial isopreneincreases from 8.2 to 37 ppbv; and initial isopreneaccounts for 22% of the total OFPs of all measured species.Its OFP value is 77% times higher than that of ethene andalmost two times as high as propene. So the conclusionwould be that after correction for losses isoprene, mainlyemitted by vegetation, has a rather large impact on ozoneformation.

Fig. 8 shows the OFPs after correction for losses at bothsites compared with the OFPs calculated using ambientmixing ratios. At both sites, the OFP for isoprene hasincreased more than other groups, and alkenes follownext to isoprene. In contrast, aromatics decrease 17% and13% at PKU site and YUFA site, respectively. But, at PKU

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Fig. 7. The top 10 VOC species in terms of OFP at PKU and YUFA sites in Beijing by their initial mixing ratios.

0.00

25.00

50.00

75.00

100.00

125.00

150.00

OFP

(pp

bv)

PKU, initialVOCs

alkanearomaticalkeneisoprene

PKU, measuredVOCs

YUFA, initialVOCs

YUFA, measuredVOCs

Fig. 8. The OFPs calculated from initial mixing ratios for isoprene and other anthropogenic VOCs.

X. Xie et al. / Atmospheric Environment 42 (2008) 6000–60106008

site, aromatics and isoprene contribute most to OFP, whileat YUFA alkenes and isoprene contribute most to OFP.

5. Conclusion

Ambient air samples collected at urban and sub-urbansites in Beijing city were analyzed for the chemicalcompositions of VOCs with a focus on isoprene, a

suggested tracer for biogenic sources. The initial mixingratios of isoprene and other VOCs species were calculatedbased on isoprene chemistry, and the OFP for initialisoprene was assessed to understand the direct isoprenecontribution in ground-level ozone formation in the city.

Estimates of the OFPs changed substantially aftercalculating initial isoprene mixing ratios. At PKU site,the OFP for initial isoprene accounts for 22% of the totalOFPs of all measured species, while initial aromatics (the

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X. Xie et al. / Atmospheric Environment 42 (2008) 6000–6010 6009

most important group) contribute 37%. At YUFA site, theOFP for initial isoprene accounts for 22% of the total OFPsof all measured species, compared to the OFPs for alkenes(most important here) of 37%.

We believe that the effects of isoprene must beconsidered when a reliable and effective strategy regardingground-level ozone abatement is developed. From a veryprimary analysis of initial isoprene and initial mixing ratiosof possible vehicle emission tracers, e.g., 1-butene, weconcluded that isoprene is largely due to biogenic emissionat both PKU and YUFA sites, probably with a minorcontribution from traffic sources at PKU as well. This meansthat ozone formation due to isoprene cannot be reduced toreach the air quality goal of the 2008 Olympic game. Itwould be beneficial if tree species with lower isopreneemission rates could be considered for the green project inBeijing. The significant contribution from isoprene to ozoneformation could also suggest the necessity of more stringentcontrols on anthropogenic VOC sources such as vehicles,gasoline evaporation and solvents.

Acknowledgments

This study was funded by China National NaturalScience Foundation program (Grant no. 40575059 andGrant no. 20637001). The authors are grateful to TitiaMeuwese of Synspec Coporation and Huang Xiaodong ofDadi Encon Company for their efforts of online VOCsmeasurements at YUFA site, Guo Song of PKU for hisefforts of gas measurements.

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