Jan 27, 2001 - We thank Cheryl Harry and Eric Shamansky for dependably ... thank Arthur Winer (UCLA) for help in the planning and implemen- tation of the ...
JOURNAL OF GEOPHYSICAL
RESEARCH,
VOL. 106, NO. D2, PAGES 1607-1621, JANUARY
27, 2001
Biogenic volatile organic compoundsat Azusa and elevated sitesduring the 1997 Southern California Ozone Study AnniReissell andJanet Arey• Air PollutionResearchCenter,Universityof California,Riverside
Abstract. Biogenicvolatileorganiccompounds (BVOC) weremeasuredat Azusaandat eitherPineMountainor Mount Baldy, elevatedsites11 km northand25 km northeastof Azusa,respectively,duringfour intensivesamplingperiodsof the 1997 SouthernCalifornia OzoneStudy. Duringthe samplingperiodstherewasa consistent patternof isolationof the mountainsitesfromvalley air massesat night,followedby transportof valley air with elevatedlevelsof 03 andNOx to the mountainsitesasthe mixing heightincreasedthroughout the day. Isoprenewasthe dominantBVOC at the mountainsiteswith afternoonconcentrationsreaching2 ppbv,andits decreaseto a low mixing ratioafter sunsetwas attributedto reactionwith NO3 radicals.At AzusatheBVOC mixingratioswerehighestin the morning with theconcentrations of monoterpenes andof methacrolein (MACR) andmethylvinyl ketone(MVK), isoprenephotooxidation products,generallyexceedingthe maximumisoprene measuredat Azusa. The highdaytimeratioof (MVK + MACR)/isoprenesuggested that nighttimedrainageflows intoAzusa,from elevatedsiteswherethe isoprenewasdepletedby chemicalreaction,may havebeenresponsible for muchof the isopreneandits photoxidation products.The dataalsoindicatedlocal isoprenesourcesat Azusaanda possiblecontribution of MVK andMACR from vehicleemissions.Instancesof high mixing ratiosof limoneneat Azusasuggested an intermittentanthropogenic source.Duringthisstudy,particularlyin early morning,BVOC are calculatedto makea significantcontributionto peroxyradical formation
at Azusa.
well.
1. Introduction
Environmental
chamber studies have shown that the
most readily monitoredprimary productsfrom the atmosGlobally, sourcesof biogenicvolatile organiccompounds phericreactionsof isoprenewith the OH radicaland 03 are (MACR) [At(BVOC), that is, non-methaneorganic compoundsemitted methylvinyl ketone(MVK) and methacrolein kinson, 1997]. from vegetation, dominate over anthropogenicvolatile organic compound (VOC) emissions [Miiller, 1992]. These BVOC, estimated at 1150 Tg C annual worldwide flux [Guentheret al., 1995], affect regional and global troposphericchemistry[Trainer et al., 1987a, 1987b,'Fehsenfeldet al., 1992]. More than half of the BVOC flux is attributedto isoprene(2-methyl-l,3-butadiene)(44%) and monoterpenes (11%) [Guenther et al., 1995], which have been shown to contribute to the formation
of elevated ozone levels in urban
and rural areas[Trainer at al., 1987b,'Chameideset al., 1988, 1992; Biesentha!et al., 1997; Starn et al., 1998a].
AlkeneBVOC are highlyreactivein the atmosphere [Carter, 1994; gtkinson, 1997] with lifetimesoften of a few hours or lessdue to their reactionswith hydroxyl (OH) radical, nitrate (NO3) radical, and ozone (03) [gtkinson and grey, 1998]. Becauseof their rapid reactionrates,the mixing ratios of BVOC are often low comparedto thoseof anthropogenic compounds,especiallyin urban areas. To thoroughlystudy the role of B VOC in urban ozone formation, it is desirable to
conductambientmeasurements of their reactionproductsas
The SouthernCalifornia Ozone Study-NorthAmericanRe-
searchStrategyfor Tropospheric Ozone(SCOS97-NARSTO) measurement programwas carriedout in summer1997 to studythe formationand transportof ozone,and to develop databasesto supportdetailedphotochemical modelingand analysis. The Los AngelesAir Basin suffersfrom severe photochemical air pollutionepisodes due to emissions from heavyurbanvehiculartraffic, meteorology dominatedby the semipermanent easternPacifichigh-pressure systemand its topography with surrounding mountainson threesidesand
opento the PacificOceanon the southwest. Subsidence withinthe high-pressure cell generates a strongelevatedtemperatureinversion,restrictingconvective mixingandventilation of air pollutantsintothe freetroposphere [Lu and Turco, 1995, 1996]. Pollutanttransportis controlledby seabreezes and winds forced by mountaintopographyand heating[Lu and Dtrco, 1994]. While anthropogenichydrocarbonsare measuredon a routinebasis,relatively few ambientmeasure-
mentsof biogenichydrocarbons or theiratmospheric reaction products havebeenconducted in thebasin[greyeta[., 1991].
'Also at Interdepartmental Programin Environmental Toxicology Previous studies have concentrated on emission measureandDepartmentof EnvironmentalSciences,Universityof Calitbmia, ments[duutiet al., 1990;grey et al., 1991;Corchnoyet al.,
Riverside.
1992;grey et al., 1995]andcalculations of emissioninventories from vegetation[Benjaminet al., 1996, 1997;Benjamin and I4/iner, 1998]. This study focuseson measurements of isoprene,its atmospheric reactionproducts MVK andMACR, and monoterpenes made during SCOS97at Azusa, a mid-
Copyright2001 by theAmericanGeophysical Union. Papernumber2000JD900517. 0148-0227/01/2000JD900517509.00 1607
1608
REISSELL
AND AREY: BIOGENIC
VOLATILE
basin receptor site, and elevated sites in the San Gabriel Mountains.
2. Experiment 2.1. Campaign, Sampling Locations, and Sampling Times
The SCOS97- NARSTO measurementprogram took place June 16 to October15, 1997. During this program,six intensivesamplingperiods,a total of 13 days of sampling, were carried out during photochemicalpollutant episodes. Samplingfor biogenicVOC was generallyconductedsimultaneouslyat three locations,and the sites chosenfor study were Azusa, Pine Mountain or Mount Baldy, and Banning (see Figure 1). Azusa is a midbasinreceptorsite for anthropogenicVOC, east of the main sourcearea of downtownLos Angeles. Samplingwas conductedat elevatedsitesimpacted by BVOC on Pine Mountain (samplingat its peak,at 1383 m on August4-6, and at 1311 m on September4-7) and Mount Baldy (sampling at 1219 m elevation), located in the San Gabriel
Mountains
11 km
north
and 25 km
northeast
of
Azusa, respectively. The Banning site is located approximately 130 km eastof downtownLos Angelesat the Banning airport; this late-basinreceptor site will not be further discussedin this paper. The resultsof the four samplingperiods duringwhich measurements were conductedat Azusa and an elevatedsite (Pine Mountain, August4-6 and September4-7; Mount Baldy, September28-29 and October 3-4, 1997) are described here.
The samplingintervalswere 3 hoursduring daytime and generally7-9 hoursduringnighttime. Typically, samplingperiods at Azusa, Pine Mountain, and Mount Baldy were 03000600, 0600-0900, 0900-1200,
1300-1600, 1700-2000, and
2000-0300 hours Pacific Daylight Time (PDT; also Local
ORGANIC
COMPOUNDS
Time, LT). VOCs were measuredat Azusa duringthe four daytimesamplingperiodsby otherinvestigators participating in SCOS97. During September4-7 nighttimesamplingfor BVOC was performedmore frequentlyat Azusa and Pine Mountainwith 3-4 hour samplingperiods:1700-2000,20002400, 2400-0300, and 0300-0600 hoursLT. 2.2. Meteorological Data
Hourly averagedtemperatureand ozonedataare available from the South Coast Air Quality Management District (SCAQMD) Azusa monitoringstationat which BVOC sam-
pling occurred.For the four intensivesamplingintervalsdiscussed here, the ambient temperatureranges and ozone maximaat Azusawereasfollows: August4-6, 21.1-41.1øC,
03 = 122ppbv;September 4-7, 20.0-38.3øC, 03 = 118ppbv; September 28-29, 20.0-36.7øC, O3= 146ppbv;October3-4, 17.8-28.9øC,03 = 91 ppbv. 2.3. Sampling
Ambient sampleswere collectedon a hydrophobicsolid adsorbentpackedin borosilicateglasstubes(250 mg Carbotrap, 20/40 mesh,Supelco,Inc.). The Carbotraptubeswere
conditioned by heatingovernightat 350øCwith a constant flow of helium throughthe tubes. After conditioningand while still warm, the adsorbenttubeswere cappedwith brass
nuts,caps(Swagelock)and Teflon ferrules(AlltechAssociates,Inc.). The tubeswere placedin cleanedglassjars which were sealed with a metal lid lined with Teflon film, and a tube
containingactivatedcharcoal(MCB Reagents)was kept in the jars duringstorage. Two samplingtubesin serieswere used,with analysesof the secondtube servingto verify that no breakthrough occurred.Jarsandcartridges werestoredin a
Pacific Ocean
Figure 1. Map of the Los AngelesAir Basinshowingsamplingsitesat Azusa(117ø 55' W longitude,34ø 08' N latitude;a midbasinreceptorsite),elevatedsitesat PineMountainandMount Baldy,andfar eastbasin siteat Banning(116ø 52' W longitude,33ø 55' N latitude). Data at Azusawere takenat the SouthCoastAir QualityManagementDistrict(SCAQMD) monitoringstation. Area elevationsare: white, 0 - 305 m; light shading,305- 914 m; mediumshading,914- 2134 m; dark shading,over2134 m.
REISSELL
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1609
refrigeratorat 4øC beforetransportation to samplingsites. compoundisomericwith the C•oH•6monoterpenes) and oof the internalstandards Tubeswere freshly desorbedprior to each intensivesampling xylene-d•o. Known concentrations period. To prevent ozone from reacting with the biogenic nonmethaneorganic compounds(NMOC) and their atmospheric reactionproductsin the adsorbentduring sampling,an ozone scrubber[Hoffmann, 1995; Calogirou et al., 1996; Helmig, 1997] consistingof eight copperplyescoatedwith manganese dioxide(Dasibi Corp.), and constructed accordingto Calogirou et al. [1996], was placed in front of the first sampling tube. Each scrubberwas testedby pumpingozone(250 ppbv) throughthe scrubberand monitoringthe mixing ratio with an ozonemonitor (Dasibi Corp.). The scrubberswere testedbefore and after eachsamplingperiod. At each site the samplingequipmentconsistedof a diaphragmpump (Thomas),massflow controllers(Tylan General), and a massflow controlunit with four channels(Tylan General). The tubeswere mountedat a height of 1.8 - 2.0 m (at Azusathe samplingequipmentwas locatedon the roof of the SCAQMD monitoringstation)and the flow rate usedwas
were introducedinto a Teflon chamber(7900 L) from a vac-
unit was checkedfor flow againsta bubble flowmeter before and after eachintensivesamplingperiod. The Carbotrapsampleswere collectedin duplicate. Blank samplestransportedto the siteswere treatedthe sameway as samples,excludingactualsampling. Collectedsampleswere storedin a cooler with blue ice and transportedto the laboratory as soon as possiblefor spiking with internal standards andthenanalyzed. Precisionwas determinedusingthe results of the duplicateCarbotrapsamplesand therefore represents the precisionof both sampling and analysis. Precisionwas
zene, toluene, and PERC.
uum rack with an MKS Baratron 1-100 Torr sensor head and
a volume-calibrated Pyrexbulb.After ambientsampling,each
tubewasspiked with100½m 3 volumeof chamber air con-
taining these standards.External calibrationsampleswere preparedby introducingknown amountsof isoprene,isoprene-ds,methacrolein,methyl vinyl ketone,tricyclene,c•pinene,camphene,[3-pinene, limonene,1,8-cineole,camphor, o-xylene-d•o,benzene,tolueneand PERC into the chamber, samplingknownvolumesontoadsorbent tubes,andanalyzing the adsorbenttube samplesbetween actual field samples. From these calibration standards,responsefactors for the compounds relativeto the internalstandards were determined with isoprene-d8 servingas the internalstandardfor isoprene, methacroleinand methylvinyl ketone,andtricycleneserving as internal standardfor c•-pinene,camphene,[3-pinene,limonene, 1,8-cineole and camphor. An estimatedresponse factor was used for quantificationof sabinene. The comwasusedasthe internalstandard for ben50 mLmin-•. Eachpump,massflowcontroller, andcontrol poundo-xylene-d•o
calculated
as the standard deviation
of the relative
difference
of measuredmixing ratios of the duplicatesamples. At the mountainsitesthe resultswere (number of samples,mixing ratio range in ppbv) isoprene+23% (48, 0.02-2.3), MACR +22% (47, 0.02-1.1), MVK +27% (47, 0.07-1.4), (x-pinene +20% (44, 0.001-0.21), limonene +22% (43, 0.002-0.06), cineole +24% (37, 0.004-0.06), and camphor +20% (44, 0.002-0.06). When excludingthe low (
D
1.00,50.0 b..
0 0
0
0
•'
•'
O,I
0
Local Time (PDT) Figure 3. Mixing ratios(ppbv) of isoprene(solidcircles),methacrolein(MACR, opencircles), and methyl vinyl ketone(MVK, squares)at the elevatedsites:(a) Pine Mountainsite, September4-7, 1997 (elevation 1311 m), (b) Pine Mountainsite,August4-6, 1997 (elevation1383 m), (c) Mount Baldy site,September2829, 1997 (elevation1219 m), and (d) Mount Baldy site,October3-4, 1997 (elevation1219 m). The symbols are plottedat the midpointof eachsamplinginterval. Table 1. CalculatedLifetimesfor SelectedBiogenicHydrocarbons With Respectto Photolysis, Reaction With the OH Radical, ReactionWith the NO3 Radical, and Reactionwith O3 Lifetime '• Due to
Biogenic Hydrocarbon
OHb
Isoprene Methacrolein Methyl vinyl ketone
1.4 hours 4.9 hours 6.9 hours
ct-Pinene Sabinene
2.6 hours 1.2 hours
13-Pinene
NO3c
1.8 hours
Limonene
49 min
1,8-Cineole
Camphor f
49 min 14 days >385 days
O3d 10 hours 5 days 1.2 days
5 min 3 min
Photolysis - 1 day• ~2 days
1.5 hours 1.5 hours
13 min
8.8 hours
3 min
40 min
12.5hours
270 days
>37 days
2.5days
>150days
>80days
aAnatmospheric lifetimeis definedasthetimerequiredfor thecompound concentration to degradeto a value of 1/e of its initial concentration.
See Table 3 for rate constant data.
bFora 12-hour daytime average OHradical concentration of2.0x 106molecule cm'3. CFor a 12-hour nighttime average NO3radical concentration of5 x 108molecule cm'3. dFor a 24-hour average 03concentration of2.1x 10•2molecule cm'3(~90ppbv). •ForoverheadSun[Aktinson, 2000].
fReissell eta/.[2000].
REISSELL
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Table 2. Ratios ofthe Sum ofthe Mixing Ratios of the IsopreneReactionProducts,MACR andMVK, to the IsopreneMixing Ratio for Daytime SamplesCollected at Azusaandthe ElevatedSites,Pine Mountain(August 4-6, 1997; September4-7, 1997) andMount Baldy (September28-29, 1997; October3-4, 1997) (MACR + MVK)/Isoprene
Sample Date
PDT
Local Time,
Azusa
Aug. 4, 1997 Aug. 4, 1997 Aug. 4, 1997 Aug. 4, 1997 Aug. 5, 1997 Aug. 5, 1997 Aug. 5, 1997 Aug. 5, 1997 Aug. 6, 1997 Aug. 6, 1997 Aug. 6, 1997 Aug. 6, 1997 Sept.4, 1997 Sept.4, 1997 Sept.4, 1997 Sept.4, 1997 Sept.5, 1997 Sept.5, 1997 Sept.5, 1997 Sept.5, 1997 Sept.6, 1997 Sept.6, 1997 Sept.6, 1997 Sept. 6, 1997 Sept.7, 1997
0600-0900 0900-1200 1300-1600 1700-2000 0600-0900 0900-1200 1300-1600 1700-2000 0600-0900 0900-1200 1300-1600 1700-2000 0600-0900 0900-1200 1300-1600 1700-2000 0600-0900 0900-1200 1300-1600 1700-2000 0600-0900 0900-1200 1300-1600 1700-2000 0600-0900
2.48 2.38 3.08 2.37 5.42 3.54 3.60 1.63 4.95 2.16 2.44 0.95 5.07 3.52 4.64 1.48 5.00 3.60 4.58 1.96 4.05 3.34 4.55 2.78 5.13
Pine
Mountain
1.13 0.13 1.15 1.99 1.07 0.26 1.34 2.64
2.99 0.47 0.63 1.43 1.34 0.50 0.58 0.38 1.59 0.16
(MACR + MVK)/Isoprene Local Time,
Azusa
Sept.28, 1997 Sept.28, 1997 Sept.28, 1997 Sept.28, 1997 Sept.29, 1997 Sept.29, 1997 Sept.29, 1997 Sept.29, 1997
0600-0900 0900-1200 1300-1600 1700-2000 0600-0900 0900-1200 1300-1600 1700-2000
3.74 2.39 2.61 1.62 3.60 2.06 3.58 1.40
0.33 1.10 0.55 0.31 0.49 0.83 0.62
Oct. 3, Oct. 3, Oct. 3, Oct. 3, Oct. 4, Oct. 4, Oct. 4, Oct. 4,
0600-0900 0900-1200 1300-1600 1700-2000 0600-0900 0900-1200 1300-1600 1700-2000
3.12 2.63 2.70 3.92 5.14 3.24 3.80 2.36
0.33 0.29 1.21 0.85 0.53 0.24 1.22 0.79
Sample Date
1997 1997 1997 1997 1997 1997 1997 1997
PDT
Mount Baldy
VOLATILE
ORGANIC
COMPOUNDS
1613
the ozone-alkenereaction,the OH yield (seeTable 3) and the concentrationof the alkene and of ozone. However, the slow
isoprene-O3reactionwill not be a sufficientsourceof OH radicalsand the mixing ratiosof the monoterpenes were low. Furthermore,if the nighttimeisoprenedecaywere due to OH radical reaction,the ratio (MVK + MACR)/isoprene should not exceedthe steadystatevalueof 2.4 notedabove,while the very slow reactionof MACR and MVK with the NO3 radical reactionwould allow the ratio to increaseas the isoprenedecreased.Thus the very high (MVK + MACR)/isopreneratios in the nighttimesamplesat Pine Mountain(for the three20002400 hoursnighttimesamplesshownin Figure 4, this ratio was 32, 20, and 9.5, respectively)are consistentwith NO3 chemistrydepletingthe isoprenebut not with OH radicalreaction.
As may be seen from Figure 3b, a low concentrationof isoprenein the first sampleafter sunset(2000-0300 hours) alsooccurredat Pine Mountain on August4 and 5. As during the September4-7 intensive,the levelsof O3at PineMountain remained elevated after sunset,and PERC ratios suggested mixing with NOx-ladenair from the valley late in the afternoon. The ct-pinene mixing ratio was also at a low, minimum value after sunset, but MACR, MVK, camphor and PERC did not reach minima until early morning. Thus the Pine Mountain data from both intensivessuggestthat NO3 radical chemistryinfluencedthe observedBVOC mixing ratios, with MACR, MVK, and PERC mixing ratio decreases after sunsetbeing attributedto dispersion,whereasisoprene and ct-pinenewere additionallysubjectto chemicalreaction with the NO3 radical.
3.2.2. Modeling NO3 chemistry at Mount Baldy. Rapid decreasesin isopreneafter sunsetwere also seen at Mount Baldy, andmeasurements of ozoneandNOx by the California Air ResourcesBoard allow NO3 radical formation at this site to be modeled. Figure 5a showsthe ozone and Figure 5b shows the NO2 data measuredat Mount Baldy for the September 28-29 sampling intensive. No significantNO (2 HNO3 Isoprene + NO3--> 0.035MVK+ 0.035MACR Isoprene + 03 -->0.16MVK+ 0.39MACR+ 0.27OH Isoprene + OH --> 0.32MVK+ 0.22MACR MVK+ NO3--> products MVK+ 03 -->products MVK+ OH -->products MACR+NO3-->products MACR+03 --->products MACR+OH --> products ot-Pinene +NO3--->products ot-Pinene + 03 --->products + 0.76OH ot-Pinene + OH -->products [3-Pinene +NO3-->products [3-Pinene + 03 -->products + 0.35OH [3-Pinene + OH -->products Limonene +NO3-->products
3.2x 10-17 b 1.2X 10'12 3.8x 10-2b 9.0X 10'12b 1.0x 10-21c 6.78x 10-13d 1.28x 10'17d 1.01x 10'løa 1.2x 10'16e 4.56x 10'18 2.02x 10-llg 3.3x 10'lSh 1.14x 10'lSf 2.86X 10'11g 6.16x 10-12d 8.66x 10-17 5.37x 10'lla 2.51x 10-12d 1.5x 10'17a 7.89x 10-lla 1.22x 10'lia
(23) (24) (25) (26) (27) (28) (29)
Limonene + OH -->products Cineole +NO3-->products Cineole + 03 -->products Cineole + OH --->products Camphor + NO3-->products Camphor + 03 -->products Camphor + OH -->products
1.71x 10'lød 1.7x 10'16i 1.5x 10-loj 1.11x 10-•i 3 x 10'16k 7 X 10-20k 4.6x 10'121
(22) Limonene + 03 -->products + 0.86OH
Initial Concentrations
moleculecm'3
NO2 03 H20
2.7x l0 II 2.7x 1012 3.9x 1017
Isoprene
2.0x 10'16cl
6.1x 10•ø
MVK MACR ot-Pinene
2.3 x 10•ø 1.1 x 10•ø 3.6 x 109
13-Pinene
1.1x 109
Limonene Cineole
8.1 x 108 5.9 x 10•
Camphor
3.2x 10•
'•First orderin s-l;second orderin cm-3molecule -I s-1. bSander etal. [2000]. CMentelet al. [1996] for 50% RH at 298 K.
dAtkinson [1997]. CUpperlimit fromRudichet al. [1996].
fAtkinson [1994]. gAtkinson et al. [ 1983];Gierczaket al.[ 1997].
hChew etal. [1998]. i.Corchnoy andAtkinson [1990]. JUpperlimit fromAtkinson[1994].
kUpper limitfromReissell etal. [2000]. IReissell etal. [2000].
Figure 7b showedsimilarprofileswith maximagenerallyat 0600-0900 hours followed by decreasesdue to OH radicalinitiatedreaction(and an increasingmixingheight)duringthe 0900-1200 hourssamplinginterval. Camphorandcineoleare not alkenes,and these longer-livedBVOC (Figure 7c) and PERC (not shown) had the same early morning (0600-0900 hours)maximabut did not decreaseas rapidly duringthe next 3 hours.
The minima
in the nonreactive
BVOC
at 1700-2000
hourssuggeststhat the mixing heightis greatestduringthis
samplinginterval,which is consistentwith the PERC data. A buildupof local monoterpeneemissions,or drainageflow of monoterpenesfrom elevatedareas,into a shallow nocturnal boundarylayer in which, as noted,NO emissionswould suppress03 (see Figure 2) and NO3 radicalformationcould explainthe early morningmonoterpene maximaseenat Azusa. The mostabundantmonoterpenes measuredat Azusawere limoneneand cz-pinene. The cz-pineneand limonenelevels were generally similar (see Figure 7b), suggestingthat the
1616
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AND AREY:
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160
'•' 120 .1:3
A
•
= O3 Azusa 0 3Mt. Baldy
(1) 80 o
N
O
40 0
ß
12
rn•.
8
O
4
z
04
08
12
16
20
24
04
08
12
16
20
24
2.4
-
q.. Isoprene
200
C
>
-160
(z-Pinene Limonene ooo
D
-
120
-
80 o
- 40
c:
o
o =
MACR
,- o-.
MVK
•
Cineole
--=--
Camphor
20
0.4
-10
•
o
o
>
0.0 o•
co
o
co
co
o•
•
co
o
0
w-
•-
04
0
0
0
•--
•-
LocalTime (PDT) Figure 5. Mount Baldy, September28-29, 1997, samplingintensive:(a) hourlyaveragedozonedata(ppbv) for Azusa and Mount Baldy, and (b) Mount Baldy hourly NO2 data (ppbv) with NO < lppbv (except on September28, 1200NO - 3 ppbv),(c) mixingratiosof isoprene(ppbv),andot-pineneand limonene(pptv), and (d) mixing ratiosof MACR, MVK (ppbv),and 1,8-cineoleandcamphor(pptv). OzoneandNOx data for Azusa and Mount Baldy suppliedby the CaliforniaAir ResourcesBoard.
emission"signature"of the area vegetationhad comparable levelsof thesemonoterpenes.During the four intensives,excluding the highest limonene value on September28, the range at Azusa for ot-pinene was 0.01-0.53 ppbv and for limonenewas 0.05-0.67 ppbv, with the maximaoccurringduring the early morning, 0300-0600 or 0600-0900 hours samples, and the minimum mixing ratiosoccurringduringthe afternoon 1300-1600 or 1700-2000 hourssamples. The 06000900 hours sampleon September28 had an extremelyhigh
could be the result of an occasional,limonene-dominatedbio. genie emissionsourcebut rather suggesta local, and intermittent,anthropogenic source. 3.3.1. (MACR + MVK)/isoprene. In contrastto the mountainsites,at Azusa the MVK and MACR mixing ratios were eachgenerallygreaterthan that of isoprene(Figure 7a), and the (MVK + MACR)/isoprene daytime ratio was frequently higher (Table 2) than the steady state value of 2.4
(3,8 ppbv) mixing ratio of limonene (verified by full
mation(from isoprene)and lossof MVK and MACR. If the
GC/MSD scan of a colocated Sample),while the ot-pinene (0.27 ppbv) and other monoterpeneswere in the usualrange. High limonene was also observedin the early morning of August 22 (2.3 ppbv, 0300-0600 hourssample)during an in-
dominant source of MVK
tensive not discussed further here because no elevated site
measurementswere made. It seemsunlikely that the very high early-morninglimonene mixing ratio peaks at Azusa
which would result from OH radical reaction-dominated and MACR
for-
was the reaction of lo-
cally emittedisoprene,suchhigh daytimeratioscouldnot occur. However, the source of the MVK and MACR at Azusa
couldstill be phot0oxidationof biogenicisopreneif transport to Azusa of air masseslargely depletedin isoprenebut not in MACR and MVK was responsible. Thus one likely source for MACR and MVK at Azusa is nighttimeshallowdrainage
REISSELL
AND AREY:
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1617
flows down the San Gabriel Mountains(includingPine riod, for the 0600-0900 hourssampleson September5, 6, and Mountain)borderingAzusa after nighttimeNO3 chemistry 7, as well as the 2000-2400 hourssampleson September4 haddepletedthe isopreneemittedduringthe day at the ele- and 5, the ratio MVK/MACR was 1 for the October3-4 intensive andwas
0.4
o
= o•-Pinene -- o.- Limonene
B
\, .--o--Sabinene,o, •',, -----13-Pinene ,'•',,
:/•,,. ,7'
0.2
''-qD'
''
''0...
,'
0.0
•
40
Camphor
C
•"ø"q ,?"q ..o-cineole " ' " ..o.. d"•",,
o•-"ø'"q
o'
,o
,.o/
Local Time (PDT) Figure 7. Mixing ratiosof BVOC at Azusaduringthe September 4-7, 1997,intensive:(a) isoprene, MACR, andMVK (ppbv),(b) monoterpenes (ppbv),and(c) camphor andcineole(noteppWscale).Thesymbols are
plottedatthemidpoint of eachsampling interval; notethatsamping began at0300onSeptember 4. riod, during which the highest CO mixing ratios of the SCOS97 studywere observed. These pollutantsare highly correlated,as would be expectedif traffic is the major source of both at Azusa. Benzeneis presentin vehicleemissionsand as seenin Figure 8b, after a late afternoonlow, benzenebuilt up duringthe nighttimeand the three benzenemaxima coincidedwith the early morningpeaksin CO andNOr. Anthropogenic vehicle emissionsof isoprene,MVK, and MACR have been proposed[McLaren et al., 1996; Biesenthaland Shepson,1997; Reimannet aL, 2000]. The NO which titrates the O3 and then buildsup at night (see Figure 2), as well as the increasein CO (Figure 8a) duringthe night, indicatesthe presenceof nighttimetraffic at Azusa. The low nighttimeinversion and the absenceof nighttime chemistrywould allow isoprenelevels to increaseduring the night, if traffic were a significantsourceof isoprene. No significantnighttimeincreasesin isoprenewere observedand, as noted previously, the 0900-1200 hourssamplesconsistentlyhad highermixing ratios of isoprenethan the 0600-0900 hours,morningpeak traffic samples. Emission
factors for MVK
and MACR
relative to CO re-
ported by Biesenthal and Shepson[1997] predict ambient mixing ratios of 0.6 ppbv of MVK and 0.3 ppbv of MACR due to vehicle emissionsfor 4 ppmv of CO (higherthan the maximummeasuredat Azusa duringSCOS97). Thusvehicle
emissionscannotaccountfor the majority of the MVK and MACR at Azusa,but for somesamplesthey couldbe signifi-
cant. Forexample,theMVK mixingratiosin thethree06000900 hourssamplesshownon Figure8b were0.46, 1.73,and 1.24ppbvonAugust4, 5, and6, respectively. Takingtheaverageof the 0600, 0700,and0800 hourshourlyaverageCO
values for eachdayandapplying thefactorof 1.4x 10-4 (mole/mole)givenby Biesenthal andShepson [1997]predicts 0.35, 0.43, and0.43 ppbvof MVK from traffic, or anthropogeniccontributions of 76%, 25%, and35%. The lackof a clear correlationbetween CO and MVK and MACR suggests that emissions from traffic are not the dominant sources of
thesecompounds; however,nontrafficanthropogenic sources of MACR and MVK cannotbe excluded. Clearly, establish-
ingthesources of theBVOC at Azusawill requireadditional data. Samplingwhenbiogenicsources are minimaland/or whenNO3chemistry is unlikelywouldbe informative, andair masstrajectoriesareimperative. 3.3.2. Significanceof BVOC at Azusa. Ozoneis formed
by the conversion of NO to NO2 (by reactionwith HO2 and organicperoxyradicalsgenerated fromthe photooxidation of VOC) with subsequent photolysis of NO2 andreactionwith 02 producing 03. The instantaneous formationof peroxy
radicals (ROO')canbetakenasa measure of thecompound's reactivity,andfor mostVOC this is the productof the com-
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Local Time (PDT) Figure 8. Azusa,August 4-6, 1997, intensive'(a) hourlyaveragedmixingratiosof CO andNO• (datasupplied by the CaliforniaAir Resources Board,2000), and (b) mixing ratios(ppbv) of benzene,isoprene,and MVK.
pound's rate constantfor reactionwith the OH radical (ko}0 and its concentration[see Chameideset al., 1992; Biesenthal et aL, 1998], but noting that not all VOC producethe same
numberof 03 molecules per ROO'formed[Carter,1994]. Utilizing the August 5, 1997, 0600-0900 hours sample as typical of a BVOC profile not dominated by limonene (ct-pinene, 0.22 ppbv; limonene,0.28 ppbv), but representative of an early morning samplewhen BVOC were highest, values of ko}•x [BVOC] are shown in Figure 9, along with those for benzene and toluene.
Note that the limonene
con-
x
centrationattributedto a local anthropogenicsourcewas over an orderof magnitudehigherthanthe limoneneconcentration utilized for Figure 9, and clearly limonene is an important VOC at Azusa. However, even for the August 5, 0600-0900 hourssampleshownin Figure 9, the sumof the reactivitiesof the BVOC (assumingthat the isopreneand monoterpenes are of biogenic origin and the the MVK and MACR are from photooxidationof biogenic isoprene)is several fold that of Figure 9. Relative reactivitiesof benzene,toluene,and the toluene. BVOC at Azusa for 0600-0900 hours' August-12 5, 1997. The Utilizing publisheddata of a summertimeVOC profile for OH rate co•2nstan•ts used wer•e k•e,ze,e = 1.23 x 10 andkto•ue,• = the urban area of Atlanta, Georgia [Chameideset al., 1992], 5.96 x 10- cm molecule- s- [Atkinson,1994]; seeTable 3 for additional rate constant data. and a summertimebasinwideaverageVOC profile for Call-
1620
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fornia's South Coast Air Basin [Lurmannand Main, 1992], Benjamin,M.T., M. Sudol,D. Vorsatz,andA.M. Winer,A spatially and temporallyresolvedbiogenichydrocarbonemissionsinventoluenerepresents3% and 7%, respectively,of the reactivity tory for the CaliforniaSouthCoastAir Basin,Atmos.Environ., calculatedas the sum of ko. x [VOC] for thesetwo urban 31, 3087-3100, 1997. sites. Becausethe VOC profiles in urbanareasare largely Biesenthal,T.A., and P.B. Shepson,Observationsof anthropogenic due to vehicle emissions,the contributionof tolueneto the re-
inputsof the isopreneoxidationproductsmethylvinyl ketoneand
methacroleinto the atmosphere,Geophys.Res. Lett., 24, 1375activity at Azusacan be assumedto be in a similarrange(i.e., 1378, 1997. 3-7%), which would put the contributionof the BVOC for the Biesenthal,T.A., Q. Wu, P.B. Shepson,H.A. Wiebe, K.G. Anlauf, August 5, 0600-0900 hours sampleat approximately8-20% and G.I. Mackay, A studyof relationshipsbetweenisoprene,its of the total reactivity. Thus,evenat this urbansitehighlyimoxidationproducts,and ozone,in the Lower FrazierValley, Atmos.Environ., 31, 2049-2058, 1997. pacted by vehicle emissions,BVOC can make significant contributionsto peroxyradicalformationand,as notedprevi- Biesenthal,T.A., J.W. Bottenheim,P.B. Shepson,S.-M. Li, and P.C. Brickell,The chemistryof biogenichydrocarbons at a ruralsitein ously [Chameideset al., 1988], the contributionof BVOC to easternCanada,J. Geophys.Res.,103, 25,487-25,498,1998. ozoneformationin urbanareascannotbe ignored. Braun, W., J.T. Herron, and D.K. Kahaner,Acuchem: A computer
programfor modelingcomplexchemicalreactionsystems,Int. J. Acknowledgments.We thankCheryl Harry andEric Shamansky for dependablyand cheerfullytakingresponsibilityfor manyaspects of the sample preparation,collection, and data verification. We thankArthur Winer (UCLA) for help in the planningand implementation of the study. We thank PatriciaMcElroy and SaraAschmann for contributingto the sampleanalysesandWilliam D. Long for fabricating critical samplingapparatus. C.H., E.S., P.M., William P. Harger, Yong Jae Chung, PatriciaT. Phousongphouang, Alvaro A1varado, Andrew Chew, and Roger Atkinson are all thanked for gamely contributingto the sampling,and Alan Bescobyis thanked for allowingus to sampleon his propertyon Mount Baldy. We wish to thank Rudy Eden of the South Coast Air Quality Management District for invaluablehelp and logisticalsupport.Bart Croes,Randy Pasekand Ash Lashgariof the California Air ResourcesBoard are thankedfor their patienteffortsand for arrangingfor the copollutant data at Mount Baldy. We also thank Roger Atkinsonfor numerous valuable discussions of the data. The Califomia
Air Resources Board
providedfinancialsupportfor this studythroughcontract95-309 and partial supportwas also providedby the University of Califomia Campus Laboratory Collaboration, grant UCCLC-20746. A.R. gratefullyacknowledgessupportfrom Maj and Tor NesslingFoundation, Helsingin SanomatCentennialFoundation,Ella and Georg EhrnroothFoundation,and Jennyand Antti Wihuri Foundation,and J.A. thanksthe Universityof CalifomiaAgriculturalExperimentStation for partialsalarysupport.
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(ReceivedMay 8, 2000; revisedAugust11, 2000; acceptedAugust25, 2000.)
