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Biogenic isoprene in the Lower Fraser Valley, British Columbia Gurren, Kristina; Gillespie, Terry; Steyn, Douw G.; Dann, Thomas; Wang, Daniel 1998

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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 103, NO. D19, PAGES 25,467-25,477,OCTOBER 20, 1998  Biogenicisoprenein the Lower Fraser Valley, British  Columbia  KristinaCurtenandTerryGillespie LandResourceScience,Universityof Guelph,Guelph,Ontario,Canada  DouwSteyn Geography, Universityof BritishColumbia,Vancouver,BritishColumbia,Canada  ThomasDann andDanielWang EnvironmentCanada, River Road, Ottawa, Ontario, Canada  Abstract. Tropospheric ozoneis formedby photochemical reactionsbetweennitrogenoxides(NO• andvolatile organiccompounds. In someregions,biogenicisoprene maybe a significant contributor to theproduction of tropospheric ozone.Thecontribution ofbiogenicisoprene is animportantaspectof regionalozonechemistry asit  represents anozoneprecursor thatcannotbe eliminated throughemissions controls.Thepurposeof thisstudywasto evaluate thecontribution of isoprene to theproduction of tropospheric ozonein theLowerFraserValley,British Columbia.Seasonal trendsanddiurnalprofileswereusedto examineisoprene's relationship with temperature, to determine its source,andto investigate thechemicalandphysicalfactorsthatlimit the ambientlevelsof isoprenepresent in theregion.Totalisoprene levelsin theLowerFraserValleywerelow, andevidencesuggested thata substantial fractionoriginated fromanthropogenic ratherthanbiogenicsources. Diurnalisoprene profilesweregenerally flat, and thetimesof thehighestconcentrations didnotcoincidewith peakNO x levelsnorwith thetimesof optimalozoneproducing meteorological conditions. Theseresultsareconsistent withthoseof previously reportedstudiesandsuggest thatbiogenicisoprenemaynotbe asimportantto thetropospheric ozonechemistryin theLowerFraserValley asit is in some southern U.S. cities.  1. Introduction  Thepurpose ofthisstudy wastoexamine theambient concentrations of isoprene throughout the year at various sites in the LowerFraser Tropospheric ozoneis a majorcomponent of urbansmogthat Valley to determine if significant ambient concentrations ofisoprene affects the health of people,crops,and natural vegetation. exist in the region;to examineisoprene's relationship with Tropospheric ozoneisformed byphotochemical reactions between temperature; and to determine whether isoprene in the region nitrogen oxides (NO•)andvolatile organic compounds (VOCs).While oqginated fromanthropogenic orbiogenic sources. Thesecond study mostNOx originates fromanthropogenic sources, VOCs may be servedto examinethe diurnalvariationin isopreneconcentrations emittedfrom both anthropogenic and biogenicsources. In recent  duringsummer ozoneepisode andnonepisode daysin theLower years, it hasbecome apparent thatbiogenic hydrocarbons, particularly Fraser Valley and investigated the chemical and physical factors that isoprene (C•Hs),maybesignificant contributors totheproduction of theobserved diurnal isoprene profiles through theuseof a tropospheric ozonein someareas.The contributions of biogenic produce of theseandpreviously published hydrocarbons areanimportant aspect of regional ozonechemistry as simpleboxmodel.Theresults studies wereusedto evaluatethe impactof biogenic isoprene in the theyrepresent anozoneprecursor thatcannotbeeliminated through Lower Fraser Valley on the regional production of tropospheric ozone. emissions controls. Previousstudies thatmeasured biogenichydrocarbon emission rates fromseveral species of vegetation showed thattheLowerFraser 2. Background Valleycontains few species of agricultural crops,naturalplants,or treesthat emit isoprene in appreciable amounts[Drewitt,1996; The Lower Fraser Valley is a roughlytriangularvalley in Drewitt et al., 1998; Curten, 1998]. This suggests that ambient southwestern BritishColumbiathatextendsfrom the Straitof Georgia  biogenic isoprene levelsin theregionshould begenerally low.It is inthewesttotheFraserCanyonin the east.The valleyis boundedby possible, however, thatthereareisoprene-emitting species of plants the Coast Mountains to the north and the Cascade Mountains to the andisinhabited by approximately 2 millionpeople[Steynet present intheareathatwerenotaccounted forintheemissions studies southeast andthatambient biogenic isoprene levelsnearthesesources couldbe al., 1997].The bulkof the anthropogenic ozoneprecursor emissions high enoughto havea significant impacton the production of arisesfrom mobilesources,primarilylight-dutyvehicles[Steynet al., 1997]. The region'snaturalvegetationis largelymade up of tropospheric ozonein theregion. trees,includingwesternredcedar•huja plicate), coastal Thispaper describes themethods andresults fromtwoanalyses of coniferous ambient isoprene levels intheLowerFraser Valley,BritishColumbia. Douglas fir (Psudotsugamenziesiissp. menziesii),and coastal Thefirststudy isananalysis of seasonal isoprene trends intheregion. hemlock(Tsugarnertensiana)[Drewitt, 1996;Drewitt et al., 1998]. Copyright1998by theAmericanGeophysical Union. Paper number98JD01214. 0148-0227/98/98JD-01214509.00  Thevalleyfloorhasa rangeof urbanandagricultural usesthatresults in considerable variationin localbiogenicemissions [Singletonet al., 1996].  Although ambient isoprene concentrations in ruralareastendto be higherthanthosein urbanareasdueto thegreaterdensityof isoprene 25,467  25,468  CURRENET AL.: BIOGENICISOPRENEIN THE LOWERFRASERVALLEY  Isopreneemittedfrom biogenicsources, on the otherhand,may emitting plants in rural areas, significantambient isoprene compounds. concentrationshave been reported in urban areas. Midday showa differentdiurnalprofilefrom anthropogenic summertimeisopreneconcentrations as high as 8 ppbvhavebeen Isoprenemixingratiosmeasuredat rural forestsites,whereareal recordedat in Atlanta,Georgia,althoughmeasurements of theorder emissionratesarehigh,aregenerally low in theearlymorning,rise throughout the dayin of 2.5 ppbvare morecommon[Cardelinoand Chameides,1995]. sharplyat sunrise,and continueto increase Ambientisoprenelevelsmeasuredin Toronto,Ontario,showedthat response to increasing temperatures [Traineret al., 1987;Fehsenfelt isoprenelevelsincreased duringthe summermonthsin response to et al., 1992].Isopreneprofilesmeasuredat urbansitesin theAtlanta, increasingemissionsfrom biogenic sources.Summer isoprene Georgia,regionshoweda differentshapefromthoseat forestsites, concentrations at this site varied,but were consistently lessthan with isoprenemixing ratiosat all but one of thesesitesroughly 1 ppbv[McLarenet al., 1996].Isopreneconcentrations measured at constantthroughoutmostof the day [Cardelinoand Chameides, in urbanandsuburban sitesin theLowerFraserValleyaregenerallyless 1995].Threeofthesitesshowedslightlyelevatedlevelsof isoprene dueto activebiogenicsources combined with than 0.4 ppbv,with suburbansitesshowingslightlyhighermixing the morning,possibly ratios (0.2-0.35 ppbv) than urban sites(<0.2 ppbv) [Canadian the relativelyshallowmixinglayer,whiletwo of the sitesshowed dramatic increases in isoprene levelslatein theafternoon, probably as CouncilofMinistersof theof theEnvironment(CCME), 1997]. Isoprene isgenerally considered to originatefrombiogenicsources. the result of the collapseof the mixing layer and decreasing of compounds thatact as isoprenesinks.The profile Studiesin Louisianaand Californiahaveshownthatthe temperature concentrations dependence of ambientisopreneconcentrations wasconsistent with measuredat the Fort McPhersonsitewas similarto profilesover at ruralsites,with isoprene levelshigherthan5 ppbvfor most laboratory-measured temperature dependence of biogenicemissions forests wasgivenfor theconsistently highlevels [National Research Council (NRC), 1991], that isoprene of theday.No explanation at thissite[CardelinoandChameides,1995]. concentrationswere negatively correlatedwith VOCs normally of isoprenemeasured associatedwith anthropogenic mobilesources[Chameideset al., 1992],andthatthediurnalprofilesof isoprene weretemporally outof 3. Methods phasewith thoseof anthropogenic compounds [Chameides et al., 1992]. 3.1 Data Collection At somesites,however,isoprenemay havean anthropogenic Thedatausedinthisanalysis wereobtainedfromthenationalfield source. Detectable levels of isoprenewere measuredat York  Universityduringthe wintermonthswhen biogenicsources were dormant under snow cover.An analysisof this data showeda significant correlation betweenisoprene and 1,3 butadiene, a well known combustionproduct,indicatingthat the isoprenelevels observed in thewintermonthsoriginatedfromanthropogenic mobile sources[McLarenet al.,1996]. Chemicalmassbalancemodelingof  sampling program database maintained bythePollutionMeasurement  Divisionof theEnvironmentProtectionService,Environment Canada  [Dann et al., 1994; CCME, 1997]. Systematicyear round measurements at several urban sites and one suburban site in the  LowerFraser Valleybeganin 1989,with severalnonurban sitesadded totheprogramin lateryears(Table1). Siteswereclassified asurban, urban street (close to street level in the urban core), or suburban (in an this dataindicatedthat the observedwinter isopreneconcentrations urbanareabutoutside theurbancore),andnotewasmadeof sitesthat could be accounted for by vehicleexhaustcontaining isoprene at a No datawere level of 0.0018% of nonmethane hydrocarbon massin the exhaust were potentiallyimpactedby industrialemissions.  availablefrom ruralsitesin the region. Field sampleswere collectedby provincialor municipal Similarly, ananthropogenic sourceof isoprene hasbeensuggested environment departments participating in theNationalAir Pollution fortheLowerFraserValley.A correlation betweenambientisoprene Surveillance program. Samples werenormallycollected overa 24 hour concentrations at severalof the samplingsitesin the Lower Fraser periodonceevery6 days,with moreintensivesamplingprograms Valleyandthe corresponding isoprene emission ratespredicted by conductedat Rocky Point Park duringthe summersof 1992 and PC-BEIS [Pierceand Waldruff,1991] showeda generaltrendof 1993. In additionto the 24 hour samples,eightsequential 3 hour increasing concentrationswith increasingambient emissions samples werecollected at varioussitesin theLowerFraserValleyon [Singletonet al.,1996], but isoprene concentrations at a sitein the selecteddaysin 1992 and 1993 [CCME,1997]. Wholeair samples urbancenter of Vancouver appeared to beanomalously high.Possible werecollected in 6 L electropolished stainless steelcanisters andwere explanations proposed to explainthereading fromthissiteincluded analyzedin EnvironmentCanada's Ottawalaboratory. Studieshave confounding effects of verylocalbiogenicsources orthatsomeof the shownthat VOC mixturescan be storedfor up to 30 daysin these measured isoprene originated fromvehicleexhaust[McLarenet al., canisters with littlechangein composition [Oliveret al., 1986]. The 1996; Singleton et al., 1996]. PC-BEIS does not include wholeair samples wereanalyzedusinga cryogenic preconcentration anthropogenic isoprene inthevehicleexhaust profile[Singleton et al., techniqueanda gaschromatograph coupledwith a flameionization 1996]. detector ora massselectivedetector.The analyticalsystemwasbased [McLaren et al., 1996].  ß  In addition toisoprene's absolute concentration, thediurnalpatterns of isopreneemissions must be considered when evaluatingits contributionto the production of tropospheric ozone.The diurnal Table 1. variationsin isopreneandotherozoneprecursor concentrations are Valley important considerations because all precursors maynotbepresent in Site appreciable concentrations in the ambientatmosphere at the same  time. Data collectedat several urban sites in the southern United States  showed thatanthropogenic hydrocarbons andNO• bothpeakedduring theearlymorningandeveningrushhours[Chameides et al., 1992]. Similarly,total nonmethane hydrocarbons andNOx mixingratios measured intheLowerFraserValleyshowedminimain the afternoon dueto enhanced mixingandpeaksduringthe eveningandmorning rush hourswhen emissionswere high and the mixed layer was shallow[CCME, 1997].  Ambient NMHC  Measurement  Sites in the Lower Fraser  Site  Industrial  Classification  Source  Kensington Park RockyPointPark  urban urban  refinery refinery  RobsonStreet  urban street  Jan. 28, 1989  SurreyEast RichmondSouth MahonPark Shellmount Street  suburban urban urban suburban  Jan. 10, 1989 Jan. 22, 1989  Lan[•ley  suburban  pipeline  Staa Date  Jan.4, 1989 Jan.4, 1989  May 23, 1990 March 18, 1990  transfer  July12,1993  CURRENET AL.: BIOGENICISOPRENEIN THE LOWERFRASERVALLEY  25,469  sunrise. The radiationprofilewasassumed to be sinusoidal, on the methodsdescribed by Winberryet aL [1988] and is further following fromzeroatsunrise toa maximum of 1600!amol m'2s'l at described byDann etal. [1994]andBottenheimet al. [1997]. Typical increasing detection limitsfor thismethodwere0.02-0.04ppbv;species values noon and returningto zeroat sunset[Oke, 1978]. Cloudcoverwas to bezeroforthenonepisodedaysandfor 2 of the3 episode thatwerebelowdetection levelweresetto zero[CCME, 1997]. The assumed reportedprecisionof the methodfor analyteswith concentrationsdays(August2, 1993 (Langley),andAugust5, 1993 (RockyPoint withrecorded meteorological dataduringPacific greaterthan 0.25 ppbvwas in the rangeof 10-15% [Dann et al., Park)).In aexordanee '93, observations of cloudcoverfor August4, 1993 (Langley)were 1994]. used to reducethe illuminationat that site. Temperature-and 3.2 SeasonalData Analysis illumination-dependent isopreneemission rateswerecalculated using the following algorithms from Guenther et al. [1991]: Seasonal trendsin isoprenemixingratioswere examinedby  combiningall availabledatafor eachstation.The relationships E = S x T'x L' (1) betweenisoprene andthe dailymeantemperature measured at the Abbotsford airportin summerandwinterwereexaminedfor each emission rate(tagm'2h'l); S is themean station,andregression equations werecalculated whereappropriate. whereE is theisoprene isoprene emission rate at 301 K (tag m '2h4);T' isthecorrection factor Forthepurposes of thisanalysis, "winter"wasdefinedastheperiod andL' is the correction factorfor illumination. between NovemberandMarch and "summer"asthe periodbetween for temperature;  May andSeptember. Thetransitional monthsof AprilandOctober wereexcluded fromthetemperature analysis [McLarenet al., 1996].  A largenumber of datapointswereavailable forRockyPointPark (n - 306),withconsiderably fewerdataavailable for otherstations.  T'= exp [T• (T•_-TsYRT,T_•s] 1 + exp [T2(T L- T3)/R TL Ts]  (2)  where Ti• is the leaf temperature= air temperature; T is •the Regressions betweenambientconcentrations of isoprene and normalizing temperature (301 K); R = 8.314 JK 4 mo14; T• = 95,100 trans-2-pentene andcis-2-butene, VOCs normallyassociated with  andT3= 311.83K. anthropogenic mobilesources, wereperformed totestthehypothesisJ mol't;T2= 231,000J mo14; thatisoprene intheLowerFraser Valleyoriginates atleastpartlyfrom L'= x- (x2- 4xfxlxL1). anthropogenic sources. Thedatawerelogarithmically transformed for 2L• theregressions toensure constant variance throughout therangeof measured concentrations. In ordertoperformthetransformations, zero  (3)  where x = fxI + L• + L2; f is the fraction of light absorbedby  readings werereplaced withconcentrations equalto halfthedetection chloroplasts (= 0.385);I istheirradiance (tamolm'2s'l);L• = 105.6 limit(0.02ppbv).Separate regressions wereperformed forthewinter tamolm'2 s'l; andL2 = 6.12 tamolm'2 s't. Correction factorsfor and summermonths.Again,considerable datawereavailablefor  RockyPointPark(n -- 359),whilefarfewerdatawereavailable for the other sites.  humidityandCO2 mixingratioswereneglectedasthesecorrections are very small.The mean arealisopreneemissionrate (S) for the  LowerFraserValleyat 301K hasbeenestimated at 226 tagm'2h4  [Drewitt, 1996]. Visual inspectionof a satelliteimageof the region revealedthatthe densityof foliagein the vicinityof theLangleysite Diurnalisoprene profilesat two sites,RockyPointParkand wasgreater thanthatsurrounding RockyPointPark.Accordingly, the Langley, wereconstructed fornonepisode daysandforselected days mean arealbiogenicisopreneemissionratefor Langleywas set at  3.3 Diurnal Data Analysis  duringthemildozoneepisode of 1993.Average nonepisode day 226 tagm'2h't forthisanalysis, whiletherateforRockyPointPark profileswerecreated bycombining datafordayswithsimilardaily wassetat 169.5tagm'2h'l. maximum temperatures, whileprofiles fortheepisode daysrepresent Hourly estimatesof anthropogenic isopreneemissions were made a singleday'sdata.  by assumingthatanthropogenic isopreneis emittedonlyby mobile sources(automobiles). Basedon the resultsof the seasonal isoprene 3.4 Box Model analysis,the total amountof anthropogenic isopreneemittedin the to be25% of the value Diurnal isopreneprofilesin the Lower FraserValley were regionoverthe courseof a daywasassumed reproduced usinga simplebox model.For the purposes of this of the total biogenicisopreneemittedin the regionon a day of analysis, isoprene advection, deposition, andentrainment fromabove moderatetemperature.Thus the grossamountof anthropogenic a valueof themixinglayerwereassumed to bezero.Undertheseassumptions,isopreneemittedin the Lower FraserValleywasassigned waspartitioned the concentration of isoprenewithin the 1 km x 1 km box was 9 x 10•øtagd'l, or225 tagm'2d4. Thistotalamount usingthe relativeallocationfactorsfrom the dependent upontheinitialconcentration of isoprene at thestartof the into hourlyemissions day,thearealanthropogenic andbiogenic isoprene source strengths, Pacific '93 VOC emissioninventory[Levelton,1996; Jiang et al. factorswere availablefor July31 to August thedepth ofthemixinglayer,andthestrength of atmospheric isoprene 1996].Relativeallocation sinks. Each of these factors will be discussed below in turn. 6, 1993.Forthenonepisodedays,averagedrelativeallocationfactors 3.4.1. Initial concentrationof isoprene.The diurnalisoprene wereusedto calculate hourlyanthropogenic emissionrates,whilethe profilesat Rocky Point Park and Langleyindicatedthat low factorsfrom specificdayswere usedfor the episodedays. concentrations of isoprene existed in theambientatmosphere during 3.4.3 Depth of mixing layer. The depthof themodelledmixing predawnhours.Isoprene concentrations measured at 0300 hours layervariedwithtimeof day.The nighttimedepthof themixinglayer (representing theaverage mixingratiofrom0300-0600hours)were was200 m. The mixinglayerbeganto growshortlyaftersunriseand usedasthe initialconcentration within the boxat the startof the day. followeda sinusoidal curveuntilits maximumdepthwasreachedat 3.4.2. Areal isoprenesource strength. Biogenicand 1500 hours.The mixinglayercollapsed1 hour beforesunsetto a anthropogenic sources of isoprene wereincludedin the model. depthof 200 m. In additionto temporalvariation,the depthof the Biogenicisoprene emission ratesare primarilydependent upon mixinglayerovera coastalregionalsovariesspatially.Air flowing temperature andillumination. Thelengthof dayandthetemperaturefrom offshoreto onshoreis modifiedby changesin temperature profileforthedaywerecalculated usingthemodelofPartonand contrasts,which producea thermalor convectiveinternalboundary Logan[1981],whichassumes a sinusoidal temperature trendfrom layer. The growthof the convectiveinternalboundarylayercan be bythefollowingequation[Venkatram,1977;Hsu, 1986]: sunriseto sunsetand an exponentialdecayfrom sunsetto the represented  25,470  CURRENET AL.: BIOGENICISOPRENE IN THE LOWERFRASERVALLEY  concentration atnoon,andretuming to zeroshortlybeforesunset. The shapeof theOH profilewaschosen to correspond withtheradiation profile asOH isformedbythephotolysis of 03. OH concentrations are For whereh isthcdepth ofthemixing layer(m);Cdisthedragcoefficient, difficultto measure,andno datawereavailablefor thisanalysis. days, peakconcentrations of OH radicals weretakento be 0• isthepotential airtemperature overland(øC); 0m isthepotential nonepisode  h = (2C•0_,•:•0•)X)ø's (7(1- 2B)ø's  (4)  airtemperature oversea(øC);X isthedistance downwind fromthe  l x106radicalscm4 (N. Bunce,Universityof Guelph,personal  shoreline (m);¾isthelapse rateabovetheboundary layer( øC/m);and  communication, 1998).  SinceO3is requiredfor theproduction of OH radicals andNO2 is  F is the entrainment coefficient.  maybedepressed underconditions Equation (4) wasusedto calculate themaximum depthof the a sinkforOH, OH concentrations of moderate O 3 and elevated NOx. The 1993 ozone episode wasmild, mixing layerforthemodeled nonepisode days.Thedistances of Rocky atRockyPointParkandLangleyranging PointParkandLangleydownwindfromtheshoreline wereestimated withpeakO3concentrations using thesimplified LowerFraserValleyshoreline depicted bySteyn from 54-72 ppbvandpeakdaytimeNOxlevelsrangingfrom3047 wastakento be and Oke[1982,Figure5], andassuming a west-southwest airflow. ppbv.For thesedays,the peakOH concentration em4 (N. Bunee,Universityof Guelph,personal Thevaluesoftheparmneters enteredintoequation(4) wereasfollows l x10s radicals communications, 1998).  Cd=0.012,0,• = 16-17øC,X = 29.4 km (RockyPointPark)and32 krn(Langl•), ¾= 0.065 øC/m,andF = 0.2. The resulting calculated maximummixinglayerdepthsvariedbetween650 and900 m. Themaximummixinglayerdepthsenteredintothemodelforthe ozoneepisodedaysof August2 (Langley),August4 (Langley),and August5 (RockyPointPark)weretakenfromthemeasured depths reported byHaydenet al. [1997].In general, themeasured depthof themixinglayerfromAugust1-5, 1993,rangedfrom500-800m in the centerof the valley. 3.4.4. Isoprenesinks. The oxidation of isoprene in theambient atmosphere isinitiated byreaction with OH andlqO3radicals andwith 03.Forthepu• ofthismodel,OH radicals wereconsidered asthe primarysinkfor isopreneduringthe day,andNO3 radicals were considered as the primarysinkfor isopreneduringthe night.The reactionbetweenisopreneand ozoneis much slowerthan that between isoprene andOH orNO3,andO3levelsaregenerally low in theLowerFraserValleyduringnonepisode daysandverylow at night; thus the reactionbetweenisopreneand O3 was neglectedin this analysis. The OH profile used in this analysiswas sinusoidal, with concentrations increasing from zero after sunrise,reachinga peak  Peakconcentrations of NO3 radicals andthe shapeof nighttime NO3 profilescanvarysubstantially fromnightto night[Plattet al., 1981;Finlayson-Pitts andPitts, 1986].Forthissimulation, NO3was increasedlinearly from zero at sunsetand reachedits peak concentration of 10 ppt 2 hoursafterthe onsetof darkness. This profilewaschosenasit roughlymatchedthe shapeof a measured profileatDeuselbaeh, WestGermany,whereNO3 levelsroseat 1900 hoursand reacheda peakat 2100 hours[Platt et al., 1981]. In addition,NO3 is likelyto be formedfrom NO2 emittedfrom the evening rushhour.Peakconcentrations ofNO3weremaintained until midnight, whereuponthey decreasedsinusoidally, reachinga concentration of 0 pptat dawn. 4. Results and Discussion 4.1 SeasonalAnalysis  Meandailyisoprene concentrations andtemperatures throughout the yearat RockyPointParkareshownin Figure1. In general, isoprene mixing ratios werelow,witha meanof0.16ppbv(n = 162)  3O  0.6  25  0.5 2O  0.4  0.3  5  m  0.2 0  0.1  i  0  50  100  i  150  200  i  250  -10  i  300  350  Julian Day  [-o--Isoprene -13-Tmean I Figure1. Seasonal isoprene andtemperature trends atRockyPointPark,1989-1995  CURRENET AL.: BIOGEN!CISOPRENEIN THE LOWERFRASERVALLEY  Table2. MeanandMaximum Isoprene Mixin•Ratios Summer  Station  Winter  Mean,  Maximum,  Mean,  Maximum,  ppbv  ppbv  ppbv  ppbv  RockyPointPark (Urban) Kensington (Urban)  0.16 (n=162) 0.15 (n=28)  Mahon Park  0.16  (Urban)  (n=15)  Richmond South  0.15  (Urban)  (n=22)  Robson Street  0.10  (Urban- Street)  (n=23)  Shellmount Street  0.15  (Suburban)  (n=41)  0.44 0.33 0.43  0.17 (n=144) 0.14 (n-20) 0.09  0.58 0.24 0.22  (n=15) 0.46  0.21  0.19  0.14  0.47  (n=14) 0.77  (n=16) 0.49  0.09  0.32  (n=52)  SurreyEast  0.10  (Suburban) Langley  (n=19) 0.22  (Suburban)  (n=lO)  0.29  0.04  0.18  0.64  (n=22) 0.02  0.05  (n=8)  25,471  trendscanbediscerned. Meansummer isoprene mixingratioswere similar atmostsites, withslightly higheraverages recorded atLangley, themosteasterly andleasturbansite.Meanwinterisoprene mixing ratiosweresimilarto or slightlyhigherthansummervaluesfor the urban sites, but lower than summer values at suburban sites.  Maximumsummer isoprene concentrations werelessthan0.5 ppbv at all stations, exceptfor Langleywherethe maximumisoprene measured was0.64 ppbv.Maximumwinterisoprene concentrations werealsolessthan0.5 ppbvforall stations, exceptforRockyPoint ParkandRobson Street. Thereading of 0.77 ppbvatRobsonStreeton January 28, 1989,wasthehig,hcst recorded in the regionandsupports thehypothesis thatanthropogenic sources anda shallowmixingdepth canoccasionally resultin veryhighambientisoprcne concentrations. The concentrations of isoprene observed atRockyPointParkand othersitesduringthewinterareprobably alsoenhanced bytheshallow mixinglayer,buttheseresults indicate activelocalsources of isoprene duringthewintermonths,particularly in urbanareas.The sourceis unlikely tobea biogenic one,asmostplantsin theregionaredormant duringthistime.Althoughsomeplantspecies in theVancouver area remaingreenthroughout themildwinter,temperatures aretoolow for substantial biogenicemissions.  Figures 2 and3 showtherelationship between meandailyisoprene in the summerand0.17 ppbv(n = 144) in thewinter.Peaksummer mixingratios andtemperatures duringthe summerandwintermonths isoprene concentrations of 0.36 ppbvor higherwererecorded on five at RockyPointPark.Figure2 showsa clearrelationship between occasions, generally onwarmdays;thehighestsummerconcentration isopreneandtemperatures greaterthan 15ø(2in the summer,while of isoprenerecordedwas 0.44 ppbv(August12, 1992). Winter Figure3 showsa completelackof correlationbetweenthe two in the concentrationsof 0.36 ppbv or more were recordedon seven winter.Biogenic emissions increase withincreasing temperature; thus occasions; the highestwinterconcentration recorded was0.58 ppbv theseresultssuggest thattheisoprene measured atRockyPointPark  (December 27, 1993). Theseresultscontrast with thoseat York in the wintermonthsdoesnot originatefrom a biogenicsource. University,wheresummerconcentrations of isoprene werehigher Similarrelationships betweenambientisopreneconcentrations and temperatureduringsummerand winter were observedat the other sites(datanot shown). isoprenemixingratiosfor all stations. Althoughthenumberof data Therelationship betweenisoprene andtemperature in thesummer  thanwinter concentrations [McLaren et al., 1996].  Table 2 shows the mean and maximum summer and winter  pointsat stationsotherthanRockyPointPark was limited,some falls into two groupings.Isoprenemixing ratiosat daily mean  0.7  0.6  0.5  0.3  0.2  '  0.1  10  15  20  25  30  Mean Daily Temperature (C)  Figure2. Therelationship between isoprene andtemperature atRockyPointParkduringsummer.  25,472  CURRENET AL.: BIOGENICISOPRENEIN THE LOWERFRASERVALLEY 0.6  0.5  0.4  0.3  0.2  00  0.1 ß  ß 0  - 0  ,  ,  -5  0  ß 4•  ,  ,  5  10  15  Mean Daily Temperature (C)  Figure:5. TheRelationship BetweenIsoprene andTemperature atRockyPointParkduring-winter.  temperatures lessthanapproximately 15øC arcvariable(likelyasthe temperature, therelationship betweenambientisoprene.concentrations resultof varying meteorological conditions on specificdays),butshow and temperature is not necessarily exponential dueto the effectsof no correlation-with temperature. Biogenicemissions at dailymean changingmeteorological conditions and the changingstrengths of temperaturesbelow 15øC (corresponding to a daily maximum isoprenesinks.There-weresufficientsummerdatafor RockyPoint temperatureof about20øC) shouldbe very small.The isoprenc Parktoperforma regression between dailymeanisoprene mixingratio measured on these days probablyoriginatesfrom the same and temperature.The increasein isopreneconcentration with anthropogenic sourceas the isoprencmeasuredduringthe -winter temperaturefor daily mean temperatures greaterthan 15øC -was months, whiletheisoprenc measured on-warmerdaysis a mixtureof approximately linear.Thusthe dailymeanisoprene mixingratioat isoprenc frombothbiogcnicandanthropogenic sources. RockyPointParkcanbecharacterized by equation(5): Table3 shows the averagesummerisoprenc mixingratioson days with mcantemperatures lessthan 15øC for allLowerFraserValley Isoprene(ppbv)= 0.11, Tm•,•< 15øC, (5) sites.Althoughdataarelimitedat sitesotherthanRockyPointPark, = 0.023x Tm• - 0.24, Tm• > 15ø(2,r• = 0.50 thesevaluesgiveroughindications of thebackground anthropogenic levelof isoprenc concentrations duringthe summerat eachof these Correlationcoefficientsfor regressionsbetween isoprene, sites. trans-2-pentene, andcis-2-butene for all sitesareshownin Table4. Isopreneconcentrations at RockyPointParkon warmsummer Isoprene-was positivelycorrelatedwith both cis-2-buteneand days(T.• >20øC,corresponding to T.• >27øC)rangedfrom trans-2-penteneat all sitesin both summerand -winter.Positive 0.14-0.44 ppbv,-while thebackground levelofisoprene oncooldays correlations-were also observedbetweenthe two anthropogenic (T•,. <]5øC)was0.l ] ppbv.Background anthropogenic isoprenecompounds. Theseresults arein contrast to thoseof Chameides et al. maytherefore account for25-78%ofthetotalobserved isoprene load [1992] for southernU.S. citiesand indicatethat an anthropogenic at RockyPointPark. sourceof isopreneexistsin the Lower FraserValley duringboth  AlthoughGuenther et al. [1991]calculated an exponentialsummer and -winter. equationfor biogenicisopreneemissionrateswith increasing Seasonaldifferences in correlations betweenisopreneandthe Table3. MeanSummer Isoprene MixingRatiosforDayswith Mean Temperatures <15 øC  Site  MeanIsoprene MixinsRatioppbv  n  RockyPointPark Kensington Park  0.11 0.08  54 11  Mahon Park Richmond South Robson Street Shellmount Street  0.14 0.11 O.10 0.08  7 5 10 22  SurreyEast  0.04  6  Lan•ey  0.10  2  anthropogenic hydrocarbons-were variable. At RockyPointPark,the station withthemostdataavailable, the correlations -werehigherin the winterthanin the summer.Thisresultcanbeinterpreted to indicate thatanthropogenic isoprene accounted for a higherfractionof thetotal isopreneload in the -winterthan in summer.This trend-wasnot consistently observedat othersites,but this may be the resultof limited data at these sites.  4.2. Diurnal Analysis 4.2.1. Ambient diurnal isoprenemeasurements.Average diurnalisoprene profiles forRockyPointParkandLangleyareshown in Figure 4. Data for nonepisode days-with similarmaximum  CURRENET AL.: BIOGENICISOPRENE IN THE LOWERFRASERVALLEY  25,473  Table 4. Correlation Coefficients for Isoprene,cis-2-Butene, and  Langley may reflectthe amountof time requiredfor the urban  trans-2-Pentene  emissions of the Vancouver core to reach this suburban area.  Station  Compound  Summer r  RockyPoint  cis-2-butene/isoprene 0.64  n  192  Winter r  0.73  n  157  Park  trans-2-pentene/Iioprene 0.68  192  0.81  157  cis-2-butene/trans-2-  0.82  192  0.75  157  cis-2-butene/isoprene O.76  29  O.63  22  Lambetal. [1985]estimated thattheisoprene flux overa northeastern  trans-2-pentene/isoprene0.62  29  0.8  22  cis-2-butene/trans-2-  29  0.82  22  cis-2-butene/isoprene 0.82  24  0.72  16  trans-2-pentene/isoprene0.75  24  0.58  16  cis-2-butene/trans-2-  24  0.72  16  24  0.39  17  0.84  pentene South  (Urban) 0.73  pentene Robson  cis-2-butene/isoprene 0.79  Street  (Urban)  trans-2-pentene/is oprene  O.86  24  O.59  17  cis-2-butene/trans-2-  O.89  24  0.24  17  cis-2-butene/isoprene O.17  21  O.49  23  trans-2-pentene/isoprene 0.57  21  0.82  23  cis-2-butene/trans-2-  21  0.37  23  cis-2-butene/isoprene 0.73  12  0.62  10  trans-2-pentene/isoprene 0.81  12  0.68  10  cis-2-butene/trans-2-  12  0.69  10  pentene  SurreyEast (Suburban)  0.37  pentene  Langley (Suburban)  To determine if the box model could account  fortypicalisoprene concentrations andprofiles observed in relatively simple systems, themodelwasrunusingdatarepresentative of a rural forest.Forthissimulation, a higherbaseemission ratewasrequired.  pentene  Richmond  events.Theozoneepisodeof 1993 was mild, andOH concentrations werelikelysuppressed dueto thehighlevelsof NOx. 4.2.2. Box mode[  (Urban)  Kensington (Urban)  It shouldbenotedthattheisoprene profilesobserved onthe 1993 episode daysmaynotbetypicalof diurnalprofilesduringmajorozone  pentene  0.90  U.S.deciduous forestwas8000•tgm'2h4 at30øC. ThisvalueforS was enteredintothe model,alongwith the temperature profileat RockyPointParkonAugust5, 1993(Tm• = 29øC) anda maximum mixing layerdepth of800m.Peakvalues of OH andNO3werelx106  radicals cm'3and10ppt,respectively. Thepredicted profile isshown in Figure5. Themodelproduces an isoprene profilesimilarto thosemeasured overforestsandpublished in theliterature [e.g.,Lambet al., 1985; Fehsenfeld et al., 1992].Thepredicted isoprene concentrations rise  sharply afterdawn,continue to risethroughout theday,andpeakata valuegreater than10 ppbvin theearlyevening. Thepeakin theearly evening istheresultof decreasing concentrations of OH radicals and thecollapse of themixinglayer. Figure 6ashows thepredicted isoprene profiles ona representative nonepisode dayatRockyPointPark.Forthenonepisode days,thebox modelsuccessfully reproduced the relativelyflat profileand low concentrations at both sites.In general,the predictedisoprene concentrations showed a veryslightpeakin themorningdueto the combined effectsof theshallowmixinglayerandlowconcentrations of OH radicals,followedby a daytimeminimumat noonthat corresponded with the OH peak.Concentrations of isoprenerose slightlyagainin the lateaftemoon in response to decreasing OH concentrations andthecollapse of themixinglayer.Risinglevelsof NO3radicals wereresponsible forthedecrease in isoprene aftersunset. Figure6b showsthepredicted isoprene profileat Langleyonthe  previously, the measured temperatures ateachsiteweregrouped,andthe averageprofileswere episodedayAugust2, 1993.As discussed profileson thesedayswerevariable,andtheboxmodelwas graphed. Thetimesindicated on thegraphsmarkthefirsthourof the isoprene at reproducing the observed pattems,although,in 3 hoursample. In general, bothRockyPointParkandLangleyshowed lesssuccessful estimations of the magnitudeof the flat profileswith low concentrations of isoprene on nonepisode days general,it madereasonable mixingratios. Themostnotabledifference betweendayswas with maximumtemperatures lessthan25øC (Figure4a). Isoprene isoprene of theisoprene profilein thelateaftemoonandevening. profiles at othersites(datanotshown)weresimilarto thosedepicted thebehavior beganto inFigure4a.Daytimeconcentrations of isoprene on nonepisode days On August2 (Langley),observedisopreneconcentrations andcontinued to risethroughout the evening. with Tm•< 25øC were lessthan0.3 ppbvat bothsites.Profilesat riseinthelateafiemoon rosein the late aftemoon,but Langley on warmerdays(Tm• > 25øC) displayedslightlyhigher On August4 (Langley),concentrations Finally,onAugust4 (RockyPointPark), concentrations throughoutthe day (0.2-0.5 ppbv). Increasesin fell sharplyin the evening. roseuntil midafternoonandthen decreased isoprene.concentrations wereobserved latein theaftemoonon some isopreneconcentrations the restof the dayandevening.The boxmodel,on the days,likelyasa resultof thecollapsing boundary layer.RockyPoint throughout similarprofileson all 3 days,with predicted Park tendedto havehigherisopreneconcentrations thanLangleyon otherhand,predicted risingin the momingand continuingto cool days (Tm• < 20øC), probablydue to higher anthropogenic isopreneconcentrations throughout theday.The highpredicted concentrations from emissionsin the urbanarea.Conversely,Langleytendedto have increase higherisoprene concentrations on warmdays(Tm•> 20øC) asa result noon to late in the aftemoonwere the resultof activeisoprene emissions increasing with temperature, combinedwith decreasing of itssuburban locationandhigherbiogenicemissions. of OH radicals. Predicted levelsof isoprene fell sharply On mildozoneepisode days(August2-5, 1993; Tm,x> 30øC), both concentrations inresponse tothemodeledincreasing concentrations of Rocky Point Park and Langley showedratherdifferentisoprene intheevening profiles (Figure4b). Isopreneconcentrations atLangleyon thesedays NO3. reached magnitudes of 0.8 ppbvor more,whilelevelsat RockyPoint Themaretwofactorsthatmayexplainthe observed afternoonand Park peakedat 0.6 ppbv.The 3 episodedaysat LangleyandRocky eveningvariationsin isopreneprofileson episodedaysandwhy the Point Park showedincreases in isopreneconcentrations afterdawn modelhad difficultyreproducing theseprofiles.First,thetime of the and elevatedconcentrations throughoutthe day. All three days collapseof the boundarylayeris importantin determining whether exhibited small moming isoprenepeaks,possiblyas a resultof isopreneconcentrations showa substantial increase(otherthan the anthropogenic momingrdsh-houremissions. At RockyPointPark, increasedue to decreasing OH radicalconcentrations) late in the this peakoccurredat 0600 hours(representing the averagemixing afternoon.If the boundarylayercollapses beforesunsetwhilelight ratiofrom06004)900hours).At Langley,the morningpeakoccurred levelsarestillsufficient for isopreneemission, ambientconcentrations at 0900 hourson bothdays.The latertime of the morningpeakat will increase asa resultof the reduceddilution.The modeledprofiles  25,474  CURRENET AL.: BIOGENICI$OPRENEIN THE LOWERFRASERVALLEY  1.6  Langley  1.4  ....  Rocky Point  1.2  1.o  0.8  0.6  >25C (n:9)  0.4  •J  20-25C (n=14) 20-25C (n=8)  d,....•• __..•'•'1 ..I...... ß ..... l'' ..... -1 -I- ...•.•... -I••. ...... ø' '''•.2.2•] (n:7) _<20c (n=3) ..  o  0.2  ..  0.0  6  12  18  24  Time (hr)  Figure 4a. Isopreneprofileson averaged nonepisode days.  1.4  1.2  1.0  0.8  0.6  0.4  0.2  0.0  i  12  18  Time (hr)  Langley, Aug 2 -' Langley, Aug 4 + Rocky Point, Aug 5] Figure 4b. Isopreneprofilesonepisode days.  24  CURRENET AL.: BIOGENICISOPRENE1NTHE LOWERFRASERVALLEY  25,475  14  12  10  I  I  6  12  24  18  Time (h) Figure 5. Predictedisopreneprofileovera ruralforest.  assumed thatthe boundary layercollapsed at the sametimethatthe biogenicsources stopped emitting,thusnegatingthiseffect. Second, the concentrationand profile of nighttimeNO3 concentrations is criticalto explainingthe behaviorof isopreneafter sunset.Measurements takenat experimental sitesin WestGermany showedthatNO3 concentrations wereveryvariableandthatprofiles changed substantially fromnightto night.NOa concentrations at these sitesrangedfrom not detectable to 280 ppt [Platt et al., 1981]. No NOa data from the Lower FraserValley were availablefor this  analysis. NOa profileswereassumed to risequicklyat sunsetandto maintaina peakconcentration of 10 pptfor severalhours.A slower increase in NOa concentrations aftersunsetwouldresultin higher eveningisoprene concentrations. 5. Conclusions  Theseasonal impme studydescribed in thispaperexaminedmean dailyimpme concentrations at severalmonitoring stations withinthe Lower FraserValley.As predicted by previousstudies whichfound  1.6  1.4  1.2  1.0  0.8  0.6 0.4  0.2  0.0  0  I  I  6  12  18  24  Time (h)  [-*- Measured + Predicted I Figure 6a. Predicted andmeasured isoprene profileatRockyPointPark;averaged dayswith 20øC<Tm•<25øC.  25,476  ½URRENET AL.: BIOGENICISOPRENEIN THE LOWERFRASERVALLEY 1.4  1.2  1.0  0.6  0.4  0.2  0.0  I  0  6  12  18  24  Time (h)  ['•- Measured + Predicted l Figure6b. Predicted andmeasured isoprene profile atLangley, August 2, 1993.  few species of isoprene-cmitting vegetation in theregion[Drewitt, concentrations ofisoprene throughout theday.Higherisoprene levels 1996; Drewitt et al., 1997; Curren,1998), dailymeanisoprene were measured duringthe mildozoneepisode daysof 1993and mixingmilosatallof theurbanandsuburbansitesexaminedwerelow  showedprofilesthat increased after sunriseandremainedelevated  throughout the year.Mean summerisoprene concentrations were throughout theday.Therewassome evidence ofa morning isoprene approximately equalin magnitude to meanwinterconcentrations at peakdueto rushhourrelated anthropogenic isoprene emissions on ud•msitesandwereslightlyhigherthanmeanwinterconcentrationsthesedays. at suburbansites.Althoughwinter isopreneconcentrations were A simple boxmodelwaswrittentoinvestigate thephysical and enhanced bytheshallow mixinglayerofthisseason, thepresence of chemical factors thatdetermine theshape andthemagnitude of isoprene in theatmosphere duringtheseason whenmostplantsare isoprene profiles intheLowerFraser Valleyandtoexplain whythe dormant indicates theexistence of activelocalnonbiogenie sources of observed urbanprofiles weresomuchflatterin shape thanthose  isoprene atthesitesstudied. Winterconcentrations didnotshowany measured atruralsitesoverforests. Themodel included temperaturerelationship with temperature, supporting the hypothesis of a and light-dependent biogenieisopreneemissions, estimated nonbiogenie winterisoprene source. Detectable levelsof isoprene anthropogenic isoprene emissions, a time-dependent mixing depth, werepresent at all sitesoncoolsummer dayswhenbiogenie sources daytime OHsinks, andnighttime NO3sinks. Themodel successfully wererelatively inactive, indicating a background anthropogenic level predicted bothisoprene concentrations andprofileshapes at urban of isopreneduring this seasonas well. Summerisoprene sites onnonepisode days. andoverforests, andwaspartially successful concentrations at temperatures greaterthan 15oC showeda dear inestimating isoprene trends onchemically complex episode days. relationship with temperature. Theseresultssuggestthat winter Theresults ofthemodelindicate thatisoprene concentrations over isoprene mixingratiosarethe resultof anthropogenic emissions of deciduous forests areconsistently highthroughout thedayasaresult isoprene,while summer concentrations are a mixture of both oftheveryhighlocal arealemission rates thatsuccessfully compete anthropogenic andbiogenie isoprene. withtheeffects of dilution dueto a growing boundary layerand  Themeasured levels ofisoprene showed strong positive correlationsdestruction by OH radicals. Isoprene emission ratesat urbanand with cis-2-butene and trans-2-.pentene, hydrocarbons normally suburban sitesin theLowerFraserValley,ontheotherhand,arean associated with anthropogenic mobilesources, af all sitesin both orderof magnitude lowerthanthoseat ruralforestsites.At urban seasons. Thisis in contrast to theresults reported for southern U.S. sites, daytime isoprene emissions fighta losing baffle withtheeffects cities andindicates thata substantial portionof boththesummertime of a deepening mixinglayeranddestruction byOH radicals; low  andwintertime ambient isoprene loadoriginates fromanthropogenic isoprene concentrations andflatdiurnal profiles aretheresult. Theflat mobilesources. Concentrations of isoprene oncoolsummer days shape ofthedaytime [soproe profiles andlowdaytime concentrations (when biogeniesources are likelyto be inactive)suggest that lessen thepotential impact ofbiogenie isoprene uponregional ozone background anthropogenic isoprene mayaccount for25-78%of the production in theLowerFraser Valleyasthetimesof itshighest totalobserved isoprene loadatRockyPointPark. concentrations (during theearlyevening) donotcoincide withpeak Thediurnalisoprene study examined thedailyvariation in isopreneNOx concentrations norwiththetimesofoptimal ozone-producing concentrations in the LowerFraserValleyand investigated the meteorological conditions. physical and chemical factorsthat definedthe observed profiles Theweight ofevidence fromthisandpreviously published research through theuseofa simple boxmodel. Onaverage, isoprene levels on suggests thatbiogenic isoprene is nota majorcontributor to the nonepisode daysat urbansitesshowedflat profilesand low production ofozone intheLowerFraser Valley. Fewspecies ofcrops,  CURRENET AL.: BIOGENIC ISOPRENEIN THE LOWER FRASERVALLEY  25,477  emissionrate variability: Observationswith eucalyptusand naturalvegetation, andtreesin the regionemitisoprenein appreciable emission ratealgorithmdevelopment, d. Oeophys. Res.,96(D6), amounts.As a result,ambientconcentrations of total isopreneare 10,799-10,808, 1991. correspondingly low throughoutthe day and in all seasons. H.A. Measurable concentrations ofisoprenein winterandon coolsummer Hayden,K.L., K.G. Anlauf,R.M. Hoff,J.W.Strapp,J.W.Bottenheing Wiebe, F.A. Frouric,and J.B Martin, The verticalchemicaland days,aswellasthepositivecorrelation of isoprene with hydrocarbons meteorological structure of theboundary lay.erin theLowerFraser knowntobeemittedfromanthropogenic mobilesources, indicatethat Valley duringPacific•)3 ,Atmos.Environ.,31(14), 2089-210;5, 1997. isoprene intheLowerFraserValleyoriginates frombothbiogenicand theheightof theconvective internalboundary anthropogenic sources;thus biogenicisopreneconstitutes only a Hsu,S.A.,Anoteonestimating layernearshore,BoundaryLayerJl/leteorol., 35, 311-316, 1996. fractionof the smalltotalisoprene load.  An analysis of isoprene's contribution to the totalreactivityof Jiang, W., D.L. Singleton,A. Dorkalam, S. Bohme,and M. Hedley, Processing the LowerFraserValley Pacific93 emissioninventory nonmethane hydrocarbons in theLowerFraserValleyshowedthat for UAM-V applications: Area and mobilesources, Rep. PETapproximately 10%or lessof theozoneproduced atLangleyresults ]3•5-965, Nat. Res.Counc.,Ottawa, Ont., Canada, Dec. 12, from reactions with isoprene[Bottenheirn et al., 1997]. Isoprene's 1996. contribution to theformationof ozonewasexpectedto be evenlessin  Lamb,B.,H. Westberg, andG. Allwine,Biogenichydrocarbon emissions from deciduousand coniferoustreesin the United States,J.Geophys. Res., 90(D1), 2380-2390, 1985. value is in goodagreement with the analysisof Biesenthalet al. B.H., Pacific93 air emissions inventorydraRreport,B.H. Levelton [ 1997], who useda separate andindependent methodto determine LeveRon,  more urban areas as a result of its lower abundance at those sites. This  & Assoc. Ltd.,Richmond, B.C.,Canada,'July 1996. that approximately 13% of the ozoneformedat anothersitein the Lower FraserValley was the productof reactions with isoprene. McLaren,P•, D.L. Singleton,J.Y.K. Lai, B. Khouw,E. Singer,Z. Wu, andH. Niki, Analysisof motorvehiclesourcesandtheir contributionto Together withtheconclusions presented above,research suggests that ambient hydrocarbon distributions at urbansitesin Torontoduring biogenic isoprene maynotbeasimportant to thetropospheric ozone the southernOntariooxidantsstudy,Atmos.Environ., 30(12), chemistry in theLowerFraserValleyasit is in southern U.S. cities. 2219-2232, 1996. NationalResearchCouncil(NRC), Committeeon Tropospheric Ozone Acknowledgements.Fundingfor thisresearch wasprovidedby •e FormationandMeasurement, Rethinkingthe OzoneProblemin NaturalSciences andEngineering Research CouncilandEnvironment Canada. Thanksto WeiminJiang•NationalResearch CouncilCanada,for providing UrbanandRegional AirPollution,N at, Acad,Press,Washington, D.C., 1991. temporalanthropogenic VOC profiles,andto Douw Steyn,Universityof BritishColumbia,andNigel Bunce,Universityof Guelph,for adviceand Oke, T.R., BoundaryLayer Climates,Methuen,New York, 1978. comments onthis manuscript. Oliver, K.D., J.D. Pleil, andW.A• McClenny,Sampleintegrityof tracelevel volatile organiccompoundsin ambientair storedin summaReferences polishedcanisters, Atmos.Environ.,20, 1403, 1986. Biesenthal, T.A.• Q. Wu, P.B. Shepson, H.A. Wiebe,K.G. Anlauf,andG.I. MacKay,A studyof relationships betweenisoprene, itsoxidation products and ozonein the Lower FraserValley, B.C., Atmos. Environ. 31(14), 2049- 2058, 1997. Bottenheim,J.W., P.C. Brickell,T.F. Dann,D.K. Wang,F. Hopper,A•J. Gallant,  K.G.  Anlauf, and H.A.  Wiebe, Non-methane  hydrocarbons andCO duringPacific'93,Atmos.Environ.31(14), 2079-2087, 1997.  Canadian Councilof Ministersof the Environment (CCME), Canadian1996 No•OC science assessment:Ground-level ozone and its precursors, reportof the dataanal.workinggroup,Ottawa,Ont., Canada, 1997. Cardelino, C./c, and W.L. Chaincities,An observation-based model for  Parton,W.J.,andJ.A.Logan,A mode!fordiurnal variation in soilandair temperature, Agric.Meteorol.,23, 205-216,1981. Pierce,T.E.,andP.S.Waldruff,PC-BEIS:A personalcomputerversionofthe BiogenicEmissionsInventorySystem/. Air WasteManage. Assoc., 41,937-941,  1991.  Platt,U., D. Perner,J. Schr6der,C. Kessler,andA• Toennissen, The diurnal  variationof NO3,d. Geophys.Res., 86(C12), 11,965-11,970, 1981.  Singleton, D., M. Hedley,W. Jiang,R. McLaren,T. Dann,andP.B. Shepson, Evaluationof isopreneemissionandchemistryfor photochemical and ozone modelling in the Lower Fraser Valley, British Columbia, paper presentedat the 89th Annual Meeting and Exhibition, Air andWasteManage.Assoc.,Nashville,Tenn.,June 23-28, 1996.  analyzingozoneprecursor relationships in theurbanatmosphere, Steyn,D.G., and T.R. Oke, The depthof the daytimemixedlayer at two J. Air WasteManage. Assoc.45, 161-180, 1995. coastal sites: A model and its validation,Boundary Layer Chameides, W.L. et al., Ozoneprecursorrelationships in the ambient Jl/leteorol., 24,161-180, 1982. atmosphere, J. Geophys. Res.,97(D5)6037-6056,1992.  Cu•  K.,Biogenic isoprene andtheproduction oftropospheric ozonein the  Steyn, D.G.,J.W.Bottenheim, andR.B.Thomson, Overview oftropospheric  ozone intheLowerFraserValleyandthePacific'93 study,Almos. Environ.,3](14), 2025-2035, 1997. Sci.,Univ.of Guelph,Guelph,*Ontario,1998. Trainer, M., E.J. Williams, D.D. Parrish, M.P. Buhr, E.J. Allwine, H.H. Dann, T., D. Wang A. Steenkamer, R. Halman,andM. Lister,Volatile Westberg. F.C. Fehsenfeld, andS.C Liu, Modelsandobservations organic compoundmeasurements in the Greater Vancouver oftheirrq•ctofnaturalhydrocarbons onrural ozone,Nature, 329, LowerFraserValley,BritiSh Columbia,Ph.D.thesis,LandResour.  Regional District (GVRD),1989-1992, Rep.PMD 94-1,Environ.  705-707, 1987.  Technol.Cent.,Environ.Can.,Ottawa,Ont.,Jan.1994. Venkatram, A., A modelfor internalboundarylayerdevelopment, Boundary Drewitt,G.,Measurement ofbiogenichydrocarbon emissions fromvegetation LayerJl/leteorol., ] 1,, 419-437, 1977. in the Lower FraserValley, BritishColumbia,M. Sc. thesis,121 pp., Vancouver., B.C., Canada,Dep. of Geogr.,Lhaiv.of B.C., 1996.  Winberry, W.A, Jr.,N.T. Murphy, andR.M.Priggan, Method TO-14and Method TO-13, in Compendium of Methodsfor the Determination of ToxicOrganic Compounds inAmbient Air,  Drewitt, G.B., K. Curreft,.D.G. Steyn,T.J. Gillespie,and H. Niki, Rep.EPA-600/4-89-O! 7,U.S.Environ. Prot.Agency, Research Measurement ofbiogenic hydrocarbon emissions fromvegetation TrianglePark,N. C., June19gg. intheLowerFraser Valley,B.C.,Atmos. Environ.,inpress,1998. Fehsenfeld, F.,etal.,Emissions of volatileorganiccompounds fromvegetation IC Cu•en andT. Gillespie,LandResource Science, Universityof Guelph, andtheimplications foratmospheric chemistry, GlobalBiochem. Guelph,ontario,CanadaN1G 2W1. (e-mail:tgillesp•lrs.uoguelph. ca). Cycles,6(4),389-430,1992. T. DannandD. Wang ETC, Environment Canada,RiverRoad,Ottawa,  Finlayson-Pitts, B.J., andJ.N. Pitts,AtmosphericChemistry:Fundamentals and ExperimentalTechniques,Wiley-Interscience New York, 1986.  Ontario, CanadaK1A 0H3.  (Received November 25, 1997;revised March31, 1998;  Guenther,A.B., R.K. Monson,and R. Fall, Isopreneand monoterpene acceptedApril 7, 1998.)  


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