The impact of rush hour traffic and mix on the ozone

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TheimpactofrushhourtrafficandmixontheozoneweekendeffectinsouthernCalifornia

H.OliverGaoa,*,DebbieA.NiemeierbaSchoolofCivilandEnvironmentalEngineering,CornellUniversity,Ithaca,NY14853,USA

bDepartmentofCivilandEnvironmentalEngineering,UniversityofCaliforniaDavis,OneShieldsAve.,Davis,CA95616,USA

AbstractTheozoneweekendeffectreferstothecounterintuitiveobservationsshowingweekendozoneconcentrationsfrequentlytobehigherthanorcomparabletothoseobservedonweekdays.Ozonedynamicsarecloselylinkedtothetiming,mag-nitudeandfleetmixoftransportationactivities,primarysourcesofozoneprecursoremissions.Toexaminetheeffectsoftrafficactivityontheozoneweekendeffect,astatisticalanalysiswasconductedoftheweeklypatternsoftimedependentlight-dutyvehicleandheavy-dutytruckvolumesobservedat27weigh-in-motionstationsinsouthernCalifornia.Theresultsshowstatisticallysignificantvariationsintrafficflowsbydayofweek,byvehicletype,andbylocationwithrespecttotheLosAngelesmetropolitanarea.Thesevariationsintraffic,whenconvertedtovariationsinrunningexhaustemis-sions,tendtosupportfourofthesevenCaliforniaAirResourcesBoard’sozoneweekendeffecthypotheses.Ó2006ElsevierLtd.Allrightsreserved.

Keywords:Ozoneweekendeffect;Weeklytrafficpattern;Nonparametricfactorialanalysis

1.IntroductionGround-levelozoneisproducedfromacomplexseriesofphotochemicalreactionsofprecursorgasesintheatmosphere(SeinfeldandPandis,1998).Ozoneprecursorsofvolatileorganiccompounds(VOC)andnitrogenoxides(NOx)areemittedfrommanyemissionssources,ofwhichtransportationemissionsarethesinglemajorcontributor(USEnvironmentalProtectionAgency,2002).AccordingtotheCaliforniaAirResourcesBoard(CARB),forinstance,transportationsourcescontribute58%ofanthropogenicVOCemissionsand73%ofNOxemissionsinCalifornia.Thevehiclemilestraveled(VMT)andtheemissionsfactors(g/mi)usedtoesti-mateemissionsvarysubstantiallybyvehicleclass.Light-dutyvehicles(LDV)producemoreVOCemissions,whilediesel-poweredheavy-dutytrucks(HDT)areincreasinglysignificantsourcesofNOxemissions.Changesinthetiming,volumeandfleetcompositionoftrafficwillthusperturbozoneprecursoremissionsandconsequently,theatmosphericozonedynamics.

1361-9209/$-seefrontmatterÓ2006ElsevierLtd.Allrightsreserved.doi:10.1016/j.trd.2006.12.001

*Correspondingauthor.

E-mailaddress:hg55@cornell.edu(H.O.Gao).

TransportationResearchPartD12(2007)83–98Intuitively,reductionsinprecursorsshouldtranslatetoreductionsinozone,andthishasgenerallybeentherationaleforapplyingozonecontrolmeasures.However,thecomplexityoftheozonesystemissuchthatlow-eredprecursorinputsdonotnecessarilyresultinreducedozoneconcentrations.Episodesofhigherorcom-parableozoneconcentrationsonweekendsrelativetoweekdaysinmajormetropolitanareasintheUSandaroundtheworldhavebeenextensivelydocumented,despitesubstantiallylowerweekendlevelsofozonepre-cursorspecies(Clevelandetal.,1974;PontandFontan,2001).Thisphenomenoniscommonlyreferredtoastheozoneweekendeffect(OWE).TheOWEraisesanimportantpolicyissueregardingozonecontrolrelatedtodefiningthetrueeffectivenessofcurrentandplannedozonecontrolmeasures;mostcontrolstrategiesrelyuponreductionsofthemajorprecursors,particularlyNOxreductions.WithoutabetterunderstandingofwhytheOWEactuallyoccurs,itisnotclearwhetherNOxreductionswouldtendtodelayorspeedattainment.Tothelayperson,reducingozoneprecursorsshouldleaddirectlytomitigatingozonepollutionandwhenitdoesnot,theunderlyingrationaleforemployingreductionstrategiesbecomesgreatlydiminished.Forexample,incertainregions,higherozoneconcentrationshavebeenobservedtobeassociatedwithlowerconcentrationsofweekendozoneprecursors(CaliforniaAirResourcesBoard,2003),whichleadsometocitetheOWEasarationaleagainstfurtherNOxreductionsataCARBhearingonLowEmissionVehicle(LEV-II)regulations(Fujitaetal.,2002).ThereissignificantambiguityassociatedwithunderstandingthecausesoftheOWE(Larsen,2003).TheCARBhaspostulatedthattherearesevenpossiblefactorscontributingtotheOWE,mostofwhichfocusontheweeklyvariationsinthetiming,magnitudeandfleetmixoftransportationactivities.Theseinclude:NOx-reduction,NOx-timing,carryoverneartheground,carryoveraloft,increasedweekendemissions,aero-solandultraviolet(UV)radiation,andozoneinhibition.TheNOx-reductionhypothesisisthattheweekenddropinHDTtrafficrelativetoLDVtrafficleadstoelevatedVOCandNOxratiosthatinturnincreaseozoneformationratesduringaccumulationinVOC-limitedareassuchassouthernCalifornia.TheNOx-timinghypothesisisthatthetimingofweekendtransportationactivityresultsinNOxemissionsthataremoreeffi-cientatproducingozone.ThecarryovernearthegroundhypothesisassumesthatLDVtrafficishigherwithcorrespondinglylowerHDTonFridayandSaturdayevenings,leadingtoovernightcarryoverofpollutantswithhigherVOC/NOxratios.Thecarryoveralofthypothesisisnotdirectlyrelatedtotrafficactivities.Itpre-sumesthatozoneandozone-formingpollutantsthatmixdownfromaboveinteractwithfreshsurfaceemis-sionsinawaymoreconducivetogreaterozoneconcentrationsonweekendsthanonweekdays.ThehypothesisofincreasedweekendemissionsassumesgreateremissionsofVOCandNOxfromallsources.Theelevatedweekendozoneconcentrations,therefore,arethoughttoreflecttheincreasedemissionsofozoneprecursors.Theaerosolandultravioletradiationhypothesesarethatlessweekendtraffic,HDTtrafficinpar-ticular,translatestolessdirectparticulatematter(PM)andNOxthatareanimportantprecursorstosecond-aryformationofPM(thatabsorbssunlightintheatmosphere).Itispossiblethatincreasedultravioletradiationintensifiesozoneformationonweekends.TheozoneinhibitionhypothesisstatesthatHDTactivityissubstantiallylessatweekendsduringtheearlymorningozoneinhibitionthanonweekdays,resultinginlowerNOconcentrationsandlessozoneinhibition.LowerNOconcentrationsonweekendmorningsresultindecreasedozoneinhibition,andthushigherinitialozoneconcentrations,onweekendmorningscomparedtoweekdaymornings.ThesehypotheseshaveyettobevalidatedasexplanationsfortheOWEandmostofthestudiesontheOWEhaveconcentratedonexaminingairqualitytrendsandpatterns,inparticularanalyzingairmonitoringdatatocharacterizetheOWE(PryorandSteyn,1995).However,giventhecomplexsecondarynatureofozone,itisdifficulttodevelopconclusiveevidencebyanalyzingairqualitydataalone.Accuratelycharacter-izingtransportationactivitiesthatgeneratethebulkofozoneprecursoremissions,providesanotherperspec-tive,andinthefewstudiesthathavebeenconductedofferlimitedinsightontheOWE.Usingweigh-in-motion(WIM)trafficcountdatainthecentralvalleyofCalifornia,DreherandHarley(1998)andMarrandHarley(2002)observedthatdailyweekendHDTvolumeswere70–80%lowerthanforweekdays,butthecorrespondingdailyLDVvolumeswereonly10%lowerwhencomparedtoweekdays.Thisobservation,however,wasqualitative.Gaoetal.(2005)examineddailytrafficvolumesinsouthernCal-iforniaandfoundthatdailyLDVvolumesonhighwaysintheareapeakonFridayanddecreasebyabout10%onweekendswhencomparedtoobservedtrafficonmid-weekdays.DailyHDTvolumes,incontrast,dropped