数密度排放因子测量

MeasurementofBlackCarbonandParticleNumberEmissionFactorsfromIndividualHeavy-DutyTrucksGEORGEA.BAN-WEISS,†MELISSAM.LUNDEN,‡THOMASW.KIRCHSTETTER,‡ANDROBERTA.HARLEY*,§DepartmentofMechanicalEngineering,UniversityofCalifornia,Berkeley,California94720-1740,AtmosphericScienceDepartment,LawrenceBerkeleyNationalLaboratory,Berkeley,California94720,andDepartmentofCivil&EnvironmentalEngineering,UniversityofCalifornia,Berkeley,California94720-1710ReceivedJuly28,2008.RevisedmanuscriptreceivedDecember30,2008.AcceptedDecember31,2008.Emissionfactorsforblackcarbon(BC)andparticlenumber(PN)weremeasuredfrom226individualheavy-duty(HD)dieseltrucksdrivingthrougha1-km-longCaliforniahighwaytunnelinAugust2006.EmissionfactorswerebasedonconcurrentincreasesinBC,PN,andCO2concentrations(measuredat1Hz)thatcorrespondedtothepassageofindividualHDtrucks.ThedistributionsofBCandPNemissionfactorsfromindividualHDtrucksareskewed,meaningthatalargefractionofpollutioncomesfromasmallfractionofthein-usevehiclefleet.Thehighest-emitting10%oftruckswereresponsiblefor∼40%oftotalBCandPNemissionsfromallHDtrucks.BCemissionswerelog-normallydistributedwithameanemissionfactorof1.7gkg-1andmaximumvaluesof∼10gkg-1.CorrespondingvaluesforPNemissionfactorswere4.7×1015and4×1016#kg-1.Therewasminimaloverlapamonghigh-emittersofthesetwopollutants:only1ofthe226HDtrucksmeasuredwasfoundtobeamongthehighest10%forbothBCandPN.MonteCarloresamplingofthedistributionofBCemissionfactorsobservedinthisstudyrevealedthatuncertainties(1σ)inextrapolatingfromarandomsampleofnHDtruckstoapopulationmeanemissionfactorrangedfrom(43%forn)10to(8%forn)300,illustratingtheimportanceofvehiclesamplesizesinemissionsstudies.Whenn)10,samplemeansaremorelikelytobebiasedduetomisrepresentationofhigh-emitters.Asvehiclesbecomecleaneronaverageinthefuture,skewnessoftheemissionsdistributionswillincrease,andthussamplesizesneededtoextrapolatereliablyfromasubsetofvehiclestotheentirein-usevehiclefleetwillbecomemoreofachallenge.1.IntroductionMotorvehicleemissionsoffineparticles(PM2.5)canadverselyaffecthumanhealth(1,2),impairvisibility,andaltertheearth’sradiativeenergybalanceleadingtoclimatechange(3).Gasolineanddieselenginesaresignificantsourcesofprimaryfineparticleemissions,especiallyinurbanareas(4).Whennormalizedtofuelconsumption,PM2.5massemissionfactorsaremorethananorderofmagnitudehigherforheavy-duty(HD)dieseltruckscomparedtolight-duty(LD)passengervehicles(almostallgasoline-fueled)(5).Blackcarbon(BC),whichaccountsformorethanhalfofPM2.5massfromdieselenginesunderload(5),isofparticularconcern.ArecentassessmentindicatesthatBCisthesecondlargestcontributortoglobalwarming(nexttoCO2)andaltersregionalprecipitationandsnowandcloudalbedos(6).StudieshavesuggestedthatreducingBCemissions,ofwhichdieselenginesareamajorsource,shouldbeanelementintheefforttocounteractglobalwarming(e.g.,(7)).BondandSun(8)note,however,thatBCreductionindevelopedcountriesisrelativelycostlyasameansformitigatingclimatechangeunlesslocalairqualityandpublichealthbenefitsareconsideredsimultaneously.Motorvehiclesemitthelargestnumberofparticlesintheultrafinemode,definedasparticleswithdiameterDP100nm(9,10).Theseparticlesaresmallenoughtopenetratedeeplyintothelung,enterthecirculatorysystem,andaccumulateinorganssuchasthebrain,heart,andliver(2).Thereisanongoingdebateaboutwhetherparticlenumber(PN),mass,orchemicalcompositionismostimportantincausingadversehumanhealtheffects(2).Currentmass-basedemissionstandardsmaynotbeoptimalinreducinghealtheffectsifparticlenumberturnsouttobethemoreharmfulfactor.Oneofthechallengesincharacterizingvehicleemissionsisextrapolatingfromasampleofvehiclestotheentirein-usepopulation.HDtrucksareespeciallytime-consumingandexpensivetotestinthelaboratory,andthereforedynamometerstudiesofHDvehicleemissionshavegenerallybeenlimitedtosmallsamplesizes(1-25vehicles).On-roadremotesensingtechniqueshavebeenusedtomeasuresnapshotsofgaseouspollutantemissionsfromlargenumbersofvehicles;remotesensingofPMemissionsisdifficultduetotheneedtorelateintegratedmeasuresofparticleopticalpropertiesbacktoparticlenumberasafunctionofsizeandchemicalcomposition.Whilemeasurementsofvehicleemissionsinroadwaytunnelscancapturealargesampleofon-roadvehicles,suchstudiesusuallyprovideonlyfleet-averageresults.Itisknownthathigh-emittingvehiclescontributedis-proportionatelytogaseouspollutantemissionsfromtheon-roadLDvehiclefleet.EmissionfactordatafromBishopandStedman(11)forNOx,CO,andhydrocarbonsshowthattheskewnessofemissionsdistributions(i.e.,therelativeim-portanceofhigh-emittersasasourceofvehicle-relatedpollution)hasbeenincreasingatthesametimethatfleet-averageemissionshavedeclinedsignificantly.Gas-phaseemissionsfromHDdieselvehicleshavebeenfoundtobeskewedtoalesserextentthanlight-dutyvehicles(12).LessisknownaboutPMemissionsfrommotorvehicles,dueinparttodifficultiesinmakingfasttime-responsemeasure-ments.VariousstudieshavereporteddistributionsofPMemissionsforLDvehicles(13-15).OtherstudieshavefocusedonPMemissionsfromlargesamplesofhigh-emittingLDvehicles(16-18).Jiangetal.(19)reporteddistributionsofBCandPM2.5emissionsfo