北部空品區臭氧不良成因探討

PhotochemicalSmog

TheMajorAirPollutantinTaiwanShawChenLiuResearchCenterforEnvironmentalChanges(RCEC)AcademiaSinicaNCUSeminarMarch6,2007AcknowledgmentOzoneResearchTeam

(sponsoredbyEPA) Dr.Chih-ChungChang張志忠(AssistantRF) Dr.Chun-KuangChou周崇光(AssistantRF) Dr.Chun-YaoLin林傳堯(PDFellow) Mr.Chien-JungShiu許乾忠(PhDcandidate) (AllaboveatRCEC) Prof.Po-HsiungLin林博雄(NTUAssist.Prof.) Prof.JuliusChang張時禹(NCUProf.) Prof.Chia-LinWang王家麟(NCUProf.)

Ozonecanbe"good"or"bad"fortheenvironment,dependingonitslocationintheatmosphere.Inthestratosphere,or"good"ozonelayerprotectslifeonEarthfromthesun'sharmfulultraviolet(UV)rays.Ground-levelor"bad"ozoneisapollutantthatisasignificanthealthhazard.Italsodamagesvegetation.Itisthemainingredientofurbansmog.

(Taiwanstandard:120ppbvover1hour;USstandard:80ppbvover8hours)WhyisTroposphericOzoneImportant?SurfaceO3isahealthhazard;canalsodamageplants.O3isanimportantgreenhousegasintheuppertroposphere.O3playsapivotalroleinthetroposphericchemistry:originofalloxidantssuchasOHandH2O2.AsimplifiedO3productionmechanism

InthepresenceofHO2(RO2)andNOx,O3canbeproducedviathefollowingcatalyticreactions HO2(RO2)+NO->NO2+OH(RO) NO2+hυ->NO+O O+O2+M->O3+M OH+CO(NMHC)+O2->HO2+CO2+RO2----------------------------------------------------------------Net: CO(NMHC)+2O2+hυ->CO2+O3+(RO)TheO3productiontendstoincreasewithHO2(RO2),NOx,hυ,OHandCO(NMHC).ButtoomuchNOxcandecreaseOHandHO2.SocanCOandNMHC.CharacteristicsofRegionalandUrbanHighOxidantsSynopticscalephenomenonsuperposedwithurbanscaleinfluenceComplexrelationshipwithprecursors,NOx&VOC(includingnaturalhydrocarbons)

1-hourAveragePeakConcentration

0-60ppb

61-79ppb

80-99ppb

100-110ppb

111-124ppb

125+ppb

DatanotavailableLAcounty:site1002BURBANKCOMMERCIALURBANANDCENTERCITYContributions(%)ofvariouspollutantstohighpollutiondaysin1993–2004(Years+1911aredenotedbydifferentcolors)

COO3SO2PM10TrendsofmonthlyaverageSO2from1993to2004

(Yearsaredenotedbydifferentcolors)JanAprJulOctTrendsofmonthlyaverageCOfrom1993to2004

(Yearsaredenotedbydifferentcolors)JanAprJulOctJanAprJulOctTrendsofmonthlyaverageNO2from1993to2004

(Yearsaredenotedbydifferentcolors)JanAprJulOctTrendsofmonthlyaverageO3from1993to2004

(Yearsaredenotedbydifferentcolors)TaipeiAverageO3(trendof10yrs:+38.7%)KaohsiungAverageO3(trendof10yrs:+41.4%)PolicyRelevantScientificQuestionsWhatisthecause(s)oftheincreasingtrendofO3inTaiwan?

HowdowecontrolhighO3?(Whatistherate-limitingprecursorofO3?)PossibleCausesofO3IncreaseintheLastDecadePhotochemicalproductionofO3hasincreased?(No!)

ButO3precursorsNOx,VOChavebeendecreasing.NOtitrationhasdecreased?(Yes!)Long-rangetransportofO3hasincreased?(Yes!)TotaloxidantOxcanbedefinedasfollows(Liu1977;Levyetal.1987)+1.5PAN+kn(RO2)n(NO)O3+NO2≈TotalOxidantsO3+NO2characterizestheregionalnatureofoxidantsbetterthanO3.O3+NO2isagoodmeasureoftherealproductionrateofO3(O3+NO2+PAN+HNO3wouldbeanevenbettermeasure).AirQualityMonitoringStationsinNorthernTaiwanTrianglesareurbanTaipeiaveragesRedisamountainstation(900m)Greenisacoastalstation***EvidenceofTitrationEffect***TrendsofDaytimeO3inTaipei

O3O3+NO2

trend(%/decade)trend(%/decade)Taipei21.6-1.7Taichung22.55.4Kaohsiung20.22.1Ilan20.47.4Yangming13.213Wanli14.010.2Lanyu14.113.1Central/Western(CW)TsuenWan(TW)TaiPo(TP)O3Central/Western(CW)TsuenWan(TW)TaiPo(TP)O3+NO22.5

±0.6%yr-1Leeetal.Geophys.Res.Lett.,25,1637-16490,1998.RecentBoundaryLayerOzoneTrendatOkinawa

PossibleCausesofO3IncreaseintheLastDecadePhotochemicalproductionofO3hasincreased?(No!)

ButO3precursorsNOx,VOChavebeendecreasing.NOtitrationhasdecreased?(Yes!)Long-rangetransportofO3hasincreased?(Yes!)PolicyRelevantScientificQuestionsWhatisthecause(s)oftheincreasingtrendofO3?

HowdowecontrolhighO3?(Whatistherate-limitingprecursorofO3?)PeakO3forvariousNOxandNMHCemissionsinsouthernTaiwan(Changetal.,2004)EKMAplotsareveryusefulforformulatingozonecontrolstrategies.EKMAplotsbasedonmodelingresultsusuallycontainlargeuncertaintiesthatexistintheemissioninventories,meteorologyparameters,photochemicalprocesses,etc.EKMAfromObservationBasedModels(OBM)canreducetheuncertaintiesbybypassingsomeoftheprocesses.TotaloxidantOxcanbedefinedasfollows(Liu1977;Levyetal.1987)+1.5PAN+kn(RO2)n(NO)O3productionratederivedfrommeasuredHO2,RO2andNO(Mihelcicetal.,2003)(Frostetal.,1998)Solidlinesaresteadystatemodelresults.Dashedlinesarethesameexceptnoisoprene.SymbolsarevaluesderivedfrommeasurementsoverthesoutheasternUS.ThesetwostudiesareveryusefulforstudyingthephotochemistryofO3production.ButtheydidnotlinkP(O3)toemissionsofprecursorsortheirearlymorningconcentrations(i.e.initialconcentrationswhichareproportionaltoemissions),asaresulttheyarenotveryusefulforformulatingozonecontrolstrategies.Inthefollowing,wewilldevelopanObservationBasedModel(OBM)approachbylinkingozoneproductionratestoinitialconcentrationsofprecursors.TotaloxidantOxcanbedefinedasfollows(Liu1977;Levyetal.1987)+1.5PAN+kn(RO2)n(NO)O3+NO2≈TotalOxidantsO3+NO2characterizestheregionalnatureofoxidantsbetterthanO3.O3+NO2isagoodmeasureoftherealproductionrateofO3(O3+NO2+PAN+HNO3wouldbeanevenbettermeasure).TheratiooftwoVOCscanbeameasureofphotochemicalprocessingbyOH

(Calvert,1976;Singh1979)

Changetal.(2005)foundtheratioethylbenzene/m,p-xylenecanbeagoodmeasureofthephotochemicalprocessingbyOH:Ethylbenzene(E)reactionrateconstantwithOHisk1=7.0x10-12.IfwefollowaLagrangiantrajectory,then

E(t)=E(0)Exp(-k1*OH*t).Form,p-xylene(X),k2=21.7x10-12

X(t)=X(0)Exp(-k2*OH*t).WhereE(0)andX(0)areinitialvaluesorearlymorningvalues.Inreality E(t)=E(0)Exp(-k1*OH*t)*F1(Transport),and X(t)=X(0)Exp(-k2*OH*t)*F2(Transport).BecauseEandXhavesamesourcesandsinks,theirratioshouldbelittleaffectedbythetransport,i.e.F1=F2.Thus

E(t)/X(t)=E(0)/X(0)Exp(-(k2-k1)*OH*t)Fromthisequation(OH*t)canbeevaluatedfromobservedE/X.

OnceweknowOH*t,then

NOx(t)=NOx(0)Exp(-k3*OH*t)whereK3=6x10-12,NOx(0)istheearlymorningNOxorinitialNOx.WecanevaluateNOx(0)and [NOx(0)-NOx(t)]=consumedNOx(t).SimilarequationscanbewrittenforVOCs.E.g. VOC1(t)=VOC1(0)Exp(-kvoc1*OH*t).Andcalculate [VOC1(0)–VOC1(t)]=consumedVOC1(t).RemoveDaLiouandChauchouandMaynonstationsR2=0.65

[O3+NO2]=0.35*[initialNOx]+0.51*[initialVOCs]+0.66PeakO3forvariousNOxandNMHCemissionsinsouthernTaiwan(Changetal.,2004)HowtocontrolO3+NO2insouthernTaiwan?Two-DOBMplotsofO3+NO2vs.initialVOCandNOxshowthatO3+NO2increasessignificantlywithVOC,butessentiallyindependentofNOx!BecausereducingNOxdecreasesthetitrationofO3,controlofNOxemissionswillincreasetheO3concentration.ThissuggeststhatO3levelsinsouthernTaiwanareVOC-limited,butmoredataareneeded.ThankYou!RemainingQuestions?Whatarethephotochemicalprocessescontributetotherelationshipbetweenthetotaloxidantanditsprecursors?Canwemeasurethephoto-stationarystateNO2/NOcorrectly?WhatistheconcentrationofHONO?Whatisitssource?Howimportantaregas-particlereactions?Howimportantarenaturalhydrocarbons,especiallyisoprene?

CausesofincreasingtrendsofO3inTaiwanfrom1994to2004AdecreaseinNOtitrationduetoreducedNOemissionscontributestoabout6ppbvincreaseinO3.Long-rangetransportofincreasingO3intheAsiancontinentcontributestoabout7ppbvincreaseinO3.SoO3concentrationsinTaiwanshouldincreasebyabout13ppbv.TheobservedincreaseinO3inTaiwanisabout9ppbv.Thedifferenceof4ppbvcanbeattributedtoreductioninO3productioninTaiwanduetoreducedemissionsofO3precursors,i.e.EPA’scontribution.Between1960and1993,backgroundO3inTaiwanprobablyincreasedbymorethan10ppbvduetolong-rangetransportofgreaterregionalandhemisphericO3.TroposphericOzone–ABriefHistoryDiscoveryoftroposphericO3:Schoenbein,1840.TroposphericO3comesfromthestratosphere:Regener,1941.O3asanairpollutant:LosAngeles,1940s.FormationofO3insmog:Haagen-Smit(1952) VOC(NMHC)+NOx+Sunlight→Ozone

ProposalofHO2(RO2)reactionsinthesmog:Leighton(1961) HO2(RO2)+NO→NO2+OH(RO) NO2+hυ+O2→NO+O3TroposphericO3comesmostlyfromphotochemicalproduction:Crutzen(1972),ChameidesandWalker(1973)WheredoesthetroposphericO3comefrom?Downwardflowfromthestratosphere.Photochemicalproduction.WheredoesO3go?Surfacedeposition.Photochemicaldestruction.KeyQuestionsAboutO3ControlWhatistherate-limitingprecursorofO3production?NOxorVOC?AssumingitisVOC,thenwhichVOCsarethemajorones?WhatarethemajorsourcesofthemajorVOCs?HOandHO2(RO2)ProductionOHandHO2(RO2)radicalsareproducedfromphotochemicalreactionsofO3andH2O(Levy,1971).Theyarealsoproducedfromphoto-dissociationofaldehydesandHONO. O3+hυ(<315nm)->O2+O(1D) H2O+O(1D)->2OH HONO+hυ->NO+HO RCHO+hυ+O2->CHO+RO2Uncertainties:(1)NMHCemissions,(2)NMHCreactions,(3)HONOformat