Science2003AkimotoGlobalairqualityandpollut

GlobalAirQualityandPollutionHajimeAkimotoTheimpactofglobalairpollutiononclimateandtheenvironmentisanewfocusinatmosphericscience.Intercontinentaltransportandhemisphericairpollutionbyozonejeopardizeagriculturalandnaturalecosystemsworldwideandhaveastrongeffectonclimate.Aerosols,whicharespreadgloballybuthaveastrongregionalimbalance,changeglobalclimatethroughtheirdirectandindirecteffectsonradiativeforcing.Inthe1990s,nitrogenoxideemissionsfromAsiasurpassedthosefromNorthAmericaandEuropeandshouldcontinuetoexceedthemfordecades.Internationalinitiativestomitigateglobalairpollutionrequireparticipationfrombothdevelopedanddevelopingcountries.Whenthefirstmeasurementsofhighconcen-trationsofCOovertropicalAsia,Africa,andSouthAmericaweremadeavailablebytheMAPS(MeasurementofAirPollutionfromSatellite)instrumentlaunchedin1981onthespaceshuttleColumbia(1),itbecameclearthatairpollutionwasaninternationalissue.Thoseimagesshowednotonlythatindustrialairpollutionfromfossilfuelcombustioncouldaffectregionalandglobalairquality,butthatemissionsfrombiomassburning(for-estfires,agriculturalwasteburning,andveg-etablefuelcombustion)wereimportantaswell,confirmingthehypothesisofCrutzenetal.(2).Thismeantthatpeopleinlessdevel-opedcountries,aswellasresidentsofindus-trializedandrapidlygrowingdevelopingcountries,couldsufferfromairpollutiongen-eratedelsewhere.AnotherillustrationoftheglobalcharacterofairpollutioncamefrommeasurementsoftroposphericozonemadebytheTOMS(TotalOzoneMappingSpectrom-eter)andSAGE(StratosphericAerosolandGasExperiment)instrumentsontheNimbus7satellite(3).Onceagain,theimpactofbiomassburningonregionalozoneconcen-trationswasdemonstrated,inadditiontothatofindustrialpollution.Morerecently,obser-vationsofvarioustroposphericairpollutantssuchasNO2,SO2,andHCHObyGOME(GlobalOzoneMonitoringExperiment)andSCHIAMACHY(ScanningImagingAbsorp-tionSpectro-MeterforAtmosphericChartog-raphY)(4)andofCObyMOPITT(Measure-mentofPollutionintheTroposphere)(5)haverevealedpollutiononaglobalscale.Edwardsetal.(6)obtainedapictureoftheprocessesaffectingtroposphericO3pro-ductionoverAfricaandtheAtlantic,com-biningthedataonTOMSO3,MOPITTCO,andGOMENO2.Aerosolsareanothercat-egoryofairpollutantsthatcanbeviewedfromsatellites.Examplesoftheglobaldis-tributionofanthropogenicandnaturalaero-solsderivedfromMODIS(Moderate-ResolutionImagingSpectrometer)ontheTERRAsatellitehavebeengivenbyRamanathanetal.(7)andKaufmanetal.(8).Nakajimaetal.(9)derivedglobaldis-tributionsofaerosolparticlenumberandcloudmicrophysicalparameters,usingtheAVHRR(AdvancedVeryHighResolutionRadiometers)remotesensingdata,andHigurashiandNakajima(10)showedthedistributionoffourmajoraerosolstypes(soildustandcarbonaceous,sulfate,andsea-saltaerosols)intheEastChinaSearegionusingSeaWiFS(Sea-viewingWideField-of-viewSensor)data.Thesestudiesshowthatsatellitedatacanbeusefulforrevealingclimaticandenvironmentalim-plicationsofglobalairpollution.Globalair-qualityissuesexistonlyinre-gardtothosepollutantswhoseatmosphericlifetimesarelongenough(ontheorderof1week)forthemtobetransportedatleasttoanothercontinent.Onesuchtracegasistro-posphericozone,apotentgreenhousegas(11)thatalsoistoxictohumans,animals,andplants.Becausetheatmosphericlifetimeofozoneis1to2weeksinsummerand1to2monthsinwinter(12),ozoneproducedinapollutedregionofonecontinentcanbetrans-portedtoanothercontinentallyearlong.Hemisphericaltransport,whosecharacteristictimescaleisabout1month,canoccurinallseasonsexceptsummer.Figure1showshowmodel-calculatedsurfaceO3duringthegrow-ingseason(MaythroughAugust)intheNorthernHemispherehasincreasedbetween1860and1993(13).Accordingtothisanal-ysis,theconcentrationofsurfaceO3overthemid-andhigh-latitudeEurasianandNorthAmericancontinentswas15to25partsperbillionbyvolume(ppbv)in1860buthasincreasedto40to50ppbveveninrelativelyremoteareas,andfrom10to15ppbvto20to30ppbvoverthemid-andhigh-latitudePa-cificOcean.Oneexampleofthespatialex-tentofglobalozonepollutionisthattheaverageconcentrationofozoneinremoteareasofEastAsiaisalreadyhighenoughtojeopardizeagriculturalandnaturalecosys-temsthere(14).Itiseasilyseen,then,howtheelevationofbackgroundlevelsofozonebylong-rangetransportcancausetheaddi-tionofozoneproducedlocallyorregionally,inamountsthatwouldnototherwisehavebeencritical,toexceedair-qualitystandardsorcriticallevels(15,16).Thismakessmallincrementsofozoneconcentrationscausedbycontributionsfromothercontinentsanissueofgreatconcern(17).TheatmosphericlifetimeofCOisalsolongenough(1to2monthsonaverage)toallowintercontinentaltransportandhemi-sphericairpollution.BecauseasignificantportionofCOpollutionisfromautomobilesandbiomassburning(13),itsintercontinentaltransportisusuallymoreeasilycapturedbyobservationthanisthatofozone(18).Be-causetheconcentrationofOHinremoteareasismainlycontrolledbyCO,andtheconcentrationofOHintheatmospheredeter-minesthelifetimesofmostatmospherictracegases,includinggreenhousegasessuchasCH4andHCFC(13),globalpollutionbyCOisworrisomebecauseofitseffectontheoxidizingcapacityoftheatmosphere.Anotherimportantaspectofglobalairpollutionistheimpactofaerosolsonclimat