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IntercontinentalTransportofAirPollution:WillEmergingScienceLeadtoaNewHemisphericTreaty?TRACEYHOLLOWAY*EarthInstitute,ColumbiaUniversity,2910Broadway,NewYork,NewYork10027ARLENEFIOREDepartmentofEarthandPlanetarySciences,HarvardUniversity,20OxfordStreet,Cambridge,Massachusetts02138MEREDITHGALANTERHASTINGSDepartmentofGeosciences,PrincetonUniversity,B-78GuyotHall,Princeton,NewJersey08544WeexaminetheemergenceofInterContinentalTransport(ICT)ofairpollutionontheagendasoftheairqualityandclimatecommunitiesandconsiderthepotentialforanewtreatyonhemisphericairpollution.ICTistheflowofairpollutantsfromasourcecontinent(e.g.,NorthAmerica)toareceptorcontinent(e.g.,Europe).ICTofairpollutantsoccursthroughtwomechanisms:(i)episodicadvectionand(ii)increasingtheglobalbackground,whichenhancessurfaceconcentrations.WeoutlinethecurrentscientificevidenceforICTofaerosolsandozone,bothofwhichcontributetoairpollutionandradiativeforcing.ThegrowingbodyofscientificevidenceforICTsuggeststhatahemispheric-scaletreatytoreduceairpollutantconcentra-tionsmaybeappropriatetoaddressclimateandairqualityconcernssimultaneously.Suchatreatycouldpavethewayforfutureclimateagreements.IntroductionPastobservationsandmodelingstudieshaveestablishedthatonenation’spollutantemissionsmayaffecttheairqualityofdownwindregions(e.g.,refs1-3).Thelate1990s,however,broughtarenewedfocusonthepotentialroleofairpollutiontransportbetweencontinentsanditsimplicationsforairpollutionmitigationeffortsthroughtraditionaldomesticcontrols.Awarenesshasgrownintheclimate,airquality,andpolicycommunitiesthatairpollutantsarebothtrans-portedonahemisphericscaleandclimaticallyimportant.HereweexaminetheemergenceofInterContinentalTrans-port(ICT)ofairpollutionontheagendasoftheairqualityandclimatecommunitiesandconsiderthepotentialforanewtreatyonhemisphericairpollution.Observationsprovideaclearindicationthatpollutionemittedatthesurfaceononecontinentcancontributetoground-levelpollutionondownwindcontinents.Modelingstudiesofthisintercontinentalflowofpollutantssuggestthatitmaybeappropriatetomanageairqualityonahemisphericscale.Atreatysuchasthe1979ConventiononLong-RangeTransboundaryAirPollution(LRTAP)offersonemechanismtomanagethisairpollutiontransportbetweencountries.Ifsuchatreatywereusedtoregulatenon-carbondioxide(CO2)greenhousegasesandblackcarbonalongwithotherspeciesofinterestforhealthandagriculture,itcouldpavethewayforfutureCO2regulations.ResearchonICTisanongoingexampleoffeedbacksbetweenscientificknowledgeandpolicyawarenessinwhichthescienceandpolicycommunitiesinfluenceoneanother.Atmosphericchemistryandclimateresearchershavecon-venedinworkshopstoaddressthescientificquestionsofhemisphericairpollutiontransportandevaluatethegrowingevidenceforICTfrombothmeasurementandmodelingstudies.Theseworkshops,outlinedinTables1and2,areinfluencingthepolicycommunity,raisingawarenessoftheissues,andincreasingthepriorityofresearchfundingforglobalscaleairpollutionresearch.Toimprovethisprocess,boththescienceandthepolicycommunitiesshouldcreateopportunitiestofostertheinteractionneededforbothcommunitiestomakeprogressinthisarea.InMarch2000,theInternationalGlobalAtmosphericChemistryProgram(IGAC)launchedtheIntercontinentalTransportandChemicalTransformation(ICTC)researchactivity,bringingtogetherinternationalmeasurementcam-paignsandmodelingeffortscontributingtoICTunder-standing.Later,U.S.EPA-sponsoredmeetingscontinuedtheobjectiveofadvancingICTsciencewhilebeginningtofulfillU.S.obligationsunderthemostrecentLRTAPprotocol.Article8ofthe1999GothenburgProtocolemphasizesthe“improvementofthescientificunderstandingofthelong-termfateofemissionsand...thepotentialforintercontinentalflowofpollutants”(4).TheICTworkshopsrepresentthefirstcasesofU.S.leadershiponLRTAP-relatedscience.Thispaperisintendedtointroducetherecentscientificevidenceforintercontinentalairpollutiontothepolicycommunity,withaparticularemphasisonozone(O3)andaerosolsandtosuggestthepolicyapplicationsofthisevidencetothescientificcommunity.Assuch,wedonotofferhereadetailedtreatmentofthepolicyissuessurroundingintercontinentalairpollutiontransport.Rather,wehighlighttheopportunityforjointadvancementofscienceandpolicydevelopmentsrelatedtoICT.EvidenceforICTofAirPollutionOverview.ICTistheflowofairpollutantsfromasourcecontinent(e.g.,NorthAmerica)toareceptorcontinent(e.g.,Europe),schematicallyshowninFigure1.Here,weoutlinerecentadvancesinourunderstandingofICTfromanalysesofobservationsandmodelsanddiscusswhethertheevidenceindicatesthataglobalairpollutiontreatycouldbesupportedbythesciencetodate.ThedistanceoverwhichICToccursishighlydependentuponmeteorologicalconditions(e.g.,windspeed,precipita-tion,frontalactivity)andthepropertiesofthepollutantitself(e.g.,solubility,reactivity).Pollutantstransportedinthefreetropospherewilltypicallybecarriedfurtherandfasterthanpollutantsintheboundarylayerduetostrongerwindsandfewerlossmechanisms.ICTofairpollutantsoccursthroughtwomechanisms:(i)episodicadvectionand(ii)increasingtheglobalbackground,whichenhancessurfaceconcentra-tions.WefocushereonaerosolsandO3toillustratetheseme