A chronology of global air quality - Journals

Air pollution has been recognized as a threat to human health since the time of Hippocrates, ca 400 BC. Successive written accounts of air ... Logintoyouraccount Email Password Forgotpassword? Keepmeloggedin NewUser InstitutionalLogin ChangePassword OldPassword NewPassword TooShort Weak Medium Strong VeryStrong TooLong Congrats! Yourpasswordhasbeenchanged Createanewaccount Email Returninguser Can'tsignin?Forgotyourpassword? Enteryouremailaddressbelowandwewillsendyoutheresetinstructions Email Cancel Iftheaddressmatchesanexistingaccountyouwillreceiveanemailwithinstructionstoresetyourpassword. Close RequestUsername Can'tsignin?Forgotyourusername? 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2020AchronologyofglobalairqualityPhil.Trans.R.Soc.A.3782019031420190314http://doi.org/10.1098/rsta.2019.0314SectionOpenAccessReviewarticlesAchronologyofglobalairqualityDavidFowlerDavidFowlerhttp://orcid.org/0000-0002-2999-2627CentreforEcologyandHydrology,Penicuik,UK[email protected]GoogleScholarFindthisauthoronPubMedSearchformorepapersbythisauthor,PeterBrimblecombePeterBrimblecombeSchoolofEnergyandEnvironment,CityUniversityofHongKong,Kowloon,HongKongGoogleScholarFindthisauthoronPubMedSearchformorepapersbythisauthor,JohnBurrowsJohnBurrowsFacultyofPhysicsandElectricalEngineering,UniversityofBremen,Bremen,GermanyGoogleScholarFindthisauthoronPubMedSearchformorepapersbythisauthor,MathewR.HealMathewR.HealSchoolofChemistry,TheUniversityofEdinburgh,Edinburgh,UKGoogleScholarFindthisauthoronPubMedSearchformorepapersbythisauthor,PeringeGrennfeltPeringeGrennfeltIVLSwedishEnvironmentalResearchInstitute,Stockholm,SwedenGoogleScholarFindthisauthoronPubMedSearchformorepapersbythisauthor,DavidS.StevensonDavidS.StevensonSchoolofGeoSciences,UniversityofEdinburgh,Edinburgh,UKGoogleScholarFindthisauthoronPubMedSearchformorepapersbythisauthor,AlanJowettAlanJowettTheBoundary,GoodleyStockRoadCrockhamHill,Kent,UKGoogleScholarFindthisauthoronPubMedSearchformorepapersbythisauthor,EikoNemitzEikoNemitzhttp://orcid.org/0000-0002-1765-6298CentreforEcologyandHydrology,Penicuik,UKGoogleScholarFindthisauthoronPubMedSearchformorepapersbythisauthor,MhairiCoyleMhairiCoyleCentreforEcologyandHydrology,Penicuik,UKGoogleScholarFindthisauthoronPubMedSearchformorepapersbythisauthor,XuejunLiuXuejunLiuhttp://orcid.org/0000-0002-8367-5833EnvironmentalScienceandEngineering,ChinaAgriculturalUniversity,Beijing,People'sRepublicofChinaGoogleScholarFindthisauthoronPubMedSearchformorepapersbythisauthor,YunhuaChangYunhuaChangNanjingUniversityofInformationScienceandTechnology,Nanjing,Jiangsu,People'sRepublicofChinaGoogleScholarFindthisauthoronPubMedSearchformorepapersbythisauthor,GaryW.FullerGaryW.FullerImperialCollegeLondon,London,UKGoogleScholarFindthisauthoronPubMedSearchformorepapersbythisauthor,MarkA.SuttonMarkA.SuttonCentreforEcologyandHydrology,Penicuik,UKGoogleScholarFindthisauthoronPubMedSearchformorepapersbythisauthor,ZbigniewKlimontZbigniewKlimontInternationalInstituteforAppliedSystemsAnalysis(IIASA),Laxenburg,AustriaGoogleScholarFindthisauthoronPubMedSearchformorepapersbythisauthor,MikeH.UnsworthMikeH.UnsworthOregonStateUniversity,Corvallis,OR,USAGoogleScholarFindthisauthoronPubMedSearchformorepapersbythisauthorandMassimoVienoMassimoVienoCentreforEcologyandHydrology,Penicuik,UKGoogleScholarFindthisauthoronPubMedSearchformorepapersbythisauthorDavidFowlerDavidFowlerhttp://orcid.org/0000-0002-2999-2627CentreforEcologyandHydrology,Penicuik,UK[email protected]GoogleScholarFindthisauthoronPubMedSearchformorepapersbythisauthor,PeterBrimblecombePeterBrimblecombeSchoolofEnergyandEnvironment,CityUniversityofHongKong,Kowloon,HongKongGoogleScholarFindthisauthoronPubMedSearchformorepapersbythisauthor,JohnBurrowsJohnBurrowsFacultyofPhysicsandElectricalEngineering,UniversityofBremen,Bremen,GermanyGoogleScholarFindthisauthoronPubMedSearchformorepapersbythisauthor,MathewR.HealMathewR.HealSchoolofChemistry,TheUniversityofEdinburgh,Edinburgh,UKGoogleScholarFindthisauthoronPubMedSearchformorepapersbythisauthor,PeringeGrennfeltPeringeGrennfeltIVLSwedishEnvironmentalResearchInstitute,Stockholm,SwedenGoogleScholarFindthisauthoronPubMedSearchformorepapersbythisauthor,DavidS.StevensonDavidS.StevensonSchoolofGeoSciences,UniversityofEdinburgh,Edinburgh,UKGoogleScholarFindthisauthoronPubMedSearchformorepapersbythisauthor,AlanJowettAlanJowettTheBoundary,GoodleyStockRoadCrockhamHill,Kent,UKGoogleScholarFindthisauthoronPubMedSearchformorepapersbythisauthor,EikoNemitzEikoNemitzhttp://orcid.org/0000-0002-1765-6298CentreforEcologyandHydrology,Penicuik,UKGoogleScholarFindthisauthoronPubMedSearchformorepapersbythisauthor,MhairiCoyleMhairiCoyleCentreforEcologyandHydrology,Penicuik,UKGoogleScholarFindthisauthoronPubMedSearchformorepapersbythisauthor,XuejunLiuXuejunLiuhttp://orcid.org/0000-0002-8367-5833EnvironmentalScienceandEngineering,ChinaAgriculturalUniversity,Beijing,People'sRepublicofChinaGoogleScholarFindthisauthoronPubMedSearchformorepapersbythisauthor,YunhuaChangYunhuaChangNanjingUniversityofInformationScienceandTechnology,Nanjing,Jiangsu,People'sRepublicofChinaGoogleScholarFindthisauthoronPubMedSearchformorepapersbythisauthor,GaryW.FullerGaryW.FullerImperialCollegeLondon,London,UKGoogleScholarFindthisauthoronPubMedSearchformorepapersbythisauthor,MarkA.SuttonMarkA.SuttonCentreforEcologyandHydrology,Penicuik,UKGoogleScholarFindthisauthoronPubMedSearchformorepapersbythisauthor,ZbigniewKlimontZbigniewKlimontInternationalInstituteforAppliedSystemsAnalysis(IIASA),Laxenburg,AustriaGoogleScholarFindthisauthoronPubMedSearchformorepapersbythisauthor,MikeH.UnsworthMikeH.UnsworthOregonStateUniversity,Corvallis,OR,USAGoogleScholarFindthisauthoronPubMedSearchformorepapersbythisauthorandMassimoVienoMassimoVienoCentreforEcologyandHydrology,Penicuik,UKGoogleScholarFindthisauthoronPubMedSearchformorepapersbythisauthorPublished:28September2020https://doi.org/10.1098/rsta.2019.0314ThisarticlehasaCorrectionCorrectionto‘Achronologyofglobalairquality’AbstractAirpollutionhasbeenrecognizedasathreattohumanhealthsincethetimeofHippocrates,ca400BC.Successivewrittenaccountsofairpollutionoccurindifferentcountriesthroughthefollowingtwomillenniauntilmeasurements,fromtheeighteenthcenturyonwards,showthegrowingscaleofpoorairqualityinurbancentresandclosetoindustry,andthechemicalcharacteristicsofthegasesandparticulatematter.Theindustrialrevolutionacceleratedboththemagnitudeofemissionsoftheprimarypollutantsandthegeographicalspreadofcontributingcountriesashighlypollutedcitiesbecamethedefiningissue,culminatingwiththegreatsmogofLondonin1952.EuropeandNorthAmericadominatedemissionsandsufferedthemajorityofadverseeffectsuntilthelatterdecadesofthetwentiethcentury,bywhichtimethetransboundaryissuesofacidrain,forestdeclineandground-levelozonebecamethemainenvironmentalandpoliticalairqualityissues.Ascontrolsonemissionsofsulfurandnitrogenoxides(SO2andNOx)begantotakeeffectinEuropeandNorthAmerica,emissionsinEastandSouthAsiagrewstronglyanddominatedglobalemissionsbytheearlyyearsofthetwenty-firstcentury.Theeffectsofairqualityonhumanhealthhadalsoreturnedtothetopoftheprioritiesby2000asnewepidemiologicalevidenceemerged.Bythistime,extensivenetworksofsurfacemeasurementsandsatelliteremotesensingprovidedglobalmeasurementsofbothprimaryandsecondarypollutants.GlobalemissionsofSO2andNOxpeaked,respectively,inca1990and2018andhavesincedeclinedto2020asaresultofwidespreademissioncontrols.Bycontrast,withalackofactionstoabateammonia,globalemissionshavecontinuedtogrow.Thisarticleispartofadiscussionmeetingissue‘Airquality,pastpresentandfuture’.1.IntroductionThepotentialsubjectareaislargeandthefocushereisonthechronologyofairpollutionbyhumanactivity,identifyingthemainissues,theircausesandtheregionalandglobaltrends.Otherpapersinthisvolume,towhichlinksaremade,providethewidercontext,thepoliciesdevelopedtoaddresstheproblemsandthepossiblefutures.Therearefourratherdifferentsourcesofevidencetoprovidethenarrativeforthisaccount.Theseincludewrittendocumentsincludingearlylegislation,directmeasurementsofatmosphericcomposition,chemistrytransportmodels,whichsimulateatmosphericcompositionchangesfromaknowledgeofemissions,meteorologyandchemicalprocessingofpollutantgases,and,finally,remotesensingoftheatmospherefromaircraftandspace.Theearlydocumentsarefascinatingandprovidehintsattheunderlyingchemistry,butareentirelylackinginquantitativedetail.Legaldocumentsindicatetheintent,but,forreasonselaboratedlater,didnotsignificantlyconstrainthedevelopingglobalissuesuntilthelaterdecadesofthetwentiethcentury.High-qualitymeasurementsofairpollutantsarerestrictedtothelast150yearsandnumericalmodellingtothelast40years,leavingconsiderablescopeforspeculationontheearlytrends.Thereisnecessarilysomesubjectivityintheselectionofinformationsourcesusedtodescribetheairpollutionchronologyoutlinedhere,assummarizedintable 1.Forthispurpose,wehavefocusedonwhatweconsidertorepresentmajormilestonesbasedon:(i)therecognitionofkeyaspectsofairpollution,(ii)ofquantitativeevidenceand(iii)ofmajorpointsofchangesinairpollutionlevels.OtherperspectivesonthetopichavebeenprovidedbyColbeck[5]andMosley[6].Themainairpollutantsofinterestexaminedherearesulfurdioxide(SO2),nitrogenoxides(NOx),ammonia(NH3),volatileorganiccompounds(VOCs),primaryparticulatematter(PM),andtheirreactionproducts,includingfineparticulatematter(PM2.5)andtroposphericozone(O3). Table 1.Componentsoftheselectedchronologyofairpollutionpresentedinthispaper. Collapse dateairpollutionevent400BCETherelationshipbetweenairandhealthdevelopedasanimportantpartofthebookAirs,watersandplacesattributedtoHippocratesfirstcenturyADWritersfromimperialRome,e.g.SenecaandFrontinus,refertotheprobablehealthimpactsofsmoke947–1279SmokeandgaseouspollutantsfromcoalburningidentifiedasaprobleminCentralAsiabyAl-Mas'ūdī(947)andinChinaduringtheSongDynasty(960–1279)1273TheSmokeAbatementAct,theearliestlegislationinEngland,prohibitsuseofcoalasitis‘prejudicialtohealth’1610TheLawofNuisance(UK):WilliamAldred'spigfarmcase1661JohnEvelynpublishedFumifugiumorTheInconvenienceoftheAerandSmoakofLondonseventeenthcenturyHarmfuleffectsofairascribedtovariouscomponents,e.g.KenelmeDigby(acids),NehemiahGrew(lead),JohnEvelyn(sulfur)andJohnHall(antimonyormercury)eighteenthcenturyGuillaumeFrançoisRouelledetectsSO2byabsorbingthegasinstrongalkalis;CarlWilhelmScheeledetectsNH3viaabsorptionwithacids1872RobertAngusSmithpublishesAirandRain:TheBeginningsofaChemicalClimatology,havingundertakenthefirstmultisite,multipollutantmeasurements1878TheUKRoyalCommissiononNoxiousVapours1894The‘greathorsemanurecrises’ofLondonandNewYork1905SmokeNuisanceActsinBengal19051952TheGreatLondonSmog;12 000dieintwoweeks[1]LosAngelessmog,chemistryandeffectsdescribed[2]1956TheUKCleanAirAct1960Extensivelocalecologicaldamagebysmelters(e.g.[3])From1967,airpollutionproblemsarerecognizedasinternationalissues1960sAcidrainextensivelydescribedbySvanteOden1972UnitedNationsStockholmConferenceconfirmsacidrainasanimportantinternationalissueinEurope1970sGround-levelozonethreattoecosystemsidentifiedinNorthAmericaandEuropefollowingearlierconcernsofeffectsoftheozoneonhumanhealth1977USAestablishesitsNationalAcidDepositionProgram(NADP)1979UNECEConventiononLongRangeTransportofAirPollution(LRTAP)established1980sForestdeclinerecognizedinEuropeandNorthAmerica1985HelsinkiProtocol:CommitmenttoreducedSO2emissionsby30%(The30%club)1980s–1990sEutrophicationofecosystemsbynitrogendepositionrecognized1991Canada-USAAirQualityAgreement1993The‘SixCities’studyinNorthAmericare-focusesattentiononthehumanhealtheffectsofairpollutionPM101995Launchofthefirstsatelliteforpassiveremotesensingatmosphericcomposition(GOME)forglobalozonemonitoring[4]1999TheUNECEGothenburgProtocoladoptedtotacklemultipollutantmultieffects(acidity,ozoneandeutrophication)2000sEmissionsofSO2andNOxinAsiaincreasinglydominateglobalemissionsandadverseeffects2010WidespreadevidenceofrecoveryfromeffectsofaciddepositioninEuropeandNorthAmericawiththedeclineinemissionsofSO2andNOx2012Beijingsmog,13thJanuary,withconcentrationsofPMandSO2similartoLondon19522015GlobalSO2emissionsreducedby15%fromthe1990peak,whileallotherairpollutantsstillincreasing2018EmissionsofSO2andNO2decliningrapidlyinChina2018PeakglobalNOxemission?GlobalemissionsofNH3andVOCcontinuetorise2020COVID-19:Theglobalpandemicdramaticallyreducesemissionsofindustrial-andtransport-relatedemissionsofSO2,NOx,VOCandprimaryPMClearwrittenevidenceshowsthatearlysocietyrecognizedathreattohumanhealthandthewiderenvironmentfromairpollution.However,theidentityofthegasesandparticlesremainedlargelyunknown,andtherewerenomeasurementstoquantifytheproblem.Earlyattemptstoregulateemissionsshowthatthelawyersintheirdayclearlyhadthemeanstoarticulatesocietaldesireforacleanerenvironment,butthelawsdevelopedwerenotsupportedbytheinfrastructurenecessarytomakethemeffective.Thelackofconsistentlanguagedescribingtheunderlyingsciencealsomakestheearlyliteraturedifficulttointerpretfromatwenty-firstcenturyperspective.Theearlyhistorytakesustotheperiodofelucidationofthecompoundspresentintheatmosphereandtoearlymeasurements,mainlyintheseventeenthtonineteenthcenturies,followingwhichdirectmeasurementsbeganinearnest.Sporadicmeasurementsofairqualitybeganinthelatenineteenthcentury,especiallybyRobertAngusSmithintheUK[7],thefirstscientisttoattemptmultisite,multipollutantinvestigationsofthechemicalclimatologyofthepollutedatmosphere.Theearlydevelopmentsinunderstandingofairpollutionweremainlybychemists,whocontinuedtheirleadershipofthemechanisticunderpinningofthesciencethroughthetwentiethcentury(e.g.[8,9]).Distributedsitestomeasureatmosphericcompositiongraduallydevelopedthroughthemid-twentiethcenturyandbythetimeacidrainbecameafocusofscientificandpoliticalinterestinthelate1960stherewerenetworksinEuropeandNorthAmericatostudythecompositionofairandprecipitationatregionalscales(e.g.[10,11]).Inaddition,localpollutionproblemsinindustrialcities,mainlyinEuropeandNorthAmerica,andaroundnotablepointsources,providedearlymeasurementsoflargelocaleffectsbysomeofthemainpollutants.Theground-basedmonitoringnetworksinplacebytheyear2000(table 1)includedregionalandglobalairchemistrymeasurements.Thethirdmainsourceoftime-seriesdatatoassessthechronologyofairpollutionistheapplicationofchemistrytransportmodels(CTMs)withglobalmeteorologicalmodelsandspatiallydisaggregatedinventoriesofpollutantemissions.Thefinalsourceofdataisthatprovidedbysatelliteremotesensing,whichhasdevelopedoverthelastthreedecades,providingglobalconcentrationfieldsforthemajorairpollutantgases(SO2,NO2,NH3,CO,andO3).ThesecomplementarysourcesareusedheretoprovideasummaryofthedevelopmentofspecificairpollutionissuesthroughthelatenineteenthandearlytwentiethcenturiesandinthelasttwodecadesrevealingsomeimportantsignsofrecoveryfromeffectsofairpollutioninEurope,NorthAmericaandEastAsia.2.Pre-1750earlyevidenceairpollutionposedarisktohumanhealthandecosystemsEarlyhumanswouldhavebeenawareofatleastsomeofthepotentialhazardsintheairtheybreathedfromtheirgeneraldiscomfortinthepresenceofsmokeandcombustiongasesclosetoopenfires.Theneedforshelterandwarmthledtofiresinsideshelters,andinconfinedstructures,theexposuretopotentiallytoxicgasesandparticlesisconsiderablyenhanced.Giventhedirectlynoxiouspropertiesofmanycombustionproducts(smell,andlachrymoseandrespiratoryeffects),itissurprisingthatsomanysocietieshaddwellingswithopenfiresandnochimneys.Thedevelopmentofthechimneyitselfcanbeseenasakeymilestoneforindoorairquality,adoptedatfirstinthelargesthousesfromthetwelfthcentury[12].Today,indoorairpollutionisanimportantcontributortoeffectsonhumanhealth.Allsubsequentanalysishere,however,isdevotedtotheoutdoorenvironment.EvidencefromGreeceshowsthattheproblemsofpollutedairoutdoorswerebeingdocumentedatleast2400yearsago.ThebookAirs,watersandplacesattributedtoHippocrates(ca400BC)suggestedallsortsofillnessasbeingrelatedtothequalityofair.Theworstitseemswasincitiesfacingdampwesterlywinds,wheretheinhabitants‘arelikelytohavedeep,hoarsevoices,becauseoftheatmosphere,sinceitisusuallyimpureandunhealthyinsuchplaces'([13],p.83).WritersalittlelaterfromImperialRomeunderstoodtheprobablehealthimpactsofsmokewithSeneca(caAD63–65)referringtotheproblemandFrontinus(caAD96)proudlydeclaringhowhiscontributiontoaqueductsandfountainshashelpedmaketheairpurer:‘thecausesoftheunwholesomeatmosphere,whichgavetheairoftheCitysobadanamewiththeancients,arenowremoved’([14],p.417).AsSenecarecordedofahealthbreakfromRome: AssoonasIescapedfromtheoppressiveatmosphereofthecity,andfromthatawfulodourofreekingkitchenswhich,wheninuse,pourfortharuinousmessofsteamandsoot,Iperceivedatoncethatmyhealthwasmending…SoIammyoldselfagain,feelingnownowaveringlanguorinmysystem,andnosluggishnessinmybrain([15],p.193).Itisnotablethatthereferencetothebrainmatchesaneffectofammonium-containingairpollutionfromnaturallyburningcoalcavesalongtheSilkRoadinCentralAsiaaslaterrecordedbytheArabgeographerAl-Mas'ūdī[16].AbookbyShenKuo(1031–1095)writtenduringtheSongDynasty(AD961–1279)providesfurtherevidenceofconcerninChinaaboutairpollutionfromcoalburning[17].Otherpost-classicalwriters,especiallyintheArabworld,contributedobservationsaboutairpollutionduringthe‘DarkAges’whenconsiderablelearningwasbeinglostinEurope[18].Ultimately,however,littlechangedthroughouttheMiddleAgesintheunderstandingofthecausesofdiseaseandpossibleroleofairpollutantsreflectingthepersistenceoftheclassicalmiasmaticconceptthatodoursandothermatterinairwerethecontrollinginfluencesforhumanhealth[19],anideagoingbacktothetimeofHippocrates.Intheseventeenthcentury,JohnEvelynpublishedFumifugiumorTheInconvenienceoftheAerandSmoakofLondon[20](figure 1).ThisiconicdocumentdescribedairpollutioninLondonandsuggestedwaysofreducingthescaleoftheproblem.Heproposedmovingindustriesincludingbrewingandlime-burningtothecountryside,welloutsidethecity.JohnGraunt,acontemporaryofEvelyn,suggestedacorrelationbetweenratesofmortalityandpollution,especiallyinfogepisodes[21].Intheabsenceofanychemicaldata,orindeedanynumericalvaluestoquantifythepollutantspresent,wehaveonlythenarrative,butitclearlyidentifiesaseriousproblemforhumanhealth.EvelynwroteofLondonin1661:‘thatthisgloriousandancientcityshouldwrapherstatelyheadincloudsofsmokeandsulphur,sofullofstinkanddarkness’. Figure1.JohnEvelynandthetitlepageofFumifugium(1661).(Onlineversionincolour.) DownloadfigureOpeninnewtabDownloadPowerPointTherewassomerecognitionofthestrong-smellingsulfurpollutantsderivedfromcoalorindustrialprocessessuchasdiscussedinFumifugiumandalsoinShakespeare'sobservationaboutthereekoflime-kilns(TheMerryWivesofWindsor,ActIII,Scene3).Lime-kilnswereusedextensivelyinEuropesinceRomantimesandwereanotedsourceofairpollution.Therewaslittleunderstandingofatmosphericchemistry,however,althoughscientificinterestbecamemoreimportantbythemid-1600s[19],withtheharmfuleffectsofairpollutantsascribedtovariouscomponentsoftheairbyKenelmeDigby(acids),NehemiahGrew(lead),JohnEvelyn(sulfur)andJohnHall(antimonyormercury).3.Thedevelopmentoflawstocontrolairpollution1273–1900TheearliestlegislationinEnglandwasthe1273SmokeAbatementAct,prohibitingtheuseofcoalasitwas‘prejudicialtohealth’[22].Somemediaevalsocietiesapproachedairpollutioncontrolbykeepingthesourcesoutsidethecitywalls,aconceptfoundinAristotle'sAthenianPoliticsandinancientRomanregulation[23].ThispracticecontinuedinmediaevalEurope,butalsoAsia,notablyinrelationtotheextensivefifteenth-centuryThuriangpotterykilns,whichwerelocatedinthenorthernleeofSiSatchanali,Thailand.Theseearlyexamplesofwhatwemaynowliketocall‘environmentallaw’includecontrolsontheburningofseacoalandthe‘forestrylaws’(protectingthevariousspeciesofgamelivingintheforests).Mostoftheseexamplesderivefromtheparticularwhimsandprejudicesofindividualrulers,oftenheavilyinfluencedbythosewithintheuppersocialechelonsofsociety.Therewasnomodernscienceinvolved:theproblemwasperhapsavisualblotonthemonarch'slandscape,anappallingsmellorapassionforhunting.Ineverycase,thecontrolorprohibitionwasimposedwithoutanyneedtoresorttothescientificknowledgebaseofthetime.IntheWesternworld,onehastolooktotheRenaissanceandthesubsequentReformationasformingthebasisfromwhicharoseourmodernmethodologiesforscientificenquiry.IntheUK,thischangeinmethodsofenquiryalongsidetheriseofindustrializationinthelatterpartoftheeighteenthcenturyanditsincreasingpaceinthenineteenthcenturyenabledthelikesofDavidHume(1711–1776),JeremyBentham(1748–1832)andJohnStuartMill(1806–1873)toarticulatesocialphilosophiessuchasutilitarianism—philosophieswhichultimatelygavemomentumtocentralizedregulationofwhatwewouldnowdescribeas‘environmentalissues'.ThedirectinterventionoftheBritishGovernmentbywayoflegislationwaslimitedthroughoutthisperiod.TheAlkaliWorksRegulationAct1863anditsAlkaliInspectorateweretheprimeexampleofgovernmentalresponsivenesstoenvironmentalmattersduringthisperiod:necessity,drivenbywidespreadandself-evidenthealthandwelfareproblems,butenactedreluctantly.Initially,itwasthe‘commonlaw’thatwasusedtocombatinstancesofpollutingactivityinEngland.TheEnglish‘civilclaimsprocedure’requiresacomplainant,adefendantandproofonabalanceofprobabilitiesthatthe‘injury’complainedofwascausedbytheactionorinactionofthedefendantbyreasonofbreachofastandardofcare—astandardestablishedandrefinedbythejudiciaryovermanydecadesonacase-by-casebasis.Thiswastherebyinevitablyastandardthatwasintrinsicallysusceptibletocircumstanceandprevailingnorms.Thisrelianceonan‘after-the-fact’procedure,coupledwitharequirementtoestablishaclear,legallyrecognizablecausallinkbetweenallegedcauseandsupposedeffect,wastopermeatetheUK'sapproachtowhatwenowtermenvironmentalregulation.Theapproachwasoneoffuturepreventionofwhatcouldbeshowntohavealreadybeenclearlyunacceptableratherthana‘precautionaryapproach’,thelatternowunderpinningmuchofcurrentthinkinginrelationtomattersoftheenvironmentandsocialwell-beinggenerally.AkeysteponthewaytodevelopingaUKlegalframeworkforairpollutionisthe‘TheLawofNuisance’.Asearlyas1610,WilliamAldred'scase,asitisknown,sawthecourtsinterveneagainstoneThomasBentonforbuildingapigsty‘sonearthehouseoftheplaintiffthattheairtherofwascorrupted’.Thecourtfoundthatlightandcleanairwereconsiderednecessaryforwholesomehabitation.Indiscussingtheissuesraised,thecourtdrewadistinctionbetween‘triflinginconveniences'thatmadelifeinconvenientoruncomfortable—whenlocationcouldbealegitimateconsiderationinmakingsuchafinding—andmaterialdamagetopropertythatdiminisheditsvaluewhenlocationwaslargelyirrelevant.Theoverallresultwasthatthecommonlawprovedincreasinglyinadequatetoaddressthesortsofissuesthatsocialphilosophersandreformerswerecomingtofocusonasincreasingurbanizationgaverisetoself-evidentpublichealthandwelfareissues.Between1800and1850,thepopulationinEnglandandWalesdoubledto16millionanddoubledagainby1900,accompaniedbydramaticchangesinthedistributionandconcentrationofthepopulationasindustrializationdrewpeoplefromruralareastowhatsoonbecamehighlyurbanizedareaswithinsanitaryhousing,disease,noxiousemissionsandfossilfuelsaddingtothetoxicmix.The‘RoyalCommissiononNoxiousVapours’of1878recordedmanyexamplesofthekindsofdamageresultingfromwhatatthattimewasuncontrolledindustry.However,theUKgovernmentwasslowtotakeremedialactionbecauseoftheimportanceofindustrytothenationaleconomy.DespitethemanufacturingcontrolsintroducedintheAlkaliAct1863,andtheestablishmentofanAlkaliInspectorate,theincreasingnumberofalkaliworks(manufacturingsodiumcarbonate,whileemittinghydrochloricacidasairpollution)meantthattheUKexperiencednomeaningfuldecreaseinemissionsofpollutants.Legislationrequirementstouse‘bestpracticablemeans',togetherwithapiecemealapproach,exacerbatedwhatwasinanyeventoftenindifferentenforcementofthelegislation.Nevertheless,lawstocontrolairpollutionthatarerecognizablymoderndiddevelopthroughthelatterpartofthenineteenthcentury,andthesealsoreflectedthesanitaryreformthatcharacterizedthebroadpublichealthconcernsofthetime[24].Asmightbeexpected,theywerecommoninEuropeandNorthAmerica,butalsofollowedimperialadministrationsacrosstheworld,sowerewellknowninIndia(e.g.SmokeNuisanceActsinBengal1905andBombay1912)andHongKong.ThewiderangeofinternationallawwasreviewedattheLondonPublicHealthCongressin1905,oftencitedastheplacewhereHenryAntoineDesVoeuxcoinedtheterm‘smog’[25]4.1750–1950UrbanairqualityandtheindustrialrevolutionDuringtheearlyphaseoftheindustrialrevolution,beginningintheUKinthelateeighteenthcenturyandspreadingthroughEuropeandNorthAmerica,arapidgrowthincoalcombustioninthedevelopingcitiessubstantiallyincreasedemissionsofSO2,NO2,NH3andsmoke(e.g.[26,27]).Theproblemofairpollutionfocusedinthisperiodonhumanhealth.Inpart,emissionswereduetoindustrialdevelopmentandrapidlyincreasingemissionsfromshortstacks.Additionalsourceswerefromdomesticemissionsbytherapidlygrowingurbanpopulationoffactoryworkerswhomostlyburnedcoalforwarmthandcooking.Ambientconcentrationswerenotmeasuredduringtheeighteenthandearlynineteenthcenturies,andvaluesareamatterofspeculation.Emissionsfromcombustionwerethemaincontributorstopoorairquality,buttheywerenottheonlypollutants.ItisimportanttomentionemissionsofNH3fromthelargeurbanpopulationofhorsesfortransport,whichwouldhaveaddedtoNH3releasedbycoalcombustion[16].Thequantityofhorsedungonurbanroadswasrecognizedasagrowingprobleminthelatenineteenthcentury,forexample100 000horsesinNewYorkproducing1000tonnesofmanuredaily(the‘greatmanurecrisis’inNewYorkandLondon;[28]),withamajorproblemprojectedintofuturedecades.Priortothetwentiethcentury,horsepopulationsweresubstantialinallmajorcities.Thepoorstateofsewagetreatment,especiallyduringtherapidexpansionoftheeighteenthandnineteenthcenturies,alsocontributedtoemissionsofNH3.TherapidreplacementofhorsedrawntransportbymotorvehiclesintheearlydecadesofthetwentiethcenturyavoidedtheproblemsforecastforcitieslikeLondonandNewYork.LittleattentionhasbeendrawntothecombinationofSO2,NOxandNH3intheurbanchemicalclimateofthenineteenthcentury,perhapsduetothelackofmeasurementsandthefocusonpollutantsfromcombustionsources.ButthepresenceoflargeemissionsofNH3wouldhavepromotedtheformationofparticulate(NH4)2SO4[29]andtherapiddepositionofSO2toterrestrialsurfaces[30].Amongthefewearlyurbanmeasurements,Smith[31],recordedconcentrationsofNHx(NHxisthesumofgaseousNH3andparticulateNH4+)inLondonof80togreaterthan1000 µg m−3,withthehighestvaluesrecordedduringfog.ThedepositionofNH3wouldalsohavecontributedtochangesinspeciesrichnessofplantcommunitiesinurbanareas[32].Thedegradationofairqualityduringtheperiod1750intothetwentiethcenturywasprimarilyinurbanareasorclosetolargeindustrialpointsources.MostmajorEuropeancitiesinthelatenineteenthcenturyhadairqualityproblems.LondonandEdinburgh,respectivelyknowncolloquiallyas‘theSmoke’and‘AuldReekie’,werenotablebutfarfromunique.AllmajorcitiesoftheUKsuffered.PopularworksofEnglishliteraturebyDickensandConanDoylecontainmanydescriptionsofdenseswirlingsmogcontributingtoanairofdangerandgloominVictorianLondon.Likewise,themajorcitiesthroughoutEurope,wherecoalprovidedthemainfuelforindustryanddomesticheating,developedsimilarairqualityproblems.ThepollutantsfromcoalcombustionincludedSO2,NO2,smokeand,toalesserextent,HClfromthechlorineincoal[33,34].UrbanconcentrationsofSO2andsmokeinthelargecitiesinthemiddledecadesofthetwentiethcenturywerecommonlybetween50and100 µg m−3,andmanyUKcitieshadannualvaluesinthisrange[35].Meteorologicalconditionsinthewintermonthsleadingtolowwindspeedandacoldsurfaceairgreatlyreducedispersionofpollution,andintheseconditions,concentrationsofsmokeandSO2couldexceed1000 µg m−3,asintheinfamous1952Londonsmogepisode[19].5.1952TheGreatLondonSmogAirpollutionwas,untilthe1950s,largelyacceptedasaconsequenceofindustrialactivity,withaperceivedwillingnesstotoleratethegrime,degradedvisibility,erosionandblackeningofvaluedbuildingsandeffectsonhumanhealth,agricultureandnaturalecosystems.Ittookamajoreventtochangethepublicandpoliticalperceptionoftheproblemandtheneedforcontrolmeasures.The1952Londonsmogresultedintheprematuremortalityofapproximately12 000people[1].ThepublicandthenmoreslowlythepoliticalreactionledtotheintroductionoftheCleanAirActin1956,some3yearsaftertheevent.ItarosefromaBilltotheUKParliamentinitiallyproposedbyaback-benchMemberofParliament(SirGeraldNabarro),andnotaninitiativeoftheGovernmentMinistersatthetime,anindicationoftheprevailingfocusonhousing,industrialgrowthandrecoveryfromtheeffectsoftheSecondWorldWar.Thelackofprioritizationformattersoftheenvironmentwasafeatureof1950sBritain,wherefoodrationingwasstillinplacein1952.However,thisActofParliamentwasaveryimportantstep,eventuallyleadingtowidespreadreductionsinemissionsofsmokeandSO2inurbanareas.DuringthethreedecadesfollowingtheLondonsmog,manyurbanpowerstationsandotherpollutingindustrialsourceswereclosed,andnew,larger,moreefficientpowerstationswereconstructedinruralareas.Theseeachtypicallyproduced2000 MWoutputofelectricalpowerandconsumed5milliontonnesofcoalannually.UKemissionsofSO2continuedtoincreasethroughthe1950stoapeakinthe1960s,mainlydrivenbyindustrialemissionsandespeciallypowergeneration.ThenewlargepowerstationswereequippedwithreasonablyeffectivecontrolsforPM,butnoneofthenewunitshadSO2removingequipmentuntilDraxin1988andRatcliffein1995.TheclosureofthelargenumberofsmallerverypollutingurbanpowerstationsandotherindustrialsourceswithshortstacksfurtherreducedemissionsofSO2andsmokeincitiesandcontributedsignificantlytotheimprovingurbanairquality.AmbientconcentrationsofsmokeandSO2declinedby60%between1962and1975inLondon,nearlyaquarterofacenturyaftertheeventthattippedthescalesinfavourofeffectiveactiononurbanairquality(figure 2). Figure2.ThedeclineinSO2andsmokeinLondonfollowingtheCleanAirAct(1956),includingdatafromthe‘bubblermethod’samplingairthroughaperoxidesolutioninwaterandultraviolet(UV)spectroscopy.(M.L.Williams,personalcommunication,2017).(Onlineversionincolour.) DownloadfigureOpeninnewtabDownloadPowerPointRecognizingneedstoreduceground-levelconcentrationsofSO2andsmoke,newpowerstationsfromthe1950swerebuiltwithincreasinglyhighchimneystacks.Thelargeruralpowerstations,manyintheTrentandOusevalleysoftheEnglishMidlandsandindustrialnorth,hadlargerstackheights,manyat200 m,promotingdispersionandreducinglocaleffects[36].SimilarchangesinpowergenerationweretakingplaceacrossEuropewhereSO2emissionspeakedinthe1980s[37].Itisnotablethatonaglobalscale,EuropeandNorthAmericawereresponsibleformost(greaterthan80%)oftheglobalSO2emissionspriorto1970(figure 3).EmissionsinNorthAmericagrewratherfasterthanthoseinEuropeandwithtallstacksalsousedinNorthAmericatodispersepollutants,tominimizelocaleffects.GlobalemissionsofNOx,non-methanevolatileorganiccompound(NMVOC)andNH3alsoincreasedrapidlyduringthelatetwentiethcentury(figure 3),asconsequencesofincreasedenergyconsumption,transport,solventsandagriculturalactivity.Thesegraphsshowthemajorshiftssince1980,whereChinaandtheAsia/PacificregionhavereplacedEuropeandNorthAmericaasthemainglobalsourcesofairpollution. Figure3.GlobalandregionalemissionsofSO2,NOx,NH3andNMVOCbetween1750and2010.AdaptedfromHoeslyetal.[37].Thedotsshowglobalestimatesofanearlierstudy(CMIP5[38]).(Onlineversionincolour.) DownloadfigureOpeninnewtabDownloadPowerPoint6.Therecognitionofregionalairqualityissuesandlong-rangetransportofairpollutionAlthoughtherehadbeenearlierconcernsregardingecologicaleffects,thepoliciestocontrolairpollutionduringthe1950sand1960swereaimedatprotectinghumanhealth,withafocusonurbanairquality.But,asnotedabove,thegradualimprovementofurbanairqualitytookplacewhilecountry-scaleemissionsofSO2wereclosetotheirmaximum,asaconsequenceofincreasingemissionsfromlargecombustionplantswithtallchimneystacks.Duringthisperiod,annualUKemissionsofSO2reachedtheirpeakof3Mt Sannually[39].AnnualEuropeanandNorthAmericanemissionsofSO2alsopeakedduringthisperiodat32Mt-S[40]and31 Mt-S[41],respectively.ThescaleandeffectsonthecountrysidefromthehighlevelsofSO2(andNO2)werenotimmediatelyrecognizedasanissue.InthePenninehillsbetweenSheffieldandManchester,theForestryCommissionwasunabletoestablishplantationsofScotsPineduetotheambientexposuretoSO2andlargeareasofcentralEnglandaswellasallofthemajorcitiesweredevoidoflichenspeciessensitivetoSO2[42].Effectsonagriculturalcropsweresubstantial,withsomecultivarsofgrassdevelopingresistancetoSO2[43],butknowledgeofthesedomesticproblemsofairpollutionwasinsufficientfortheUKGovernmenttointroducefurtherlegislationtoreduceemissions.TheCleanAirActwasregardedasadequate,andtherewerenolimitsontheoverallscaleofSO2emissions,justarequirementtousestackheightstallenoughtominimizetheconcentrationsdownwindatthesurfacefollowingthephilosophyofBestPracticalMeans[44].Policyprioritiesatthisstagewerefirmlyfocusedonhumanhealth.ElsewhereincontinentalEurope,tallstacksandlargepowerplantslocatedoutsidethecitieswerealsoregardedaseffectivepoliciestominimizeeffectsonhumanhealth.AreashighlypollutedbySO2wereextensiveoverEngland,Germany,easterncentralEuropeandtheLowCountries(figure 4).Ecologicaleffectswerenotconsideredsufficientlyimportanttointroducefurthercontrolmeasures,eventhoughitwasknownthatsomeindustrialprocesses,especiallysmelting,producedstrikingexamplesoflocaldamagefromSO2andmetaldeposition.Forexample,emissionsfromtheSudburysmelterineasternCanadaduringtheearlytwentiethcenturycausedextensiveareasofnaturalvegetationtobedestroyedbythecombinationofexposuretoverylargeconcentrationsofSO2andlargedepositionratesofarangeofmetals[45,46].Smelterswerepresentinmanycountriesglobally,andlargeexposurestoSO2andmetaldepositionwerecommonintheirproximitywithexamplesinSlovenia,Peru,Canada,USA,Russia,China,France,PolandandZambia[47]. Figure4.AnnualmeanEuropeanSO2concentrations(µg m−3)in1970,ataroundthetimeofpeakSO2emissions,modelledusingEMEP4UKwith1970emissionsand2012meteorology(M.Vienoetal.,personalcommunication,2020).(Onlineversionincolour.) DownloadfigureOpeninnewtabDownloadPowerPoint7.EarlyevidenceofairpollutiontransportfrommeasurementsTherewereseventeenth-centuryanalysesofrainfall,possiblythefirstbyOleBorchinDenmark.Thesebecamemorecommonlyundertakenbyagriculturalistsduringthe1800s[19]andincreasinglyusedworldwide[10,48,49],providingearlyevidenceofinter-countryexchangeofpollutantsfromobservationsofcontaminatedsnowfall.ThedepositionofurbansulfateinLondonrainfallwasdeterminedbyRobertAngusSmithin1869–1870[7].Thenumberofestimatesoftheconcentrationsofsubstancesinairincreasedrapidlythroughthenineteenthcentury.Russell[50]measuredtotalPMgravimetricallyincentralLondonat120,360and860 µg m−3infine,dullandfoggyweather,respectively[51].Althoughmeasurementsofcarbondioxidewerefrequent(aslistedinCallendar[52]),thisoccurredonlyoccasionallyfortracegasessuchasammonia[31,53].Theearlytwentiethcenturysawthedevelopmentofthedepositgaugethatmeasuredwetdepositionandwhateverelsefellintothelargeglassbowl[54]andtheuseofleadperoxidecandlestodeterminedepositedsulfurdioxide[55].ThewidespreadoccurrenceofSO2ledtoincreasinglysophisticatedmethodsthatinvolveddrawingairthroughDreschelbottles(bubblers)containingsolutionsofiodine,hydrogenperoxideordisulfitomercurate[56].Table 2showsthedateswhenregionalnetworkstomeasureatmosphericcompositionwereintroduced,revealingthatonlytheEuropeanAirChemistryNetwork(EACN)wasinplacepriortothepeakinEuropeanemissionsofSO2.Attheglobalscale,monitoringofglobaltrendsinbackgroundatmosphericcompositionisnowcoordinatedthroughtheGlobalAtmosphericWatch(GAW)network[58],whoseoriginscanbetracedbacktotheBackgroundAirPollutionMonitoringNetwork(BAPMoN)firstestablishedin1974[59]. Table 2.Long-termmonitoringactivitiesinrelationtoacidrainandotherpollutants(adaptedfromGrennfeltetal.[57]). 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activityandtimegeographicalcoverageandnumberofsitesprogrammecentrewebpagecommentsatmosphereEACN(IMInetwork)1955–1976Europe>100sitesStockholmUniversitysomesitescontinuedwithinEMEPafter1976(L.Granat,personalcommunication,2019)WMOGAW/BAPMoN1964–global>200sitesWorldMeteorologicalOrganizationhttp://www.wmo.int/pages/prog/arep/gaw/gaw_home_en.htmlEMEP1977–EuropeandECEregionofAsiaapproximately350NorwegianInstituteforAirResearch(NILU)http://www.emep.int/NADP1977–USAapproximately260sitesUniversityofWisconsin-Madisonhttp://nadp.slh.wisc.edu/CAPMoN(incl.APN)1978–Canadaapproximately25sitesEnvironmentCanadahttps://www.canada.ca/en/environment-climate-change/services/air-pollution/monitoring-networks-data/canadian-air-precipitation.htmlEANETEastAsiaAsiaCenterforAirPollutionResearch(ACAP)http://www.eanet.asia/MaleDeclaration2003–SouthAsia15sitesOriginallytheSouthAsiaCooperativeEnvironmentProgramme;nowAsianInstituteofTechnologyhttp://www.sacep.org/programmes/male-declarationhttp://www.rrcap.ait.asia/maleEcosystemsICPForests1985–Europe5000plotsand500intenseplotsThünenInstituteofForestEcosystemshttp://icp-forests.net/ICPWaters1985–EuropeandNorthAmericaapproximately250sitesNorwegianInstituteforWaterResearchhttp://www.icp-waters.no/ICPMaterialEuropeandNorthAmericaapproximately40sitesRiseKIMABAB,Swedenhttp://www.corr-institute.se/icp-materials/web/page.aspxICPIntegratedMonitoringEuropeapproximately50sitesFinnishEnvironmentInstitutehttp://www.syke.fi/nature/icpimICPVegetationEuropeCentreforEcology&Hydrology,UKhttps://icpvegetation.ceh.ac.uk8.Long-rangetransportofairpollutionTheNorwegianplaywrightHenrikIbsen'splay‘Fire’(Brand)showedthatpeoplewereawareoflong-rangetransportofpollutantsinthenineteenthcentury[60]: Worsetimes,worsesinsthroughthenightoffutureflashesofBritain'ssuffocatingcoaldustisslowlydescendingoverthecountrysidesoilingallthatisgreenstranglingallthatstrivestogrowcreepinglowandmixedwithpoisonstealingsunandlightfromthevalleypeltingdownasrainofashes.Evenintheearly1950s,itwasknownthatthemajorityofUKsulfuremissionswereexportedfromtheUKcoastline,asMeetham[61]demonstratedusingnationalmonitoringdataandasimpleatmosphericmassbalance.However,themagnitudeoftheeffectsoflong-rangetransportofairpollutantsfromthemajoremittingcountriesinEuropeonthenetimportingcountrieswasnotconsideredanimportantissueuntilthelate1960sand1970s.ThepotentialfortransboundarytransportwithinEuropecanbereadilyvisualizedfromthedatainfigure 4,giventypicalboundarylayerwind-speedsof10 m s−1andanatmospherice-foldinglifetimeforSOxofafewdays.9.1960sAcidrainSwedishscientistSvanteOdeninitiallyadvancedtheideathatthelong-rangetransportofsulfurandacidityinEuropefromthemajorsulfuremittingcountries(UK,Germany,FranceandPolandinparticular)wasresponsibleforwidespreadacidificationoffreshwatersandlossoffishpopulationsinScandinavia[62].MonitoringdataforairandprecipitationestablishedbyEgnerandcolleaguesfrom1955(theEACN)providedthevitalchemicaldatashowingboththegeographicalpatternsandtrendsinconcentrationsofthemajorionsinprecipitation(e.g.[10]).TheEACNdatashowedacidityandsulfateinprecipitationincreasingsteadilythroughthe1950sand1960s[63].OdenattractedconsiderableSwedishinterestwiththeseideas,muchofitcritical.Theideaswerenewandmanyaspectsoftheatmosphericchemistryandphysicsofthecompoundsinvolvedwerepoorlyunderstood.However,theevidencewaspersuasive,andfurtheranalysisbySwedishcolleaguesprovidedstrongsupportforhisarguments.AUnitedNationsconferenceontheHumanEnvironmentinStockholmin1972advancedthewidercasethatthepollutionofonecountrybyanotherthroughemission,atmospherictransportanddepositionwasunacceptable[64],buildingontheevidenceoflong-rangetransportofsulfurcompoundswithinEuropeandtheeffectsinScandinavia.TheStockholmConferenceof1972wasaturningpointinenvironmentalscience.Thereislittledoubtthatlong-rangetransportandeffectsofpollutantshadbeentakingplacefordecades;infact,manyhadnotedtheinter-countrytransportofpollutantsinEurope(e.g.[60]).ItwasonlyfollowingOden'sanalysisthatinternationalscientificandpoliticalattentionwasdrawntothesubjectandmonitoringandprocessstudiesdemonstratedthescaleandecologicalsignificance.TheinitialreactionofthemajorpollutingcountriesinEuropewasmixed.Allrecognizedtheneedtoquantifythescaleofinter-countrytransportofthemajorpollutants,theunderpinningatmosphericchemistryandphysics,andtheeffectsonecosystems.However,thelegalinstrumentsneededtobedeveloped,andthesupportingmonitoringandanalysistoolswerestilllacking.Theindustrialnationshaddesignedtallstackstodispersethepollutantswithoutconsideringpossibleeffectsoutsidetheirjurisdiction.ThecombinationofthescienceandpoliticalbackgroundisdescribedindetailbyGrennfeltetal.[57].AnearlierdescriptionofthepoliticalbackgroundintheUKbyRose[65]providesfurtherinsightsandistypicalofthelargenumberofpublicationsonthepoliticsofacidrain.ResearchandmonitoringactivitiesexpandedacrossEuropeandtheessentialdetailsoftheemissions,atmosphericchemistryanddepositionwerepresentedatDubrovnikin1977[66].ThefirstmajorinternationalconferenceonacidrainwasheldinColumbus,OH,USAin1975,beginninganimportantseriesofmeetingsonthesubjectapproximatelyevery5yearsfrom1975to2015.Table 3provideslinkstothisseriesofmeetingswhichshowsthedevelopingglobalscaleofairpollutionissuesthroughthelatterdecadesofthetwentiethcentury,beginningwithacidrain.TheresultsoftheNorwegianSNSFresearchprogrammeintothecausesandeffectsofacidrainwerepresentedatthesecondinternationalAcidRainconferenceinSandefjordin1980[68].ClearlinksbetweensulfuremissionsintheindustrialnationsofEuropeandlong-rangetransportto,anddepositionandeffectswithin,NorwayandmorewidelyinScandinaviaweredemonstrated.Theeffectswereprimarilyonfreshwaters,withlargedeclinesinfishpopulationsinthemostacidifiedregions.TheearlystudiesinScandinaviadidnotshownegativeeffectsonforestsofthelong-rangetransportofpollutants.DiscussionmeetingsattheRoyalSocietyofLondonreportedthePathwaysofpollutantsintheatmospherein1979(Phil.Trans.A,vol.290)andTerrestrialEffectsofdepositedsulfurandnitrogencompoundsin1984(Phil.Trans.B,vol.305). Table 3.Theseriesofinternationalconferencesonaciddepositionshowingthebroadeningofissuesandscalefrom1976to2016. Collapse dateissuelocationreferencetoproceedings1976acidrainColumbus,OH,USADochinger&Seliga[67]1980acidrainSandefjord,NorwayDrabløs&Tollan[68]1985aciddeposition,forestdeclineMuskoka,Ontario,CanadaMartin[69]1990aciddeposition,eutrophication,ozoneGlasgow,UKLast[70]1995aciddeposition,eutrophication,ozone,criticallevelsGothenburg,SwedenGrennfelt[71]2000aciddeposition,eutrophication,ozone,recoveryTsukuba,JapanSatake[72]2005aciddeposition,eutrophication,ozone,recoveryPrague,CzechRepublicBrimblecombeetal.[73]2011aciddeposition,eutrophication,ozone,recoveryBeijing,China2016aciddeposition,eutrophication,ozone,recoveryRochester,NY,USAAherneetal.[74]Bythelate1970s,therequirementtoreduceEuropeanemissionsespeciallyofSO2,andtherebyreducedepositionofacidifyingcompounds,inScandinavia,wasclear.ThisledtotheestablishmentoftheConventiononLong-rangeTransboundaryAirPollution(LRTAP)asamajorinternationalframeworktoaddresstheproblem[75].Thisrequiredextensivemonitoringofthechemicalcompositionofairandprecipitationandassociatedmeteorologicalvariables,thedevelopmentofatmospherictransportanddepositionmodelstoquantifythenettransferofpollutantsbetweencountries(theEuropeanMonitoringandassessmentProgramme(EMEP)),andaframeworkforinterpretationandnegotiationoftheissuesbetweenthecountries.ThedevelopmentoftheLRTAPConventionhasprovedaveryeffectiveprocesstobringtogethertheprocess-basedscience,themonitoringandthemodelling(withintheEMEP)andpolicydevelopment,ultimatelyleadingtointernationalagreementstoreduceemissionsofSO2andsubsequentlyNO2,VOCandotherairpollutants.Likethehuman–health-orientedmeasurementprogrammesinstigatedintheUSA,theLRTAPprogrammerecognizedtheneedtosimultaneouslymeasureawiderangeofconstituents,butalsorecognizedtheneedtomakemeasurementsoverlongtimeperiodstoovercometheconsiderableinter-annualvariabilityinmeteorologicalconditions[76].TheLRTAPprogrammealsointroducedtheexplicitgoalofmakingmeasurementstosupportassociatedatmosphericmodelling.Intheearly1980s,agroupofEuropeancountriesconsideredareductioninSO2emissionsappropriate,butintheabsenceofcountry-specificcontributionstotheecologicaldamageinScandinavia,anarbitraryagreementtoreduceemissionsby30%wasproposed.Many,butnotallcountries,supportedthemeasure,formingthe30%club.Argumentswerepresentedthatthecostsofcontrolweresubstantiallygreaterthanthebenefits.Forexample,itwasstated(incorrectly)that‘aciddepositionisamilliondollarproblemwithabilliondollarsolution’[77].Nevertheless,the30%clubwasthebasisforthefirstSulphurProtocol,signedinHelsinkiin1985,whichstipulatedareductioninsulfuremissionsof30%between1980and1993.Freshwateracidificationandadeclineinfishpopulationsweretheinitialfocusandtheevidencethatthecausewaslong-rangetransportofpollutants,mainlysulfur,wascompelling.TherewasasecondaryfocusonthehealthofforestsinScandinaviabutheretheevidencewasnotpersuasive.ThescientificandpoliticalinterestinacidraininScandinaviaandmorewidelyinEuropestimulatedinterestelsewhere,especiallyNorthAmerica,wheresimilarlylargeincreasesinemissionsofSO2hadoccurred(figure 3).TheUSAhadestablisheditsownNationalAtmosphericDepositionProgram(NADP)in1977.ThecombinationofalargesourceareaintheOhioRivervalleyandalargeareadownwindwithgeologyandecosystemssensitivetoacidificationsoonledtotherecognitionofproblemsfromlong-rangetransportofpollutantssimilartothoseidentifiedinEurope.Thefactthatsubstantialareasofacid-sensitiveecosystemswerelocatedinCanadaaddedapoliticaldimensionsimilartothatinEurope,withonecountrybeingresponsibleforecologicalproblemsinaneighbouringterritory.10.1980sForestdeclineBy1980acidrain,ormorecorrectlyaciddeposition,recognizingtheimportanceofbothwetanddrydepositiontothetotalinputtotheground[78],wasestablishedasaninternationalissue,andallindustrialcountriesengagedinresearchandmanyinthedevelopmentofcontrolmeasures.InterestinaciddepositioninEuropewasgreatlystimulatedintheearly1980sbyadeclineinthehealthofforestsinthemostpollutedregions[79].ThemostdamagedforestswerethoseintheuplandsinborderregionsoftheCzechRepublic,PolandandtheGermanDemocraticRepublicwheredie-backoftheforestwasextensive.InpartsofGermany,especiallytheHarzMountains,thetree-linemoveddownthehillsasdamageathighelevationwhichwasafeatureoftheproblemprogressedtolowerlevels.ThelargeareasofforestdeclinethroughoutGermany(Waldsterben)becameadefiningenvironmentalissueofthelatetwentiethcentury[32].Thecausesofforestdeclinewerehotlydebatedandcontentious[79].Themaincausalagentsappearedtobeaciddepositionandozone,butexcessivenitrogendepositionandmetalswerealsopossiblecontributorsand,atmanyofthesitesofforestdecline,exposurestolargeinputsofacombinationofthesepollutantswerecommon.Whilemanypublicationsaddresstheproblem,thereisnoconsensustodateontheproportionsoftheobserveddamageattributabletoeachofthepollutantsandmechanismsofdamage.ForestdeclinewasanimportantpartoftheaciddepositionstoryinNorthAmerica,andaparticularspecies,redspruce,showedwinterinjurythatwasshowntobeassociatedwiththeexposuretoacidiccloud-waterandsulfateintheAppalachians[80].Fromthemid-1980s,sulfurandaciddepositiondeclinedsteadilyinEuropeandNorthAmerica,withrecoveryinatmosphericcompositionprecedinganysignsofecosystemrecovery[81,82].WhiletherewereclearlinksbetweenaciddepositionandforestdeclineinbothEuropeandNorthAmerica,ground-levelozonewasalsoimplicatedinbothregions[83,84].11.Ground-levelozoneThebroadeningoftheecologicalfocusfromfreshwaterstoforestsandtheexpansionofthenumberofpollutantsimplicatedineffectswasanimportantdevelopment.Inexpandingtherangeofeffectsandpollutants,theregionalscalewasalsoexpandingconsiderably.Ozoneisformedwithintheatmospherefollowingphotolysisofoxygeninthestratosphere,andsomeistransferredintothetroposphereandcontributestoozoneatthegroundlevel[85].However,ozoneisalsoproducedthroughthephotochemicaldegradationofcarbonmonoxideandVOCsinthepresenceofNO2,andtheissueofozoneinsurfaceairiscommonlyreferredtoasground-levelozonetodistinguishitfromstratosphericozoneissues.ItwasfirstrecognizedasaproblemforhumanhealthandvegetationinCalifornia,andespeciallytheLosAngelesbasin,whereitwasfirstdescribedbyMiddletonetal.[86].ThepresenceofozoneconcentrationsthatposedarisktovegetationandhumanhealthoverEuropewasdemonstratedintheearly1970s[87].OverEurope,thebackgroundconcentrationofozonehasincreasedbyapproximatelyafactoroftwosincepre-industrialtimes[88],andepisodesofelevatedozonewereshowntobewidespreadinEuropeinthe1980s[89].TheeffectsofozoneonnaturalvegetationandcropsarediscussedbyStevensetal.[32]andEmberson[90],respectively.ThediscoveryofdamagingozoneconcentrationsinEuropeandNorthAmericagreatlyincreasedtherecognitionofphotochemicaloxidantsinregionalairqualityissuesinthe1970sand1980s.ThefocusofcontrolmeasuresthereforebroadenedfromsulfurandnitrogenoxidestoincludeVOCsinmanyotherindustrialcountries[57].Therelativelylonglifetimeofozoneinthetroposphere(approx.20days)andphotochemicalproductionoverregionalscalesmakesground-levelozoneacontinentalandhemisphericscalepollutant[91].TheindustrialregionsofEuropeandNorthAmericaexperiencedfrequentsummerepisodesofozoneinthe1970sand1980swithconcentrationsexceeding200 µg m−3.Thecontrolmeasurestodatehaveallbeencountryorregionalinscale,andwhileimportantprogresshasbeenmadeinreducingpeakvalues,especiallyinCalifornia,butalsoacrossmuchoftheUSAandEurope,ozoneremainsasubstantialthreattocrops,naturalvegetationandhumanhealth[32,90].Methaneisplayingamajorroleintheformationofbackgroundozone,andthereisincreasinginterestintakingpolicyactionstocontrolmethaneemissionsasitisbothagreenhousegasandanozoneprecursor[92].12.1990Eutrophication:theeffectsofnitrogendepositiononecosystemsAsanunderstandingofaciddepositiondeveloped,andground-levelozonewasrecognizedasanadditionalregional-scaleairpollutionissue,theimportanceofnitrogencompoundsgrew.Nitrogencompoundswerealwaysapartofaciddeposition,evenwhendepositedinreducedformasNH3orNH4+inprecipitation,astheprotonsgeneratedinsoilfollowingmicrobialoxidationtonitratecreateacidity[93].However,theNetherlandsandtheUKwerefirsttoobservewidespreadchangesinbotanicalspeciescompositionofheathlands[94].Itwassoonshownthatthechangeswerebeingdrivenbynitrogendepositionfromtheatmosphereandbyammoniainparticular.Asalwaysinecology,thestoryisalittlemorecomplex,asthereplacementofheather-dominatedheathlandsbygrasslandintheNetherlandswasmediatedbytheheatherbeetle,buttheunderlyingdriverofchangewasthedepositionofnitrogencompoundsfromtheatmosphere[95].TheeutrophicationofecosystemsbynitrogendepositionhasbeenshowntoreducespeciesrichnessofgrasslandsoverregionalscalesinEurope[96,97].Closetolivestocksourcesofammonia,thechangesinfloracanbesubstantial[98]andtheformofthenitrogendepositedhasbeenshowntobeanimportantfactorinthescaleofeffects,withgaseousammoniabeingmoredamagingtoheatherthanwet-depositedNO3−orNH4+[16,99].SimilareffectsofdepositednitrogenonecosystemshavebeenreportedinNorthAmericaandChina.Thescaleofeffectsofpollutantsonecosystemsquantifiedattheturnofthetwenty-firstcenturyshowedthat24%ofglobalforestswereexposedtophytotoxicexposuresofozone[100].ThedevelopmentoftheCriticalLoadsapproachandintegratedassessmentmethodsprovedvaluableinstrumentsinthedevelopmentofpoliciestomaximizetheecologicalbenefitsofcontrolmeasureswithintheLRTAPConvention[57].13.1990sHumanhealthregainsthefocusofpoliticalattentiononairqualityAsnotedabove,theearlyevidenceofairpollutioneffectswerelargelyhumanhealth-relateduntilthediscoveryofacidraineffectsinScandinaviainthelate1960s.Therecognitionofeffectsoflong-rangetransportandthedepositionofpollutantschangedthescientificand,forawhile,thepoliticalattention.Thebroadeningofthescienceinterestintoground-levelozoneandeutrophicationwereimportantinthescienceandeffects,andledtocontrolsontheprecursorpollutantemissionsinEuropethroughLRTAPprotocols.InNorthAmerica,effortstocontroltheprecursorgasesfollowedadifferentcontrolprocess,butachievedsimilarreductionsinemissionsoverthelongerterm.EmissionsofSO2inEuropeandNorthAmericahavebeenreducedin2016byapproximately90%fromtheirpeakvaluesinthe1970sand1980s,respectively(figure 3).However,intheearly1990s,apublicationshowingassociationsacrosssixUScitiesbetweenhumanmortalityandmorbidityandlevelsofairpollutants,especiallyPM,changedthepoliticalandscientificfocusofeffects[101].Subsequentpublicationsonhumanhealtheffectsofpollutantsfollowingsimilarepidemiologicalapproachesrevealedthescaleofeffectsonhumanhealththroughoutthedevelopedanddevelopingnations.CurrentestimatesarethatoutdoorconcentrationsofPM2.5aloneareresponsibleforannualburdensof4.2millionprematuredeathsand100milliondisability-adjustedlife-yearslostglobally[102].ThesepublicationsshowedairpollutiontobeoneofthemajorglobalcausesofprematuremortalityanddrewattentiontothehumanhealtheffectsofpollutantsatmuchsmallerconcentrationsthanhadbeenimplicatedintheLondonsmogof1952.Thisrefocusedscientificandpoliticalattentiononairpollutantsbacktohumanhealth.Theunderlyinglogicofthechangeinfocusisunderstandable,giventhelargenumbersofindividualsandthesocietalcostsofpoorhealthandmortality.Bycomparison,effectsofpollutantsonnaturalecosystems,whicharealwaysdifficulttovalue,andonagriculturalandforestcropsaresmallerinvaluethanthoseonhumanhealth.Forthesereasons,ecosystemeffectshavebecomeasecondaryconsiderationforthepolicymakers.Humanhealthhasbeentheprimaryfocusforthecontrolofairpollutionsincethelate1990s.CleanairlegislationinEurope,NorthAmerica,Japanandotherdevelopedcountriestargetsbothambientlevelsandemissionsources.Nevertheless,themulti-impacteffectsofPM,NO2andO3onhumanhealthandmanagedandnaturalecosystemsmeanthatUNECE-LRTAPprotocolsstillfulfilacrucialrole[57].14.ParticulatematterThechronologypresentedheredescribesthedevelopmentofairqualityissuesastheyaroseratherthanprovidinganarrativeforeachpollutant.However,itisimportanttodrawattentiontoPManditsroleincurrentairqualityproblems.PMfeaturesintheearliestreportsofairpollution,althoughterminologyhasbeeninconsistentandoftenpoorlydefinedwithtermsincludingsmoke,soot,fume,hazeanddust,frequentlyusedsomewhatindiscriminatelythroughtheliterature.PM,describedindetailbyHarrison[103],inthisissue,referstothesumofallsolidandliquidparticlessuspendedinairandisacomplexmixtureofsize,spanningatleastfourordersofmagnitude(1–10 000 nm)andwithalargerangeofchemicalcomposition.Thelatterreflectsthewidevarietyofcontemporarysourcesandverybroadlycomprisescarbonaceousparticlesemitteddirectlyfromcombustion,dustsfromindustrialprocessesandwithin-atmosphereconversionsofinorganic(SO2,NOxandNH3)andorganic(VOC)gasesintoPM.PMisthemaincontributortohumanhealtheffectsbysomemargin,anditisalsotheforminwhichmostofthelong-rangetransportofsulfurandnitrogen-containingpollutantsoccurs.PMcontributestochangesintheEarth'senergybalancebothbyabsorption(e.g.blackcarbon)andbydispersionandreflectionofradiation.ManyofthelinksbetweenairqualityandclimatechangearethereforeduetointeractionsbetweenPMandtheradiativebalanceandthusclimate[104].Similarly,manyoftheeffectsofpollutantsonecosystemsareduetothedepositionofPMeitherdirectlybydrydepositiononfoliarsurfacesorthroughoccultorwetdeposition[105].Smogincludesbothparticulateandgaseouscomponents,butthevisibilityeffectsaredominatedbyPM.GiventhecontributionofPMtothechemicalclimatologyoftheatmosphereoverthedevelopedandrapidlydevelopingcountriesandthecontributionofPMtoeffectsonhumanhealth,itislikelythatPMwillcontinuetodominatecontrolmeasuresforsomedecadestocome.15.2010–2020AirqualitygloballyEmissionsofmostprimarypollutantshavedeclinedinEurope,NorthAmericaandJapanfromthe1990suntilthepresentwiththegreatestprogressinSO2,butevenNO2andVOCemissionshavedecreasedmorethan50%fromtheirpeaksintheseregions.Bycontrast,duringtheperiod1990–2010,emissionshaveincreasedinEastandSouthAsia,andelsewhere,sothatreductionsinglobaltotalemissions,evenforSO2,aremodest,withareductionof15%fromthepeakin1990(figure 3)[37].ForNOxemissions,theglobaltotalcontinuedtoriseandallthereductionsinemissionsinEurope,NorthAmericaandelsewherehavebeencounterbalancedbyincreaseselsewhereandmainlyinAsia(figure 3).ForNH3andVOC,thecaseissimilartothatforNOx,withtheglobaltotalsteadilyincreasing[37].ThelargeincreasesinemissionsofallprimarypollutantsinSouthandEastAsiahavebeenwidelyreportedanddescribedbyZhengetal.[106].AirqualityinAsianmegacitiesshowsvaluesforPM,SO2andNO2inepisodeconditionsthataresimilartothehighlypollutedatmosphereofLondoninthesmogepisodesofthe1950s,forexampletheBeijing‘haze’eventsinJanuary2012.Theglobalburdenofairpollutantshasthereforecontinuedtoincreaseintothefirsttwodecadesofthetwenty-firstcentury.ThefocusofpoliticalattentionremainsfirmlyonhumanhealthduetothePMandNO2exposureinurbanareasofthedevelopedanddevelopingworld.ThedistributionofambientPM2.5concentrationsexperiencedbydifferentregionalpopulationspresentedinfigure 5showshowthecurrentglobalairpollutionhealthburdenisdisproportionatelybornebycountriesinEastandSouthAsia,ratherthanthecountriesthatwereafflictedintheearlystagesoftheIndustrialRevolution.Evenso,themajorityoftheworld'spopulationliveinlocationswherelevelsofambientPM2.5exceedtheWHOguidelinevalue. Figure5.Distributionsofthepopulationasafunctionofannual(2013)averageambientPM2.5concentrationfortheworld's10mostpopulouscountriesandtherestoftheworld.DashedverticallinesindicateWorldHealthOrganizationInterimTargets(IT)andtheAirQualityGuideline(AQG).Source:Braueretal.[107].(Onlineversionincolour.) DownloadfigureOpeninnewtabDownloadPowerPointItisimportanttonotethatthisfocusonhumanhealthdeflectsattentionfromthecontinuedwidespreadexceedancesofthresholdsforeffectsofpollutantsonmanagedandnaturalecosystems[90].16.SatelliteremotesensingTheneedtoobservetroposphericpollutiongloballydrovethedevelopmentofpassiveandsomeactiveremotesensingtechniquestomeasurethetroposphericburdenofkeypollutantstracegasesandaerosol.In1981,1984andtwicein1994theMAPS(MappingPollutionwithSatellites),agascorrelationradiometerinstrumentflewfortypically9daymissionsonthespaceshuttleandmeasuredmiddleanduppertroposphericCObetweenapproximately55°Nandapproximately55°S.AmoreadvancednadirgascorrelationinstrumentMOPITT(MeasurementsofPollutioninTheTroposphere)measuringinboththethermalandshort-waveinfraredhasnowmadeover20yearsofmeasurementsoftroposphericCOandsomeCH4fromtheNASATerra,whichwaslaunchedattheendof1999.Thetotalcolumnsandverticalprofilesofozone,O3,dataproductsfromNASATOMSandSBUVonNimbus7andlaterTOMSandSAGEIIdata[108,109]wereusedtoretrieveO3,O3amountsanddistributionswithafocusinthetropics.From1984,theSCIAMACHY(SCanningImagingAbsorptionspectroMeterforAtmosphericCHartographY)projectwasdevelopedandproposedtoESAin1988.ThisledtothesmallerGOME(GlobalOzoneMonitoringExperiment),beingflownonESAERS-2(1995–2011,[4])andSCIAMACHYonESAENVISAT(2002–2012,[110,111]).Bothsatellitesflewinpolarorbitswithequatorcrossingtimesof10.30and10.00,respectively,andmeasuredinnadirviewinggeometrytheupwellingradiationinthesolarspectralregionatthetopoftheatmosphere.TheUVandvisiblenadirmeasurementsofGOMEandSCIAMACHYhavebeenexploitedtoretrievetroposphericcolumnsofNO2,O3,SO2,HCHO,CHO.CHO,BrO,IOandH2O[4]incloud-freeregionsandaboveclouds.TheSCIAMACHYshort-waveinfraredspectralmeasurementsenabledCOcolumnsandforthefirsttimethetotaldrycolumnmixingratiosofCH4andCO2tobedeterminedglobally.SatellitemeasurementsrevealedthegrowthinemissionsinAsiaandthedeclinesinEuropeandNorthAmericaduringtheperiod1996–2004[112].SatelliteremotesensinginthesolarspectralregionalsoprovidesglobalfieldsfortroposphericSO2,CO,HCHOandCHO.CHO.ThelaunchoftheinstrumentsAIRSonNASAandIASI,aCNESFTIRonEUMETSATMetOpAB,hasledtothedetectionofNH3(see[113,114]).AcombinationofdatafromtheGOME,SCIAMACHYandOMIinstrumentsprovidesclearevidenceoftheincreaseinNO2inEasternChinabetween1995and2010andthesubsequentdeclinefrom2010to2018,asshowninfigure 6. Figure6.TrendsinthetroposphericNO2columnoverEastChinabetween1995and2018(A.RichterandJ.P.Burrows,personalcommunication,2020).(Onlineversionincolour.) DownloadfigureOpeninnewtabDownloadPowerPointSatelliteremotesensingalsoprovidesmeasuresofPM,aerosolopticalthickness,e.g.MODISonNASATerra(1999–present)andAqua(2002–present)[107].17.2020Areweattheglobalpeakofairqualityproblems?Globalemissionsofsulfurhavedeclinedsincethepeakin2000,andrecenttrendsinChinasince2012showareductioninemissionsapproaching50%(figures 3and7).Suchareductionrepresentsremarkableprogress,relativetothetimeittooktoreduceemissionsinEuropeandNorthAmericabyasimilaramount(approx.20years).EmissionsofNOxinChinahavealsodeclined,byapproximately25%overthelast8years[115]andfigure 6,althoughsurfaceozonehascontinuedtoincrease[116].ItisthereforepossiblethattheworldhaspassedthepointofmaximumemissionsofseveralmajorgaseousairpollutantsasacombinationoffurthercontrolsinNorthAmerica,EuropeandEastAsiadrivedownglobaltotals.ClimatechangepoliciesdirectedtowardsreduceduseofcoalandoilareexpectedtocontributefurtherreductionsinemissionsofSO2andNO2overcomingdecades[117].However,therearegoodreasonstobecautious,becauseemissionsofammonia,animportantcontributortoPMandeutrophication,continuetorise,andpossiblefeedbacksbetweenemissionsofthesegasesandclimatemaydriveoverallemissionsupwards[118,119].GlobalemissionsofCH4andVOCalsocontinuetorise,andinthecaseofbiogenicemissions,itispossiblethatchangesinclimateandthewidespreadplantingofnewforestsmayaccelerateglobalemissionsofbiogenicVOC(BVOC).DecisionsoverthespecieschosenfortreeplantingtoincreasecarbonsequestrationwillalsoneedtobemadetosimultaneouslyensurethatBVOCemissionsdonotincrease.Atpresent,itremainsinconsistentininternationalpolicythatlanduse,land-usechangeandforestryarerecognizedasareastocountascarboncreditsintheUNFrameworkConventiononClimateChange,butwhenitcomestotherevisedGothenburgProtocolundertheLRTAPConvention,theaccompanyingBVOCemissionsareconsidered‘natural’andareexcludedfromtheemissionscommitments.BoththebenefitsforcarbonandthepossibledisbenefitsforBVOCwillneedtoberecognizedinfutureinternationalagreements. Figure7.Annualemissionsof(a)SO2,(b)NOxand(c)NMVOCinChinabetween2010and2017(adaptedfromZhengetal.[115]).(Onlineversionincolour.) DownloadfigureOpeninnewtabDownloadPowerPointDespitethewidespreadelevatedlevelsofPM2.5illustratedinfigure 5,datafromtheglobalburdenofdiseaseproject(figure 8)indicatethatgloballytheworldmaynowbeonadownwardtrendofdeathratesfromoutdoorPM2.5andfromground-levelozone. Figure8.AnnualdeathratesattributedtooutdoorPM2.5,outdoorground-levelozoneandindoorpollutionfromsolidfuels1990–2017.Source:www.ourworldindata.org/air-pollution/basedondatafromtheGlobalBurdenofDiseaseproject.(Onlineversionincolour.) DownloadfigureOpeninnewtabDownloadPowerPointGiventhecurrentscaleofeffectsofairpollutiononhumanhealthandecosystemsanduncertaintiesinmeasurementsandmodelling,itisprematuretocelebratethedownturninglobalemissionsoftwoofthemostimportantairpollutants(SO2andNOx).However,thetemporalpatterninemissionsofpollutantsdisplayedintheEnvironmentalKuznetsCurve[120],withincreasingeffortstocontrolemissionsaseconomiesmature,continuestobeconsistentwithobservationsaspartsofAsianowshowsubstantialreductionsinemissions,atleastforpollutionarisingfromcombustionsources.Furthermore,thereisareasonableexpectationthatmeasurestocombatclimatechangeandincreasetheuseofrenewableenergyindevelopingregions,especiallyAfrica,maysubstantiallymitigateemissionsofairpollutantsastheireconomiesdevelop.18.ConcludingremarksInprovidingachronologyofwhathasbecomeaverylargeandcomplexfield,thisnarrativehas,ofnecessity,beenselective.Ithasalsobeenexcessivelybriefondevelopmentsoverthelasttwodecadesduringwhichtherangeofissues,geographicalscaleandverydifferenttrendsindifferentareasoftheworldobscuresthewiderpicture.Thesuggestionthattheworldhaspassedthepeakinairpollutionproblemsisastrongstatement,andmayprovetobeincorrect.However,theevidencefromSO2andNOxemissionsispersuasiveforthemajoremittingcountriesinEurope,NorthAmericaandalsoforEastAsia.Ifsimilarlystrongcontrolswereappliedtoammoniaemissions,whicharecertainlypossibletechnically,thecurrentproblemswiththenitrogencyclecouldalsobeaddressed[121].ItislessclearwhenorhowglobalVOCemissionsmaybecontrolled,buttheseemissionswillbecomelessimportantiftheworldmovestowardsalowerNOxchemicalclimate.19.COVID-19TheDiscussionmeetingattheRoyalSocietyinLondon11thand12thNovember2019tookplacejustbeforethefirstcaseoftheSars-COVID-19wasreportedinChinaonthe17thNovember2019.ByJune2020,6.3millioncaseshadbeenreportedacross188countriesandterritoriesresultingin376 000deaths.Lockdownmeasureshaveledtomajoreffectsonindustrialandtransportactivitiesandreducedemissionsofmanyoftheprimarypollutantscontributingtopoorairquality.Whileitistoosoontoprovideadetailedanalysis,therearemanypreliminaryreports,includingsurfacemeasurementsfrommonitoringnetworksandsatelliteremotesensing.Inthemajorcities,reducedcombustion-relatedemissionsarerevealedbyCO2fluxmeasurements,withreductionsof55%incentralLondon[122].ThereductionsinurbanNO2intheUKduringthefirstweeksoflockdownof20–30%[122]aresimilartoreductionsinothermajorcitiesacrossthedevelopedworld.SimilarreductionshavebeenobservedincolumnNO2satelliteremotesensing(J.P.Burrows,personalcommunication,2020).TheeffectsonPM2,5aremuchsmallerandmorevariablethaneffectsonNO2,withsomeCOVID-19-affectedcitiesinChinareportingreductionsinPM10similarinscaletoreductionsinNO2butforashorterperiod[123].TheanalysisfortheUKduringlockdownsuggestedreductionsinpersonalexposureinLondontoPM2.5intherange5–25%,dependingonthemodeoftravel,buteffectsonambientPMaresmallandveryvariable.Theglobalscaleofthepandemicproducedacleareffectonglobalemissionsofcombustion-relatedemissionsofpollutants,withexpectedhealthandenvironmentalbenefitsduemainlytoreducedNOxemissions.Whetherthesebenefitsleadtolonger-termreductionsinemissionsismuchlessclearastransportandindustrialemissionsgrowfollowingthewidespreadpopulationlockdown.ItseemslikelythataneffectofCOVID-19willbetoreducenetacidityandincreasethegaseousalkalinefraction[16]astransportandcombustionemissionarereduced,butwithlittleanticipatedreductioninNH3emissionsfromagriculture.Whilethismaybeassociatedwithhealthbenefits,additionaladverseeffectsof‘alkalineair’onecosystemswillalsoneedtobeconsidered.DataaccessibilityThebulkofthepaperistextandreview,buttherearedataprovidedforthispaperwhichhavenotbeenpublished.Thedataareshowninfigures 2,4and6.Thesourcesforthedataarelistedinthelegendsforeachfigureandthedatamaybeobtainedfromthenamedsources.Authors'contributionsD.F.,M.R.H.,A.J.andP.B.wrotethecoresectionsofthemanuscriptwithcontributionsandcriticalrevisionsprovidedbyP.G.,D.S.S.,E.N.,M.C.,M.H.U.,M.A.S.,X.L.,Y.C.,Z.K.andG.W.F.M.V.providedtheEMEP4UKmodellingandJ.B.providedthesatelliteremotesensinganalysis.CompetinginterestsWedeclarewehavenocompetinginterests.FundingWereceivednofundingforthisstudy.AcknowledgementsTheauthorsgratefullyacknowledgeconstructivecommentsfromanonymousreviewers.ThecontributionsbyM.C.,M.A.S.,E.N.andM.V.weresupportedbytheUKNaturalEnvironmentResearchCouncil(NERC)NationalCapabilityawardNE/R016429/1,UK-SCAPE.WegratefullyacknowledgesupportfromUKRIforsupportoftheGlobalChallengesResearchFund(GCRF)SouthAsianNitrogenHub(M.A.S.,E.N.,M.V.andD.S.S.)andtheGEF/UNEPproject‘TowardstheInternationalNitrogenManagementSystem’(M.A.S.andM.V.).FootnotesOnecontributionof17toadiscussionmeetingissue‘Airquality,pastpresentandfuture’.©2020TheAuthors.PublishedbytheRoyalSocietyunderthetermsoftheCreativeCommonsAttributionLicensehttp://creativecommons.org/licenses/by/4.0/,whichpermitsunrestricteduse,providedtheoriginalauthorandsourcearecredited.References1.BellML,DavisDL.2001ReassessmentofthelethalLondonfogof1952:novelindicatorsofacuteandchroniccons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MonksP,RavishankaraA,vonSchneidemesserEandSommarivaR(2021)Opinion:Papersthatshapedtroposphericchemistry,AtmosphericChemistryandPhysics,10.5194/acp-21-12909-2021,21:17,(12909-12948) AmiraslaniF(2022)ClimateChangeandUrbanCitizens:TheRoleofMediainPublicisingtheConservationofGreenSpacesandMitigationofAirPollution,Conservation,10.3390/conservation2020014,2:2,(219-232) SokhiR,MoussiopoulosN,BaklanovA,BartzisJ,CollI,FinardiS,FriedrichR,GeelsC,GrönholmT,HalenkaT,KetzelM,MaragkidouA,MatthiasV,MoldanovaJ,NtziachristosL,SchäferK,SuppanP,TsegasG,CarmichaelG,FrancoV,HannaS,JalkanenJ,VeldersGandKukkonenJ(2022)Advancesinairqualityresearch–currentandemergingchallenges,AtmosphericChemistryandPhysics,10.5194/acp-22-4615-2022,22:7,(4615-4703) GiechaskielB,MelasA,MartiniGandDilaraP(2021)OverviewofVehicleExhaustParticleNumberRegulations,Processes,10.3390/pr9122216,9:12,(2216) RelatedarticlesCorrectionto‘Achronologyofglobalairquality’May24,2021,12:00:00AMPhilosophicalTransactionsoftheRoyalSocietyA:Mathematical,PhysicalandEngineeringSciences ThisIssue30October2020Volume378Issue2183Discussionmeetingissue‘Airquality,pastpresentandfuture’organisedandeditedbyDavidFowler,JohnPyle,MarkSuttonandMartinWilliams ArticleInformationDOI:https://doi.org/10.1098/rsta.2019.0314PubMed:32981430Publishedby:RoyalSocietyPrintISSN:1364-503XOnlineISSN:1471-2962History: Manuscriptaccepted22/06/2020Publishedonline28/09/2020Publishedinprint30/10/2020 License:©2020TheAuthors.PublishedbytheRoyalSocietyunderthetermsoftheCreativeCommonsAttributionLicense http://creativecommons.org/licenses/by/4.0/,whichpermitsunrestricteduse,providedtheoriginalauthorandsourcearecredited. 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