Unit7 Air Pollution Ⅰ.docx
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Unit7 Air Pollution Ⅰ.docx
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Unit7AirPollutionⅠ
AirPollutionⅠ
Globalwarming
GlobalwarmingistheincreaseintheaveragetemperatureofEarth’snear-surfaceairandoceanssincethemid-20thcenturyanditsprojectedcontinuation.Accordingtothe2007FourthAssessmentReportbytheIntergovernmentalPanelonClimateChange(IPCC),globalsurfacetemperatureincreasedby0.74±0.18℃(1.33±0.32℉)duringthe20thcentury.Mostoftheobservedtemperatureincreasesincethemiddleofthe20thcenturyhasbeencausedbyincreasingconcentrationsofgreenhousegases,whichresultfromhumanactivitiessuchastheburningoffossilfuelanddeforestation.Globaldimming,areductionofsunlightreachingthesurfaceasaresultofincreasingatmosphericconcentrationsofhuman-madeparticulates,haspartiallycounteredtheeffectsofwarminginducedbygreenhousegases.
Climatemodelprojectionssummarizedinthe2007IPCCreportindicatethattheglobalsurfacetemperatureislikelytoriseafurther1.1to6.4℃(2.0to11.5℉)duringthe21stcentury.Theuncertaintyinthisestimatearisesfromtheuseofmodelswithdifferingsensitivitytogreenhousegasconcentrationsandtheuseofdifferingestimatesoffuturegreenhousegasemissions.Anincreaseinglobaltemperaturewillcausesealevelstoriseandwillchangetheamountandpatternofprecipitation,probablyincludingexpansionofsubtropicaldeserts.WarmingisexpectedtobestrongestintheArcticandwouldbeassociatedwithcontinuingretreatofglaciers,permafrostandseaice.Otherlikelyeffectsofthewarmingincludemorefrequentoccurrenceofextremeweathereventsincludingheatwaves,droughtsandheavyrainfallevents,speciesextinctionsduetoshiftingtemperatureregimes,andchangesinagriculturalyields.Warmingandrelatedchangeswillvaryfromregionaroundtheglobe,thoughthenatureoftheseregionalchangesisuncertain.
Thescientificconsensusisthatglobalwarmingisoccurringandismostlytheresultofhumanactivity.Thisfindingisrecognizedbythenationalscienceacademiesofallthemajorindustrializedcountriesandisnotrejectedbyanyscientificbodyofnationalorinternationalstanding.AccordingtoarecentGalluppoll,peopleinmostcountriesaremorelikelytoattributeglobalwarmingtohumanactivitiesthantonaturalcauses.ThemajorexceptionistheU.S.,wherenearlyhalfofthepopulation(thelargestpercentageofanycountry)attributesglobalwarmingtonaturalcauses.
TheKyotoProtocolisaimedatstabilizinggreenhousegasconcentrationtopreventa“dangerousanthropogenicinterference”.AsofNovember2009,187stateshadsignedandratifiedtheprotocol.Proposedresponsestoglobalwarmingincludemitigationtoreduceemissions,adaptationtotheeffectsofglobalwarming,andgeoengineeringtoremovegreenhousegasesfromtheatmosphere.
Greenhousegases
Thegreenhouseeffectistheprocessbywhichabsorptionandemissionofinfraredradiationbygasesintheatmospherewarmaplanet’sloweratmosphereandsurface.ItwasproposedbyJosephFourierin1824andwasfirstinvestigatedquantitativelybySvanteArrheniusin1896.
Naturallyoccurringgreenhousegaseshaveameanwarmingeffectofabout33℃(59℉).Themajorgreenhousegasesarewatervapor,whichcausesabout36-70percentofthegreenhouseeffect;carbondioxide(CO2),whichcauses9-26percent;methane(CH4),whichcauses4-9percent;andozone(O3),whichcauses3-7percent.Cloudsalsoaffecttheradiationbalance,buttheyarecomposedofliquidwateroriceandsohavedifferenteffectsonradiationfromwatervapor.
HumanactivitysincetheIndustrialRevolutionhasincreasedtheamountofgreenhousegasesintheatmosphere,leadingtoincreasedradiativeforcingfromCO2,methane,troposphericozone,CFCsandnitrousoxide.TheconcentrationsofCO2andmethanehaveincreasedby36%and148%respectivelysince1750.Theselevelsaremuchhigherthanatanytimeduringthelast800,000years,theperiodforwhichreliabledatahasbeenextractedfromicecores.LessdirectgeologicalevidenceindicatesthatCO2valueshigherthanthiswerelastseenabout20millionyearsago.Fossilfuelburninghasproducedaboutthree-quartersoftheincreaseinCO2fromhumanactivityoverthepast20years.Therestofthisincreaseiscausedmostlybychangesinland-use,particularlydeforestation.
Overthelastthreedecadesofthe20thcentury,GDPpercapitaandpopulationgrowthwerethemaindriversofincreasesingreenhousegasemissions.CO2emissionsarecontinuingtoriseduetotheburningoffossilfuelsandland-usechange.Emissionsscenarios,estimatesofchangesinfutureemissionlevelsofgreenhousegases,havebeenprojectedthatdependuponuncertaineconomic,sociological,technological,andnaturaldevelopments.Inmostscenarios,emissionscontinuetoriseoverthecentury,whileinafew,emissionsarereduced.Theseemissionscenarios,combinedwithcarboncyclemodelling,havebeenusedtoproduceestimatesofhowatmosphericconcentrationsofgreenhousegaseswillchangeinthefuture.UsingthesixIPCCSRES“marker”scenarios,modelssuggestthatbytheyear2100,theatmosphericconcentrationofCO2couldrangebetween541and970ppm.Thisisanincreaseof90-250%abovetheconcentrationintheyear1750.Fossilfuelreservesaresufficienttoreachtheselevelsandcontinueemissionspast2100ifcoal,oilsandsormethaneclathratesareextensivelyexploited.
Thepopularmediaandthepublicoftenconfuseglobalwarmingwiththeozonehole,i.e.,thedestructionofstratosphericozonebychlorofluorocarbons(CFCs).Althoughthereareafewareasoflinkage,therelationshipbetweenthetwoisnotstrong.Reducedstratosphericozonehashadaslightcoolinginfluenceonsurfacetemperatures,whileincreasedtroposphericozonehashadasomewhatlargerwarmingeffect.
Ozonedepletion
Ozonedepletiondescribestwodistinctbutrelatedphenomenaobservedsincethelate1970s:
asteadydeclineofabout4%perdecadeinthetotalvolumeofozoneinEarth’sstratosphere(theozonelayer),andamuchlargerspringtimedecreaseinstratosphericozoneoverEarth’spolarregions.Thelatterphenomenonisreferredtoastheozonehole.Inadditiontothesewell-knownstratosphericphenomena,therearealsospringtimepolartroposphericozonedepletionevents.
Thedetailsofpolarozoneholeformationdifferfromthatofmid-latitudethinning,butthemostimportantprocessinbothiscatalyticdestructionofozonebyatomichalogens.Themainsourceofthesehalogenatomsinthestratosphereisphotodissociationofman-madehalocarbonrefrigerants(CFCs,freons,halons.)Thesecompoundsaretransportedintothestratosphereafterbeingemittedatthesurface.Bothtypesofozonedepletionwereobservedtoincreaseasemissionsofhalocarbonsincreased.
CFCsandothercontributorysubstancesarereferredtoasozone-depletingsubstances(ODS).SincetheozonelayerpreventsmostharmfulUVBwavelengths(280-315°nm)ofultravioletlight(UVlight)frompassingthroughtheEarth’satmosphere,observedandprojecteddecreasesinozonehavegeneratedworldwideconcernleadingtoadoptionoftheMontrealProtocolthatbanstheproductionofCFCs,halons,andotherozone-depletingchemicalssuchascarbontetrachlorideandtrichloroethane.Itissuspectedthatavarietyofbiologicalconsequencessuchasincreasesinskincancer,cataracts,damagetoplants,andreductionofplanktonpopulationsintheocean’sphoticzonemayresultfromtheincreasedUVexposureduetoozonedepletion.
Theozoneholeanditscauses
TheAntarcticozoneholeisanareaoftheAntarcticstratosphereinwhichtherecentozonelevelshavedroppedtoaslowas33%oftheirpre-1975values.TheozoneholeoccursduringtheAntarcticspring,fromSeptembertoearlyDecember,asstrongwesterlywindsstarttocirculatearoundthecontinentandcreateanatmosphericcontainer.Withinthispolarvortex,over50%ofthelowerstratosphericozoneisdestroyedduringtheAntarcticspring.
Asexplainedabove,theprimarycauseofozonedepletionisthepresenceofchlorine-containingsourcegases(primarilyCFCsandrelatedhalocarbons).InthepresenceofUVlight,thesegasesdissociate,releasingchlorineatoms,whichthengoontocatalyzeozonedestruction,TheCl-catalyzedozonedepletioncantakeplaceinthegasphase,butitisdramaticallyenhancedinthepresenceofpolarstratosphericclouds(PSCs).
Thesepolarstratosphericclouds(PSC)formduringwinter,intheextremecold.Polarwintersaredark,consistingof3monthswithoutsolarradiation(sunlight).Thelackofsunlightcontributestoadecreaseintemperatureandthepolarvortextrapsandchillsair.Temperatureshoveraroundorbelow-80℃.Theselowtemperaturesformcloudparticles.TherearethreetypesofPSCclouds:
nitricacidtrihydrateclouds,slowlycoolingwater-iceclouds,andrapidcoolingwater-ice(nacerous);thatprovidesurfacesforchemicalreactionsthatleadtoozonedestruction.
Thephotochemicalprocessesinvolvedarecomplexbutwellunderstood.Thekeyobservationisthat,ordinarily,mostofthechlorineinthestratosphereresidesinstable“reservoir”compounds,primarilyhydrochloricacid(HCl)andchlorinenitrate(ClONO2).DuringtheAntarcticwinterandspring,however,reactionsonthesurfaceofthepolarstratosphericcloudparticlesconvertthese“reservoir”compoundsintoreactivefreeradicals(ClandClO).ThecloudscanalsoremoveNO2fromtheatmospherebyconvertingittonitricacid,whichpreventsthenewlyformedClOfrombeingconvertedbackintoClONO2.
TheroleofsunlightinozonedepletionisthereasonwhytheAntarcticozonedepletionisgreatestduringspring.Duringwinter,eventhoughPSCsareattheirmostabundant,the
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