Unit7 Air Pollution Ⅰ.docx

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