30Acid-Rain

AcidRainAcidrainisaseriousenvironmentalproblemthataffectslargepartsoftheworld.Thissectionprovidesinformationaboutacidrain'scausesandeffects,howwemeasureacidrain,andwhatisbeingdonetosolvetheproblem.•Whatisacidrainandwhatcausesit?•Howdowemeasureacidrain?•Whatareacidrain'seffects?•Howdowereduceacidrain?WhatisAcidRainandWhatCausesIt?Acidrainisabroadtermusedtodescribeseveralwaysthatacidsfalloutoftheatmosphere.Amoreprecisetermisaciddeposition,whichhastwoparts:wetanddry.Wetdepositionreferstoacidicrain,fog,andsnow.Asthisacidicwaterflowsoverandthroughtheground,itaffectsavarietyofplantsandanimals.Thestrengthoftheeffectsdependonmanyfactors,includinghowacidicthewateris,thechemistryandbufferingcapacityofthesoilsinvolved,andthetypesoffish,trees,andotherlivingthingsthatrelyonthewater.Drydepositionreferstoacidicgasesandparticles.Abouthalfoftheacidityintheatmospherefallsbacktoearththroughdrydeposition.Thewindblowstheseacidicparticlesandgasesontobuildings,cars,homes,andtrees.Drydepositedgasesandparticlescanalsobewashedfromtreesandothersurfacesbyrainstorms.Whenthathappens,therunoffwateraddsthoseacidstotheacidrain,makingthecombinationmoreacidicthanthefallingrainalone.Prevailingwindsblowthecompoundsthatcausebothwetanddryaciddepositionacrossstateandnationalborders,andsometimesoverhundredsofmiles.Scientistsdiscovered,andhaveconfirmed,thatsulfurdioxide(SO2)andnitrogenoxides(NOx)aretheprimarycausesofacidrain.IntheUS,About2/3ofallSO2and1/4ofallNOxcomesfromelectricpowergenerationthatreliesonburningfossilfuelslikecoal.Acidrainoccurswhenthesegasesreactintheatmospherewithwater,oxygen,andotherchemicalstoformvariousacidiccompounds.Sunlightincreasestherateofmostofthesereactions.Theresultisamildsolutionofsulfuricacidandnitricacid.HowDoWeMeasureAcidRain?AcidrainismeasuredusingascalecalledpH.Thelowerasubstance'spH,themoreacidicitis.PurewaterhasapHof7.0.Normalrainisslightlyacidicbecausecarbondioxidedissolvesintoit,soithasapHofabout5.5.Asoftheyear2000,themostacidicrainfallingintheUShasapHofabout4.3.WhatAreAcidRain'sEffects?Aciddepositionhasavarietyofeffects,includingdamagetoforestsandsoils,fishandotherlivingthings,materials,andhumanhealth.Acidrainalsoreduceshowfarandhowclearlywecanseethroughtheair,aneffectcalledvisibilityreduction.HowDoWeReduceAcidRain?WhatsocietycandoWhatindividualscandoWhatSocietyCanDoAboutAcidDepositionThereareseveralwaystoreduceaciddeposition,rangingfromsocietalchangestoindividualaction.Understandaciddeposition'scausesandeffectsCleanupsmokestacksandexhaustpipesUsealternativeenergysourcesRestoreadamagedenvironmentTakeactionasindividualsUnderstandaciddeposition'scausesandeffectsTounderstandaciddeposition'scausesandeffectsandtrackchangesintheenvironment,scientistsfromstategovernmentsandacademicstudyacidificationprocesses.TheycollectairandwatersamplesandmeasurethemforvariouscharacteristicslikepHandchemicalcomposition,.andtheyresearchtheeffectsofaciddepositiononhuman-madematerialssuchasmarbleandbronze（青铜）.Finally,scientistsworktounderstandtheeffectsofsulfurdioxide(SO2)andnitrogenoxides(NOx)-thepollutantsthatcauseaciddepositionandfineparticles-onhumanhealth.Tosolvetheacidrainproblem,peopleneedtounderstandhowacidraincausesdamagetotheenvironment.Theyalsoneedtounderstandwhatchangescouldbemadetotheairpollutionsourcesthatcausetheproblem.Theanswerstothesequestionshelpleadersmakebetterdecisionsabouthowtocontrolairpollutionandthereforehowtoreduce-oreveneliminate-acidrain.Sincetherearemanysolutionstotheacidrainproblem,leadershaveachoiceofwhichoptionsorcombinationofoptionsarebest.CleanupsmokestacksandexhaustpipesAlmostalloftheelectricitythatpowersmodernlifecomesfromburningfossilfuelslikecoal,naturalgas,andoil.aciddepositioniscausedbytwopollutantsthatarereleasedintotheatmosphere,oremitted,whenthesefuelsareburned:sulfurdioxide(SO2)andnitrogenoxides(NOx).CoalaccountsformostUSsulfurdioxide(SO2)emissionsandalargeportionofNOxemissions.Sulfurispresentincoalasanimpurity,anditreactswithairwhenthecoalisburnedtoformSO2.Incontrast,NOxisformedwhenanyfossilfuelisburned.ThereareseveraloptionsforreducingSO2emissions,includingusingcoalcontaininglesssulfur,washingthecoal,andusingdevicescalledscrubberstochemicallyremovetheSO2fromthegasesleavingthesmokestack.Powerplantscanalsoswitchfuels;forexampleburningnaturalgascreatesmuchlessSO2thanburningcoal.Certainapproacheswillalsohaveadditionalbenefitsofreducingotherpollutantssuchasmercuryandcarbondioxide.Understandingtheseco-benefitshasbecomeimportantinseekingcost-effectiveairpollutionreductionstrategies.Finally,powerplantscanusetechnologiesthatdon'tburnfossilfuels.Eachoftheseoptionshasitsowncostsandbenefits,however;thereisnosingleuniversalsolution.Similartoscrubbersonpowerplants,catalyticconvertersreduceNOxemissionsfromcars.ThesedeviceshavebeenrequiredforovertwentyyearsintheUS,anditisimportanttokeepthemworkingproperlyandtailpipe（排气管）restrictionshavebeentightenedrecently.UsealternativeenergysourcesThereareothersourcesofelectricitybesidesfossilfuels.Theyinclude:nuclearpower,hydropower,windenergy,geothermalenergy,an