ACIDRAIN—LABACTIVITY
AcidRain
PurposeUsegasgeneratorstoproducethe
componentsofacidrain,and determinewhichtype
ofgascausesthegreatestchangeinpHwhen
dissolvedin water.
Background
Whatis acid rain? Acidrainisabroad term
usedtodescribe severalwaysthatacids fall out
ofthe atmosphere.Amore precisetermisacid
deposition,whichhas twoparts:wet deposition and dry deposition.
Wet depositionrefersto acidicrain,fog, andsnow. Asthis acidicwaterflows overand throughtheground,itaffectsavarietyofplantsandanimals,anditcan alsoaccelerateerosion.Thestrengthoftheeffectsdependsonmany factors, includinghowacidicthewater is;thechemistryandbuffering capacity ofthesoilsinvolved; and the types offish,trees,and other living things thatrely onthe water.
Drydepositionreferstoacidicgasesandparticles.Abouthalfofthe acidityintheatmosphere fallsbacktoEarththroughdrydeposition. Thewindblowstheseacidicparticlesandgasesontobuildings,cars, homes, andtrees.Dry-depositedgasesandparticlescanalsobe washedfromtreesandothersurfacesbyrainstorms.When that happens,therunoffwateraddsthoseacidstotheacidrain,makingthe combinationmoreacidicthan thefallingrainalone.Whenthe environmentcannotneutralizetheacidbeingdeposited,damage occurs.
Whatcausesacidrain?Scientists have discoveredthatsulfurdioxide (SO2)andnitrogen oxides(nitricoxide, nitrogenoxide, and nitrousoxide, collectively knownasNOx)aretheprimarycausesofacidrain.Acidrainoccurs whenthesegasesreactintheatmospherewithwater, oxygen,and otherchemicalstoformvariousacidiccompounds.Sunlightincreases therateofmostofthesereactions.Theresult isamildsolutionof sulfuricacid and nitricacid.
In the US, about two-thirds of all SO2andone-quarterofallNO2 comesfromelectricpowergenerationthatreliesonburningfossilfuels such as coal. Othersourcesinclude automobile exhaust,furnaces, andmetalsmelters.
Acidrainisaglobalpollutionissue.Prevailingwindsblowthe compoundsthatcausebothwetanddryaciddepositionacrossstate andnationalborders,andsometimesoverhundredsofmiles.
Howisacidrainmeasured?AcidrainismeasuredusingascalecalledpH.The lowera substance's pH,themoreacidicitis. Pure waterhasapH of 7.0. Normalrainisslightlyacidicbecausecarbondioxidedissolvesinto it, so ithasapHofbetween5.5and6.0. (See Chart on next page.)
Effectsof Acid RainAcidraincauses acidification oflakes and streams andcontributesto damageoftrees at highelevations(forexample,redsprucetrees above 600 m)andmany sensitiveforestsoils.Inaddition, acidrain acceleratesthedecay of building materials(such aslimestoneand marble),metals(suchasbronze),automotivepaint, andother coatings.PriortofallingtotheEarth,SO2andNOxgasesandtheir particulatematterderivatives,sulfatesandnitrates,contributeto visibility degradation and harm publichealth.
A
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Acidrainproducesfluctuations inwatersystemscausingaquatic lifetoexperiencechemical “shock”effects.(Forexample, asthe pHdropsto5.5, plankton, certaininsects,andcrustaceansbegin todieandtrouteggsdon’thatchwell.)Acidrain reducescrop productivityandforestgrowthrates.Acidrainacceleratestherate atwhichheavymetalsareleached(dissolved)fromsoils,rocks, andsedimentsofwaterways.
Effectsof acid rain onlakes streams Theecologicaleffects of acidrainare mostclearlyseen inthe aquatic, or water,environments,suchas streams,lakes, and marshes.Acidrainflowstostreams,lakes,andmarshesafter fallingon forests,fields, buildings, androads. Acidrainalsofalls directlyonaquatichabitats.Most lakesandstreamshaveapH between6 and 8, althoughsome lakes are naturallyacidiceven withouttheeffectsofacidrain.Acidrainprimarilyaffectssensitive bodiesof water, whichare locatedinwatersheds that have soils withalimitedabilitytoneutralizeacidiccompounds(called buffering capacity). Lakesandstreamsbecomeacidic (pHvalue goes down) whenthe wateritselfanditssurroundingsoil cannot buffertheacidrainenoughtoneutralizeit.Inareaswherebuffering capacityislow,acidrainalsoreleases aluminumfrom soilsinto lakesandstreams; aluminum ishighlytoxic to many species of aquaticorganisms.
Sometypesofplantsandanimalsareableto tolerateacidic waters.Forexample, frogscantoleratewaterthatismoreacidic(haslowerpH)thantrout can tolerate.Others, however,areacid- sensitiveandwillbelostasthepHdeclines.Generally,theyoung ofmost speciesaremoresensitivetoenvironmentalconditions thanadults.AtpH5,most fisheggscannothatch. At lowerpH levels,someadult fishdie.Someacidlakeshavenofish.
Effectsof acid rain onforests Aftermanyyearsof collectinginformationon the chemistry and biology offorests,researchers are beginningtounderstand howacidrainworksonthe forestsoil,trees,andother plants. Researchersnow knowthatacidraincauses slowergrowth, injury,or deathofforests.Acidrain hasbeenimplicatedinforestandsoildegradationinmanyareasofthe eastern United States,particularlyhigh elevation forestsofthe AppalachianMountainsfromMainetoGeorgiathatincludeareassuch as theShenandoahandGreatSmokyMountainNationalParks.Of course,acidrainisnottheonlycauseofsuchconditions.Otherthings thataddstress,suchasairpollutants,insects,disease,drought,or verycoldweather,alsoharmtreesandplants.Inmostcases,in fact, the impacts of acidrainontreesoccur due tothecombined effects of acidrainandtheseotherenvironmentalstressors.
Effectsofacidrainonecosystems Together, biologicalorganisms andthe environmentinwhichtheylive are calledanecosystem. Theplants and animalsliving withinan ecosystemarehighlyinterdependent.Forexample,frogsmaytolerate relativelyhigh levelsofacidity,butiftheyeat insects likethemayfly, theymaybeaffectedbecause partoftheirfoodsupplymaydisappear. Becauseoftheconnectionsbetweenthemanyfish,plants,andother organisms living inanaquaticecosystem, changesinpHor aluminum levelsaffectbiodiversityaswell.Thus,aslakesandstreamsbecome moreacidic,thenumbersandtypesoffishandotheraquaticanimals andplantsthatliveinthesewatersdecrease.
Acidraineffectsaround the world Asoftheyear2000,themostacidicrainfallingintheUShadapHof about4.3. Acidrainisnotanewphenomenon;observationsofhighlyacidic rainfallfromindustrializedareas inGreat Britainand Sweden date back morethan100years.
Inareassuchas thenortheasternUnitedStates,wherebuffering capacityofthesoil ispoor,some lakesnowhaveapHof lessthan5. Oneofthemostacidic lakesreportedisLittleEchoPondinFranklin, NewYork.LittleEchoPondhasapHof4.2.
Acidrainhasseverely affectedGerman forests.In 1983 theGerman MinisterforAgricultureadmittedthat34%ofGermanforestswere dyingfromacidrainpollution. In1988thispercentagehadrisen to52%ofGermanforestland.Atleast70%oftheoaktreesinthese forests are affected.