1492Ch10

Manahan,StanleyE.PARTICLESINTHEATMOSPHEREEnvironmentalChemistryBocaRaton:CRCPressLLC,200010PARTICLESINTHEATMOSPHERE____________________________________________________10.1.PARTICLESINTHEATMOSPHEREParticlesintheatmosphere,whichrangeinsizefromaboutone-halfmillimeter(thesizeofsandordrizzle)downtomoleculardimensions,aremadeupofanamazingvarietyofmaterialsanddiscreteobjectsthatmayconsistofeithersolidsorliquiddroplets.1Anumberoftermsarecommonlyusedtodescribeatmosphericparticles;themoreimportantofthesearesummarizedinTable10.1.Particlesaboundintheatmosphere.EventheArctic,remotefromsourcesofindustrialpollution,isafflictedwithan“ArcticHaze”ofairborneparticlesfromOctobertoMayeachyear.Particulatesisatermthathascometostandforparticlesintheatmosphere,althoughparticulatematterorsimplyparticles,ispreferredusage.Table10.1.ImportantTermsDescribingAtmosphericParticlesTermMeaningAerosolColloidal-sizedatmosphericparticleCondensationaerosolFormedbycondensationofvaporsorreactionsofgasesDispersionaerosolFormedbygrindingofsolids,atomizationofliquids,ordispersionofdustsFogTermdenotinghighlevelofwaterdropletsHazeDenotesdecreasedvisibilityduetothepresenceofparticlesMistsLiquidparticlesSmokeParticlesformedbyincompletecombustionoffuel©2000CRCPressLLCParticulatemattermakesupthemostvisibleandobviousformofairpollution.Atmosphericaerosolsaresolidorliquidparticlessmallerthan100µmindiameter.Pollutantparticlesinthe0.001to10µmrangearecommonlysuspendedintheairnearsourcesofpollutionsuchastheurbanatmosphere,industrialplants,highways,andpowerplants.Verysmall,solidparticlesincludecarbonblack,silveriodide,combustionnuclei,andsea-saltnuclei(seeFigure10.1).Largerparticlesincludecementdust,wind-blownsoildust,foundrydust,andpulverizedcoal.Liquidparticulatematter,mist,includesraindrops,fog,andsulfuricacidmist.Particulatemattermaybeorganicorinorganic;bothtypesareveryimportantatmosphericcontaminants.Figure10.1.Burstingbubblesinseawaterformsmallliquidaerosolparticles.Evaporationofwaterfromaerosolparticlesresultsintheformationofsmallsolidparticlesofsea-saltnuclei.Someparticlesareofbiologicalorigin,suchasviruses,bacteria,bacterialspores,fungalspores,andpollen.Inadditiontoorganicmaterials,organismsmaycontributetosulfateparticulatematterintheatmosphere.Marinebiologicalsourcesmaycontributesignificantlytoatmosphericaerosols.2Biogenicmaterialsreactinginandonthesurfaceofsea-saltaerosolsproducesomesignificantatmosphericchemicalspecies,suchashalogenradicals,andinsodoinginfluencecyclesinvolvingatmosphericsulfur,nitrogen,andoxidants.3Asdiscussedlaterinthischapter,particulatematteroriginatesfromawidevarietyofsourcesandprocesses,rangingfromsimplegrindingofbulkmattertocomplicatedchemicalorbiochemicalsyntheses.Theeffectsofparticulatematterarealsowidelyvaried.PossibleeffectsonclimatearediscussedinChapter14.Eitherbyitself,orincombinationwithgaseouspollutants,particulatemattermaybedetrimentaltohumanhealth.Atmosphericparticlesmaydamagematerials,reducevisibility,andcauseundesirableestheticeffects.Itisnowrecognizedthatverysmallparticleshaveaparticularlyhighpotentialforharm,includingadversehealtheffects.InnewambientairqualitystandardsforparticulatematterissuedinJuly1997,theU.S.EnvironmentalProtectionAgencyregulatedparticles2.5µmindiameterorlessforthefirsttime.Forthemostpart,aerosolsconsistofcarbonaceousmaterial,metaloxidesandglasses,dissolvedionicspecies(electrolytes),andionicsolids.Thepredominantconstituentsarecarbonaceousmaterial,water,sulfate,nitrate,ammoniumnitrogen,andsilicon.Thecompositionofaerosolparticlesvariessignificantlywithsize.The©2000CRCPressLLCverysmallparticlestendtobeacidicandoftenoriginatefromgases,suchasfromtheconversionofSO2toH2SO4.Largerparticlestendtoconsistofmaterialsgeneratedmechanically,suchasbythegrindingoflimestone,andhaveagreatertendencytobebasic.10.2.PHYSICALBEHAVIOROFPARTICLESINTHEATMOSPHEREAsshowninFigure10.2,atmosphericparticlesundegoanumberofprocessesintheatmosphere.Smallcolloidalparticlesaresubjecttodiffusionprocesses.Smallerparticlescoagulatetogethertoformlargerparticles.Sedimentationordrydepositionofparticles,whichhaveoftenreachedsufficientsizetosettlebycoagulation,isoneoftwomajormechanismsforparticleremovalfromtheatmosphere.Theotherisscavengingbyraindropsandotherformsofprecipitation.Particlesalsoreactwithatmosphericgases.CondensationofatmosphericwaterandothervaporsFigure10.2.Processesthatparticlesundergointheatmosphere.Particlesizeusuallyexpressesthediameterofaparticle,thoughsometimesitisusedtodenotetheradius.Therateatwhichaparticlesettlesisafunctionofparticlediameteranddensity.Thesettlingrateisimportantindeterminingtheeffectoftheparticleintheatmosphere.Forsphericalparticlesgreaterthanapproximately1µmindiameter,Stokes’lawapplies,ν=gd2(ρ1-ρ2)(10.2.1)18η©2000CRCPressLLCwhereνisthesettlingvelocityincm/sec,gistheaccelerationofgravityincm/sec2,ρ1isthedensityoftheparticleing/cm3,ρ2isthedensityofairing/cm3,andηistheviscosityofairinpoise.Stokes’lawcanalsobeusedtoexpresstheeffectivediameterofanirregularnonsphericalparticle.ThesearecalledStokesdiameters(a