1492Ch11

Manahan,StanleyE.GASEOUSINORGANICAIRPOLLUTANTSEnvironmentalChemistryBocaRaton:CRCPressLLC,200011GASEOUSINORGANICAIRPOLLUTANTS____________________________________________________11.1INORGANICPOLLUTANTGASESAnumberofgaseousinorganicpollutantsentertheatmosphereastheresultofhumanactivities.ThoseaddedinthegreatestquantitiesareCO,SO2,NO,andNO2.(ThesequantitiesarerelativelysmallcomparedtotheamountofCO2intheatmosphere.ThepossibleenvironmentaleffectsofincreasedatmosphericCO2levelsarediscussedinChapter14.)OtherinorganicpollutantgasesincludeNH3,N2O,N2O5,H2S,Cl2,HCl,andHF.Substantialquantitiesofsomeofthesegasesareaddedtotheatmosphereeachyearbyhumanactivities.Globally,atmosphericemissionsofcarbonmonoxide,sulfuroxides,andnitrogenoxidesareoftheorderofonetoseveralhundredmilliontonsperyear.11.2.PRODUCTIONANDCONTROLOFCARBONMONOXIDECarbonmonoxide,CO,causesproblemsincasesoflocallyhighconcentrationsbecauseofitstoxicity(seeChapter23).Theoverallatmosphericconcentrationofcarbonmonoxideisabout0.1ppmcorrespondingtoaburdenintheearth’satmos-phereofapproximately500millionmetrictonsofCOwithanaverageresidencetimerangingfrom36to110days.MuchofthisCOispresentasanintermediateintheoxidationofmethanebyhydroxylradical(seeSection9.8,Reaction9.8.17).FromTable9.1itmaybeseenthatthemethanecontentoftheatmosphereisabout1.6ppm,morethan10timestheconcentrationofCO.Therefore,anyoxidationprocessformethanethatproducescarbonmonoxideasanintermediateiscertaintocontributesubstantiallytotheoverallcarbonmonoxideburden,probablyaroundtwo-thirdsofthetotalCO.DegradationofchlorophyllduringtheautumnmonthsreleasesCO,amountingtoperhapsasmuchas20%ofthetotalannualrelease.Anthropogenicsourcesaccountforabout6%ofCOemissions.TheremainderofatmosphericCOcomesfromlargelyunknownsources.Theseincludesomeplantsandmarineorganismsknown©2000CRCPressLLCassiphonophores,anorderofHydrozoa.Carbonmonoxideisalsoproducedbydecayofplantmatterotherthanchlorophyll.Becauseofcarbonmonoxideemissionsfrominternalcombustionengines,thehighestlevelsofthistoxicgastendtooccurincongestedurbanareasattimeswhenthemaximumnumberofpeopleareexposed,suchasduringrushhours.Atsuchtimes,carbonmonoxidelevelsintheatmospherehavebecomeashighas50-100ppm.Atmosphericlevelsofcarbonmonoxideinurbanareasshowapositivecorrelationwiththedensityofvehiculartraffic,andanegativecorrelationwithwindspeed.Urbanatmospheresmayshowaveragecarbonmonoxidelevelsoftheorderofseveralppm,muchhigherthanthoseinremoteareas.ControlofCarbonMonoxideEmissionsSincetheinternalcombustionengineistheprimarysourceoflocalizedpollutantcarbonmonoxideemissions,controlmeasureshavebeenconcentratedontheauto-mobile.Carbonmonoxideemissionsmaybeloweredbyemployingaleanerair-fuelmixture,thatis,oneinwhichtheweightratioofairtofuelisrelativelyhigh.Atair-fuel(weight:weight)ratiosexceedingapproximately16:1,aninternalcombustionengineemitsverylittlecarbonmonoxide.Modernautomobilesusecatalyticexhaustreactorstocutdownoncarbonmonoxideemissions.Excessairispumpedintotheexhaustgas,andthemixtureispassedthroughacatalyticconverterintheexhaustsystem,resultinginoxidationofCOtoCO2.11.3.FATEOFATMOSPHERICCOItisgenerallyagreedthatcarbonmonoxideisremovedfromtheatmospherebyreactionwithhydroxylradical,HO•:CO+HO•→CO2+H(11.3.1)Thereactionproduceshydroperoxylradicalasaproduct:O2+H+M→HOO•+M(11.3.2)HO•isregeneratedfromHOO•bythefollowingreactions:HOO•+NO→HO•+NO2(11.3.3)HOO•+HOO•→H2O2+O2(11.3.4)ThelatterreactionisfollowedbyphotochemicaldissociationofH2O2toregenerateHO•:H2O2+hν→2HO•(11.3.5)MethaneisalsoinvolvedthroughtheatmosphericCO/HO•/CH4cycle.©2000CRCPressLLCSoilmicroorganismsacttoremoveCOfromtheatmosphere.Therefore,soilisasinkforcarbonmonoxide.11.4.SULFURDIOXIDESOURCESANDTHESULFURCYCLEFigure11.1showsthemainaspectsoftheglobalsulfurcycle.ThiscycleinvolvesprimarilyH2S,(CH3)2S,SO2,SO3,andsulfates.Therearemanyuncertaintiesregardingthesources,reactions,andfatesoftheseatmosphericsulfurspecies.Onaglobalbasis,sulfurcompoundsentertheatmospheretoaverylargeextentthroughhumanactivities.Approximately100millionmetrictonsofsulfurperyearenterstheglobalatmospherethroughanthropogenicactivities,primarilyasSO2fromthecom-Figure11.1.Theglobalatmosphericsulfurcycle.Fluxesofsulfurrepresentedbythearrowsareinmillionsofmetrictonsperyear.Thosemarkedwithaquestionmarkareuncertain,butlarge,probablyoftheorderof100millionmetrictonsperyear.bustionofcoalandresidualfueloil.Thegreatestuncertaintiesinthecyclehavetodowithnonanthropogenicsulfur,whichenterstheatmospherelargelyasSO2andH2Sfromvolcanoes,1andas(CH3)2SandH2Sfromthebiologicaldecayoforganicmatterandreductionofsulfate.Thesinglelargestsourceofnaturalsulfurdischargedtotheatmosphereisnowbelievedtobebiogenicdimethylsulfide,(CH3)2S,frommarinesources.2AnyH2SthatdoesgetintotheatmosphereisconvertedrapidlytoSO2bythefollowingoverallprocess:H2S+3/2O2→SO2+H2O(11.4.1)©2000CRCPressLLCTheinitialreactionishydrogenionabstractionbyhydroxylradical,H2S+HO•→HS•+H2O(11.4.2)followedbythefollowingtworeactionstogiveSO2:HS•+O2→HO•+SO(11.4.3)SO+O2→SO2+O(11.4.4)Theprimarysourceofanthropogenicsulfurdioxideiscoal,fro