1492Ch12

Manahan,StanleyE.ORGANICAIRPOLLUTANTSEnvironmentalChemistryBocaRaton:CRCPressLLC,200012ORGANICAIRPOLLUTANTS____________________________________________________12.1.ORGANICCOMPOUNDSINTHEATMOSPHEREOrganicpollutantsmayhaveastrongeffectuponatmosphericquality.Theeffectsoforganicpollutantsintheatmospheremaybedividedintotwomajorcate-gories.Thefirstconsistsofdirecteffects,suchascancercausedbyexposuretovinylchloride.Thesecondistheformationofsecondarypollutants,especiallyphotochemicalsmog(discussedindetailinChapter13).Inthecaseofpollutanthydrocarbonsintheatmosphere,thelatteristhemoreimportanteffect.Insomelocalizedsituations,particularlytheworkplace,directeffectsoforganicairpollutantsmaybeequallyimportant.Thischapterdiscussesthenatureanddistributionoforganiccompoundsintheatmosphere.Chapter13dealswithphotochemicalsmogandaddressesthemechanismsbywhichorganiccompoundsundergophotochemicalreactionsintheatmosphere.LossofOrganicSubstancesfromtheAtmosphereOrganiccontaminantsarelostfromtheatmospherebyanumberofroutes.Theseincludedissolutioninprecipitation(rainwater),drydeposition,photochemicalreact-ions,formationofandincorporationintoparticulatematter,anduptakebyplants.Reactionsoforganicatmosphericcontaminantsareparticularlyimportantindeterminingtheirmannerandratesoflossfromtheatmosphere.Suchreactionsarediscussedinthischapter.Foresttreespresentalargesurfaceareatotheatmosphereandareparticularlyimportantinfilteringorganiccontaminantsfromair.Foresttreesandplantscontacttheatmospherethroughplantcuticlelayers,thebiopolymer“skin”ontheleavesandneedlesoftheplants.Thecuticlelayerislipophilic,meaningthatithasaparticularaffinityfororganicsubstances,includingthoseintheatmosphere.Astudyoftheuptakeoforganicsubstances(lindane,triadimenol,bitertanol,2,4-dichlorophen-oxyaceticacid,andpentachlorophenol)byconiferneedles1hasshownthatthepro-©2000CRCPressLLCcessconsistsoftwophases:(1)adsorptionontotheneedlesurfacesand(2)transportthroughthecuticlelayerintotheneedleandplant.Uptakeincreaseswithincreasinglipophilicityofthecompoundsandwithincreasingsurfaceareaoftheleaves.Thisphenomenonpointstotheimportanceofforestsinatmosphericpurification.GlobalDistillationandFractionationofPersistentOrganicPollutantsOnaglobalscale,itislikelythatpersistentorganicpollutantsundergoacycleofdistillationandfractionationinwhichtheyarevaporizedintotheatmosphereinwarmerregionsoftheEarthandcondenseandaredepositedincolderregions.2Thetheoryofthisphenomenonholdsthatthedistributionofsuchpollutantsisgovernedbytheirphysicochemicalpropertiesandthetemperatureconditionstowhichtheyareexposed.Asaresult,theleastvolatilepersistentorganicpollutantsaredepositedneartheirsources,thoseofrelativelyhighvolatilityaredistilledintopolarregions,andthoseofintermediatevolatilityaredepositedpredominantlyatmidlatitudes.Thisphenomeononhassomeimportantimplicationsregardingtheaccumulationofpersistentorganicpollutantsinenvironmentallyfragilepolarregionsfarfromindustrialsources.12.2.ORGANICCOMPOUNDSFROMNATURALSOURCESNaturalsourcesarethemostimportantcontributorsoforganicsintheatmosphere,andhydrocarbonsgeneratedandreleasedbyhumanactivitiesconsituteonlyabout1/7ofthetotalhydrocarbonsintheatmosphere.Thisratioisprimarilytheresultofthehugequantitiesofmethaneproducedbyanaerobicbacteriainthedecompositionoforganicmatterinwater,sediments,andsoil:2{CH2O}(bacterialaction)→CO2(g)+CH4(g)(12.2.1)Flatulentemissionsfromdomesticatedanimals,arisingfrombacterialdecompositionoffoodintheirdigestivetracts,addabout85millionmetrictonsofmethanetotheatmosphereeachyear.Anaerobicconditionsinintensivelycultivatedricefieldsproducelargeamountsofmethane,perhapsasmuchas100millionmetrictonsperyear.Methaneisanaturalconstituentoftheatmosphereandispresentatalevelofabout1.4partspermillion(ppm)inthetroposphere.Methaneinthetropospherecontributestothephotochemicalproductionofcarbonmonoxideandozone.Thephotochemicaloxidationofmethaneisamajorsourceofwatervaporinthestratosphere.Atmospherichydrocarbonsproducedbylivingsourcesarecalledbiogenichydrocarbons.Vegetationisthemostimportantnaturalsourceofnon-methanebiogeniccompounds.Acompilationoforganiccompoundsintheatmosphere3documentedatotalof367differentcompoundsthatarereleasedtotheatmospherefromvegetationsources.Othernaturalsourcesincludemicroorganisms,forestfires,animalwastes,andvolcanoes.Oneofthesimplestorganiccompoundsgivenoffbyplantsisethylene,C2H4.Thiscompoundisproducedbyavarietyofplantsandreleasedtotheatmosphereinitsroleasamessengerspeciesregulatingplantgrowth.Becauseofitsdoublebond,©2000CRCPressLLCethyleneishighlyreactivewithhydroxylradical,HO•,andwithoxidizingspeciesintheatmosphere.Ethylenefromvegetationsourcesshouldbeconsideredasanactiveparticipantinatmosphericchemicalprocesses.Mostofthehydrocarbonsemittedbyplantsareterpenes,whichconstitutealargeclassoforganiccompoundsfoundinessentialoils.Essentialoilsareobtainedwhenpartsofsometypesofplantsaresubjectedtosteamdistillation.Mostoftheplantsthatproduceterpenesareconifers(evergreentreesandshrubssuchaspineandcypress),plantsofthegenusMyrtus,andtreesandshrubsofthe