1492Ch09

Manahan,StanleyE.THEATMOSPHEREANDATMOSPHERICCHEMISTRYEnvironmentalChemistryBocaRaton:CRCPressLLC,20009THEATMOSPHEREANDATMOSPHERICCHEMISTRY____________________________________________________9.1.THEATMOSPHEREANDATMOSPHERICCHEMISTRYTheatmosphereconsistsofthethinlayerofmixedgasescoveringtheearth’ssurface.Exclusiveofwater,atmosphericairis78.1%(byvolume)nitrogen,21.0%oxygen,0.9%argon,and0.03%carbondioxide.Normally,aircontains1-3%watervaporbyvolume.Inaddition,aircontainsalargevarietyoftracelevelgasesatlevelsbelow0.002%,includingneon,helium,methane,krypton,nitrousoxide,hydrogen,xenon,sulfurdioxide,ozone,nitrogendioxide,ammonia,andcarbonmonoxide.Theatmosphereisdividedintoseverallayersonthebasisoftemperature.Ofthese,themostsignificantarethetroposphereextendinginaltitudefromtheearth’ssurfacetoapproximately11kilometers(km),andthestratospherefromabout11kmtoapproximately50km.Thetemperatureofthetroposphererangesfromanaverageof15˚Catsealeveltoanaverageof-56˚Catitsupperboundary.Theaveragetemperatureofthestratosphereincreasesfrom-56˚Catitsboundarywiththetroposphereto–2˚Catitsupperboundary.Thereasonforthisincreaseisabsorptionofsolarultravioletenergybyozone(O3)inthestratosphere.VariousaspectsoftheenvironmentalchemistryoftheatmospherearediscussedinChapters9–14.Themostsignificantfeatureofatmosphericchemistryistheoccurrenceofphotochemicalreactionsresultingfromtheabsorptionbymoleculesoflightphotons,designatedhν.(Theenergy,E,ofaphotonofvisibleorultravioletlightisgivenbytheequation,E=hν,wherehisPlanck’sconstantandνisthefrequencyoflight,whichisinverselyproportionaltoitswavelength.Ultravioletradiationhasahigherfrequencythanvisiblelightandis,therefore,moreenergeticandmorelikelytobreakchemicalbondsinmoleculesthatabsorbit.)Oneofthemostsignificantphotochemicalreactionsistheoneresponsibleforthepresenceofozoneinthestratosphere(seeabove),whichisinitiatedwhenO2absorbshighlyenergeticultravioletradiationinthewavelengthrangesof135-176nanometers(nm)and240-260nminthestratosphere:©2000CRCPressLLCO2+hν→O+O(2.3.1)TheoxygenatomsproducedbythephotochemicaldissociationofO2reactwithoxygenmoleculestoproduceozone,O3,O+O2+M→O3+M(2.3.2)whereMisathirdbody,suchasamoleculeofN2,whichabsorbsexcessenergyfromthereaction.Theozonethatisformedisveryeffectiveinabsorbingultravioletradiationinthe220-330nmwavelengthrange,whichcausesthetemperatureincreaseobservedinthestratosphere.Theozoneservesasaveryvaluablefiltertoremoveultravioletradiationfromthesun’srays.Ifthisradiationreachedtheearth’ssurface,itwouldcauseskincancerandotherdamagetolivingorganisms.GaseousOxidesintheAtmosphereOxidesofcarbon,sulfur,andnitrogenareimportantconstituentsoftheatmosphereandarepollutantsathigherlevels.Ofthese,carbondioxide,CO2,isthemostabundant.Itisanaturalatmosphericconstituent,anditisrequiredforplantgrowth.However,thelevelofcarbondioxideintheatmosphere,nowatabout360partspermillion(ppm)byvolume,isincreasingbyabout1ppmperyear.AsdiscussedinChapter14,thisincrease.inatmosphericCO2maywellcausegeneralatmosphericwarming—the“greenhouseeffect,”withpotentiallyveryseriousconsequencesfortheglobalatmosphereandforlifeonearth.Thoughnotaglobalthreat,carbonmonoxide,CO,canbeaserioushealththreatbecauseitpreventsbloodfromtransportingoxygentobodytissues.Thetwomostseriousnitrogenoxideairpollutantsarenitricoxide,NO,andnitrogendioxide,NO2,collectivelydenotedas“NOx.”ThesetendtoentertheatmosphereasNO,andphotochemicalprocessesintheatmospherecanconvertNOtoNO2.Furtherreactionscanresultintheformationofcorrosivenitratesaltsornitricacid,HNO3.Nitrogendioxideisparticularlysignificantinatmosphericchemistrybecauseofitsphotochemicaldissociationbylightwithawavelengthlessthan430nmtoproducehighlyreactiveOatoms.Thisisthefirststepintheformationofphotochemicalsmog(seebelow).Sulfurdioxide,SO2,isareactionproductofthecombustionofsulfur-containingfuelssuchashigh-sulfurcoal.Partofthissulfurdioxideisconvertedintheatmospheretosulfuricacid,H2SO4,normallythepredominantcontributortoacidprecipitation.HydrocarbonsandPhotochemicalSmogThemostabundanthydrocarbonintheatmosphereismethane,CH4,releasedfromundergroundsourcesasnaturalgasandproducedbythefermentationoforganicmatter.Methaneisoneoftheleastreactiveatmospherichydrocarbonsandisproducedbydiffusesources,sothatitsparticipationintheformationofpollutantphotochemicalreactionproductsisminimal.Themostsignificantatmosphericpollutanthydrocarbonsarethereactiveonesproducedasautomobileexhaustemissions.InthepresenceofNO,underconditionsoftemperatureinversion(see©2000CRCPressLLCChapter11),lowhumidity,andsunlight,thesehydrocarbonsproduceundesirablephotochemicalsmogmanifestedbythepresenceofvisibility-obscuringparticulatematter,oxidantssuchasozone,andnoxiousorganicspeciessuchasaldehydes.ParticulateMatterParticlesrangingfromaggregatesofafewmoleculestopiecesofdustreadilyvisibletothenakedeyearecommonlyfoundintheatmosphereandarediscussedindetailinChapter10.Someatmosphericparticles,suchasseasaltformedbytheevaporationofwaterfromdropletsofseaspray,arenaturalandevenbeneficialatmosphericconstituents.Verysmallparticlescalledc