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Chapter 11.pdf

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Department
Chemistry
Course
CHEM 302
Professor
Michael Wheeler
Semester
Fall

Description
199CHAPTER 11OXIDIZINGPOWEROFTHETROPOSPHERETheatmosphereisanoxidizingmediumManyenvironmentallyimportanttracegasesareremovedfromtheatmospheremainlybytoxiccombustiongasesoxidationgreenhousegasessuchasCH4suchasCOagentsforstratosphericOdepletionsuchasHCFCs3andothersOxidationinthetroposphereisofkeyimportancebecausethetropospherecontainsthebulkofatmosphericmass85seesection23andbecausegasesaregenerallyemittedatthe surfaceandThemostabundantoxidantsintheEarthsatmosphereareO2OTheseoxidantshavelargebondenergiesandarehence3relativelyunreactiveexcepttowardradicalsOonlytowardhighly2unstableradicalsWithafewexceptionsoxidationofnonradicalorOisnegligiblyslowWorkintheatmosphericspeciesbyO231950sfirstidentifiedtheOHradicalasastrongoxidantinthestratosphereOHreactsrapidlywithmostreducednonradicalspeciesandisparticularlyreactivetowardHcontainingmoleculesOProductionduetoHabstractionreactionsconvertingOHtoH21D section 1021of OH is by reaction of water vapor with O1R1hnOODO321MOMR2OD1O2OHOD HR32Wesawinchapter10howOHoxidizesanumberoftracegasesinofOHthestratosphereAsimpleexpressionforthesourcePOHfromreactionsR1R3canbeobtainedbyassumingsteadystate1forODLaboratorystudiesshowthatR2ismuchfasterthanOmixingratiosfoundintheatmosphereallowingforR3attheH2simplification2kk113PO2kODHOHOOH3232kMkHO2321112kk13OHO32kM22001CriticaltothegenerationofOHistheproductionofODatoms1byR1Until1970itwasassumedthatproductionofODwouldbenegligibleinthetropospherebecauseofneartotalabsorptionofcolumnoverheadItwasthoughtthatUVradiationbytheO3oxidationofspeciesemittedfromtheEarthssurfacesuchasCOrequiredtransporttothestratospherefollowedbyandCH4reaction with OH in the stratosphereHR4COOHCO2OHCHHOCHR5432ThismechanismimpliedlongatmosphericlifetimesforCOandbecauseairtakesonaverage510yearstotravelfromtheCH4tropospheretothestratospheresection444andthestratosphereaccountsforonly15oftotalatmosphericmassInthe1960sconcernemergedthataccumulationofCOemittedbyfossilfuelcombustion would soon represent a global air pollution problem111THE HYDROXYL RADICAL1111 Tropospheric production of OHAmajordiscoveryintheearly1970swasthatsufficientOHisinfactproducedinthetropospherebyreactionsR1R3toallowforwithinthetroposphereoxidationofspeciessuchasCOandCH4AcalculationoftherateconstantforR1atsealevelisshowninFigure111astheproductofthesolaractinicfluxtheabsorption1crosssectionforOandtheODquantumyieldTropospheric31DtakesplaceinanarrowwavelengthbandproductionofObetween300and320nmradiationofshorterwavelengthsdoesnotpenetrateintothetropospherewhileradiationoflongerAlthoughtheproductionofwavelengthsisnotabsorbedbyO31DinthetroposphereisconsiderablyslowerthanintheOstratospherethisiscompensatedintermsofOHproductionbythe23Omixingratiosinthetroposphere1010timeshigherlargerH2thaninthestratosphereModelcalculationsinthe1970saccountingforthepenetrationofUVradiationat300320nmfound63moleculescmtroposphericOHconcentrationsoftheorderof10resultinginatroposphericlifetimeforCOofonlyafewmonthsandallayingconcernsthatCOcouldaccumulatetotoxiclevelsCrudemeasurementsofOHconcentrationsinthe1970sconfirmed
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