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4.01 Ozone, Hydroxyl Radical, and Oxidative Capacity

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4.01 Ozone, Hydroxyl Radical, and Oxidative Capacity 4.01 Ozone, Hydroxyl Radical,and OxidativeCapacity R.G.Prinn Massachusetts InstituteofTechnology,Cambridge, MA, USA 4.01.1 INTRODUCTION 1 4.01.2 EVOLUTIONOFOXIDIZING CAPABILITY 3 4.01.2.1 Prebiotic Atmosphere 4 4.01.2.2 Pre-industrialAtmosphere 5 4.01.3 FUNDAMENTAL REACTIONS 5 4.01.3.1 Troposphere 5 4.01.3.2 Stratosphere 7 4.01.4METEOROLOGICAL INFLUENCES 8 4.01.5HUMAN INFLUENCES 9 4.01.5.1 IndustrialRevolution 9 4.01.5.2 FutureProjections 10 4.01.6 MEASURING OXIDATION RATES 11 4.01.6.1 DirectMeasurement 11 4.01.6.2 IndirectMeasurement 13 4.01.7 ATMOSPHERIC MODELS AND OBSERVATIONS 16 4.01.8CONCLUSIONS 16 REFERENCES 17 4.01.1 INTRODUCTION overall rateofthisprocess asthe “oxidation capacity” ofthe atmosphere.Without thiseffi- The atmosphereisachemically complexand cient cleansingprocess,the levels ofmany dynamic systeminteractinginsignificant ways emitted gasescouldriseso high thattheywould withthe oceans,land, andlivingorganisms. A radicallychange the chemicalnatureofour keyprocess proceedinginthe atmosphereis atmosphereandbiosphereand, through the oxidation ofawidevariety ofrelatively reduced greenhouseeffect,our climate. chemicalcompoundsproduced largely bythe Oxidation becameanimportant atmospheric biosphere.Thesecompoundsinclude hydrocar- reaction on Earthonce molecularoxygen(O 2 ) bons(RH),carbon monoxide (CO),sulfur dioxide from photosynthesishad reached sufficiently (SO2 ),nitrogenoxides(NOx ),andammonia high levels.ThisO 2 couldthenphotodissociate (NH3 )amongothers. Theyalso include gases inthe atmosphereto giveoxygenatoms,which associated withadvancedtechnologiessuch as combinewithO2 to form ozone(O 3 ). WhenO 3 hydrofluoro carbonsandhydrochlorofluoro- absorbsUV lightatwavelengthsless than carbons. Asummary ofthe composition ofthe 310 nm,itproducesexcited oxygenatoms atmosphereatthe start ofthe twenty-first century (O( 1 D)) which canattack watervaporto isgiveninTable1. Dueto theirkeyrole, produce the hydroxyl free radical(OH). Itis oxidation reactions aresometimesreferred to as the hydroxyl radicalthat,aboveall,defines nature’s atmospheric “cleansing”process,andthe the oxidativecapacityofour O 2 -and 1 2 Ozone, Hydroxyl Radical,andOxidativeCapacity Table1 Gaseous chemicalcomposition ofthe atmosphere(1 ppt 102 12; 1ppb 102 9 ; 1ppm 102 6 ). ¼ ¼ ¼ Constituent ChemicalformulaMolefraction indry airMajor sources NitrogenN 2 78.084%Biological OxygenO 2 20.948%Biological Argon Ar0.934%Inert Carbon dioxide CO2 360 ppm Combustion,ocean,biosphere Neon Ne 18.18ppm Inert Helium He 5.24ppm Inert MethaneCH 4 1.7 ppm Biogenic, anthropogenic HydrogenH 2 0.55 ppm Biogenic, anthropogenic, photochemical Nitrous oxide N 2 O0.31 ppm Biogenic, anthropogenic Carbon monoxide CO 50–200 ppbPhotochemical,anthropogenic Ozone(troposphere)O 3 10–500 ppbPhotochemical Ozone(stratosphere)O 3 0.5–10 ppm Photochemical NMHC C x H y 5–20 ppbBiogenic, anthropogenic Chlorofluorocarbon 12 CF2 Cl 2 540ppt Anthropogenic Chlorofluorocarbon 11 CFCl 3 265ppt Anthropogenic Methylchloroform CH3 CCl 3 65ppt Anthropogenic Carbon tetrachloride CCl 4 98ppt Anthropogenic NitrogenoxidesNOx 10 ppt–1ppm Soils,lightning, anthropogenic Ammonia NH3 10 ppt–1ppbBiogenic Hydroxyl radicalOH 0.05ppt Photochemical Hydroperoxyl radicalHO2 2ppt Photochemical Hydrogenperoxide H 2 O 2 0.1–10 ppbPhotochemical Formaldehyde CH2 O0.1–1ppbPhotochemical Sulfur dioxide SO2 10 ppt–1ppbPhotochemical,volcanic, anthropogenic Dimethyl sulfide CH3 SCH3 10–100 ppt Biogenic Carbon disulfide CS2 1–300 ppt Biogenic, anthropogenic Carbonyl sulfide OCS 500 ppt Biogenic, volcanic, anthropogenic Hydrogensulfide H 2 S5–500 ppt Biogenic, volcanic Source: Brasseur etal. (1999) andPrinn etal. (2000). H 2 O-rich atmosphere(Levy,1971; Chameides Table2 Globalturnoveroftropospheric trace gases andWalker,1973; Ehhalt,1999; Lelieveld etal., andthe fraction removed byreaction withOHaccording 1999; Prather etal.,2001). Asofearly 2000s, to Ehhalt (1999).The meanglobalOH concentration 6 its globalaverage concentration isonly , 106 wastakenas1 10 cm 2 3 (Prinn etal.,1995) 2 3 14 £ 1Tg 1012g . radicals cm or , 6parts in10 bymolein ð ¼ Þ the troposphere(Prinn etal.,2001). But its Trace gasGlobal Removal Removal influence isenormous andits lifecycle emission rate byOH byOH complex. For example, itreacts withCO, (Tg yr2 1 ) (%) (Tg yr2 1 ) usually within , 1s,to produce CO2 (andalso ahydrogenatom,which quickly combineswith CO 2,800 85 2,380 CH4 530 90 477 O 2 to form hydroperoxyfree radicals,HO 2 ). C 2 H 6 20 90 18 Anotherkeyplayerisnitric oxide (NO),which Isoprene570 90 513 canbe producedbylightning, combustion,and Terpenes1405070 nitrogenmicrobes,andundoubtedly became NO2 1505075 present atsignificant levels inthe Earth’s SO2 300 30 90 atmosphereassoon asO 2 andN2 becamethe (CH3 ) 2 S30 90 27 dominant atmospheric constituents. NO can reactwitheitherO 3 or HO2 to form NO2 . The reaction ofNOwithHO 2 isspecialbecause itregeneratesOH givingtwomajor sources The importance ofOHasaremovalmechanism 1 (O D H 2 O 2OH andNO HO2 NO2 for atmospheric trace gasesisillustrated inTable2, ð Þþ ! þ ! þ OH)for thiskeycleansingradical. TheNO 2 whichshows the globalemissions ofeach gasand isalso important becauseitphotodissociates the approximatepercentageofeachofthese eveninvioletwavelengthstoproduce oxygen emitted gaseswhich isdestroyed byreaction with atoms andthus O 3 .Hence, therearealso two OH (Ehhalt,1999). major sourcesofO3 (photodissociation ofboth Evidently,OHremovesatotalof , 3 : 65 15 £ O 2 andNO 2 ). 10 gofthe listed gaseseach year,or an Evolution ofOxidizingCapability 3 amount equaltothe totalmass ofthe substantiallyovergeologicalandevenrecent atmosphereevery 1 : 4 106 yr: Without OH times. But becausenaturaloranthropogenic our atmospherewouldh£ aveavery different combustion ofbiomass or fossilfuelisaprimary composition,asitwouldbedominated bythose source ofgasesdrivingthe oxidation processes, gaseswhich arepresently trace gases. andbecauseCOandNOarebothproducts of BecausebothUVradiation fluxesandwater combustion with(usually) oppositeeffects on OH vapor concentrations arelargest inthe tropics levels,the systemisnot asunstableasitcould andinthe southern hemisphere, the levels of otherwisebe. OH generally havetheirmaximainthe lower BecauseO3 isapowerful infrared absorberand troposphereintheseregionsandseasons emitter(a“greenhouse”gas),andbecausemany (Figure1). Also,becausemuch O 3 isproduced othergreenhousegases,notably CH4 ,are inandexported frompolluted areas,its destroyed principally byreaction withOH, there concentrations generally havemaximainthe aresignificant connections betweenatmospheric northern hemispheremid-latitude summer oxidation processesandclimate.Addedto these (Figure1). greenhousegasrelated connections isthe factthat Evenfrom the very brief discussion above, itis the first stepinthe production ofaerosolsfrom clearthatthe levels ofOHinthe atmosphereare gaseousSO2 ,NO 2 ,andhydrocarbons isalmost not expected to remainsteadybut to change always the reaction withOH.Aerosols playa rapidly on awide variety ofspace andtimescales. keyroleinclimatethrough absorption and/or Inthe loweratmosphere, OH sourcescanbe reflection ofsolarradiation. decreased or turned off byloweringUVradiation Theseoxidation processesalso dominateinthe (nighttime, winter,increasingcloudiness, chemistry ofairpollution,wherethe occurrence of thickeningstratospheric O 3 layer),loweringNO harmful levels ofO3 andacidic aerosols depends emissions,andloweringH2 O(e.g.,inacooling on the relativeandabsolutelevels ofurban climate). Its sinkscanbe increased byincreasing emissions ofNO, CO, RH, andSO 2 .Inthe marine emissions ofreducedgases(CO, RH, SO2 ,etc.). andpolartroposphere, reactivecompoundsof Hence, OH levels areexpected to havechanged chlorine, bromine, andiodineprovideasmall but significant additionalpathwayfor oxidation besidesOH,asdiscussed byvon Glasow and Crutzen(see Chapter4.02). 200 120 Thischapterreviews the possibleevolution of 60 0 4 80 oxidizingcapacity overgeologic time, the funda- ) 400 60 mentalchemicalreactions involved,andthe Pa H influencesofmeteorologyandhumanactivity on e( 600 atmospheric oxidation. The measurement of ssur e oxidation rates(i.e.,oxidativecapacity),andthe r P complexinteractivemodels ofchemistry,trans- 800 4 30 40 0 port,andhumanandotherbiospheric activities thatarenow beingdeveloped to help understand 1,000 atmospheric oxidation asakeyprocess inthe (a) 90˚ S60˚ S30˚ S 0˚ 30˚ N60˚ N 90˚ N Earthsystemarealsodiscussed. 200 0 5 4.01.2 EVOLUTION OF OXIDIZING 1 CAPABILITY 400 ) Adiscussion ofthe evolution ofO3 andOH Pa H overgeologictimeneedstobe placed inthe e( 600 20 context ofthe evolution ofthe Earthsystem ssur e r (see Table3). TheEarthaccreted between4.5and P 800 15 4.6 billion years (Gyr)beforepresent (BP). 1 5 0 Enormous inputsofkineticenergy,ice, and 1,000 hydrated minerals duringaccretion wouldhave 90˚ S60˚ S30˚ S0˚ 30˚ N60˚ N 90˚ N led to amassiveHO(steam) atmospherewith (b) 2 Latitude extremely high surface temperatures(Lange and Figure1 (a)Ozonemolefractions (inppb)and Ahrens,1982). Thisearliest atmospheremayalso (b)hydroxyl radicalconcentrations (in10 5 radicals havebeenpartly or largely removed bythe cm 2 3 )asfunctions oflatitude andpressure(inhPa) subsequent Moon-forminggiant impact. Dueto from amodelofthe atmosphereinthe 1990s (source the apparent obliteration ofrocksmuch older WangandJacob, 1998). than4Gyr on Earth, very littleisknowndirectly 4 Ozone, Hydroxyl Radical,andOxidativeCapacity Table3 Schematic showingtimeinbillions ofyears beforepresent,major erasandassociated biospheric andgeospheric evolutionary events,andhypothesized
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