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

h Pa influencesofmeteorologyandhumanactivity on 600 atmospheric oxidation. The measurement of oxidation rates(i.e.,oxidativecapacity),andthe P r e ssur e( complexinteractivemodels ofchemistry,trans- 800 4 0 30 40 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 5 4.01.2 EVOLUTION OF OXIDIZING 1 0 CAPABILITY 400 ) Adiscussion ofthe evolution ofO3 andOH h Pa overgeologictimeneedstobe placed inthe 600 20 context ofthe evolution ofthe Earthsystem (see Table3). TheEarthaccreted between4.5and P r e ssur e( 800 15 4.6 billion years (Gyr)beforepresent (BP). 1 0 5 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 atmospheric composition. Gasesarelisted inorderof theirhypothesized concentrations andthosewhosesourcesand/or sinksaredominated bybiologicalprocessesare denoted withanasterisk.Notethe transition from anabiotic to anessentially biologically controlled atmosphereover geologic time. TimeMajor evolutionary events Hypothesized atmospheric compositions (Gyr BP)

Earthaccretesandmassive [H2 O, CO2 ,CO, N 2 ,H2 ] steamatmosphereforms Surface cools,volcanism begins? Oceanforms,continentalweathering, [CO2 ,CO, N 2 ,H2 O, H 2 ] andcarbonateprecipitation begin 4Archeanerabegins,oldest rocks Lateheavy bombardment Sediment subduction andvolatile [N2 ,CO 2 ,CO, H 2 O, H 2 ,CH 4 ,HCN, NOx ] recyclingbegins? First livingorganisms evolve, prokaryotes 3First protosynthetic organisms evolve: [N2 ,H2 O, CO2 ,*O 2 ,CH 4 ] cyanobacteria, stromatolites Proterozoic erabegins:scarce fossils 2Aerobic biospherewithnitrogen [*N2 ,*O 2 ,H2 O, *CO2 ,*CH4 ,*N 2 O] andmethanemicrobesevolves 1Eucaryotesevolve Metazoans appear Invertebratesappear Paleozoic erabegins,ubiquitous fossils Fishesappear[*N2 ,*O 2 ,H2 O, *CO2 ,*CH4 ,*N 2 O] Landplants appear Amphibians appear Mesozoic (reptilian) erabegins Fission ofPangeansupercontinent Cretaceous–Tertiary collision Cenozoic (mammalian) erabegins 0Present [*N2 ,*O 2 ,H2 O, *CO2 ,*CH4 ,*N 2 O, *CF2 Cl 2 ,etc.]

about the Earth’s earliest atmosphere.Itishypo- 4.01.2.1 Prebiotic Atmosphere thesized that,asthe Earthcooled, the massive Beforethe evolution ofthe photosynthetic H 2 Oatmospherecondensed formingoceans source ofO2 ,the atmospherewasrelatively andamuch less massiveCO 2 /CO/N 2 /H 2 O/H 2 atmospherethenevolved. chemically neutralorevenreducing.Its reduction The next big evolutionary step,involvingthe statewouldhavedepended significantly on the aqueousweatheringofrocksandremovalof quantitiesofreduced gasesproducedfrom volca- nic sources(e.g.,sourcesinwhich primordialiron, CO2 ascarbonates,had occurred , 3.8Gyr ago (Holland, 1984). Thus,the early Archaeanatmos- sulfur,carbon,andorganic compoundswere thermally oxidized byH OandCO producing pherewasprobably largely dominated byN 2 with 2 2 H ,CO, andCH (Prinn,1982; LewisandPrinn, smalleramounts ofCO 2 ,CO, H 2 O, H 2 ,CH 4 , 2 4 HCN, andNO x (from lightning). The evolution of 1984)). Intheseprebiotic atmospheres,small the first life forms (procaryotes),andspecifically amountsofO2 canbe produced fromH 2 O photosynthetic cyanobacteria, wouldhave photodissociation (Levine, 1985),andsmall allowed the slow accumulation ofbiologically quantitiesofNOcanbe formed byshock heating, produced O 2 inthe atmosphereinthe late bylightning, andlarge collisions (Chameides Archaeanperiod.Similarly,the early evolution andWalker,1981; Prinn andFegley,1987; ofmethaneandnitrogenbacteria alongwiththe Fegley etal.,1986). evolution ofthe aerobic biosphereled, apparently However,itisreasonableto conclude thatthe early inthe Proterozoic,to anN 2 /O 2 /H 2 O/CO2 / evolution ofthe powerful oxidizingprocesses CH4 /N 2 Oatmospherewithall ofthesegases,with involvingO3 andOHinthe present atmosphere the exception ofH2 O, beinglargely controlled by had to awaitthe evolution ofphotosynthetic O 2 . biologicalsourcesandbiologicaloratmospheric Beforethat,atmospheric chemicaltransformations chemicalsinks. wereprobably drivenbysimplephotodissociation FundamentalReactions 5 anddissolution reactions andbyepisodic pro- indicatingCOmolefractionsof , 60 102 9 cesseslike lightningandcollisions. The trans- (Antarctica)and , 90 102 9 (Greenland),£ which formation fromreducing/neutraltooxidizing arenotmuch less thant£ hoseobserved today. As atmospherewasitselfalong-term competition wewill discuss inSection 4.01.3.1,oxidation of betweenabiotic sourcesofreducedgases,anaero- each CO moleculeoughttoproduce oneO3 bic microbialprocesses,andfinally the aerobic molecule, so thatpresumingOHlevels inthe biologicalprocesseswhich cameto dominatethe 1880s to be similartothoseinthe 1990s implies biosphere.Once the landbiosphereevolved, the similarO 3 production ratesfrom CO. burningofvegetation ignited bylightning provided anadditionalsource oftrace gases (see Chapter4.05). 4.01.3 FUNDAMENTAL REACTIONS 4.01.3.1 Troposphere 4.01.2.2 Pre-industrialAtmosphere Aswenoted inSection 4.01.1,the ability ofthe We expectthatOH andO3 changedevenunder troposphereto chemically transform andremove the approximately constant levels ofatmospheric trace gasesdependsoncomplexchemistrydriven O 2 inthe Cenozoic era.For example, the glacial– bythe relatively small flux ofenergetic solarUV interglacialcyclesshouldhavesignificantly radiation thatpenetratesthrough the stratospheric changedatmospheric H 2 O(inferred to be low in O 3 layer(Levy,1971; ChameidesandWalker, ice ages) andCH 4 (observed to be low inice 1973; Crutzen,1979; Ehhalt etal.,1991; ages).But theseparticularchangeswouldhave Logan etal.,1981; Ehhalt,1999; Crutzenand partially offsettingeffects on OH (bylowering Zimmerman,1991). Thischemistryisalsodriven bothOHproduction andOHremovalratesinice byemissions ofNO, CO, andhydrocarbonsand ages). Andunfortunately,the lackofdirect leadstothe production ofO3 ,which isoneofthe information about the abundance ofthe key important indicatorsofthe oxidizingpowerof short-lived gases(CO, NOx ,nonmethanehydro- the atmosphere.But the most important oxidizeris carbons (NMHC),andstratospheric O 3 )in the hydroxyl free radical(OH),andakeymeasure paleoenvironmentsmake itdifficult to be quan- ofthe capacity ofthe atmosphereto oxidizetrace titativeaboutpast OH changes(Thompson, gasesinjected into itisthe localconcentration of 1992). Oneobservationalrecordoverthe past hydroxyl radicals. millennium which isuseful inthisrespectis Figure2summarizes,withmuch simplification, the analysisofhydrogenperoxide (H 2 O 2 )in the chemistry involved inproduction andremoval Greenlandpermafrost,which indicatesstable ofO3 andOH(Prinn,1994). Thischemistry is H 2 O 2 from the years 1300 to 1700 followed by remarkablebecauseitinvolvescompounds a50% increasethrough to 1989(however,most thatalso influence climate, airpollution,and ofthe increasehasoccurred since 1980andisnot acid rain. seeninAntarctic permafrost(Sigg andNeftel, The greenhousegasesinvolved hereinclude 1991; AnklinandBales,1997)). H 2 O, CH4 ,andO3 .Primary pollutants emitted IncreasesinH 2 O 2 duringindustrialization mainly asaresult ofhumanactivity (including couldsignaladecreaseinOH dueto net fossil-fuelcombustion,biomass burning, and conversion ofHO 2 to H 2 O 2 ratherthanrecycling land-use)include RH, CO, andnitrogenoxides ofHO 2 back to OH.Staffelbach etal. (1991) (NO, NO2 ). Reactivefree radicals or atoms arein haveused CH2 Omeasurements inGreenlandice two categories:the very reactivespecies,such as corestosuggest thatpre-industrialOH levels O( 1 D)andOH, andthe less reactiveones,such as 3 were30% higherthanthe levels inthe late HO2 ,O( P),NO, andNO 2 . twentiethcentury. Indeed, avariety ofmodel The atmospheric cleansingroleofOHisvery studieswhich assumelow pre-industrialemis- evident inFigure2. WhenOH reacts withanRH, sionsofCO, NOx ,andNMHC compared to the the latterisoxidized inaseriesofsteps mostly to levels inthe twentiethcentury suggest thatthe OH CO.Thesesteps consumeOHbut mayalso levels inthe years 1200–1800 were , 10–30% produce HO2 .Inturn,OHoxidizesCO to CO2 , higherthanthe levels atthe endofthe twentieth anditalso oxidizesthe gasesNO2 andSO 2 to century(Thompson,1992).Ozonelevels in nitric (HNO3 )andsulfuric (H 2 SO4 )acids,respect- the 1880s canbe inferred fromiodometric ively. The primary OH source involveswater measurements (VolzandKley,1988;Marenco vapor which reacts withthe very reactivesinglet etal.,1994)indicatingmolefractions aslow as oxygenatom,O( 1 D),thatcomesfrom photo- 2 9 7 – 12 10 ; which arelowerbyafactor of3 dissociation ofO3 bysolarUV radiation at orð morÞ ef£ rom thoseobserved inthe 1990s. Such wavelengthsless than310 nm.Typically,OH low levels ofO3 aredifficult to reconcilewithCO withinasecondofits formation oxidizesother measurements inice cores(Haan etal.,1996) compoundseitherbydonatingoxygenorby 6 Ozone, Hydroxyl Radical,andOxidativeCapacity

Stratosphere

UV O 2 NO O 3 2 OH UV NO O HNO 2 3 H 2 SO4 O HO 1 2 2 O( D) OH OH N 2 O Lightning CFCs

OH OH CO2 H 2 O RH CO CO2 SO2 (CH3 ) 2 S

Hydrosphere Biosphere Hydrosphere

Greenhousegases Reactivefree radical/atom Primary pollutants Less reactiveradicals Naturalbiogenicspecies Reflectiveaerosols

Figure2 Schematic showingprincipleoxidation processesinthe troposphereinNOx -rich air(afterPrinn, 1994). InNOx -poor air(e.g.,remotemarineair),recyclingofHO 2 to OH isachieved byreactions ofO3 withHO 2 or byconversion of2HO2 to H 2 O 2 followed byphotodissociation ofH2 O 2 .Inamorecomplete schematic, nonmethanehydrocarbons (RH)wouldalso reactwithOHto form acids,aldehydesandketonesin addition to CO.

removinghydrogenleavinganHatom or organic catalyzed reactions creatingOHandO3 inthe free radical(R). ThenRandHattachrapidly to troposphereareshown inEquations (1)–(3), molecularoxygentoform hydroperoxy radicals (4)–(9),and(10)–(15): (HO2 )ororganoperoxy radicals (RO2 ),which are O UV O O 1 D 1 relatively unreactive.If thereisnoefficient wayto 3 þ ! 2 þ ð ÞðÞ rapidly recycleHO 2 back to OH, thenlevels of OH arekept low. 1 O D H 2 O 2OH 2 Addingthe nitrogenoxides(NO andNO 2 ), ð Þþ ! ð Þ which havemany human-related sources,sig- –––––––––––––––––––––––––––––––––––––––– nificantly changesthispicture.Specifically,NO Neteffect : O 3 H 2 O O 2 2OH 3 reacts withthe HO2 to form NO2 ,andreform þ ! þ ð Þ OH.UV radiation thendecomposesNO2 at and wavelengthsless than430 nm to form O 3 and reform NO, so the nitrogenoxidesarenot OH CO H CO2 4 consumed inthisreaction. Inastudyof þ ! þ ð Þ relatively polluted airinGermany,Ehhalt H O HO 5 (1999) estimated thatthe production rateof þ 2 ! 2 ð Þ OH bythissecondary pathinvolvingNOis5.3 HO NO OH NO 6 timesfasterthanthe aboveprimary path 2 þ ! þ 2 ð Þ involvingO( 1 D). Notethatthe reaction ofNO withHO 2 doesnot actasasinkfor the so-called NO UV NO O 7 2 þ ! þ ð Þ odd hydrogen(the sum ofthe H, OH, andHO 2 concentrations). Rather,itdeterminesthe ratioof O O 2 O 3 8 OH to HO2 .Inthe Ehhalt (1999) study,the þ ! ð Þ [HO2 ]:[OH] ratiowas44 :1. Thisisdueto,and –––––––––––––––––––––––––––––––––––––––– indicativeof, the much greaterreactivity ofOH Neteffect : CO 2O CO O 9 compared to HO2 .Tosummarize, the principal þ 2 ! 2 þ 3 ð Þ FundamentalReactions 7 and Also,ifthe temperaturesofoceans areincreas- ing, weexpectincreased watervapor inthe lower OH RH R H O 10 þ ! þ 2 ð Þ troposphere.Becausewatervapor ispart ofthe primary source ofOH, climatewarmingalso R O RO 11 þ 2 ! 2 ð Þ increasesOH.Thisincreasecouldbelowered or raised dueto changesincloudcoveraccompanying RO2 NO RO NO2 12 the warmingleadingto moreor less reflection of þ ! þ ð Þ UV back towardspace.Risingtemperaturealso NO2 UV NO O 13 increasesthe rateofreaction ofCH 4 withOH, þ ! þ ð Þ thus loweringthe lifetimesofbothchemicals. O O 2 O 3 14 The oppositeconclusions applyif trace gas þ ! ð Þ emissions increaseor the climatecools. Finally, –––––––––––––––––––––––––––––––––––––––– decreasingstratospheric O 3 canalsoincrease tropospheric OH. Neteffect : OH RH 2O RO H O O þ þ 2 ! þ 2 þ 3 15 ð Þ Intheoreticalmodels,the globalproduction of 4.01.3.2 Stratosphere tropospheric O 3 bythe abovepathwaybeginning withCOistypicallyabout twicethatbeginning The O 3 inthe stratosphereismaintained bya withRH.For mostenvironments,itisthese distinctively different setofchemicalreactions overallcatalytic processesthatpumpthe thanthe troposphere.The flow diagramin majority ofthe OH andO3 into the system. If Figure2showshow the drivingchemicals like the concentration ofNO 2 becomestoo high, watervapor,chlorofluorocarbons,andnitrous however,its reaction withOHto form HNO3 oxide, aretransported up from the troposphere ultimately limits the OH concentration (see to the stratosphere, wheretheyproduce chlorine-, Section 4.01.4). The globalimpactofthe nitrogen-, andhydrogen-carryingfree radicals, abovetropospheric OH production mechanisms which destroyO 3 .Thereisaninteresting hasbeensummarized inTable2. Hough and paradox here: the nitrogenoxide free radicals, Derwent (1990) haveestimated the global which areresponsiblefor destroyingasignificant production rateoftropospheric O 3 bythe fraction ofthe O 3 inthe stratosphere, arethe 2 1 abovereactions to be 2,440Tg yr inpre- samefree radicals thatareproducingO3 inthe industrialtimesand5,130 Tg yr2 1 inthe 1980s tropospherethrough the chemistryoutlined withthe increasecaused byman-made NOx , earlier(Figure3). hydrocarbons,andCO. Whenthe ratioofnitrogenoxide free radicals to Anupdateto the above1980s calculation O 3 islow,asitisinthe stratosphere, the neteffect (Ehhalt,1999) givesaproduction rateof isfor O 3 to be destroyed overall,whileifthis 4,580Tg yr2 1 .Acomparison ofseveraltropo- ratioishigh, asitisinmuch ofthe troposphere, spheric O 3 models showed achemicalproduction thenO 3 isproducedoverall (Crutzen,1979). 2 1 raterange for O 3 of3,425–4,550Tg yr The stratosphereischemicallyvery activedue (Lelieveld etal.,1999). Since the troposphere to its high O 3 andUVlevels. But the densities contains , 350Tg ofO3 (Ehhalt,1999),the andusually temperaturesaremuch lowerthan chemicalreplenishment timefor O 3 (defined as inthe lowertroposphereso thatonly asmall the tropospheric content divided bythe tropo- fraction ofthe destruction ofthe gaseslisted spheric chemicalproduction rate)is28–37 d.This inTable2occursinthe stratosphere.The ismuch longerthanthe replenishment timefor stratospherecontains , 90% ofthe world’s O 3 2 1 OH, , 1s,andindicatesthe enormous reactivity of andexports , 400–846Tg yr ofO3 to the OH relativeto O 3. troposphereto augment the chemicalproduction How stableisthe oxidizingcapability ofthe discussed inSection 4.01.3.1 (Lelieveld etal., troposphere?How mightithavechanged over 1999). geologic timeevenafteroxygenreached near Thereisalso animportant linkbetweenstrato- today’s levels?If emissions ofgasesthatreact spheric chemistry andclimate.Theprecursor withOHsuch asCH4 ,CO, andSO 2 areincreasing gasesfor the destructivefree radicals inthe then,keepingeverythingelseconstant,OH stratospherearethemselvesgreenhousegases,as levels shoulddecrease.Conversely,increasing isthe stratospheric O 3 itself.Therefore, while NOx -emissions from combustion (ofbiomass in increasesinthe concentrations ofthe source the past (see Chapter4.05)augmented byfossil gaseswill increasethe radiativeforcingof fueltoday) shouldincreasetropospheric O 3 (and warming, theseincreaseswill,atthe same thus the primary source ofOH),aswell asincrease time, lead to decreasesinstratospheric O 3 ,thus the recyclingrateofHO 2 to OH (the secondary loweringthe radiativeforcing.Once again,asin source ofOH). the troposphere, thereareimportant feedbacks 8 Ozone, Hydroxyl Radical,andOxidativeCapacity

UV UV O 2 O 3 O O 2 O 2

HNO3 N 2 O UV ClONO2 CH4 etc. Cl OH NO HOCl CFCl 3 ClO HNO3 N 2 O 5 HO2 NO2 H 2 O CF2 Cl 2 (ClO) 2 H O 2 2 HCl H 2 O HCl

O 2 TROPOSPHERE

Source/GreenhousegasDestructiveatom/free radical Benign reservoirgas

Figure3 Schematic showingmajor chemicalprocessesformingandremovingO3 inthe stratosphere(afterPrinn, 1994). UV photodissociaton oflargely natural(N 2 O, CH4 ,H2 O)andexclusively man-made (CFCl 3 ,CF 2 Cl 2 ) source gasesleadstoreactiveHO x ,NO x ,andClO x free radicals which catalytically destroy O 3 .The catalysts reversibly form reservoircompounds(HNO3 ,etc.),someofwhich livelongenough to be transported down to the troposphere. betweenchemicalprocessesandclimatethrough 4.01.4METEOROLOGICAL INFLUENCES the various greenhousegases. Itisnot just man- made chemicals thatcanchange the stratospheric The oxidativecapability ofthe atmosphereis O .Changesinnaturalemissions ofN OandCH , not simplyafunction ofchemistry. Convective 3 2 4 stormscancarry short-lived trace chemicals from andchangesinclimatethatalterH 2 Oflows into the stratosphere, undoubtedly led to changesinthe the planetary boundary layer(the first fewhundred to fewthousandmeters) to the middleandupper thickness ofthe O 3 layerovergeologic timeeven troposphereinonly afewtoseveralhours.This afterO 2 levels reached levels nearthoseinthe early 2000s. caninfluence the chemistry oftheseupperlayers Thereisalso astronglinkbetweenthe thickness insignificant ways bydelivering, e.g.,reactive hydrocarbonstohigh altitudes. Conversely,the ofthe stratospheric O 3 layerandconcentrations of OH inthe troposphere.BecauseO( 1 D)canonly be occurrence ofvery stableconditions inthe boundary layercaneffectively trapchemicals producedfromO 3 atwavelengthsless than 310nm,andthe current O 3 layereffectively nearthe surface for many days,leadingto polluted absorbsall incomingUVatless than290 nm, air. Larger-scalecirculations serveto carry gases thereisonly avery narrowwindow of290– aroundlatitude circlesontimescalesofafew 310nmradiation thatisdrivingthe major weeks,betweenthe hemispheresontimescalesof oxidation processesinthe troposphere.Decrease ayear,andbetweenthe troposphereandstrato- inthe amount ofstratospheric O 3 ,which has sphereon timescalesofafewyears. occurred since about 1980,canthereforeincrease Aconvenient measureofthe stability ofa the tropospheric OH byincreasingthe flux of tracechemicalisits localchemicallifetime, radiation less than310 nm which reachesthe which isdefined asits localconcentration (e.g.,in troposphereto produce O( 1 D)(Madronich and mol L 2 1 )divided byits localremovalrate(e.g.,in Granier,1992). Bythe samearguments,ifthe mol L 2 1 s 2 1 ). Concentrations ofchemicals whose stratospheric O 3 layerwasvery much thickerin lifetimes(dueto chemicaldestruction or depo- the past,itwouldhaveremoved agreatdealofthe sition)arecomparableto,or longerthan,the above “cleansing”capacity ofthe Earth’s troposphere transporttimeswill be profoundly affected by leadingto much higherlevels ofgaseslike CO transport. Asasimpleexample, inasteady andCH 4 . (constant concentration) statewithahorizontal HumanInfluences 9 wind u ; the concentration i ofchemical i whose the oxidizingreactions inthe atmosphere.First, ½ Š lifetimeis t i decreasesdownwindofasource region lightningandthundershocksserveto thermally (located athorizontalposition x 0) accordingto convert atmospheric N 2 andO2 to 2NO, leadingto ¼ avery important naturalsource ofNO .Second, 2 x x i x i 0 exp the formation ofclouddroplets isaided by(and ½ Šð Þ¼½Šð Þ uti consumes) aerosols (ascloudcondensation  ð Þ  Specifically,the concentration of i decreases nuclei),andthe clouddroplets thenserveas sinksfor solublespecieslike H 2 O 2 ,HNO3 ,NO x , byafactor ofeoverthe distance scale uti : Note alsothat,ifthe transport time x = u ismuch less SO2 ,andH2 SO4 .For someoxidation reactions, e.g.,oxidation ofSO 2 to H 2 SO4 ,thisdissolution than t i ; the concentration remains essentially constant andthe speciescanbe considered to leadstoanadditionalnongaseous pathwayfor oxidation generallyconsidered comparableto be almost inert. For oxidation byOH ofNO 2 and SO to form the acidsHNO andHSO ,the time the gas-phaseprocessesfor SO2 .Finally,the 2 3 2 4 formation ofconvectiveclouds,andparticularly t istypically3d 2 : 6 105 s ; so the e-folding i ð £ Þ the extensiveanvils,leadstoreflection of distance uti isonly , 800 kmfor atypical u of 3ms 2 1 .Alternatively,for the oxidation ofCO UV radiation enhancingthe photo-oxidation 6 processesabovethe cloudandinhibitingthem byOH to form CO2 , t i 3months 7 : 9 10 s ; so ut 2 : 4 104 kmfor¼ the same u ð .In£asimplÞ e below (e.g.,WangandPrinn,2000). i ¼ £ waythisillustrateswhyNO2 andSO 2 arelocalor regionalpollutants,whereasCO isahemispheric pollutant. Inthe aboveexample, if the chemicallifetime 4.01.5HUMAN INFLUENCES t i ismuch less thanthe transport time x = u thenwe 4.01.5.1 IndustrialRevolution couldignoretransportandsimply consider chemicalsourcesandsinkstodetermineconcen- The advent ofindustrialization since 1850has trations. However,for the chemicals controlling led to significant increasesinthe globalemis- oxidation reactions,caremust be takenin sions ofNO x ,CO, NMHC, andCH 4 (Wangand defining t i : Specifically,sometimesmembers of Jacob, 1998;Prather etal.,2001).Biomass a“chemicalfamily” areconverted from oneto burningassociated withhumanlanduseprovides anotheronvery shorttimescalesrelativeto anadditionalsource ofthesetrace gases(see transport,whereasthe totalpopulation ofthe Chapter4.05). Also,the observed temperature family isproducedandremoved on longer increaseof0.6 ^ 0.2 8 Cinthistimeperiod timescales. (Folland etal.,2001)shouldhaveled to increases For example, inthe family composed ofNOand intropospheric H 2 Oandpossibly changesin NO2 (called the NOx family) the reactions shown globalcloudcover(ofunknownsign).Anthro- inEquations (16)–(18)rapidly interconvert NO pogenic aerosols,bothscatteringandabsorbing, andNO 2 on timescalesmuch less thantypical havealsoincreased dueto humanactivity transport timesanddosowithoutaffectingthe (Penner etal.,2001). Theseaerosol increases total NOx NO NO2 : shoulddecreaseUVfluxesbelow the cloudtops ½ Š¼½ Šþ½ Š dueto boththeirdirectopticaleffects (absorbing NO O NO O 16 þ 3 ! 2 þ 2 ð Þ or reflecting)andtheirindirecteffects as cloudcondensation nuclei, which shouldincrease NO h n NO O 17 the reflectivity ofwaterclouds. Finally,depletion 2 þ ! þ ð Þ ofstratospheric O 3 ,which hasoccurred dueto O O 2 M O 3 M 18 man-madehalocarbons,will increasetropo- þ þ ! þ ð Þ spheric UV irradiation (Madronich andGranier, 1992). NO2 OH M HNO3 M 19 þ þ ! þ ð Þ All ofthesechangesshouldhaveaffected O 3 However,the removalofNO x bythe reaction andOHconcentrations,sometimesoffsettingand showninEquation (19) ismuch slower(e.g., sometimesaugmentingeach other,asdiscussed , 3d). Thus,wecanusethe chemicalsteady- earlier.Table4summarizesthe expected stateapproximation (which equateschemical changesinOH andO3 relativeto pre-industrial production to chemicalloss only) to definethe times. Wherequantitativeestimatesarenot ratioofNOto NO2 .We musthowevertake full available, only the signs ofexpected changes accountoftransportincalculating NO : areindicated.Noteinparticularthatfor OH the ½ x Š Similarargumentsholdfor the “odd oxygen” positiveeffects ofNO x increaseshavebeenmore O O O and“odd hydrogen” thanoffsetbythe negativeeffects ofCO, ð½ x Š¼½ Šþ½ 3 ŠÞ HOx H OH HO2 families. NMHC, andCH 4 increases,so the netchange ð½ BesŠide¼½sbeiŠþ½ngaŠvþ½igorousŠÞverticaltransport inOH isamodest 10%decrease(Wang mechanism,moist convection also directly affects andJacob, 1998;see also Levy etal.,1997). 10 Ozone, Hydroxyl Radical,andOxidativeCapacity Table4 Effects ofanthropogenically drivenorobserved changesintrace gasemissions,concentrations,or meteorologicalvariablesbetweenpre-industrialandpresent timesonthe oxidizingcapability ofthe atmosphere (expressed whereavailableaspercentage changes(D )in[OH] or [O3 ]from pre-industrialtopresent).

Present/pre-industrial D [OH]/[OH] D [O3 ]/[O3 ] a NOx 4.7 36% 30% COa 3.2 2þ 32%b þ 10%b NMHCa 1.6 þ a CH4 2.1 2 17% 13% Combineda 2 10%c þ 63%c þ H 2 O(T) . 1 . 0 . 0 Clouds d . 1, , 1 , 0, . 0 , 0, . 0 Scat. aeroe . 1 , 0 , 0 Abs. aero f . 1 , 0 , 0 g Strat. O 3 , 1 . 2% . 0 a WangandJacob(1998). Individualeffectofeach specific chemicalcomputed insensitivity runs withpre-industrialconditions except for b c the specific chemicalwhich isatpresent levels. CO andNMHCeffects combined. CO, NMHC, CH4 ,andNO x effects combined. Totaleffectnot equaltosum ofindividualeffects dueto interactions. d Cloudcoverchangesunknown. Effects shown for eitheranincreaseor a decrease. e Specificallyaerosols withhigh singlescatteringalbedos (e.g.,sulfates) which reflectUV radiation back to space. f Specificallyaerosols withlow singlescatteringalbedos (e.g.,black carbon) which absorbUVradiation. g Madronich andGranier(1992) andKrol etal. (1998).

Table5 Percentagesbywhich eitheremissions ofNO x ,CO, NMHC (plus oxidized NMHC),andCH 4 ,or concentrations oftropospheric OH, or column amounts oftropospheric O 3 ,areprojected to change between2000 and 2100 for various IPCC scenarios.

ScenarioNOx CO NMHC CH4 OH O 3 (%) (%) (%) (%) (%) (%)a A1B 26 90 37 2 11 2 10 7.7 A1T 2þ 12 þ 137 2þ 9.2 2 15 2 18 þ 5.5 A1FI 243 þ 193 198 128 2 14 þ 47 A2 þ 241 þ 165 þ 143 þ 175 2 12 þ 46 B1 2þ 42 þ2 59 2þ 38 2þ 27 5 2þ 8.6 B2 91 128 21 85 2þ 16 23 A1p þ 27 þ 138 þ 15 2þ 13 2 18 þ 11 A2p þ 238 þ 140 þ 133 163 2 12 þ 46 B1p þ 3.4 2þ 8.3 þ 2.0 þ 9.2 2 3 þ 2.6 B2p þ 86 100 þ 13 þ 46 2 11 þ 19 IS92a þ 124 þ 64 þ 125 þ 95 2 11 þ 29 þ þ þ þ þ Source: Prather etal. (2001). a Ozonechangesdecreased by25%toaccount for modelingerrors asrecommended byIPCC.

Since combustion isacommon,and(except for 4.01.5.2 FutureProjections CH4 )adominant source for all thesegases affectingOH, thenthe offsettingincreasesof The importance ofoxidation processesin climateledthe IntergovernmentalPanelon NOx ,CO, andhydrocarbons meanthatOH levels arenot very sensitiveoverall to past increasesin ClimateChange (IPCC)toinvestigatethe effects combustion. Note, however,thatif futureair on O 3 andOHof11 IPCC scenarios for future anthropogenic emissions ofthe relevant O and pollution controls lowerNOx emissions more 3 thanCO andhydrocarbon emissions,thenOH OH precursors (Prather etal.,2001). Inall ofthe decreasesshouldresult. scenarios,except one, OH wasprojected to For O 3 ,incontrast to OH, the effects ofNO x , decreasebetween2000 and2100 by3–18%, CO, andRHemission increasesareadditiveand whileO3 wasprojected to increaseby2.6–46% insum arecalculated to havecaused , 63% (Table5). The smallest decreaseinOH and increaseinO 3 (WangandJacob, 1998). The smallest increaseinO 3 wasseenasexpected inthe actualincreasemaybe evenlarger,since scenarioinwhich emissions ofNO x ,CO, NMHCs, observations (albeitnot very accurate)ofO3 andCH 4 wereprojected to change byonly duringthe 1800s (Marenco etal.,1994)suggest 3.4%, 2 8.3%, 2.0%,and 9.2%,respect- evenlowervaluesthanthosecomputed byWang iþ vely,between2000þ and2100. Aþ lso asexpected, andJacob(1998). the greatest OH decreaseswereseenintwo MeasuringOxidation Rates 11 )

– 3 12 forecasts. We will usehereonesuch model(Prinn high etal.,1999)thatintegratesacomputablegeneral reference 11 low equilibrium economicsmodelwithacoupled

r a di ca l s c m chemistry andclimatemodel,andanecosystem 5 1 0 10 model. The economicsmodelautomatically (in accountsfor the strongcorrelations between emissions ofairpollutants (gasesandaerosols) 9 andgreenhousegasesdueto theircommon production processes(e.g.,combustion ofcoal, c on en tr a t ion 8 oil,gasoline, gas,biomass).

OH 19802000 2020 20402060 2080 2100 (a) Year Results from thismodelfor (i)ahigh- probability reference emissionsforecast and 40 (ii)alower-probability high andlow emissions high reference forecastsareshowninFigure4(WangandPrinn,

pp b ) low 1999). Emissions drivingtheseprojectionsare (in 35 summarized inPrinn etal. (1999);reference anthropogenic CO, NOx ,andCH 4 emissions

f r ac t ion increaseby , 18%, , 95%,and , 33%,respect- 30 ively,between2000 and2100 intheseprojections. mole 3

O Evidently,theseemission projections from the 25 economicsmodel(whichconsistently models all 198022000 020 20402060 2080 2100 ofthe relevant industrialandagriculturalsectors (b) Year producingthesegases) lead to projections of significant depletion ofOHandenhancement of Figure4 (a)Predicted futuretropospheric average O 3 between2000 and2100 withmagnitudes concentrations ofOHand(b)molefractions ofO3 for comparableto the largerofthe changes three scenarios offutureemissions ofO3 andOH predicted from the IPCC scenarios (Table5). precursors (aswell asgreenhousegases) predicted inan economicsmodel(Prinn etal.,1999). The natural systemmodelreceivingtheseemissionsincludes simultaneous calculations ofatmospheric chemistry andclimate(WangandPrinn,1999). 4.01.6 MEASURING OXIDATION RATES Givenits importance asthe primary oxidizing chemicalinthe atmosphere, agreatdealofeffort scenarios inwhich small increasesoreven hasbeengiventomeasuringOHconcentrations decreasesinNOx emissions (OH source)were andtrends. Two approaches,directandindirect, accompanied byrelatively very large increasesin havebeentakentoaddress thismeasurement need. CO emissions(OH sink). The greatest O 3 increaseswereseenintwo scenarios where emissions ofall four O 3 precursors increased 4.01.6.1 DirectMeasurement very substantially. Finally,the oneanomalous The directaccuratemeasurement oflocalOH scenario(inwhich OH increased by5%andO3 concentrations hasbeenoneofthe major technical decreased by8.6%) involved significant decreases challengesinatmospheric chemistry since the inemissions ofall four precursors (whichisvery early 1980s. Thisgoalwasfirst achieved inthe unlikely). stratosphere(e.g.,Stimpfle andAnderson,1988), Theaboveresults makeitvery clearthat but the troposphereproved moredifficult (Crosley, forecasts ofthe futureoxidation capacity ofthe 1995). Nevertheless,early long-baselineabsorp- atmospheredependcriticallyonthe assumed tion methodsfor OH wereadequateto test some emissions.The IPCC did notassignprobabilities basic theory (e.g.,Poppe etal.,1994). Current to its emission scenarios but itisapparent that successful directmethodsinclude differential someofthesescenarios arehighly improbablefor opticalabsorption near-UVspectroscopywith oxidant precursoremissions. Integrated assess- longbaselines(e.g.,Mount,1992; Dorn etal., ment models,which coupleglobaleconomic and 1995;Brandenburger etal.,1998),laser-induced technologicaldevelopment models withnatural fluorescence afterexpansion ofairsamples Earthsystemmodels provideanalternative (e.g.,Hard etal.,1984, 1995;Holland etal., approach to the IPCC scenarioapproach withthe 1995),andavariety ofchemicalconversion addedadvantagethatobjectiveestimatesof techniques(Felton etal.,1990; ChenandMopper, individualmodeluncertaintiescanbe combined 2000; Tanner etal.,1997). withMonteCarlo approachestoprovide more The long-baselinespectroscopic techniques objectiveways ofdefiningmeans anderrors in requirepathlengthsinairof3–20 km,which 12 Ozone, Hydroxyl Radical,andOxidativeCapacity canbe achieved byasinglepass through air absorption coefficients isthenused, alongwiththe (e.g.,Mount andHarder(1995)used a10.3 km trace gasandphotodissociation ratemeasure- pathto achieveasensitivity of5 105 ments,to predictOH concentrations for compari- radicals cm 2 3 ),or multiplereflections inl£ ong son withobservations. Examplesofresults from cells (e.g.,Dorn etal. (1995)used 144 passes the MaunaLoaObservatory Photochemistry through a20 mcell to observesixOH absorption Experiment (MLOPEX)andthe Photochemistry linesaround308nm). Thelaser-induced fluor- ofPlant-emitted CompoundsandOHRadicals in escence methodsuseeither282nmor308nm NorthEastern Germany Experiment (POPCORN) photonstoexciteOH, which thensubsequently areshowninFigures5and6,respectively. emits at308–310 nm (e.g.,Brune etal. (1995); The 1992 MLOPEX experiment wasableto Holland etal. (1995)use308nm laserlight samplebothfree tropospheric air(associated with andachievedetection limits of 1 – 3 105 downslopewindsatthishigh mountainstation) radicals cm 2 3 ). ð Þ £ andboundary layerair(upslopewinds).Itis The common chemicalconversion techniques evident from Figure5thatagreement between 34 useaflow reactor inwhich ambient OH reacts observed (usingthe SO2 method)andcalculated withisotopicallylabeled SO2 or CO to yield OH concentrations wasalways goodinthe free observableproducts (e.g.,Felton etal. (1990) used troposphere, but inthe summerthe calculated 14 14 CO withradioactive CO2 detection,while boundary layerOH wasabout twice thatobserved 34 EiseleandTanner(1991)used SO2 with (Eisele etal.,1996). Since the measurement 34 H 2 SO4 detection byion-assisted mass spec- accuracywasargued to be much betterthanfactor trometry). of2,thissuggestsamissingOHsinkintheir Thesevarious directOH measurement tech- model,which theysuggest mightbe dueto niquesenablecriticaltests ofcurrent theoretical unmeasuredhydrocarbonsfromvegetation or models offast photochemistry. Thisisachieved by undetected oxidation products ofanthropogenic simultaneously measuringNO, NO2 ,CO, H 2 O, compounds. O 3 ,RH, andotherrelevant trace species,UV The1994POPCORN experiment inrural fluxes,the frequencyofphotodissociation ofO3 to Germany compared OH measurementsusing produce O( 1 D),andthe concentrations ofOH(and laser-induced fluorescence to calculations ina sometimesHO2 ). Amodelincorporatingthe best regionalairchemistry model. Figure6(a)shows estimatesofthe relevant kinetic rateconstants and the measured OH concentrations asafunction

10 7 10 7 (a) (d)

10 6 10 6

7 7 ) 10 10

– 3 (b) (e)

10 6 Measured 6 Measured Calculated

(mole c u le s m 10 w/oMay7 Calculated [OH] 10 7 10 7 (c) (f)

10 6 10 6

105 67891011 121314 15 161718 67891011 121314 15 161718 Time of day(h) Time of day(h)

Figure5 Hourly average measured andcalculated OH concentrations duringspring(a)–(c)andsummer(d)–(f) of1992 inMLOPEX: (a),(d)all time;(b),(e)free tropospheric aironly; (c),(f)boundary layeraironly (after Eisele etal.,1996). May7measurements wereanomalous. MeasuringOxidation Rates 13

14

12

10 – 3

c m 8 6

/ 1 0 6

[OH] 4

2

0 (a) 06:00 09:00 12:00 15:00 18:00

20 2.0

16 1.6 J(O 1 D) –1 s – 3

12 1.2 –5 c m 6 D ) / 1 0 / 1 0 8 0.8 1 J ( O [OH] [OH] 4 0.4

0 0.0 12:30 12:45 13:00 13:15 13:30 13:45 14:00 (b)Daytime /h(UT)

Figure6 (a)Diurnalvariation ofOHconcentrations duringPOPCORN (August 16,1994)and(b)highertime resolution version of(a)showingfast responseofOHconcentrations to J(O 1 D)variations (solid linesarethree-point runningaverages) (afterHofzumahaus etal.,1996). oftime.Notethe expected strongdiurnal clearairenvironmentsinvestigated inthese variation inOH dueto the daily cycleinUV experiments. radiation.AlsoshowninFigure6(b)isa The powerandgreatimportance ofthe insitu demonstration ofthe expected strongcorrelation directmeasurement isthatinconjunction with betweenthe measured frequencyofphotoproduc- simultaneous insitu observations ofthe other 1 1 tion ofO( D)from O 3 (J(O D)) andthe measured componentsofthe fast photo-oxidation cycles, OH (see Section 4.01.3.1). The stronghigh- theyprovide afundamentaltest ofthe photo- frequencyvariations inOH evident inFigure6(a) chemicaltheory. However,the very short lifetime arerealandaredueto strongly correlated andenormous temporalandspatialvariability of variations insolarUV fluxescaused bytransient OH, alreadyemphasized inSection 4.01.1,make cloudcover. The agreement betweenobserved itimpossiblepractically to usethesedirect andcalculated OH wasquitegoodinPOPCORN techniquestodetermineregionaltoglobalscale but,asinMLOPEX, therewasalsoatendency OH concentrations andtrends. Itistheselarge- for the modeltooverpredictOH (Hofzumahaus scaleaveragesthatareessentialtounderstanding etal.,1996; Ehhalt,1999). Part ofthe discre- the regionaltoglobalchemicalcyclesofall ofthe pancymaybe attributableto the combined effects long-lived (morethanafewweeks) trace gasesin inthe modelingofrateconstant errors and the atmospherewhich reactwithOHastheir measurement errors inthe observed RH, CO, primary sink.For thispurpose, anindirect NO,NO 2 ,O3 ,andH2 Oused inthe model. Poppe integratingmethodisneeded. etal. (1995)haveestimated thismodelingerror to be ^ 30%. 4.01.6.2 IndirectMeasurement Overall,consideringthe difficultiesinboth the measurements andthe models,the agree- The large-scaleconcentrations andlong-term mentsbetweenobservation andtheoryare trendsinOH caninprinciplebemeasured encouragingatleast for the relatively cleanand indirectly usingglobalmeasurements oftrace 14 Ozone, Hydroxyl Radical,andOxidativeCapacity gaseswhoseemissions arewell known andwhose convenient (but not essential) to solvefor x t primary sinkisOH.The best trace gasfor this usingadiscreterecursiveoptimallinearfilter purposeisthe industrialchemical1,1,1-trichloro- such asthe discreteKalmanfilter(DKF). The ethane(methylchloroform,CH 3 CCl 3 ). First,there DKF hasthe specific useful propertythatit areaccuratelong-term measurements ofCH 3 CCl 3 providesanobjectiveassessment ofthe uncer- t beginningin1978inthe ALE/GAGE/AGAGE taintyinestimatesof x aseach CH3 CCl 3 network(Prinn etal.,1983,2000,2001)and measurement isused andthus ofthe usefulness beginningin1992 inthe NOAA/CMDL network ofeach measurement. Application ofthe DKF (Montzka etal.,2000). Second, methylchloroform requiresaCTM to compute H : hasfairly simpleendusesasasolvent,and Whilethe derivation ofthe measurement voluntary chemicalindustry reports since 1970, equation usesLagrangianconcepts, H canbe alongwiththe nationalreportingprocedures equallywell derived usinganEulerianCTM. underthe MontrealProtocol inmorerecent Severalintuitiveconceptsexist regardingthe years,haveproducedvery accurateemissions important effects ofobservationalerrors and estimatesfor thischemical(McCulloch and CTM errors on the valueofthe observations in Midgley,2001). improvingandloweringthe errors inthe OH Othergaseswhich areuseful OH indicators estimates(Prinn,2000). (Thelatterpaperis include 14 CO, which isproducedprimarily by contained inateachingmonographdesigned to cosmic rays (Volz etal.,1981; Mak etal .,1994; introduce researchers to theseinversetechniques Quay etal.,2000). Whilethe accuracyofthe 14 CO applied to the biogeochemicalcycles.) Appli- production estimates,andespecially the frequency cation ofoptimallinearfilteringrequirescareful andspatialcoverage ofits measurements,donot attention to boththe physicsofthe problem match thosefor CH3 CCl 3 ,its lifetime(2 months) expressed inthe “measurement” and“system” ismuch shorterthanfor CH3 CCl 3 (4.9 yr),so it (or “state-space”) equationsormodels,andthe providesestimatesofaverage concentrations of sourcesandnatureofthe errors inthe observations OH thataremoreregionalthanCH3 CCl 3 .Another andCTMs. Thisapproach wasthe oneadopted for useful gasisthe industrialchemicalchlorodi- analysisofCH 3 CCl 3 databythe ALE/GAGE/ fluoromethane(HCFC-22,CHClF 2 ). ItyieldsOH AGAGEscientists fromthe inception ofthe concentrationssimilartothosederived from experiment. CH3 CCl 3 but withless accuracydueto greater The useofCH 3 CCl 3 hasestablishedthatthe uncertaintiesinemissions andless extensive globalweighted averageOHconcentration in measurements (Miller etal.,1998). Theindustrial the troposphereis,overthe 1978–2000period, 6 2 3 gasesCH2 FCF3 (HFC-134a),CH 3 CCl 2 F(HCFC- , 10 radicals cm (Prinn etal.,1987,1992, 141b),andCH 3 CClF 2 (HCFC-142b)arepoten- 1995, 2001; Krol etal.,1998;Montzka etal., tially useful OH estimators but the accuracyof 2000). The weightingfactor for thisaverage is theiremission estimatesneedsimprovement formallythe rateconstant k (which istemperature (Simmonds etal.,1998;HuangandPrinn,2002). dependent) ofthe reaction ofOHwithmethyl- The mostpowerful methodsfor determining chloroform,multiplied bythe CH3 CCl 3 concen- OH concentrations from trace gasdatainvolve tration,[CH3 CCl 3 ].Practically,thismeans that solution ofaninverseprobleminwhich the the averageisweighted towardthe tropical observablesareexpressed asLagrangianline lowertroposphere.Asimilaraverage concen- integrals,andthe unknown OH concentrations tration isderived from 14 CO (Quay etal.,2000), (expressed asfunctionsofspace andtime)arethe although the weightinghereisnot temperature integrands. The inverseproblemofinterest dependent. consistsofdeterminingan“optimal” estimatein Whilethe average OH concentration appears to the Bayesiansenseofthe unknownOH concen- be fairlywell defined bythismethod, the trations from imperfectconcentration measure- temporaltrendsinOH aremoredifficult to ments ofsayCH3 CCl 3 overspace andtime.The discern since theyrequirelong-term measure- unknownOH concentrations arearrayed ina ments,andvery accuratecalibrations,model t “state”vector x andthe CH3 CCl 3 measurement transports,andaboveall,avery accuratetime errors arearrayed ina“noise”vector. Approxi- seriesofCH 3 CCl 3 emissions (Prinn etal.,1992). matingthe lineintegralbyasummation leadsto Inthe firstattemptattrenddetermination, the observed CH3 CCl 3 concentrations being Prinn etal. (1992) derived anOH trendof expressed asthe noisevector plus amatrixof 1.0 ^ 0.8%yr 2 1 between1978and1990. “partialderivatives” H multiplied by x t : H Dueprimarily to asubsequent recalibration of ð Þ expressesthe sensitivity ofthe chemicaltransport the ALE/GAGE/AGAGE CH3 CCl 3 measure- model(CTM)CH3 CCl 3 concentrations to ments,the estimated 1978–1994trendwas changesinthe statevector elements (i.e.,to lowered to 0.0 ^ 0.2%yr 2 1 (Prinn etal.,1995). changesinOH). Giventhe discretetime Inasubsequent reanalysisofthe samemeasure- seriesnatureofCH 3 CCl 3 measurements,itis ments(but not withthe sameemissions), MeasuringOxidation Rates 15 Krol etal. (1998)derived atrendof0.46 ^ two beingthe dominant contributors to OH trend 0.6% yr2 1 .Thereasons for thisdifference, errors. includingdifferencesinmodels,dataprocessing, Becausethesesubstantialvariations inOH were emissions,andinversemethods,havebeen not expected from current theoreticalmodels, intensely debated (Prinn andHuang, 2001; Krol theseresults havegenerated much attention and etal.,2001). However,whenthe Prinn etal. scrutiny. Recognizingthe importance oftheir (1995)methodisused withthe sameemissions as assumed emissions estimatesintheircalculations, Krol etal. (1998),ityieldsatrendof0.3% yr2 1 , Prinn etal. (2001) also estimated the emissions inreasonableagreement withtheirvalue(Prinn required to provide azero trendinOH.These andHuang, 2001). required emissionsdifferbymany standard Inthe latest analysisapplied to the entire1978– deviationsfromthe best industryestimates 2000ALE/GAGE/AGAGECH 3 CCl 3 measure- particularlyfor 1996–2000(McCulloch and ments,Prinn etal. (2001) showed thatglobalOH Midgley,2001). Theyarealsoatoddswith levels grewby15 ^ 22% between1979 and1989. estimatesofEuropeanandAustralianemissions But,the growthratewasdecreasingatastatisti- obtained from measurements ofpolluted airfrom callysignificant rateof0.23 ^ 0.18%yr 2 1 ,so that thesecontinentsreachingthe ALE/GAGE/ OH beganslowlydecliningafter1989toreach AGAGE stations (Prinn etal.,2001). Indeed, if levels in2000 thatwere10 ^ 24%lowerthanthe theserequired zero-trendemissions areactually 1979values(see Figure7). Overall,the weighted the correctones,thenthe phase-out ofCH 3 CCl 3 globalaverageOHconcentration was 9 : 4 ^ consumption reported bythe party nations to the 1 : 3 105 radicalscm 2 3 withconcentra½ tions MontrealProtocol must seriously be inerror. This 14 Š^ £ 35%lowerinthe northern hemispherethan topic isexpected to be studied intensively inthe inthe southern hemisphere.The Prinn etal. (2001) future.The goodnews isthatasthe emissions of analysisincluded, inaddition to the measurement CH3 CCl 3 reduce to essentiallyzero (asthey errors,afullMonteCarlo treatment ofmodel almost areinthe early 2000s),the influence of errors (transport,chemistry),emission uncertain- theseemission errorsonOH determinations ties,andabsolutecalibration errors withthe last becomesnegligible.

12 Globe 11

10

9

8 ) – 3 7

1978198019821984 19861988 199019921994 19961998 2000

r a di ca l s c m (a) 5 (1 0 16 [OH] 14 Southernhemisphere 12

10

8

6 Northernhemisphere 4 1978198019821984 19861988 199019921994 19961998 2000 (b) Year

Figure7 (a)Annualaverageglobaland(b)hemispheric OH concentrationsestimated fromCH3 CCl 3 measurements (afterPrinn etal.,2001). Uncertainties(onesigma)dueto measurement errors areindicated by thick error bars anduncertaintiesdueto bothmeasurement,emission,andmodelerrors areshown bythinerror bars. Also shown arepolynomialfits to theseannualvalues(solid lines),andpolynomials describingOHvariations which wereestimated directly from CH3 CCl 3 measurements (dotted lines). 16 Ozone, Hydroxyl Radical,andOxidativeCapacity Presumingthattheyarecorrect,the above concentrations. Theyconcludedthatfurther positiveandnegativeOHtrendsarenot simply workisneeded on simulatingseasonalCH4 explained bythe measured trendsintrace gases emissionsfromwetlandsandrice paddies. involved inOH chemistry. Thereisnoclear Spivakovsky etal. (2000) extensively tested negativeor positiveglobal-scaletrendinthe theirOH calculations usingCH 3 CCl 3 ,CHClF 2 , major OH precursor gasesO 3 andNO x between andothertrace gasdataandconcluded thattheir 1978and1999,whilelevels ofthe dominant OH 3DCTM gavegoodagreement except inthe two sinkCOdecreased,ratherthanincreased, inthe tropicalwinters (possibleOHunderestimates) and northern hemisphere(Prather etal.,2001). Atthe southern extratropics(possibleOHoverestimate). sametime, concentrations ofanotherOH sink, Theynoted the greaterusefulness ofCH 3 CCl 3 for CH4 ,haverisensignificantly during1978–1999. regionalOH tests,asits emissions arebecoming Prinn etal. (2001) havesuggested thatincreasesin negligible. tropicalandsubtropicalcountriesofhydrocarbons Models havealsobeentested usingpaleodataas andSO 2 thatreactwithOH, alongwithadecrease noted inSections 4.01.2.2 and4.01.5.1.Wangand ofNO x emissions indeveloped countries,couldbe Jacob(1998)noted theirsignificant overestima- involved. tion ofO3 levels inthe 1800s anddiscussed the Also,agrowthinlevels ofanthropogenic possibleeffects ofincorrectly modeled surface aerosols couldincreaseheterogeneousOH deposition andtroposphere–stratosphereexchange destruction.Such agrowthinaerosols (both inproducingthisdiscrepancy. Mesoscale3D absorbingandreflecting)couldalso lowerUV models,which couplethe dynamicsofconvection fluxes,through directandindirectreflection and withcloudmicrophysics,andfast photochemistry absorption,thus loweringOH.Neglectofthese havealsobeentested against observations;e.g., aerosol effects,aswell asurban-scalechemistry, WangandPrinn (2000) concludedthatNOx which canremoveNO x locally,mayalso be the producedfrom lightningcandepleteO3 inthe reason whythe higherOH levels inferred inthe tropicalPacific uppertroposphereasisoften southern hemispherearenot simulated inmany of observed, but thereweredifficultiesinsimulating the models inuseinthe early 2000s. the observed high O 3 concentrations inthe middle Itisclearthatfurtherstudyonpossiblecauses troposphereintheseregions. ofOHvariationsofthe typesuggested from the Underpinningthesetestsofour understanding CH3 CCl 3 analysisisnecessary.Towardthisend, arealarge numberofobservationalexperiments the lackoflong-term global-coveringmeasure- usingsurface,aircraft,balloon andsatelliteplat- ment ofO3 ,NO x ,SO 2 ,hydrocarbons,andaerosols forms,andrangingfrom short-term campaigns to isavery serious impediment to our understanding multidecadalmeasurement networks. Lelieveld ofOHchemistry. etal. (1999) andEhhalt (1999) havereviewed someofthe relevant experiments focused on tropospheric oxidants. Theynoted the advances madeindirectNOx andHO x observations for 4.01.7 ATMOSPHERIC MODELS AND testingphotochemicalmodels.Thompson etal. OBSERVATIONS (2003) havereviewed the gaps intropospheric O 3 Amajor approach for testingour knowledge of observations andhavepresented animportant new atmospheric oxidation processeshasinvolved a datasetfor the southern hemispheretropicsthat careful comparison betweentrace gasandfree will provide newtests ofchemicalmodels. radicalobservations andthe predictions from three-dimensional(3D)CTMsthatincorporate the latest understandingofhomogeneousand 4.01.8CONCLUSIONS heterogeneous fast photochemistry. Whilethese 3DCTMssimulatequalitatively many features Oxidation processeshaveplayed amajor rolein ofthe observations,therearediscrepancies the evolution ofEarth’s atmosphere.Observations thatpoint out the need for additionalresearch. oftrace gasesandfree radicals inthe atmosphere Someillustrativeexamplesarebrieflydiscussed in1978–2003,andofchemicals inice cores here. recordingthe composition ofpast atmospheres, Berntsen etal. (2000) noted thattheir3DCTMs areprovidingfundamentalinformation about indicateincreasesinuppertropospheric O 3 inthe theseprocesses. Also,basic laboratory studiesof 1980s,whereasobservations show alevelingoff chemicalkinetics,whilenot reviewed here, have or adecrease.Theypoint to the possibleroleof played anessentialroleindefiningmechanisms unmodeled stratospheric O 3 changeswhich andrates. Models havebeendeveloped for fast mightexplainthisdiscrepancy. 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