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Carbon Kinetic Isotope Effect in the Oxidation of Methane by The JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 95, NO. D13, PAGES 22,455-22,462, DECEMBER 20, 1990 CarbonKinetic IsotopeEffect in the Oxidationof Methaneby the Hydroxyl Radical CttRISTOPHERA. CANTRELL,RICHARD E. SHE•, ANTHONY H. MCDANmL, JACK G. CALVERT, JAMESA. DAVIDSON,DAVID C. LOWEl, STANLEYC. TYLER,RALPH J. CICERONE2, AND JAMES P. GREENBERG AtmosphericKinetics and PhotochemistryGroup, AtmosphericChemistry Division, National Centerfor AtmosphericResearch, Boulder, Colorado The reactionof the hydroxylradical (HO) with the stablecafix)n isotopes of methanehas been studied as a functionof temperaturefrom 273 to 353 K. The measuredratio of the rate coefficientsfor reaction with•ZCHn relative to •3CH•(kn•/kn3) was 1.0054 (20.0009 at the95% confidence interval), independent of temperaturewithin the precisionof the measurement,over the rangestudied. The precisionof the present valueis muchimproved over that of previousstudies, and this resultprovides important constraints on the currentunderstanding of the cyclingof methanethrough the atmospherethrough the useof carbonisotope measurements. INTRODUCTION weightedaverage of the sourceratios must equal the atmospher- Methane (CH•) is an importanttrace gas in the atmosphere ic ratio, after correctionfor fractionationin any lossprocesses. [Wofsy,1976]. It is a key sink for the tropospherichydroxyl The primaryloss of CI-I4in the troposphereis the reactionwith radical. Methane contributesto greenhousewarming [Donner the hydroxylradical: and Ramanathan,1980]; its potentialwarming effects follow only CO2 and H20. Methaneis a primary sink for chlorine (R1) CHn + HO ---)CH3 + HzO atomsin the stratosphereand a majorsource of watervapor in the upper stratosphere. The concentrationof CI-I4 in the The rate coefficient for this reaction has been studied extensive- tropospherehas been increasing at a rateof approximately1% ly (seereview by Ravishankara[1988]), but dataindicating the per year, at leastover the pastdecade [Rasmussen and Khalil, effect of isotopesubstitution in methaneare scarce. Fraction- 1981; Blake et al., 1982; Steeleet al., 1987; Blake and Rowland, ation occursin the atmospherebecause the rate coefficientfor 1988]. Ice core measurementsindicate a rapid increasebegan reaction(R1) is slightlylarger for the•2Hn relativeto •3CHn. a few hundredyears ago [Craig and Chou, 1982; Rasmussen This secondary(i.e., involvingisotopic substitution at a position and Khalil, 1984]. The reasonsfor this increasehave not been other than the direct reactioncenter and involving an atom not established,but it hasbeen suggestedthat it could be due to an splitoff in thereaction) kinetic isotope effect is expectedto be increasein sourceemissions, a decreasein the atmosphericloss small, and indeedprevious studies near room temperaturehave rate, or both. found an effect of 1% or less[Rust and Stevens,1980; Davidson Severalapproaches have been applied in orderto understand et al., 1987]. thesottrees and sinks of atmosphericmethane (see discussion by We attemptedto improvethe precisionof the value for the Ciceroneand Oremland[ 1988]). One way to studythe methane carbonkinetic isotope effect in reaction(R1) (i.e., the ratio of budget is through the use of stable isotopesof carbon as therate coefficients for thereaction of •H nwith hydroxyl as proposedby Stevensand Rust [1982]. Measurements of comparedto the reactionof •3CHn)near room temperature. in methanein remote backgroundair and in methanesources Additionally,we investigatedthe temperature dependence of this have been used with data of fluxes from these sources to rate coefficient ratio. estimaterelative source strengths and to provideinput to models of atmosphericmethane [e.g., Stevens and Engelkemeir,1988; Tyler et al., 1988; Quay et al., 1988; Wahlenet al., 1989]. For The kineticisotope effect in reaction(R 1) hasbeen studied in example,an approachsuch as the followingis used. Methane thislaboratory previously [Davidson et al., 1987]. Many of the from each sourcecan be characterizedby a range of carbon experimentaldetails reported in that studyapply here. Differ- isotoperatios. Thesemeasured ratios are comparedwith the encesfrom our earlier studywill be pointedout. The experi- isotoperatio in atmosphericmethane. In principle,the mass ment involved the reactionof a mixture of methanecontaining both stablecarbon isotopes, nominally at relativeatmospheric abundances,with the hydroxyl radical. We measuredthe methaneconcentration and isotopic composition before and after •Nowat Instituteof N•clearSdences, DSI1L Lower Hutt, New a reactionperiod. The relationbetween the amountof methane Zealand. converted,the changein the ratio of stablecarbon composition ßIow at GeosciencesDepartment, University of Calif•a, Inrine. and the ratio of rate coefficientsfor reaction (R1) for the two speciesis givenby (seederivation in Davidsonet al. [1987]): Copyright1990 by the AmericanGeophysical Union. ln(A) Papernumber 90JD01651. - __. - 0148 -0227/90/90JD-01651$05.00 k,, ln(A)+ ln{($,+ 1000)/($ o+1000)} 22,456 CANTRELLETAL.: KINETICISOTOPE EFFF. L"T FOR CH4 + OH where/q2is the rate coefficientfor reaction(R1) for methane photolysislamp. The filter inhibitedphotolysis of ozonein the containingcarbon-12 and k•3 is for carbon-13methane; A is the Chappiusbands which producesground state oxygen atoms. fractionof methaneremaining at timet; and$ is a measureof Groundstate oxygen does not reactwith watervapor to produce the ratio of carbon-13to carbon-12(in per rail, or partsper hydroxyl radical. thousanddifference), at thestart of an experiment(•Jo) and the The reactionmixture was continuouslystirred throughout the end(•J0, def'med as follows: courseof a run with a stainlesssteel bellows pump. The volumeof the pumpand connecting robing was minimized; less than 0.05% of the reaction mixture was outside of the cell at x 1000 (2) any given time. The entirecell mixture was circulatedevery 5 to 10 min. The cell was typicallycirculated for 10 rain before the first samplewas extracted, and for 10 rain after thephotoly- sis lamp was turnedoff beforethe final samplewas taken. whereR, andR,• standfor theratio of carbon-13to carbon-12 The following procedurewas used to prepare a reaction in a sample,x, andin a standard,respectively. In thiscase, the mixture.A volumeof liquidwater (0.2 to 0.8 cm•) wasadded standardis PeeDeeBelemnite, which is the commonlyaccepted through a septurn to an evacuated,temperature regulated referencefor stable carbon isotopework. The choice of stainlesssteel cell (approximately48 L volume),which has been standardhas no effecton the final kineticisotope effect derived. describexipreviously [Shetter et al., 1987]. When the pressure In theliterature, the reciprocal of TI is oftenused, designated withinthe cell reachedthe expectedlevel (3 to 6 x 10•? moleculescm3), a knownpressure of methane(Linde Research Grade,greater than 99.99% purity), measured with a capacitance k,3 In{(•J +1000)/(•J +1000)} (3) manometer (MKS Insmnnents), was added to a calibrated kn In(A) volumeand swept into thecell with helium(Linde UHP, greater than99.999% purity). Heliumwas used as the bath gas because The accuracy of the measuredkinetic isotope effect is it is ineffectivein quenchingthe excited oxygen atoms produced sensitiveto errorsin determiningthe extent of conversion(A) in reaction(R2). The ratio of water to methanein the cell was and the isotoperatios ($). This sensitivityis minimizedat kept ashigh as possible([H20]/[CH,] o = 600 to 1700) in order relativelylarge fractional conversions, because of the increased to minimizeloss of methanedue to reactionwith excitedoxygen amount of isotopic fractionationwhich occurs. Hydroxyl atoms(O(•D)): radicalswere produced in sufficientamount to remove50-90% of the methanein 24 hours. This designis relativelyinsensitive (R4) CHn + O(•D)-• -• products to contaminantsfound in the commerciallyprepared methane (such as light allcanes)because only the unreactedmethane is The rate coefficientfor reaction(1t3) is about 1.4 timesthat for analyzed(a problemwith ethaneis possiblebecause it may be reactionreaction (R4). Thereaction rate for O(•D) with H20 is cryogenicallytrapped with methanein the isotopeanalysis, about 800 to 2400 times that for reactionwith CI-I•. Reaction discussedbelow). (R4) almostcertainly has a kinetic isotopeeffect which is less We used the same hydroxyl radical sourceas our previous than that in reaction (R1) [Davidsonet al., 1987]; thus any study [Davidsonet al., 1987]. Ozone was photolyzedin the contributionto the methaneloss from reaction (R4) would bias Hartleyband to produceexcited oxygen atoms which react with the measuredfractionation in theseexperiments. In order to water vapor.. monitorthe possible extent of reaction(R4), nitrousoxide (USP US Medical, greaterthan 99.0% purity) was addedwith a (R2) 0 3 + hv -• 02 + O(•D) reactivitytoward O(•D) approximately equal to that of theinitial amountof methanein the cell ((1.0 to 2.5) x 10• molecules (R3) O(•D)+ H,_O-• 2 HO cm-3). Nitrousoxide reacts with O(•D), but not with the hydroxylradical. The concentrationof N20 throughouta given Ozone was added continuouslyduring a mn by passing experimentalrun was monitoredthrough the use of Fourier helium througha trap of silicagel, whichhad previouslybeen transforminfrared spectroscopy (discussed below). The lossof loaded with ozone producedthrough corona dischargeof nitrousoxide, if any, couldthen be usedto apply a first order oxygen.The silicagel trapwas maintained at 195 K with a dry correctionto themethane loss due to reactionwith O(•D).
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