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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 95, NO. D13, PAGES 22,455-22,462, DECEMBER 20, 1990

CarbonKinetic IsotopeEffect in the Oxidationof Methaneby the Hydroxyl

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- (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 . 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 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 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]. 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• (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 (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). ice/ethanolmixture. A typicalflow of 5.0 standardcm 3 min 4 Oxygen(Linde UHP, greaterthan 99.99% purity) was added causeda pressureincrease in the cell of approximately150 torr to the cell in orderto insurethat methylradicals (CH3), onc.9 during a 24-hour experiment. The ozonewas addedin this formed,would be convertedto stableproducts (CO or COo fashionto maintaina low steadystate concentration (< 10 •3 rather than be convertedback to methane,o,r be associated:to cm-3) whichresulted in efficientuse of thehydroxyl radicals form . Back reaction to methane and ethane could produced;more OH radicalsreacted with methanerather than providealternate pathways for isotopefractionation. with ozone. We calculateda steadystate hydroxyl radical concentrationof 2 to 6 x 10• cm'3 from the rate of conversion (R5) CH3 + 02 + M --> CH302+ M of methane. The cell was illuminated through a quartz window with Typicallyan oxygen concentration of 3 x 10•? cm 3 was radiationfrom a 300-W Xenon arc (ILC Corporation)filtered used. Reaction(1t5) alongwith the reactionof methyl radicals througha Coming glass•ter 7-54. The [fiter was cooledby with ozone, assuresthat no back reactions occur. circulatingtap water through the filter mount. This arrangement minimizedpossible effects of heatingthe reaction mixture by the (R6) CH3 + O3 -• CH30 + 02 CANTRELLET AL: KINETICISOTOPE E•'ECT •OR CH4 + OH 22,457

Nitrogendioxide ((3 to 5) x 10•3 molecules cm '3) was added for dilutiondue to theaddition of heliummonitored by pressure in order to maintaina steadystate of (NO), which measurementsat thebeginning and end of thephotolysis period. speedsthe conversionof methaneto CO and CO: through The relative standard deviation of the IR measurements reaction(R7): (3-5%) was larger than that for the gas chromatographic determinations. The methaneremaining as determinedfrom (R7) CHaO: + NO --• CHaO + NO: spectroscopicmeasurements (FTIR) as comparedto GC mea- surementsis shownin Figure 1. The leastsquares fit of the The methoxy radicals formed react with oxygen to form data yields a slopeof 0.98 (+ 0.06), which is not significantly ,which subsequentlyphotolyzes or reacts with different from unity. hydroxyl. The isotoperatio determinationswere performed in a fashion The extent of conversion of methane and was similar to that discussedby Tyler [1986; Tyler et al., 1988] and monitoredby infraredspectroscopy. A BOMEM modelDA3.01 Davidsonet al. [1987], usinga new high-volume,fast-flow rate Fouriertransform spectrometer system was interfaced to thecell. combustiontrain to convertCI-I4 to CO:. A majorimprovement An infrared beam from a heated SiC rod was collimated and over the earlier systemis in the use of an oven catalyst of passedthrough a CaF: beamsplitter. The beam was then platinizedalumina (1% loading)at 800øC. Carbondioxide and transferredto the cell via matchingoptics through BaF: win- water are removedprior to the entry of the samplegas stream dows. The White-type internal multiple pass optics were into the the ovenusing a seriesof four multipleloop trapsat arrangedto yield 24 passesor a total opticalpath of 48.6 m. liquidnitrogen temperature (77 K). SincePt-alumina is porous The beamexited the cell andwas detected by a liquidN:-cooled andpotentially subject to high blanks,it is conditionedperiodi- InSb detector. This beamsplitterand detectorcombination cally by flushingwith dry zero air. Our carbonblank is less limitedenergies to thosegreater than 1800 cm4; however, this that 1 I.tliter of CO: for 300 L of cleanzero air processed.The regionallows quantificafion of methane,nitrous oxide, as well recoveryof carbondioxide produced from the conversionof ascarbon dioxide and , if desired.The spectral methaneis greaterthan 99%, evenwith relativelysmall samples resolution,0.125 cm 4, wassufficient to resolveand analyze such as thosefor this experiment(20 to 50 I.tL). The mass individualrotational lines, and also operate in a regionof linear spectrometerused in this study was a Finnigan-Mat Delta-E relationshipbetween measured absorbance and speciesconcen- model, whichresulted in overallprecision of betterthan 0.1%o tration. The initial spectnnn(100 interferometerscans) was by the useof a speciallydesigned cold fingerinlet system.The takenbefore samples were withdrawnfor GC andMS analysis. working standardfor this study was Oztech-002 (Oztech Gas The Beer-Lambertlinearity for methanewas verified for 10 Co.) whichis--30.011%o relative to PDB carbonate.The individuallines which were free of spectraloverlap with water valuesare reportedwith respectto PDB carbonate,with usual or formaldehyde.These lines were due to the •hEH4 species correctionsfor background,leakage, capillary fractionation, and only, but most of the methanein the cell containscarbon-12 •70. The separationand procedure would not (about99%), andonly a smallfractionation takes place during distinguishcarbon originating in ethanefrom that in methane, the courseof a run {,'less'than i %). Thereforeit is expected'that thereforea run was chosenfor analysisof light allcanes. All theratio [•h2H4]•/[•h2Ha] 0would be close to, but systematically including ethane were below detectionlimits, andcould low by less than 1% comparedto {[•ZCH4]•+ constitute no more that 0.1% of the methane concentration, and [•CI-h]•}/{[•h2I-h]o+ [•CI-h]o}, which is the factor,A, in thuspresent no interferencein the isotoperatio analysis. equation(1). This erroris smallcompared to the scatterof the CI-Iadetermination by this technique.Since only the fractional methaneremaining is requiredfor this study,the line integra- RESULTS AND DISCUSSION tions were not converted to absolutemethane concentrations,but insteadthe areasfrom the spectrumat the endof the experiment Temperatureand ConversionDependence of km/kts andthose from the beginningwere ratioed, and an averageof all The resultsof this studyare analyzedin severalways. First 10 lines was used for the factor A, discussedabove. This factor a rate coefficientratio (k•!k•) wascalculated for eachexperi- must be correctedfor the amountof sampleremoved for the ment from equation(1). Thesedata are presentedin Figure2 initial GC and MS measurements(determined though pressure and Table 1. The averageand 95% confidenceintervals for the measurements). roomtemperature recommendations for/q•/k•3 of Davidsonet al. Samplesof the reactionmixture were withdrawnbefore and [1987] and Rust and Stevens[1980] are also shown (lower afterphotolysis for gaschromatographic (GC) methaneanalysis confidenceinterval for Davidsonet al. and upper confidence andmass spectrometric (MS) isotopiccomposition. The 7.6 L intervalfor Rustand Stevens), The horizontallines are averages electropolishedstainless steel sample bulbs with stainlesssteel of all the datafrom the presentwork. The resultsof thisstudy bellowsvalves were evacuated prior to samplecollection. The arebetween the two previous studies. The kineticisotope effect methanemixing ratio was determinedby gas chromatography showsno temperaturedependence, from 273 to 353 K. A linear and flame ionization detection [Zimmerman et al., 1988]. The regressionof /q•/k•s versus temperatureyields slopes of methanemixing ratio in the sampleswas comparedto two -0.0(0)0(O per degreeand 0.000001 per degreeusing the GC standardmixtures, 3.88 ppmv (NBS SRM1660a) and 71.83 andFTIR conversiondeterminations, respectively. The standard ppmv (SpecialtyGas Products). Absoluteconcentration is not deviationsof theseslopes are factorsof 10 and6 largerthan the requiredfor this study,only the ratio of methanebefore and slopes,respectively; they are not statisticallysignificant. after photolysis. Typically 10 individualchromatograph rims The kineticisotope effect in this reactionis independentof wereperformed and averaged. The upperlimit of theuncertain- temperatureover the range studied,and so the resultsat all ty in the measurementhas been estimated at 0.4 ppbv [Zimmer- temperatureswere averaged. The GC and FTIR conversions man et al., 1988]. The final methanemixing ratio wascorrected were analyzedseparately, yielding values for the kineticisotope 22,458 Ca2rmœLLET AL: KINETICISOTOPE Evvvxgr • CH4 + OH

lOO ' I ' ! ' I ' I "

80

60

40

2o

0 0 20 40 60 80 100 Percent CH4 Remaining, GC

Fig. 1. Methaneconversion extent determined by infraredspectroscopy (FTIR) versusthat determinedby gaschromatography (GC). A fit to the data yields a slopeof 0.98 (+ 0.06, 95% confidenceinterval).

effectof 1.0056(ñ 0.00091)and 1.0058 (ñ 0.0011),respectively N•O. If one acceptsthe N•O lossas real, and assumesthat the (the error limits are the 95% confidenceintervals). kineticisotope effect for reaction(R4) is 1.000, then the values The data from this study were treated in another fashion of • would increaseto about 1.0055 (from 1.0054). This small (althoughnot independent)in order to demonstrateto lack of correctionwas not applieddue to the uncertaintyin the amount dependenceof kn//q3on extentof conversion.If equation(1) is of N•O depletion. rearrangedto collect the In(A) terms together,the following expressionis derived: Comparisonwith PreviousMeasurements Our new result is about one-half the calculated fractionation • ßIn (4) of methanecarbon isotopes as determinedby Davidsonet al. In(A)- (1-•l) $o+ 1000 [ 1987]. Thepresent result has an uncertaintywhich is nearlyan I•J,+10001 order of magnitude smaller than Davidson et al. [1987]. Possible reasons for this difference will be discussed below. where • is the ratio of rate coefficients,k•//q3. Thus a fit of The present result is in agreementwith the results of In(A) versusthe logarithmof the term in largebrackets yields a Davidsonet al. [1987] and Rust and Stevens[1980] within the slopewhich is relatedto •! {•! = sloped(l+slope)}.The dataare large uncertaintiesassociated with thosestudies (•! = 1.010 ñ presentedas In(A) versusthe logarithmicterm on the right of 0.007 and 1.003 ñ 0.007 at the 95% confidencelevel; note the equation(4) in Figure3. The resultof a fit (constrainedto have confidence interval for Rust and Stevens is calculated from the an intercept,Ao equalto unity) yieldsa value for • of 1.0054 data,not as reportedby them). (ñ 0.0009, at the 95% confidenceinterval), which is essentially It hasbeen pointed out that the individual•! valuesfrom the identical to the results of the other treatments. This treatment Davidsonet al. studyfall into two groups[Craig et al., 1988; demonstratedthe derivedkinetic isotope effect is independentof Stevensand Engelkemeir,1988]. At leasttwo factorsmay have the extent of conversion. contributedto the large spreadof values from the study of The extent of reaction(R4) was monitoredwith nitrousoxide Davidsonet al. [1987] as comparedto the presentone. The as describedabove. There was no loss of N20 within the precisionof the extent of methaneconversion was greatly uncertaintyof the measurements(2% ñ 5%, 95% confidence improvedin thepresent study (less than 1%) ascompared to the level). A 2% lossof N20 wouldimply a lossof about2.7% of previousstudy (about 9%). Also, in this studyisotope ratio methane due to reaction (R4), becausethe rate coefficient for measurementswere performedon the methaneat the beginning O(•D) reactionwith CH n is 1.34times that for reactionwith of everyrun. In theprevious experiment, seven determinations CANTRELLET P&..: I•IN•TIC ISOTOPE EFFF_,C"T FORCH 4 + OH 22,459

! ! ! ! ! ! ! ! ! ! ! L? • 22,460 CANTRELLET AL.: KINETICISOTOPE EFFECT FOR CH 4 + OH

1.0gO

FTII 1.015 GCf Thiswork Davidson et al. (1987) Rust and Stevens (1980) 1.OLO

005

1.000

0.995 , I , I , I , , , , , -20 0 20 40 60 80 100 Temperature (øC) Fig. 2. Kineticisotope effect versus temperature for this study. Also shownare the roomtemperature results of Davidso• • al. [ 1987] and Rust and Stevens [ 1980].

were performedand averaged. While one would expect the requiredto performaccurate simulations on a secondarykinetic isotoperatio of the unreactedmethane to be easilycharacterized isotopeeffect suchas occursin this reaction. and constant,some variability was found in the presentstudy (averageinitial per value equals -35.9 + 0.5%o). The amount Implicationsfor the AtmosphericMethane Budget of methanemeasured in the initial sampleswas alwaysslightly What does the value for the ratio of the two rate constants tell higher (0 to 3%) than the amountadded to the cell, possibly us aboutthe 13CH4]1:lCH4 ratio in thesources of atmospheric enteringthe cell with the additionof water. This contaminant methane? We performeda simplified calculationfollowing methanewas probably with a •(•aC)value Craig et al. [1988], with an extensionto includepossible soil of about-47 as comparedto the cylindermethane, presumably sink fractionationas discussedby King et al. [ 1989]. of thermogenicorigin, with a •(•3C)value of about-36. It is We take also possiblethat fractionationof methanecould occur in the low pressurevacuum line used for additionof gasesto the reactioncell. In the presentstudy, use of the averageinitial per mil valueresults in the sameaverage kinetic isotopeeffect with a slightly larger standarddeviation. + •'• ß + ß If the Davidsonet al. studyhad a problemwith variabilityin •= Vl,.• •l •l 1000 the isotopiccontent of the unreactedmethane and the per mil valueswere systematicallyless negative than the averageof the measuredvalues, then a systematicallyhigh valuefor thekinetic whereF.•, is the fractionof methanethat is removedchemically isotopeeffect would be measured.We do not know if this was by troposphericOH and F• is the fractionremoved by soils. the case. Equation(5) is only an approximation. A more complete analysismust recognizeseveral factors, among those: that the Modeling the Carbon Kinetic IsotopeEffect spatialand temporal distributions of methanesources and sinks Although Ehhalt et al. [1989] found that primary kinetic are not identical,and that the sources,sinks and atmospheric isotopeeffects in simplesystems (H 2 andHD with HO) canbe amountsare not necessarilyin steadystate. calculatedquite successfully using the BEBOVIB-IV computer With our valuefor tl,•, (1.0054),and 15•= = -47.7%O[Craig program[Burton et al., 1977], the use of this programfor the et al., 1988]this equation yields 15,,,•,• =-52.2%O, assuming F,,,a carbonkinetic isotopeeffect in the methane-hydroxylreaction is negligible. With a value of 15.,•per mil after Lowe et al. gave a range of values (1.00 to 1.04). These valuesdepend [1989], we obtain15,o•, =-1.4%O. To accountapproximately upon the shapeassumed for the potentialenergy surface of the for the fact that atmosphericmethane is increasing in activatedcomplex. A more sophisticatedmethod apparently is concentration, i.e., that sources exceed sinks, and for the CANTRELLET AL.: KINETICISOTOPE EFFECT FOR CH 4 + OH 22,461

lOO ' I

lO ...... 1 ß 0040 1.0055 - -- 1.0070 O GC A FTIR

I • I , I I I , 0.000 0.004 0.008 0.012 0.016 6 + looo In t 6 + lOOO o

Fig.3. Treatmentof equation (4) described in test to estractaverage kinetic isotope effect from this study. Shown for reference are linescorresponding tokinetic isotope values of 1.0040,1.0055, and 1.0070. The best fit yieldsa kineticisotope effect of 1.0054(+ 0.0009, 95% confidenceinterval).

possibilitythat •5.,•is alsochanging (decreasing at a rate of the precisionof the results. This new valuemay be usedto about0.05%0 yr-•), Craig et al.would add about 0.4%0 to the two constrainthe atmosphericmethane budget using isotope ratio distinctvalues of fi,o•, above,yielding -51.8 and-51.0%o. In studies,and suggestsmore measurementsof methanesources eithercase, because most biogenic methane is considerablymore andsinks are necessary in orderto fully understandatmospheric depletedin •3Cthan -52%0, it is apparentthat there must be methane. very significantsources of heaviermethane from, for example natural gas processes,coal mining, biomassburning and Acknowledgments.We acknowledgethe funding of the NASA possiblymethane hydrate deposits. The earlier value for •! from InterdisciplinaryGrants Program in EarthSciences (W16184 mod. 5) for Davidsonet al. [1987]would have implied that 8,o,• = -55 or supportof two of us (S.C.T.and R.J.C.). We alsoacknowledge Ed -56%0. To the extentthat this value of •,o,• hasbeen used as Dlugokencky,Geoffrey Tyndall, and Michael Keller for helpful a constraintin deducingbudgets of atmosphericmethane (see commentsin the InChon of thismanuscript. The NationalCenter for AtmosphericResearch is sponsoredby the NationalScience Wahlenet al. [1989] andCicerone and Oremland[1988]) these Foundation. budgetsmust be adjustedto allow for relativelymore sources that are enrichedin •3C. The inclusionof •!,oaof King et al. (1.016 to 1.026) and reasonablees,timates of the size of REFERENCES [Ciceroneand Oremland,1988], suggeststhat 8,o• couldbe Blake, D.R., and F.S. Rowland, Continuingworldwide increasein morenegative by about1%0 (i.e., 8,,_,•is -52 to -53%0). troposphericmethane, 1978 to 1987, Science,239, 1129-1131,1988. Blake, D.R., E.W. Mayer, S.C. Tyler, Y. Makide,D.C. Montague,and CONCLUSIONS F.S. Rowland,Global in•e in atmosphericmethane concentrations between1978 and 1980, Geophys.Res. Lea., 9, 477-480, 1982. We performeda studyof thetemperature dependence of the Burton,G.W., L.B. Sims,J.C. Wilson, and A. Fry, J. Am. Chem.Soc., stablecarbon kinetic isotope effect in theoxidation of methane 99, 3371, 1977; QuantumChemistry Program Exchange, Indiana by hydroxyl.The average effect measured in ourstudy is/q•/k•s University,Chemistry Department, Bloomington, IN, ProgramNo. = 1.0054 (+ 0.0009, 95% confidenceinterval). This value is 337. largerthan that of Rustand Stevens [1980] andlower than that Cicerone, R3., and R.S. Oreroland, Biogeochemicalaspects of of Davidsonet al. [1987], but with muchbetter precision. The atmosphericmethane, Global Biogeochem. Cycles• 2, 299-327,1988. ratiois independentof temperaturefrom 273 to 353 K, within Craig,H., C.C. Chou,J.A. Welhan, C. NL Stevens,and A. Engelkemeir, 22,462 CANTRELLET AL.: KINETICISOTOPE EFF• FORCH 4 + OH

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