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Experimentally Determined Kinetic Isotope Effects in The GEOPHYSICAL RESEARCH LETTERS, VOL. 27, NO. 12, PAGES 1715-1718,JUNE 15, 2000 Experimentally determinedkinetic isotopeeffects in the reaction of CH4 with CI: Implicationsfor atmosphericCH4 StanleyC. Tyler, Henry O. Ajie, Andrew L. Rice, andRalph J. Cicerone Earth SystemScience Department, University of California, Irvine Ernesto C. Tuazon Air PollutionResearch Center, University of California, Riverside Abstract. We report experimental values for the carbon and agreement[Atkinson et al., 1999]. While generally considered hydrogenkinetic isotope effects (KIEs) in the reactionof CH4 only a minor loss processin the troposphere,(1) becomesmore with C1 at temperaturesbetween 273 and 350 K. Isotoperatio important in the stratospherewhere it converts C1 atoms into massspectrometry wasutilized to measure 13CH4/12CH4 and HC1. Its importancein the marine boundarylayer (MBL) is not CH3D/CH4 ratioson samplestaken from a 5870- L reaction yet established because of uncertainty in MBL C1 radical chamber.At 299K, kc12/kcl3 = 1.0621+0.0001(2a) andkh/k d = concentrations. Overall, the effect of C1 has been calculated to 1.474+0.020 (2a). For both KIEs, the ratio decreased with accountfor from 2 to 4% of all atmosphericCH 4 loss[e.g., increasing temperature over the range studied. These results Gupta et al., 1996]. agree well with experimentalstudies using tunable diode laser Both measured and theoretically calculated values for the absorptionspectroscopy and FTIR absorptionspectroscopy and kinetic isotope effects (KIEs) in (1) are much larger than for a with a recenttheoretically-calculated set of values. similarreaction involving CH 4 andOH, the principalsink for atmosphericCH 4. This makes(1) of increasedimportance to researchersusing isotopicmeasurements to study the CH4 1. Introduction budget. Thus far, studiesof KIEs in (1) report a wide range of values, making their use in atmospheric budget calculations Methane (CH4) is an importantradiative trace gas[e.g., somewhat problematic. Here we report an experimental Donner and Ramanathan,1980] which also plays an important investigation of C and H KIEs in reaction (1) and their role in troposphericand stratosphericchemistry [e.g., Cicerone temperature dependences with the goal of reaching some and Oremland,1988]. Measurementsof CH4 in ice cores consensus values for them. indicatethat in the past 200 years the mixing ratio has increased from ca. 0.7 ppm to its presentvalue of ca. 1.78 ppm [Etheridge et al., 1998]. The well-documentedgrowth in CH4 mixingratio 2. Experimental Methods of as much as ca. 1%/yr during the past two decades [e.g, Reactionstook place in a 5870-L Teflon coated chamberat Dlugokencky et al., 1998] concerns researchers and policy the Air Pollution ResearchCenter at the University of California, makerstrying to understandglobal atmosphericchange and its Riverside. A detailed description of the reaction chamber is potential impacts on humankind. The exact causes for the given in Winer et al. [1980]. In brief, light from a 24 kW xenon increasein CH4 mixingratio andthe variationsin its annual arc lamp was usedto photolyzeC12 and therebyinitiate the growthrate are not known although studies of CH4 sourcesand reactionbetween atomic C1 and CH 4. A pyrexfilter eliminated sink processesare many. wavelengths<300 nm and prevented the production of O(1D). Measurementsof CH4 isotopiccomposition in ambientair Temperaturecontrol was achievedby an insulatingjacket system andin CH4 sourcesare a usefultool for constrainingCH 4 budget locatedinside the chamberwalls. Ethylene glycol was circulated estimates. Isotopicfractionation of C and H during CH4 in the jacket to maintain temperaturewithin __.0.6K over the 273 formationleads to individualCH 4 sourceswith characteristic to 349 K temperaturerange. Two Teflon coated stirring fans rangesof isotopicvalues. In steadystate, the flux-weightedsum were used to maintain constant temperature and to avoid of the isotopic signatures from all sources should equal stratification of air in the chamber. A Nicolet 7199 Fourier atmospheric values once the atmospheric isotopic value is transform infrared (FTIR) spectrometerinterfaced to a White- adjustedto account for isotope fractionation by chemical sink type three mirror multiple reflectionsystem was usedto monitor processes[Rust and Stevens,1980]. changesin CH4 concentrations. One suchsink process is the reactionof CH4 with chlorine The partialpressures of CH4 andC12 were measuredinto radicals: calibrated Pyrex bulbs (5 L and 2 L respectively) with CH4 + C1--> CH3 + HC1 (1) capacitance manometers (MKS Baratron, Boulder, CO) and Therate constant, kc1= 1.0x10-13 crn3molecules -1 s' 1 at 298 K, introducedinto the chamberby flushing each bulb in line with for this reaction has been studied extensively with good zeroair (air strippedof all constituentsexcept N 2, 0 2, andAt, Scott-MarinInc., Riverside, CA) following methodsdescribed in Copyright2000 by the AmericanGeophysical Union. Tua•onet al. [1992]. Typical chamberconcentrations of CH4 (99.999%, MathesonGas Products,Inc.) and C12 (>99.5%, Papernumber 1999GL011168. MathesonGas Products, Inc.) were in therange of (2.1-3.8)x1015 0094-8276/00/1999GL011168505.00 molecules/cm3 and 1.5-2.6 x 1015molecules/cm 3, respectively, 1715 1716 TYLERET AL.:KINETIC ISOTOPE EFFECTS IN THEREACTION OF CH 4 WITH CL with the balancegas of zero air between2.2-2.8 x 1019 Equation (2) holds for either the carbonor hydrogenKIE. molecules/cm3. Pressure measurements within the chamber were ForC, {x = kc12/kc13,and150 and 15t refer to 1513C-CH4 attime 0 madewith a 0-1000 Tort MKS Baratroncapacitance manometer. and t respectively,while for H, {x is the ratio of the two rate Initialcell pressures were typically 790 to 800 Ton:. Theoxygen constants,kh/k d, wherek h andk d arethe rate constants for CH4 eliminatedCH 3 radicalrecombination and protonabstraction andCH3D respectively. In thiscase •5 o and•5 t referto •SD-CH 4. reactionsthat might interfere with the desired CH 4 isotopesignal There is one notable difference, however. In the reaction of by reactingwith CH 3 to produceCH30 2. Totalphotolysis time CH3 D withC1 there are two reactions that contribute tothe decay was typically 70 to 90 minutes,consuming 50 to 75% of the of the reactants. One reaction is the direct abstraction of initiallypresent CH 4. deuteriumresulting in DC1and CH 3 asproducts, while the other To insurethat no sidereactions were influencing the reaction, reactionis an abstractionof a protonresulting in the products a static box model photochemicalsimulation was run to 83% HC1and CH 2 D. Therate constant, kd, determined in this study is consumptionof CH4. It useda 20 stepmechanism to describe in fact the sum of these two rate constants. reactionsof CH4 in irradiatedC12-CH4-air mixtures and included reactionsof C1 atoms and OH radicals (the latter formed from 3. Results and Discussion CH3OOH) with CH4 and its productsHCHO, CH3OOH, CH3OH,HC1, and H20 2 [Atkinsonet al., 1999]. At theend of Results of the first room temperature(299 K) carbon KIE thecomputer simulation less than --3% of theCH 4 consumptionexperiment are displayed in Figure 1. Following the form of was attributableto reactionwith speciesother than C1 atoms. equation(2), thefraction of remainingCH 4 is plottedagainst the Gas samples (12 L STP) were withdrawn from the cell at logarithmofthe ratio of the two 1513C-CH4 terms for a reaction regular intervals throughout the reaction using a Teflon run to 76% completion. A curve fit to the data using a least diaphragmpump (KNF Neuberger,Inc., Princeton,NJ). Gas was squares regression method with residual sum of squares collected into evacuated 6-L electropolishedstainless steel uncertainty analysis results in a calculated carbon KIE of canistersfor analyses.CH 4 mixingratio was determined using a kc12/kcl3= 1.0621_+0.0004 (20,R 2 = 0.99999).We repeated this HewlettPackard 5880 Gas Chromatographwith flame ionization experimentin a reactionrun only to 48% completionto checkthe detector.Samples were measured against reference gas CH 4 reproducibilty of our experimental methods. In the second preparedby Scott-Marin,Inc. whichhad been calibrated using a carbonKIE experimentat T = 299 K, a similar curve fit resulted NOAA/CMDL standard [Lang et al, 1990]. Measurement in a calculatedKIE of kc12/kc13 = 1.0621-+0.0007 (20,R 2 = uncertaintywas 0.25-0.50% over the concentration range of CH4. 0.99991). Based on theseresults, subsequent reactions were run Preparationof the samplesfor isotopicanalysis used either of only until approximately50% of CH4 wasreacted in orderto two combustionvacuum lines designed to separateCH 4 from keep experimentsrelatively short. otherreaction constituents and to convertit to CO2 andH20. In every experimentalreaction run except the first one, both Vacuumline designsand procedures are describedin Tyleret al. the carbon and hydrogen KIEs were studied simultaneously. [1997,1999]. To analyzeD/H ratios,the H20 fromCH 4 was Figure 2 displaysthe hydrogenKIE resultsat room temperature reducedto H2 by reactingit withzinc shot at 500øCfollowing the from the secondexperimental reaction run (48% completionrun). methodof Colemanet al. [1982]. Least squaresregression of the curve resultedin a hydrogenKIE Isotopicmeasurements were madeon a Finnigan-Matmodel ofkh/k d- 1.47_+0.02(2c5,R 2 = 0.993). 252 isotoperatio mass spectrometer. Working C andH isotope Additional determinations were made at 273 K, 323 K, and standardswere from OztechGas Company(Dallas, TX). Ratios 349 K. The resultsfrom theseexperiments
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