Rate Coefficients for the Reactions of Hydroxyl Radicals with Methane and Deuterated Methanes Tomasz Gierczak, ² Ranajit K. Talukdar, Scott C. Herndon, ‡ Ghanshyam L. Vaghjiani, § and A. R. Ravishankara* ,‡ National Oceanic and Atmospheric Administration, Aeronomy Laboratory, 325 Broadway, Boulder, Colorado 80303, and CooperatiVe Institute for Research in EnVironmental Sciences UniVersity of Colorado, Boulder, Colorado 80309 ReceiVed: NoVember 21, 1996; In Final Form: February 11, 1997 X The rate coefficients for the reaction of OH with CH 3 D(k 1 ), CH 2 D 2 (k 2 ), CHD 3 (k 3 ), CD 4 (k 4 ), and CH 4 (k 5 ) as well as that of OD with CH 4 (k 6 ) have been measured using the pulsed photolytic production of OH followed by its detection via pulsed laser induced fluorescence. k 1 -k 4 and k 6 were measured between ∼220 and ∼415 K, while k 5 was measured down to 195 K. The measured rate coefficients do not strictly obey the Arrhenius expression. However, below 298 K, they can be represented by the expressions (in cm 3 molecule -1 s -1 ): k 1 ) (3.11 ( 0.44) × 10 -12 exp[-(1910 ( 70)/T]; k 2 ) (2.3 ( 1.2) × 10 -12 exp[-(1930 ( 250)/T]; k 3 ) (1.46 ( 0.22) × 10 -12 exp[-(1970 ( 70)/T]; k 4 ) (1.00 ( 0.22) × 10 -12 exp[-(2100 ( 120)/T]; k 5 ) (1.88 ( 0.11) × 10 -12 exp[-(1695 ( 30)/T]; k 6 ) (1.68 ( 0.12) × 10 -12 exp[-(1640 ( 40)/T]. The obtained values of the rate coefficients and kinetic isotope effects are compared with values previously measured or calculated by other groups. The atmospheric implications of this data are briefly discussed. Introduction Methane is one of the most important and the most abundant trace organic gases in the atmosphere. It is one of the main reactants for the OH radical, which is the primary oxidant in the troposphere. Hence, CH 4 controls the abundance of OH in the troposphere. Oxidation of methane leads to ozone produc- tion. The importance of methane increases in the remote, clean troposphere, where it could be practically the only hydrocarbon. Because it absorbs well in the atmospheric infrared window and because of its large abundance, methane is one of the primary greenhouse gases contributing as much as 20% to the radiative forcing of the industrial atmosphere. 1 Transport of methane from the troposphere to the stratosphere, followed by its oxidation, provides the stratosphere with a large fraction of the water vapor. In addition, oxidation of methane in the presence of sufficient concentrations of nitrogen oxides leads to further production of OH and, hence, acts as an amplifier of HO x species. For these reasons, methane is considered one of the most important constituents of the Earth’s atmosphere. Methane is produced by natural and human influenced biological activity as well as via fossil fuel usage. 2,3 Our ability to predict the future abundance of CH 4 in the atmosphere requires a quantitative knowledge of the sources and sinks of this molecule. The major process for the removal of methane from the atmosphere is its reaction with the OH radical. Other minor pathways include soil uptake. Because the atmospheric concentration of CH 4 is very well measured and its main process, i.e., its reaction with the OH radical, is quantified, the total flux of CH 4 into the atmosphere is reasonably well established. However, quantification of the individual sources of CH 4 remains elusive. The reason for this difficulty is the diffuse nature of the sources, whose emissions are individually small and variable in time. One of the approaches employed to constrain the source strengths has been to use isotopic signatures of various emissions and the isotopic composition of the atmospheric methane. 2-5 It has been suggested that the use of deutero-isotopomers of methane will potentially yield more information than the 12 C, 13 C isotopomers. 4 One piece of information that is essential for this exercise is the atmospheric isotopic fractionation of methane, which is almost completely due to the differences in the rate coefficients for the reactions of OH with different isotopomers. This method has been successfully employed using 12 C, 13 C, and 14 C isotopomers. However, in these cases, the atmospheric fractionation is very small. On the other hand, D to H substitution in the methanes leads to larger differences in the fractionation and the differences in these abundances have been recently measured. 4,6,7 One of the major pieces of information needed for an analysis of methane budget using the measurements of isotopomers is accurate rate coefficients for the reaction of OH with deuterated methanes relative to that for the OH + CH 4 reaction at atmospheric temperatures. The rate coefficient for the reaction of OH with CH 4 is also needed to quantify the OH production in the lower stratosphere and upper troposphere, where the temperatures are often below 200 K, and where the methane oxidation acts as an amplifier of HO x . Rate coefficients at such low temperatures for the reaction of OH with CH 4 are currently poorly defined. Further, CD 4 has been used as a tracer of atmospheric motion and its lifetime is needed to evaluate the time scales over which it is a conserved tracer. In addition to their importance in the atmosphere, the OH+ CH 4 reaction has been a test bed for evaluating chemical kinetic theories and evaluating the capabilities to compute rate coef- ficients. Isotopically labeling the reactant does not change the potential energy surface for the reaction and, hence, provides an opportunity to test the ability of theorists to calculate rate * Author to whom the correspondence should be addressed at: NOAA, R/E/Al-2, 325 Broadway, Boulder, CO 80303. ² On leave from Department of Chemistry, Warsaw University, ul. Zwirki i Wigury 101, 02-089 Warsaw, Poland. ‡ Also associated with the Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 30809. § Hughes STX, Phillips Laboratory, PL/RKFT, 10E. Saturn Blvd., Edwards AFB, CA 93524. X Abstract published in AdVance ACS Abstracts, April 1, 1997. 3125 J. Phys. Chem. A 1997, 101, 3125-3134 S1089-5639(96)03892-3 CCC: $14.00 © 1997 American Chemical Society