Journal of Atmospheric Chemistry 23: 37-49, 1996. @ 1996 Kluwer Academic Publishers. Printed in the Netherlands. 37 Ab Initio Study on Carbon Kinetic Isotope Effect (KIE) in the Reaction of CH4 + Cl l NORIYUKI TANAKA* YITIAN XIAO and ANTONIO C. LASAGA Center for the Study of Global Change and Department of Geology and Geophysics, Yale University, PO. Box 208109, New Haven, CT 06511-8109, U.S.A. (Received: 29 June 1994; in final form: 5 December 1994) Abstract. Recent studies suggest that the destruction of methane by Cl* in the marine boundary layer could be accounted for as another major sink besides the methane destruction by OH l . High level ab initio molecular orbital calculations have been carried out to study the CH4 + Cl l reaction, the carbon Kinetic Isotope Effect (KIE) is calculated using Conventional Transition-State Theory (CTST) plus Wigner and Eckart semiclassical tunneling corrections. The calculated KIE is around 1.026 at 300 K and has a small temperature variation. This is by far the largest KIE among different processes involving atmospheric methane destruction (e.g., OH l , soil). A calculated mass balance of atmospheric methane including the KIE for the CH4 + Cl l reaction is found to favor those methane budgets with enhanced biological methane sources, which have relatively lighter carbon isotope composition. Key words: Ab initio, methane, chlorine, kinetic isotope effect 1. Introduction Methane is one of the major greenhouse gases in the atmosphere and its concen- tration has increased by 40% in the past 50 years (Stevens, 1988). To understand the global methane cycle becomes critical for the study of future climate change. Many studies have tried to establish a global methane budget through biostatistical analysis based on field flux measurements of various ecosystems. At best, however, current estimates of the methane budget based on these techniques have at least a factor of two uncertainty (Wahlen, 1993; Fung et&., 1991; Cicerone and Oremland, 1989; Houghton et al., 1990). Additional constraints on the relative magnitude of methane sources have been based on the stable isotope signal of methane (Rust, 1981; Stevens and Rust, 1982) and there has been an increasing emphasis on this approach (Tyler, 1986, 1987, 1992; Stevens and Engelkemeir, 1988; Wahlen et al., 1989, 1990; Quay et al., 1988, 1991). To calculate the isotopic budget, the isotopic fractionations of atmospheric methane removal due to different processes must be known. At the present time, only the isotopic fractionations arising from methane destruction by OH* and from soil oxidation have been considered in the model calculations. However, a recent study by Pszenny et al. (1993) suggests that the * Present address: Laboratory of Marine and Atmospheric Geochemistry (MAG), Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060, Japan.