Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 4 3 2004 10.5194/acpd-4-2283-2004 http://www.atmos-chem-phys-discuss.net/4/2283/2004/ http://www.atmos-chem-phys-discuss.net/4/2283/2004/acpd-4-2283-2004.html http://www.atmos-chem-phys-discuss.net/4/2283/2004/acpd-4-2283-2004.pdf 2283 2300 2004-05-04 Hemispheric average Cl atom concentration from ¹³C/¹²C ratios in atmospheric methane U. Platt ulrich.platt@iup.uni-heidelberg.de W. Allen D. Lowe Institut für Umweltphysik, University of Heidelberg, INF 229, D-69120 Heidelberg NIWA, Private Bag 14-901, Kilbirnie, Wellington, New Zealand Methane is a significant atmospheric trace gas in the context of greenhouse warming and climate change. The dominant sink of atmospheric methane is the hydroxyl radical (OH). Recently, a mechanism for production of chlorine radicals (Cl) in the marine boundary layer (MBL) via bromine autocatalysis has been proposed. The importance of this mechanism in producing a methane sink is not clear at present because of the difficulty of in-situ direct measurement of Cl. However, the large kinetic isotope effect of Cl compared with OH produces a large fractionation of ¹³C compared with ¹²C in atmospheric methane. This property can be used to estimate the likely size of the methane sink attributable to MBL Cl. By taking account of the mixing of MBL air into the free troposphere, we estimate that the global methane sink due to reaction with Cl atoms in the MBL could be as large as 19 Tg yr^−1, or about 3.3% of the total CH₄ sink.