Hemispheric average Cl atom concentration from 13C/12C ratios in atmospheric methane

[摘要] Methane is a significant atmospheric trace gas in the context of greenhousewarming and climate change. The dominant sink of atmospheric methane is thehydroxyl radical (OH). Recently, a mechanism for production of chlorineradicals (Cl) in the marine boundary layer (MBL) via bromine autocatalysishas been proposed. The importance of this mechanism in producing a methanesink is not clear at present because of the difficulty of in-situ directmeasurement of Cl. However, the large kinetic isotope effect of Cl comparedwith OH produces a large fractionation of ¹³C compared with ¹²C inatmospheric methane. This property can be used to estimate the likelyminimum size of the methane sink attributable to MBL Cl. By taking accountof the mixing of MBL air into the free troposphere, we estimate that theglobal methane sink due to reaction with Cl atoms in the MBL could be aslarge as 19Tgyr^-1, or about 3.3% of the total CH₄ sink.However, its impact on the methane stable carbon isotope budget is large andwarrants further attention.

[学科分类] 大气科学