Methane carbon isotope effects caused by atomic chlorine in the marine boundary layer: Global model results compared with Southern Hemisphere measurements

Abstract

Recent measurements of the apparent kinetic isotope effect (KIE) of the methane (CH 4 ) atmospheric sink in the extratropical Southern Hemisphere (ETSH) have shown the apparent KIE to be larger in magnitude than expected if the sink were the hydroxyl radical (OH • ) alone. We present results from simulations using the U.K. Met Office's Unified Model (UM) to evaluate whether atomic chlorine (Cl • ) in the marine boundary layer (MBL) could give this effect. We modify the UM to include sources of 12 CH 4 and 13 CH 4 , soil and stratospheric sinks, and a tropospheric OH • sink. Also included is a Cl • sink in the MBL with a large seasonal cycle and a constant mean value (Cl • mean ) in latitude. We show that analysis of the simulated seasonal cycles in CH 4 mixing ratio and δ 13 C give an accurate estimate of the OH • KIE at ETSH midlatitudes. The apparent KIE of the combined OH • and Cl • sink increases in magnitude as Cl • mean increases. The experimentally measured values of apparent KIE in the ETSH midlatitudes of −15‰ in 1994–1996 and −7‰ in 1998–2000 are attained with MBL Cl • mean values of 28 × 10 3 atoms cm −3 and 9×10 3 atoms cm −3 , respectively (although we consider the latter to be a lower bound). We suggest that 18×10 3 atoms cm −3 is a reasonable midrange estimate of Cl • mean in the MBL. This value results in a Cl • sink strength of 25 Tg y −1 (range 13–37 Tg y −1 ) and an enrichment in δ 13 C of atmospheric CH 4 by 2.6‰ (range 1.4–3.8‰). This sink strength is significant but has not yet been included in global CH 4 budgets.