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

Recent measurements of the apparent kinetic isotope effect (KIE) of the methane (CH4) 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 center dot) alone. We present results from simulations using the U. K. Met Office's Unified Model (UM) to evaluate whether atomic chlorine (Cl-center dot) in the marine boundary layer (MBL) could give this effect. We modify the UM to include sources of (CH4)-C-12 and (CH4)-C-13, soil and stratospheric sinks, and a tropospheric OH center dot sink. Also included is a Cl-center dot sink in the MBL with a large seasonal cycle and a constant mean value (Cl-mean(center dot)) in latitude. We show that analysis of the simulated seasonal cycles in CH4 mixing ratio and delta C-13 give an accurate estimate of the OH center dot KIE at ETSH midlatitudes. The apparent KIE of the combined OH center dot and Cl-center dot sink increases in magnitude as Cl-mean(center dot) increases. The experimentally measured values of apparent KIE in the ETSH midlatitudes of -15 parts per thousand in 1994-1996 and -7 parts per thousand in 1998-2000 are attained with MBL Cl-mean(center dot) values of 28 x 10(3) atoms cm(-3) and 9 x 10(3) atoms cm(-3), respectively (although we consider the latter to be a lower bound). We suggest that 18 x 10(3) atoms cm(-3) is a reasonable midrange estimate of Cl-center dot mean in the MBL. This value results in a Cl-center dot sink strength of 25 Tg y(-1) (range 13-37 Tg y(-1)) and an enrichment in delta C-13 of atmospheric CH4 by 2.6% ( range 1.4-3.8 parts per thousand). This sink strength is significant but has not yet been included in global CH4 budgets.