Modeling the effects of methane source changes on the seasonal cycles of methane mixing ratio and δ¹³C in Southern Hemisphere midlatitudes

The apparent kinetic isotope effect (ɛ A ) inferred from measurements of methane (CH 4 ) and δ 13 CH 4 in the extratropical Southern Hemisphere (ETSH) is significantly larger than expected if the sink were caused by hydroxyl radical (OH • ) alone. If the OH • sink were the primary driver for the observed seasonal cycles in CH 4 and δ 13 CH 4 in the ETSH, published laboratory data suggest an ɛ A of the order −3.9‰. However, a value of the order −11‰ has been inferred from measurements in the background troposphere. Atomic chlorine (Cl • ) in the marine boundary layer (MBL) could explain the difference, but an alternative hypothesis is that the large ɛ A in the ETSH is due to seasonally varying CH 4 emissions. We test this by sequentially omitting or doubling CH 4 emissions from each seasonally varying source in the UMeth model (Unified Model with Methane) and evaluating the corresponding effects on ɛ A at Baring Head (41.40°S). If only the OH • sink is included, the maximum change in ɛ A is about 1.7‰. When both OH • and a mean Cl • concentration of 18 × 10 3 atoms cm −3 in the MBL are included, the maximum change in ɛ A is about 1.1‰. For realistic changes in the CH 4 sources over a few years (∼10% of the simulated changes), we estimate that changes in ɛ A are of the same order as the uncertainties in the derivation of ɛ A from time series of CH 4 and δ 13 CH 4 over a comparable period. We conclude that realistic changes in seasonally varying CH 4 sources do not significantly affect ɛ A or previously derived estimates of the MBL Cl • sink using ɛ A .