Modeling atmospheric delta(CH4)-C-13 and the causes of recent changes in atmospheric CH4 amounts

Inclusion of kinetic isotope effects (KIEs) of methane (CH4) sinks (gaseous OH and Cl, and soil microbes) has a significant effect on modeled distributions of delta(13)C of atmospheric CH4. For a given scenario of surface sources and corresponding delta(13)C values of individual CH4 sources, the KIE due to soil uptake enriched delta(13)C by 1.18 parts per thousand in the model's northern hemisphere (NH) (40 degrees N) and 1.16 parts per thousand in the southern hemisphere (SH) (40 degrees S) under steady state conditions in January. The KIE due to CH4 oxidation by stratospheric Cl radicals further enriched these delta(13)C values at the surface by 0.99 parts per thousand and 1.03 parts per thousand respectively. In the vertical direction, during January at 50 degrees N, inclusion of a KIE due to Cl enriched delta(13)C at 18 km by 0.95 parts per thousand compared to the corresponding surface value, whereas the enrichment was only 0.31 parts per thousand when this KIE was omitted. These results suggest that modeling of delta(13)C distributions should include KIEs due to CH4 oxidation by soil and stratospheric chlorine radicals. It is shown that possible oxidation of CH4 in marine boundary layer by Cl radicals can significantly enrich delta(13)C. However, if a recent theoretical value for the KIE of the Cl and CH4 reaction is correct, then the impact of this reaction is less than the figures quoted above. In the model, monthly variations in OH concentration and interhemispheric exchange transport cannot reproduce the observed seasonal amplitude variation of delta(13)C in either the NH or SH. It is argued that seasonal variations in individual CH4 fluxes are primarily responsible for this discrepancy. We show that increasing Cl radical concentrations due to continued release of anthropogenic chlorocarbons enrich the delta(13)C values. The effects of an increase in tropospheric OH concentration due to stratospheric ozone depletion and a cooling of the troposphere due to the eruption of Mt. Pinatubo, with a lowering of water vapor concentration and reduction in isoprene emissions, on delta(13)C and surface CH4 mixing ratios are investigated. Other model simulations with adjusted surface CH4 fluxes have been performed to study the postulated explanations for recent changes in CH4 surface mixing ratios and delta(13)C values. A modified version of the Oslo two-dimensional global tropospheric photochemical model was used for all simulations.