Isotopic Abundances in the Atmosphere and Sources

Abstract

The ultimate goal of isotopic studies of atmospheric CH4 is to contribute to the understanding of the atmospheric CH4 cycle by determining the relative fluxes from various categories of sources and the causes of the increasing concentration (Stevens and Engelkemeir, 1988; Quay et al.,1988; Wahlen et al., 1989). Because the large number of generic anthropogenic source types makes it impossible to determine their relative strengths based on carbon-13 data alone, Stevens and Engelkemeir (1988) and Craig et al. (1988) used the isotopic data to calculate the flux of the source with the greatest uncertainty, namely biomass burning, making use of the estimated fluxes for the other sources from emission inventories. This method determined the flux and isotopic composition of the natural sources from the concentration and isotopic composition of CH4 in old polar ice cores assuming the same lifetime as now. The lifetime was mostly determined by the fluxes based on the emissions inventories. This approach does not use the lifetime as a constraint nor contribute to the knowledge of the major sources, which have significant uncertainties in the estimates based on emissions inventories. A better approach is to start with the constraint of the lifetime value based on the methyl chloroform cycle (see Mayer et al., 1982; Khalil and Rasmussen, 1983; Prinn et al., 1987; Cicerone and Oremland, 1988). Then it is possible to calculate the fluxes of the two most isotopically different sources, providing an estimate based on emissions inventories for one of the anthropogenic sources is used as a constraint. A source is chosen that introduces the least error, namely landfills, which is one of the smallest and has an isotopic composition closest to the average.