Tracing sources of atmospheric methane using clumped isotopes.

We apply a recently developed measurement technique for methane (CH 4 ) isotopologues * (isotopic variants of CH 4 - 13 CH 4 , 12 CH 3 D, 13 CH 3 D, and 12 CH 2 D 2 ) to identify contributions to the atmospheric burden from fossil fuel and microbial sources. The aim of this study is to constrain factors that ultimately control the concentration of this potent greenhouse gas on global, regional, and local levels. While predictions of atmospheric methane isotopologues have been modeled, we present direct measurements that point to a different atmospheric methane composition and to a microbial flux with less clumping (greater deficits relative to stochastic) in both 13 CH 3 D and 12 CH 2 D 2 than had been previously assigned. These differences make atmospheric isotopologue data sufficiently sensitive to variations in microbial to fossil fuel fluxes to distinguish between emissions scenarios such as those generated by different versions of EDGAR (the Emissions Database for Global Atmospheric Research), even when existing constraints on the atmospheric CH 4 concentration profile as well as traditional isotopes are kept constant.