Contributions of natural and anthropogenic sources to atmospheric methane variations over western Siberia estimated from its carbon and hydrogen isotopes

Aircraft measurements of carbon and hydrogen isotopic ratios of atmospheric CH₄ (δ¹³CH₄ and δD-CH ₄), with the respective precisions of 0.08‰ and 2.2‰, as well as CH₄ concentration were made at 1 and 2 km altitudes over western Siberia during 2006-2009. δ¹³CH₄ and δD-CH₄ were almost always lower at lower altitudes, while the CH₄ concentration was higher, implying strong sources on the ground with low isotopic values. δ¹³CH₄ showed a clear seasonal minimum in the late summer, while seasonality of CH₄ and δD-CH₄ was ambiguous due to the local disturbances. By inspecting the relationships between the CH₄ concentration and isotopes, we found that isotopic source signatures in the winter (December-April) are -41.2 ± 1.8 and -187 ± 18‰ for δ¹³CH₄ and δD-CH₄, respectively, and the corresponding values in the summer (June-October) are -65.0 ± 2.5 and -282 ± 25‰. These values indicate predominant CH₄ emissions from fossil fuel facilities in the winter and wetlands in the summer. It was also found that the shorter-term CH₄ variations are more influenced by fossil CH₄ than that from wetlands. The finding presumably reflects the fact that the former is released from limited areas such as leakage from fossil fuel facilities, while the latter is released from a vast expanse of wetland. By employing a CH₄ emission data set used in an atmospheric chemistry transport model, we calculated seasonal isotopic changes of CH₄ sources in western Siberia and compared them to the estimates obtained in this study. The results indicated that the seasonal change in the CH₄ emission data set is reasonable, at least in terms of a ratio of fossil to biogenic emissions.