Effects of natural and anthropogenic CH4 sources on variations of atmospheric CH4 over western Siberia identified by its carbon and hydrogen stable isotopes

Western Siberia is thought to be important for the global CH4 budget due to the presence of large natural and anthropogenic CH4 sources. We have conducted aircraft observations of CH4 concentration at altitudes of 0.5-7 km over the region once a month since 1993. Deviations of the CH4 concentration from the best-fit curves obtained at the respective altitudes indicated that the lowest layer below 2 km was generally influenced by the local sources. In order to examine effects of the sources on the observed CH4 variations, measurements of carbon and hydrogen stable isotopic ratios of atmospheric CH4 (δ13CH4 and δD-CH4) at altitudes of 1 and 2 km were also made during 2005-2009. In most observation flights, the CH4 concentration was higher at 1 km than that at 2 km with lower δ13CH4 and δD-CH4 values. These vertical gradients, which were pronounced in summer, imply the presence of strong CH4 sources at the ground surface with low δ13CH4 and δD-CH4 values. δ13CH4 at 1 km showed its seasonal minimum in late summer, while the CH4 concentration and δD-CH4 did not show such clear seasonality, because they were masked by large variability. By inspecting relationships between the CH4 concentration and δ13CH4 (δD-CH4) (Miller/Tans plot), we examined the CH4 sources influencing on atmospheric CH4 variations in various timescales. δ13CH4 and δD-CH4 values of the CH4 source in winter (December-April) were estimated to be -41.2±1.8‰ and -187±18‰, respectively, while those in summer (June-October) to be -65.0±2.5‰ and -282±25‰. Since these isotopic values for respective seasons were consistent with those of CH4 originated from fossil and biogenic sources, fossil fuel and wetlands would be the primary CH4 sources in winter and summer, respectively. It was also indicated from various Miller/Tans plots that fossil source brought relatively short-term variations of atmospheric CH4 while wetlands did long-term variations, which might be explained by spot emission of fossil CH4 such as ventilation or leakage from pipelines and gas delivery stations and by broad emission from vast areas of wetlands in this region. We calculated seasonal changes of δ13CH4 and δD-CH4 of the western Siberian source based on CH4 emission scenario employed in an atmospheric chemistry transport model, in which CH4 emission from boreal wetlands was enhanced in summer, and those values were compared with δ13CH4 and δD-CH4 values of the CH4 sources estimated in this study. The results indicated that the present CH4 emission scenario was reasonable in terms of a ratio of biogenic and fossil CH4 emissions.