Identifying emission sources of CH in East Asia based on in-situ observations of atmospheric δC-CH and CH.

Methane (CH) is the second most important greenhouse gas influenced by human activity. The increase in atmospheric CH concentrations contributed ~23 % to the anthropogenic radiative forcing (Saunois et al., 2020). The current anthropogenic CH emissions trajectory implies that large emissions reductions are needed to meet the target of the Paris Agreement (Nisbet et al., 2019). For effective regulation of CH, it is important to identify spatiotemporal emission sources, in particular those from East Asia - one of the largest CH emitters. In this study, we present in-situ observations of atmospheric CH concentrations (i.e., dry air mole fractions in part per billion (ppb)) and carbon isotopic compositions of CH made during 2017-2020 at the Gosan station (GSN, 33.3°N, 126.2°E, 72 m a.s.l) which is representative of regional background conditions in East Asia. The annual growth rate of the observed CH baseline concentrations was 11 ± 1 ppb yr. The enhanced pollution concentrations of CH showed seasonally distinctive correlations with the corresponding δC-CH. The CH source isotopic signature for winter derived based on both the Keeling and Miller-Tans approaches was -40.7 ± 3.4 ‰, suggesting dominant thermogenic sources (e.g., coal and/or gas combustion), whereas the source signature for summer was estimated as -54.1 ± 1.2 ‰, which seemed to represent both microbial sources (e.g., rice paddies) and fossil fuel sources of CH emissions. Based on the δC-CH source signatures, we were able to infer that the proportional contribution of microbial sources to CH summer emissions was ranges from 45 to 79 %. The finding indicates that microbial sources account for a substantial portion of CH summer emissions, consistent with estimates of 74-80 % derived from the observed correlation between CH and CH, which serves as a complementary tracer for fossil fuel sources.