Identifying emission sources of CH₄ in East Asia based on in-situ observations of atmospheric δ¹³C-CH₄ and C₂H₆

Methane (CH₄) is the second most important greenhouse gas influenced by human activity. The increase in atmospheric CH₄ concentrations contributed ~23 % to the anthropogenic radiative forcing (Saunois et al., 2020). The current anthropogenic CH₄ 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 CH₄, it is important to identify spatiotemporal emission sources, in particular those from East Asia - one of the largest CH₄ emitters. In this study, we present in-situ observations of atmospheric CH₄ concentrations (i.e., dry air mole fractions in part per billion (ppb)) and carbon isotopic compositions of CH₄ 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 CH₄ baseline concentrations was 11 ± 1 ppb yr^-1. The enhanced pollution concentrations of CH₄ showed seasonally distinctive correlations with the corresponding δ¹³C-CH₄. The CH₄ 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 CH₄ emissions. Based on the δ¹³C-CH₄ source signatures, we were able to infer that the proportional contribution of microbial sources to CH₄ summer emissions was ranges from 45 to 79 %. The finding indicates that microbial sources account for a substantial portion of CH₄ summer emissions, consistent with estimates of 74-80 % derived from the observed correlation between CH₄ and C₂H₆, which serves as a complementary tracer for fossil fuel sources.