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[Spatio-temporal variation of atmospheric CH4 concentration and its driving factors in monsoon Asia.]

Based on the ground-based observations from seven atmospheric background stations during 2009 to 2018 in monsoon Asia (including BKT station in Indonesia, LLN and WLG stations in China, RYO and YON stations in Japan, TAP station in Republic of Korea, and UUM station in Mongolia), we analyzed the temporal and spatial variation of atmospheric CH4 concentration and its driving factors using harmonic model and maximal information-based nonparametric exploration. The results showed that the CH4 concentration in monsoon Asia varied from 1853.04 to 1935.61 nmol·mol-1, higher than that in Mauna Loa (MLO) station (1838.33 nmol·mol-1) in Hawaii, USA. The CH4 concentration decreased from north to south, with the highest value in TAP station (1935.61 nmol·mol-1) in Republic of Korea and RYO station (1907.19 nmol·mol-1) in Japan. The average seasonal amplitude at YON station in Japan was the largest (108.20 nmol·mol-1); while that at WLG station in China was the smallest (29.48 nmol·mol-1). The seasonal amplitude of TAP station in Republic of Korea changed faster at the rate of 4.49 nmol·mol-1·a-1. Except for WLG and TAP stations, CH4 concentrations were low in summer and high in winter. From the long-term perspective, the CH4 concentration at LLN (7.68 nmol·mol-1·a-1) and WLG (7.56 nmol·mol-1·a-1) stations in China exhibited the most obvious growth trend. Compared with wind speed, temperature and precipitation had greater impact on CH4 concentration, which were negatively associated with CH4 concentration. Local CH4 emission at some stations had a significant positive effect on CH4 concentration.