Abstract. To quantify CH4 emissions at policy-relevant spatial scales, the Korea Meteorological Administration (KMA) started monitoring its atmospheric levels in 1999 at Anmyeondo (AMY), and expanded monitoring to Jeju Gosan Suwolbong (JGS) and Ulleungdo (ULD) in 2012. The monitoring system consists of a Cavity Ring Down Spectrometer (CRDS) and a new cryogenic drying method, with a measurement uncertainty (68 % c.i.) of 0.7–0.8 ppb. To determine the regional characteristics of CH4 at each KMA

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... ation, we assessed the CH4 level relative to local background (CH4xs), analyzed local surface winds and CH4 with bivariate polar plots, and investigated CH4 diurnal cycles. We also compared the CH4 levels measured at KMA stations with those measured at the Mt. Waliguan (WLG) station in China and Ryori (ROY) station in Japan. CH4xs followed the order AMY (55.3±37.7 ppb) > JGS (24.1±10.2 ppb) > ULD (7.4±3.9 ppb). Although CH4 was observed in well mixed air at AMY, it was higher than at other KMA stations, indicating that it was affected not only by local sources but also by distant air masses. Annual mean CH4 was highest at AMY among all East Asian stations, while its seasonal amplitude was smaller than at JGS, which was strongly affected in the summer by local biogenic activities. From the long-term records at AMY, we confirmed that the source of CH4xs changed from the past (2006 to 2010) to recent (2016 to 2020) years in East Asia. Especially in northern China, CH4xs was mainly attributed to fossil fuel combustion or biomass burning during 2006–2010, but mainly to biogenic activities during 2016–2020, as indicated by decreasing δ13CH4 from the northern China. CH4 emissions in the southern part of China and in South Korea were enhanced by biogenic signals during 2016–2020. We confirmed that long-term high-quality data can help understand changes in CH4 emissions in East Asia.