Keywords:atmospheric CH4, atmoshperic CO2, synoptic variation
Atmospheric mixing ratios of greenhouse gases including CO2 and CH4 have been monitored at Hateruma Island, Japan (HAT; lat. 24.1°N, long. 123.8°E) for more than 20 years by the National Institute of Environmental Studies (NIES). The observed CO2 and CH4 show secular increasing trends and seasonal variations, typical of a regional background sites, and consist of synoptic variations (ΔCO2 and ΔCH4) with duration ranging from several hours to several days. These synoptic variations were contain signals arising from the regional emissions from the continental East Asia, especially China, and were observed more often during late fall to early spring because of the suppression of the continental emissions signal during the East Asian monsoon in summer. Since the observed ΔCO2 and ΔCH4 in wintertime generally showed fairly good correlations, the variation ratio (ΔCH4/ΔCO2 ratio) can be used to constrain the regional emission ratio. In previous study (Tohjima et al., 2014), based on the fact that the fossil fuel CO2 emissions were relatively well determined, we obtained the increasing rate of the CH4 emission from China (about 1 Tg-CH4 yr-2) in 2000s from the comparison of theΔCH4/ΔCO2 ratios between the observation and simulation. In this study, we revisited the temporal change in ΔCH4/ΔCO2 ratio at HAT and investigate the recent change in the CH4 emissions from China. The observed ΔCH4/ΔCO2 ratio showed decrease for the entire period (1997-2019) from about 13 ppb/ppm to 8 ppb/ppm while its decreasing rate showed a gradual slowdown and appeared to reach bottom after 2010. Such change in the decreasing rate is mostly attributed to the slowdown of the increase in the fossil fuel CO2 emissions from China. We calculated ΔCO2 and ΔCH4 by using a Lagrangian Particle Dispersion Model (LPDM) and several CH4 and CO2 flux maps based on bottom-up and top-down approaches. Then we scaled the CH4 emissions from China so as to match the calculated ΔCH4/ΔCO2 ratio with the observation. The results suggested that the linear increasing trend of the CH4 emissions from China for 2010s is significantly reduced in comparison with that for the 2000s.