Long-term changes in CH4 emissions: Comparing ΔCH4/ΔCO2 ratios between observation and proved model in East Asia (2010–2020).

To understand the changes in CH 4 emissions in East Asia, mainly in China, we analyzed the synoptic scale variability (SSV) of CO 2 and CH 4 mole fractions at Anmyeondo (AMY; 36.53°N, 126.32°E, 46 m above sea level) in South Korea using near-surface in-situ observations and an atmospheric chemical transport model (ACTM) to investigate the role of prescribed surface fluxes on SSV. The SSV of the tracers at AMY was mainly caused by emission variations from eastern and northeastern China during the winters in 2010–2020, as indicated by the simulated concentration footprints at AMY observed using the WRF-STILT model. The estimated SSV of ΔCH 4 /ΔCO 2 mole fraction ratios from the observations was 4.2–6.2 ppb ppm−1, with a mean of 5.7 ± 0.93 ppb ppm−1 during the winters in 2010–2020. We also calculated the CH 4 /CO 2 emission ratio from China's total annual fossil-fuel emissions, excluding seasonal sources estimated from the Emission Database for Global Atmospheric Research. We found that the ratio was overestimated by ∼98.1% compared to observations, which was likely driven either due to the overestimation of CH 4 or underestimation of CO 2. We discerned that the model-simulated SSV of the ΔCH 4 /ΔCO 2 ratios generally reproduced the observations when the CH 4 FixCoal simulation case was used. This implies that CH 4 from coal was a major contributor to the observed variations in the winter period ΔCH 4 /ΔCO 2 ratios. However, the model did not reproduce the observed ratio in 2012—this discrepancy requires further study. The trend of ΔCH 4 /ΔCO 2 was a slight decrease over time, which was attributed to the earlier onset of CH 4 emissions relative to fossil-fuel CO 2 , predominantly due to the reduction of CH 4 emissions from coal. In 2013, a relatively low tracer ratio was noted, which was caused by the large contribution of CO 2 from biomass burning. Overall, the observed AMY ΔCH 4 /ΔCO 2 ratios provide an efficient approach for validating the existing estimates of East Asian sectoral emissions of CH 4. • Synoptic scale variability (SSV) of CH 4 and CO 2 at Anmyeondo, South Korea was mainly caused by emission variations from eastern and northeastern China. • Simulated wintertime SSV of CH 4 a from coal was well matched with observations at Anmyeondo during 2010–2020. • EDGAR v6 China anthropogenic CH 4 /CO 2 emission ratio was overestimated against observation based SSV ΔCH 4 /ΔCO 2 mole fractions ratios. • Decline trend of wintertime SSV ΔCH 4 /ΔCO 2 mole fractions ratios Anmyeondo was attributed to a slower growth rate of China anthropogenic CH 4 emission compared to CO 2. • Observed low ΔCH 4 /ΔCO 2 ratio in 2013 was most likely caused by large contribution CO 2 from biomass burning. [ABSTRACT FROM AUTHOR]