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 CH4 emissions in East Asia, mainly in China, we analyzed the synoptic scale variability (SSV) of CO2 and CH4 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 ΔCH4/ΔCO2 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 CH4/CO2 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 CH4 or underestimation of CO2. We discerned that the model-simulated SSV of the ΔCH4/ΔCO2 ratios generally reproduced the observations when the CH4 FixCoal simulation case was used. This implies that CH4 from coal was a major contributor to the observed variations in the winter period ΔCH4/ΔCO2 ratios. However, the model did not reproduce the observed ratio in 2012—this discrepancy requires further study. The trend of ΔCH4/ΔCO2 was a slight decrease over time, which was attributed to the earlier onset of CH4 emissions relative to fossil-fuel CO2, predominantly due to the reduction of CH4 emissions from coal. In 2013, a relatively low tracer ratio was noted, which was caused by the large contribution of CO2 from biomass burning. Overall, the observed AMY ΔCH4/ΔCO2 ratios provide an efficient approach for validating the existing estimates of East Asian sectoral emissions of CH4.