Evaluation of model-simulated atmospheric methane over South Korea using ground-based and aircraft observations

Methane (CH₄), a Kyoto Protocol-regulated greenhouse gas produced by multiple pathways, remains challenging to characterize regionally due to spatiotemporal limitations of current measurements. In this study, we present a regional-scale CH₄ modeling framework using the GEOS-Chem atmospheric chemistry-transport model over South Korea and evaluate the model performance with ground and aircraft observations. Model performance was evaluated using monthly mean CH₄ concentrations observed at three ground-based monitoring sites (Anmyeondo, Gosan, and Ulleungdo) by the Korea Meteorological Administration (KMA) from 2017 to 2022. The nested regional-scale modeling was conducted at a horizontal resolution of 0.25°×0.3125° using boundary conditions from coarser 2°×2.5° global simulations driven by Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2) meteorological fields. The model showed good agreement with surface observations, with a root mean square error (RMSE) of 19.63 ppb (0.99% of the mean CH₄) and a correlation coefficient (r) of 0.84. Validation against vertical profiles showed a systematic underestimation when evaluated across all observation-model pairs. The RMSE was 32.55 ppb (1.65% of the mean CH₄), with a r of 0.77, with the level of agreement relatively declining at higher altitudes. In addition, we used the KMA aircraft observations to identify regional-scale characteristics in CH₄ concentrations over South Korea often influenced by dominant emissions. The findings are expected to provide a scientific basis for future investigations of the spatiotemporal variability of CH₄ and its key drivers over South Korea.