Identifying Major CO2 and CH4 Emission Sources with the STILT Model in the Seoul Metropolitan Area During Winter 2024, Including the ASIA-AQ Campaign

Abstract

To investigate urban enhancements of greenhouse gases (GHGs), CO₂ and CH₄ were measured in Seoul from January to March 2024 including the Airborne and Satellite Investigation of Asian Air Quality (ASIA-AQ) campaign. Mean concentrations of both CO₂ and CH₄ exceeded those reported for other major urban cities during winter. Identified four high-concentration episodes occurred under elevated CO and low wind speeds, consistent with suppressed ventilation after long-range transport. The footprint analysis indicated that the contribution of South Korea was dominant in most episodes indicating mainly influenced by local emission sources; however, episode 4 showed a relatively larger upwind (other) contribution suggesting the long-range transport. The contribution of emission sectors indicated that buildings/heating and power generation dominated ΔCO₂ (subtracted by background concentration) variability, whereas waste management led ΔCH₄, with secondary contributions from agriculture, buildings, and fuel exploitation. Despite different major emission sectors, ΔCO₂ and ΔCH₄ converged with a narrow range with high correlation coefficient (R), reflecting co-located anthropogenic sources during wintertime. A diurnal variation of both ΔCO₂ and ΔCH₄ peaked in the morning, then reached local minimum in mid-afternoon with increasing PBL height. However, the diurnal variation of the simulated ΔCO₂ and ΔCH₄ from the Stochastic Time-Inverted Lagrangian Transport (STILT) model showed late-afternoon minimum compared to measured values because the STILT overestimates the afternoon vertical gradient of CO₂, likely owing to biases in diagnosed PBL height. This approach might provide scientific evidence to policy makers to attain the carbon neutral by suitable regulation for major emission sources when more dense spatial resolution of bottom-up inventory and meteorological fields are available.