A high-resolution regional CTDAS-WRF-Chem framework for constraining methane emissions over East Asia

As a major methane hotspot, East Asia urgently requires high-resolution, reliable emission maps to support mitigation strategies. We applied and evaluated a regional inversion framework by coupling the CarbonTracker Data Assimilation Shell (CTDAS) with WRF-Chem. This framework was used to perform inversions across East Asia (13.3°N–49.3°N, 99.8°E–150.2°E) for 2022 based on ground-based atmospheric methane observations, using a 9 km nested grid focused on the Yangtze River Delta (YRD) in eastern China. Pseudo-observation tests show robust recovery of the prescribed “true” emissions across three well-constrained regions: the YRD, Korea, and northern Japan (≥37°N). Among eight tested parameters and input datasets, the number of ensemble member accounts for most of the uncertainty reduction. In the YRD sectoral inversion, major sources (fuel exploitation, waste, natural, and agricultural emissions) are effectively corrected, whereas minor contributors (<6%, livestock and others) remain weakly constrained. Based on the in-situ inversion, we find that prior methane emissions over East Asia are generally overestimated. In the YRD, posterior annual emissions are reduced by 8.4%–22.4% (0.13–0.35 Tg a⁻¹) relative to the prior across the four provinces. The prior appears to underestimate emissions in summer, whereas it overestimates emissions in the other seasons. The strongest seasonal adjustment occurs in winter, with reductions of 27.3%–39.6%. In other Asian regions, inversion output shows that EDGARv8 underestimates northern Japan by 8.3% and overestimates Korea by 8.6%. This study provides the first benchmark of the CTDAS–WRF-Chem system for East Asia. Future works will assimilate multiple observation types over a longer period to deliver more comprehensive reference for mitigation planning, especially in China.