NMVOC emission optimization in China through assimilating formaldehyde retrievals from multiple satellite products

Abstract. Non-methane volatile organic compounds (NMVOCs) serve as key precursors to ozone and secondary organic aerosols. Given that China is a major source of NMVOCs, the emission inventory is crucial for understanding and controlling atmospheric pollution. Mainstream inventories are constructed using bottom-up approaches, which cannot accurately reflect the spatiotemporal characteristics of NMVOCs, resulting in poor model outcomes. This study performed monthly optimization of NMVOC emissions in China by assimilating formaldehyde retrievals from the latest satellite products. A semi-variogram spatial analysis is conducted before assimilation, highlighting the advantages of using Tropospheric Monitoring Instrument (TROPOMI) and Ozone Mapping and Profiler Suite (OMPS) formaldehyde products for estimating high-resolution NMVOCs compared to Ozone Monitoring Instrument (OMI) retrievals. The emission optimization is performed based on a self-developed 4DEnVar-based system. A positive increment of NMVOC emissions was obtained by assimilating OMPS formaldehyde, with annual anthropogenic emissions rising from 22.40 to 41.32 Tg, biogenic emissions increasing from 16.56 to 28.01 Tg, and biomass burning emissions rising from 0.29 to 0.65 Tg. Our model simulations, driven by the posterior inventories, demonstrate superior performance compared to the prior. This is validated through comparisons against the independent satellite measurements and the surface ozone measurements. The RMSE of the posterior formaldehyde columns decreased from 0.49 to 0.45 ×10¹⁶ molec/cm² nationwide. In the severe-polluted NCP, it was improved effectively, reaching levels comparable to TROPOMI, with the RMSE dropping from 0.52 to 0.37 ×10¹⁶ molec/cm². Validation using surface ozone observations also yielded favorable results, especially in NCP.