Top-down NOx estimation from TROPOMI observation over East Asia

Atmospheric nitrogen oxides play a crucial role as a precursor of nitrate and ozone formations. East Asia is still one of the significant contributors to the global NO_x budget. Therefore, we evaluated the accuracy of bottom-up and top-down NO_x emissions by comparing tropospheric NO₂ columns simulated from the CMAQ with those from the TROPOMI over East Asia for four months. The CMAQ simulation using the UNIMIX inventory illustrates an overestimation of NO₂ columns by approximately 28-53% in Central East China (CEC) compared to the TROPOMI observation. In contrast, the South Korean (SK) region shows an underestimation of 8-28% for the same period, except for an overestimation of 18% in April. To improve the accuracy of NO_x emissions in East Asia, we estimated top-down NO_x emissions using an algorithm based on the Finite Difference Mass Balance (FDMB) method with TROPOMI observation data. The top-down approach indicated a decrease of 4-32% in emissions for the CEC region and an increase of 9-24% for the SK region compared to bottom-up estimates. Utilizing the top-down NO_x emission in the CMAQ simulation demonstrated enhanced spatial and temporal consistency with the TROPOMI-observed NO₂ columns. Additionally, we evaluated the accuracy of the top-down NO_x emission from a comparison between the simulation and independent in-situ observations of the AirKorea network.