| Summary: | This study focuses on the estimation of top-down NO<sub>x</sub> emissions over East Asia, integrating information on the levels of NO<sub>2</sub> and NO, wind vector, and geolocation from Ozone Monitoring Instrument (OMI) observations and Weather Research and Forecasting (WRF)-Community Multiscale Air Quality (CMAQ) model simulations. An algorithm was developed based on mass conservation to estimate the 30 km × 30 km resolved top-down NO<sub>x</sub> emissions over East Asia. In particular, the algorithm developed in this study considered two main atmospheric factors—(i) NO<sub>x</sub> transport from/to adjacent cells and (ii) calculations of the lifetimes of column NO<sub>x</sub> (τ). In the sensitivity test, the analysis showed the improvements in the top-down NO<sub>x</sub> estimation via filtering the data (τ ≤ 2 h). The best top-down NO<sub>x</sub> emissions were inferred after the sixth iterations. Those emissions were 11.76 Tg N yr<sup>−1</sup> over China, 0.13 Tg N yr<sup>−1</sup> over North Korea, 0.46 Tg N yr<sup>−1</sup> over South Korea, and 0.68 Tg N yr<sup>−1</sup> over Japan. These values are 34%, 62%, 60%, and 47% larger than the current bottom-up NO<sub>x</sub> emissions over these countries, respectively. A comparison between the CMAQ-estimated and OMI-retrieved NO<sub>2</sub> columns was made to confirm the accuracy of the newly estimated NO<sub>x</sub> emission. The comparison confirmed that the estimated top-down NO<sub>x</sub> emissions showed better agreements with observations (R<sup>2</sup> = 0.88 for January and 0.81 for July).
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