Comparison of two top-down NOx emission estimates over East Asia using TROPOMI observation

Atmospheric nitrogen oxides (NOₓ) are key precursors of nitrate aerosols and tropospheric ozone, and East Asia remains one of the largest contributors to the global NOₓ budget. In this study, we derived and evaluated top-down NOₓ emission estimates over East Asia using two mass-balance-based approaches constrained by TROPOMI tropospheric NO₂ column observations. The first approach follows Han et al. (2020), which explicitly accounts for the lifetime of column-integrated NOₓ and grid-scale inflow and outflow of NOₓ molecules within a refined mass balance framework. The second approach is based on the widely used Finite Difference Mass Balance Approach (FDMA). Three-dimensional air quality simulations were conducted using the CMAQ model for representative periods in July and October 2022 and January and April 2023, with each episode simulated for seven consecutive days. The two independently derived top-down NOₓ emission datasets were implemented in the model and compared against simulations driven by conventional bottom-up inventories. For all seasons, CMAQ simulations using both top-down emissions reproduced the spatial and temporal variability of TROPOMI-observed NO₂ columns more accurately than those using bottom-up emissions alone. Although regional discrepancies were found among South Korea (SK), Central East China (CEC), and the entire modeling domain, the Han et al. (2020)-based method generally exhibited higher agreement and stronger correlations with satellite observations than the FDMA-based approach. The derived top-down emissions showed substantial deviations from bottom-up estimates, with region- and season-dependent increases or decreases. For example, monthly NOₓ emissions over China ranged from approximately 695–808 GgN month⁻¹ in bottom-up inventories, while the Han et al. (2020) and FDMA approaches yielded values up to ~746–859 GgN month⁻¹ and ~820–1041 GgN month⁻¹, respectively. Similar seasonal contrasts were identified over the Korean Peninsula and Japan. Further evaluation of the top-down NOₓ emissions will be conducted using independent surface observations to provide additional constraints on their reliability and applicability for air quality management.