Local and transboundary contributions to NO _<i>y</i> loadings across East Asia using CMAQ-ISAM and a GEMS-informed emission inventory during the winter–spring transition

Abstract. We investigated source contributions of nitrogen oxides (NOx) emissions to reactive nitrogen species (NOy) loadings across East Asia during the 2022 winter–spring transition. Using the Community Multiscale Air Quality (CMAQ) model and its Integrated Source Apportionment Method (ISAM), we conducted air quality simulations, leveraging top-down estimates of NOx emissions adjusted by the Geostationary Environment Monitoring Spectrometer (GEMS) tropospheric nitrogen dioxide (NO2) columns. After the Bayesian inversion, the inventoried NOx emissions increased by 50 % in South Korea and 33 % in China compared to the a priori estimates, which substantially reduced the model's prior underestimation of surface NO2 concentrations from −32.75 % to −13.01 % in South Korea and from −10.26 % to −3.04 % in China. We compared local and transboundary contributions of NOx emissions to NOy concentrations across East Asia. Local contributions showed a declining trend, from 32 %–43 % in January to 23 %–30 % by May, while transboundary contributions consistently increased from 16 %–33 % in January to 27 %–37 % by May. North China consistently contributed over 10 % to East Asia's NOy loadings. East China and South Central China were significant contributors to each other's NOy budget by 9 %–12 %. South Central China transboundary contributions consistently outweighed local contributions by 5 %, indicating vulnerability to pollution transport. South Korea, initially the least influential, contributed 1 %–4 % to transboundary NOy concentrations in January. This increased to 6 %–7 % by May, becoming comparable to other regions' contributions. These behaviors of NOy were driven by distinct synoptic settings, where strong wintertime northwesterly winds directed pollutants southeastward, while their weakening in spring led to more multidirectional transport patterns, allowing pollutants to spread more broadly across the regions.