Local and transboundary contributions to nitrogen loadings across East Asia using CMAQ-ISAM and GEMS-informed emissions inventory during the winter-spring transition 

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