| Summary: | This study examined the impact of foreign SO<sub>2</sub> emission changes on the aerosol direct radiative effects (ADRE) in South Korea. Simulations that applied basic emissions (BASE) and simulations that applied reduced SO<sub>2</sub> emissions from foreign sources (R_FSO2) were performed, respectively, using the Weather Research and Forecasting (WRF)-Community Multiscale Air Quality (CMAQ) two-way coupled model. In addition, the difference between the two experimental results was calculated (i.e., R_FSO2 minus BASE) to quantitatively identify the impact of foreign SO<sub>2</sub> emission reduction. The reduction in foreign SO<sub>2</sub> emissions caused a decrease in the concentration of SO<sub>2</sub> flowing in from overseas to South Korea. As a result, a clear decrease in SO<sub>4</sub><sup>2−</sup> concentration was shown mainly in the southwest coast of South Korea. The difference in PM<sub>2.5</sub> concentration in South Korea according to the foreign SO<sub>2</sub> emission reduction did not correspond to the difference in SO<sub>4</sub><sup>2−</sup> concentration; it was determined in a complex way by the changes in SO<sub>4</sub><sup>2−</sup> concentration caused by SO<sub>2</sub> concentration changes, and the subsequent series of changes in NO<sub>3</sub><sup>−</sup> and NH<sub>4</sub><sup>+</sup> concentrations. The differences in SO<sub>4</sub><sup>2−</sup> and PM<sub>2.5</sub> concentrations caused by the foreign SO<sub>2</sub> reduction also affected the ADRE changes in South Korea. The distribution of ADRE difference between the two experiments was not consistent with the distribution of PM<sub>2.5</sub> concentration difference, but it was very similar to the distribution of SO<sub>4</sub><sup>2−</sup> concentration difference. These results imply that the ADRE of South Korea is not simply proportional to PM<sub>2.5</sub> concentration and may be determined by concentration changes of SO<sub>4</sub><sup>2−</sup>.
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