## 4 Summary and Conclusions  
It is important to find out the changes of the levels of air pollutants in Northeast Asia with respect to the changes of the emissions of air pollutants. In this study, we analyzed long-term trends of the chemical composition of fine particles and gaseous species in Seoul, a megacity, and on Baengryeong Island, a background area located in the Yellow Sea of Republic of Korea (South Korea) from 2012 to 2019. Using the Mann-Kendall trend test and Sen's slope, long-term changes of the concentrations of air pollutants were analyzed. Changes in major ion components in the atmosphere and the equivalence ratios were examined in relation to changes in the emissions of air pollutants in this area. Compared with previous studies using short-term measurement data or modeling results, this study used long-term actual high temporal resolution measurement data both in Seoul and on Baengryeong Island. In Seoul, most of the air pollutants, including PM2.5, showed a decreasing trend over the past 8 years. The major air pollutants including PM2.5 on Baengryeong Island also showed a decreasing trend; however, NO3\({}^{-}\) showed a tendency to increase. Compared to Baengryeong Island, Seoul has a larger decrease in SO4\({}^{2-}\) but a smaller decrease in SO2. To consider the change in emissions around Seoul metropolitan area, the SO2 emissions in the Chungcheongnam-do region, which has large-scale coal-fired power plants and industrial complexes, is on the rise, and this may affect the SO2 concentration of Seoul. Both in Seoul and on Baengryeong Island, when the concentration of PM2.5 increased, the proportion of NO3\({}^{-}\) increased more than the SO4\({}^{2-}\) proportion, and NOR increased  
Figure 12: Changes in the SOR and NOR according to RH on Baengryeong Island: (**a**) SOR; (**b**) NOR. more rapidly than SOR. That is, the contribution of NO3- rather than SO42- to the high-concentration PM2.5 has been increasing. As explained in the study by [PERSON] et al. (2020), according to the changes in the air pollutant emissions in this region, NO3- formed the majority of the aerosol proportions in this region. Seoul and Baengnyeong Island are rich in NH4+ in the atmosphere, and due to the SO2 emission reduction, the balance of ammonia-nitric acid-sulfuric acid has changed and the concentration of NO3- is increasing. This trend is more obvious on Baengnyeong Island than in Seoul due to the lower local emissions on Baengnyeong Island. As a result of this study, it was possible to confirm that concentrations of air pollutants and the majority of aerosols affecting PM2.5 concentrations in Northeast Asia have changed according to the change in emissions in this region. Conceptualization, N.-K.K. and J.-Y.L. ; methodology, N.-K.K. and J.-Y.L. ; software, N.-K.K. ; validation, N.-K.K. and J.-Y.L. ; formal analysis, N.-K.K. ; investigation, N.-K.K. ; resources, J.-Y.L. ; data curation, N.-K.K. and H.-J.S. ; writing--original draft preparation, N.-K.K. ; writing--review and editing, [PERSON]-P.K. and [PERSON]-Y.L. ; visualization, N.-K.K. ; supervision, Y.-P.K. ; project administration, J.-Y.L. ; funding acquisition, H.-J.S., [PERSON]-P.K. and [PERSON]-Y.L. All authors have read and agreed to the published version of the manuscript. This research was funded by the Ministry of Science and ICT (2020M3G1A1114537), the Ministry of Science and ICT (2019M1A2A2103953a) and the National Institute of Environmental Research, Korea (NIER-2021-03-02-013). Not applicable. Not applicable. Not available. This research was supported by the FRIEND (Fine Particle Research Initiative in East Asia Considering National Differences) Project through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (2020M3G1A1114537), Technology Development Program to Solve Climate Changes through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (2019M1A2A2103953a) and the National Institute of Environmental Research, Korea (NIER-2021-03-02-013).