## 3 Results and Discussion
### The Equivalent Ratios of Ion Components in Seoul and on Baengnyeong Island  
In order to calculate the equivalence ratio, the mass concentrations of \(\text{SO}_{4}{}^{2-}\), \(\text{NO}_{3}{}^{-}\) and \(\text{NH}_{4}{}^{+}\) (unit: \(\upmu\text{g m}^{-3}\)) were recalculated into the equivalent concentrations (unit: \(\upmu\text{eq m}^{-3}\)) considering molecular weight and valency. Tables 4 and 5 show the annual changes in equivalence ratios of \(\text{NH}_{4}{}^{+}\), \(\text{SO}_{4}{}^{2-}\) and \(\text{NO}_{3}{}^{-}\) in Seoul and Baengnyeong Island. Both in Seoul and on Baengnyeong Island, the ratios of \(\text{NH}_{4}{}^{+}\) to \(\text{SO}_{4}{}^{2-}\) increased; that is, the \(\text{SO}_{4}{}^{2-}\) concentration decreased compared to \(\text{NH}_{4}{}^{+}\). On the other hand, the ratios of \(\text{NH}_{4}{}^{+}\) to \(\text{NO}_{3}{}^{-}\) decreased, because the \(\text{NO}_{3}{}^{-}\) concentration increased compared to \(\text{NH}_{4}{}^{+}\) over the last 8 years. Until 2014, the ratios of \(\text{NH}_{4}{}^{+}\) to the sum of \(\text{SO}_{4}{}^{2-}\) and \(\text{NO}_{3}{}^{-}\) in Seoul and Baengnyeong were below 1, so they were all in a weakly acidic state. However, since  
Figure 9: Changes in the SOR and NOR according to PM2.5 concentration in Seoul: (**a**) SOR; (**b**) NOR. 2015, the NH\({}_{4}\)+-rich state has been maintained. This is consistent with the existing research results of [6,8] carried out on Baengnyeong Island. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Year**} & \(\mathbf{NH_{4}}\)\({}^{+}\) (\(\mathbf{\mu_{0}}\)\(\mathbf{m}\)\({}^{-3}\)) & \(\mathbf{NH_{4}}\)\({}^{+}\) (\(\mathbf{\mu_{0}}\)\(\mathbf{m}\)\({}^{-3}\)) & \(\mathbf{NH_{4}}\)\({}^{+}\) (\(\mathbf{\mu_{0}}\)\(\mathbf{m}\)\({}^{-3}\)) \\  & / \(\mathbf{SO_{4}}\)\({}^{2}\) (\(\mathbf{\mu_{0}}\)\(\mathbf{m}\)\({}^{-3}\)) & / \(\mathbf{NO_{3}}\)\({}^{-}\) (\(\mathbf{\mu_{0}}\)\(\mathbf{m}\)\({}^{-3}\)) & / \(\mathbf{\Sigma}\)\(\mathbf{SO_{4}}\)\({}^{2}\) and \(\mathbf{NO_{3}}\)\({}^{-}\) (\(\mathbf{\mu_{0}}\)\(\mathbf{m}\)\({}^{-3}\)) \\ \hline
2012 & 1.327 & 1.757 & 0.9586 \\
2013 & 1.3679 & 1.5337 & 0.9092 \\
2014 & 1.4881 & 1.8863 & 0.9747 \\
2015 & 1.6142 & 1.833 & 1.0837 \\
2016 & 1.5469 & 1.6618 & 1.0213 \\
2017 & 1.8686 & 1.4178 & 1.0275 \\
2018 & 1.8945 & 1.5126 & 1.0583 \\
2019 & 2.3263 & 1.5887 & 1.0981 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: The annual equivalent ratio in Seoul. Figure 10: Changes in the SOR and NOR according to PM2.5 concentration on Baengnyeong Island: (a) SOR; (b) NOR. As explained in the study by [PERSON] et al. [3], the SO\({}_{2}\) emissions in China have decreased since 2011, the SO\({}_{4}\)\({}^{2-}\) concentration in Northeast Asia has also decreased, and the surplus total NH\({}_{4}\)+ came to react with NO\({}_{3}\)\({}^{-}\). For this reason, when high PM2.5 events have occurred in recent years, the NO\({}_{3}\)\({}^{-}\) concentration has increased significantly instead of SO\({}_{4}\)\({}^{2-}\). By analyzing long-term data measured in Seoul and on Baengnyeong Island, we were able to confirm these trends once again. By analyzing the long-term data measured in Seoul and on Baengnyeong Island, it was possible to confirm the change in the concentration of air pollutants in the downwind area according to the change in China's emissions. Figure 11: Changes in the SOR and NOR according to RH in Seoul: (**a**) SOR; (**b**) NOR.