## 3 Results and Discussion
### SOR and NOR in Seoul and on Baengnyeong Island According to PM2.5 Concentration and RH  
Figure 8 shows the changes in SOR and NOR in Seoul and on Baengnyeong Island from 2012 to 2019. SOR and NOR are indicators of the degree of oxidation of sulfur and nitrogen [7; 36; 37]. In both SOR and NOR, Baengnyeong Island showed higher values than Seoul throughout the measurement period. This means that the degree of the partitioning status of sulfur and nitrogen species to the particle phase in the atmosphere of Baengnyeong Island is greater than Seoul. As shown in Section 3.1, the concentrations of both SO4\({}^{2-}\) and NO3\({}^{-}\) in Seoul were higher than Baengnyeong Island. Nevertheless, the smaller SOR and NOR values in Seoul mean that Seoul has higher SO2 and NO2 concentrations compared to Baengnyeong Island. Therefore, in the case of Seoul, as consistently  
Figure 4: Changes of the proportions of major aerosol species in PM2.5: (**a**) Seoul; (**b**) Baengnyeong Island.  
mentioned in Section 3.2, it can be expected that the local influence is significant compared to Baengnyeong Island.  
Figure 5: Changes in the concentrations of major ion components according to PM2.5 concentration in Seoul.  
Figure 6: Changes in the concentrations of major ion components according to PM2.5 concentration on Baengryeong Island.  
Figures 9 and 10 show the annual changes of SOR and NOR classified by the PM2.5 concentration level at both sites. Generally, the SOR in Seoul increased until the PM2.5 concentration was smaller than 125 \(\upmu\)g m\({}^{-3}\), but SOR values did not increase greatly with higher concentration levels of PM2.5. However, NOR in Seoul tends to increase gradually with the increase of PM2.5 concentration, even in the higher concentration levels over 125 \(\upmu\)g m\({}^{-3}\).  
On Baengnyeong Island, similar to Seoul, the SOR increases until the concentration of PM2.5 reaches 125 \(\upmu\)g m\({}^{-3}\), but it tended to decrease after the peak. The values of SOR were larger than that of Seoul. The NOR of Baengnyeong Island, similar to Seoul, showed a linear increase as the concentration of PM2.5 increased. The fact that the SOR and NOR values of Baengnyeong Island were larger than those of Seoul is consistent with the results shown in Figure 8. It is thought that the aerosols in Bangnyeong were of a greater age than Seoul.  
The tendencies of SOR are determined by the concentration of SO\({}_{2}\) and the state of SO\({}_{4}\)\({}^{2-}\) that either formed at the measurement area or migrated from outside. In the case of NOR, even when NO\({}_{2}\) stopped increasing, the concentration of NO\({}_{3}\)\({}^{-}\) continued to increase as the PM concentration increased. This is thought to be due to the steady inflow of NO\({}_{3}\)\({}^{-}\), which oxidizes already even there is a high PM2.5 event in addition to the effect of local emissions. This trend was the same for both Seoul and Baengnyeong Island. In addition, the larger increase of NOR than SOR when a high concentration of PM2.5 occurred might be interpreted to the larger contribution of NO\({}_{3}\)\({}^{-}\) than SO\({}_{4}\)\({}^{2-}\) in the high PM2.5 events in recent years, as shown in Section 3.3.  
Figure 8: The changes in SOR and NOR in Seoul and Baengnyeong: (**a**) SOR; (**b**) NOR.  
Figure 7: The changes in SO\({}_{4}\)\({}^{2-}\) and NO\({}_{3}\)\({}^{-}\) concentrations: (**a**) Seoul; (**b**) Baengnyeong Island.  
Figures 11 and 12 show the changes in SOR and NOR according to RH changes in Seoul and Baengnyeong Island. As observed in many previous studies [38], SOR at both sites showed a tendency to increase as RH increased. The increase in SOR according to changes in RH was larger on Baengnyeong Island than in Seoul. The effect of humidity seems to be more evident than that of Seoul, because Baengnyeong is an island region. However, neither NOR in Seoul nor on Baengnyeong Island showed a clear correlation with RH.