## 3 Results and Discussion
### Possible Sources of Secondary Inorganic Species  
During the summer, the total concentrations of secondary inorganic species (\(\sum\)SIS = SO\({}_{4}\)\({}^{2-}\)+ NO\({}_{3}\)\({}^{-}\) + NH\({}_{4}\)\({}^{+}\)) were almost the same at the two sites (see Table 1), but their contribution to the PM\({}_{2.5}\) was a little higher at the CNU site (mean: 45.0%, range: 11.1-74.0%) than at the BKO site (mean: 41.1%, range: 8.4-71.7%). Among the SIS, SO\({}_{4}\)\({}^{2-}\) was the most significant contributor to the PM\({}_{2.5}\), constituting 29.6% (5.4-47.5%) and 25.7% (4.3-50.6%) of the PM\({}_{2.5}\) at the CNU and BKO sites, respectively. During winter, the concentrations of the t\(\sum\)SIS and NO\({}_{3}\)\({}^{-}\) and their contributions to the PM\({}_{2.5}\) increased at both the sites compared to those seen during summer, but the contribution of SO\({}_{4}\)\({}^{2-}\) decreased. The average contributions of the \(\sum\)SIS, NO\({}_{3}\)\({}^{-}\), and SO\({}_{4}\)\({}^{2-}\) concentrations to PM\({}_{2.5}\) were, respectively, 51.3% (28.5-73.1%), 23.2% (9.4-44.1%), and 16.6% (6.9-28.9%) at CNU, and 50.2% (24.3-76.0%), 22.3% (9.3-44.7%), and 17.1% (7.2-29.4%) at BKO. Previous studies indicated that the NO\({}_{3}\)\({}^{-}\)/SO\({}_{4}\)\({}^{2-}\) ratio can be used as an indicator of mobile versus stationary emission sources [27; 28; 53; 54; 55]. A higher NO\({}_{3}\)\({}^{-}\)/SO\({}_{4}\)\({}^{2-}\) ratio indicates a greater effect of mobile sources than stationary sources. In this study, the NO\({}_{3}\)\({}^{-}\)/SO\({}_{4}\)\({}^{2-}\) at the BKO site varied from 0.06 to 1.09 (mean: 0.38) in summer and from 0.51 to 3.99 (mean: 1.47) in winter. At the CNU site, the ratio was in the range of 0.05-0.79 (mean: 0.29) and 0.57-4.06 (mean: 1.55) in the summer and winter, respectively. The relatively low NO\({}_{3}\)\({}^{-}\)/SO\({}_{4}\)\({}^{2-}\) ratio in summer at both sites suggests a greater influence of stationary sources than mobile sources [54]. However, the high NO\({}_{3}\)\({}^{-}\)/SO\({}_{4}\)\({}^{2-}\) ratio, which ranges between 2 and 5, observed in Los Angeles, USA, indicates a significant contribution of mobile sources in downtown LA [53]. Considering that there are no major emission sources of SO\({}_{2}\), such as coal- and oil-fired power plants and industrial facilities, around the sampling sites or in Gwangju metropolitan city, it is probable that the particulate SO\({}_{4}\)\({}^{2-}\) observed at the sites during the summer originated primarily from atmospheric processing during the long-range transportation of air pollutants from other areas, rather than the local production of SO\({}_{4}\)\({}^{2-}\) from SO\({}_{2}\). However, it should be considered that the high O\({}_{3}\) and high RH conditions during the summer may have increased the gas-phase homogeneous and/or aqueous phase reactions of SO\({}_{2}\) to produce SO\({}_{4}\)\({}^{2-}\) particles at both sites. Our previous studies conducted in Gwangju also demonstrated a significant influence of long-range transported aerosols on the elevated SO\({}_{4}\)\({}^{2-}\) concentrations [11; 12; 24; 27; 28]. A detailed explanation of this issue is given below in Section 3.4.