## 3 Results and Discussion
### Air Analysis  
Figure 3 and Table S2 presents the statistics of the data obtained in 2017 at the intensive air quality monitoring station. The results of the analysis of the ion components of PM2.5 showed generally high concentrations of \(\mathrm{SO_{4}}^{2-}\) and \(\mathrm{NH_{4}}^{+}\) in summer. Although the \(\mathrm{SO_{4}}^{2-}\) and \(\mathrm{NH_{4}}^{+}\) concentrations increased in summer, the PM2.5 concentration decreased. Moreover, the mean ratio of \(\mathrm{SO_{4}}^{2-}\) and \(\mathrm{NH_{4}}^{+}\) in PM2.5 was 23.98 and 11.83% in summer, respectively. The \(\mathrm{SO_{4}}^{2-}\) concentration in summer increased because of the photochemical activity that can increase the fraction of sulfate in total sulfur (\(\mathrm{[SO_{4}}^{2-}]/\mathrm{[SO_{2}]}+\mathrm{[SO_{4}}^{2-}]\)), which is consistent with the results of previous studies [43; 44; 45] that reported the concentration to be greater in summer (18%) than in winter (11%). In contrast, the \(\mathrm{NO_{3}}^{-}\) concentration was the largest in winter (13.13%) for all the cases, followed by that in spring, fall, and summer. These results are consistent with those from a previous study [43]. Figure 3: _Cont._  
Figure 2: Annual average concentrations of particulate matter with an aerodynamic diameter \(\leq\) 2.5 \(\upmu\)m (PM2.5) (**left**) and particulate matter with an aerodynamic diameter \(\leq\) 10 \(\upmu\)m (PM10) (**right**) according to wind direction (see Supplementary Materials Table S1 for more detail). EC is a primary pollutant generated only by the combustion process [46], whereas OC is generated primarily or secondarily in the form of aerosols [47]. In the present study, the annual mean concentrations of OC and EC were 2.91 and 0.62 ug/m\({}^{3}\), respectively, in 2017. Regarding seasonality, OC concentrations were high in spring, whereas EC concentrations were high in winter. The annual mean OC/EC ratio was 5.62 (5.52-5.73, 95% CI), which was low in winter (4.98 (4.89-5.07, 95% CI)) and high in spring (6.78 (6.46-7.10, 95% CI)). Compared to the results of previous research, the OC and EC concentrations measured in Cheongju (44.2 ug/m\({}^{3}\) PM2.5) between October 1995 and August 1996 were 4.99 and 4.44 ug/m\({}^{3}\), respectively [43]. The OC and EC concentrations in Sihwa (35.4 ug/m\({}^{3}\) PM2.5) were 9.8 and 1.8 ug/m\({}^{3}\), respectively [48]. The OC and EC concentrations in Seoul (72.5 ug/m\({}^{3}\) PM2.5) in October and November 2011 were 18.0 and 8.13 ug/m\({}^{3}\), respectively [21]. The OC and EC concentrations in Incheon between 2009 and 2010 (42.56 ug/m\({}^{3}\) PM2.5) were 8.04 and 1.79 ug/m\({}^{3}\), respectively [49]. These concentrations (reported by previous studies) are higher than those presented in this study, and the difference can be attributed to the lower PM2.5 concentration compared with that of previous studies, which led to the lower concentrations of its components. In this study, the OC and EC contents in PM2.5 were 16.4 and 3.3%, respectively. In previous studies, OC contents ranged from 11.3 [43] to 27.7% [48], and EC contents ranged from 4.2 [49] to 11.2% [21]. Hence, the annual mean of the OC and EC proportions in PM2.5 is considerably similar to that presented in this study. Heavy metal concentrations in all cases were similar to those measured in Milan in 2002 (119-342 ng/m\({}^{3}\) K, 6-14 ng/m\({}^{3}\) Ti, 3-7 ng/m\({}^{3}\) V, 5-18 ng/m\({}^{3}\) Mn, 120-309 ng/m\({}^{3}\) Fe, 2-9 ng/m\({}^{3}\) Ni, 1-3 ng/m\({}^{3}\) As, and 13-55 ng/m\({}^{3}\) Pb) [50], and in four cities in Spain (6.32-53.81 ng/m\({}^{3}\) Ti, 5.54-95.29 ng/m\({}^{3}\) V, 3.20-13.77 ng/m\({}^{3}\) Mn, 3.09-17.20 ng/m\({}^{3}\) Ni, 26.40-48.80 ng/m\({}^{3}\) Zn, 0.39-1.03 ng/m\({}^{3}\) As, and 6.32-23.91 ng/m\({}^{3}\) Pb) [51]. However, the heavy metal concentrations measured in this study in Ulsan in 2017 were lower than those reported in downtown Incheon from June 2009 to May 2010 and in Gwangju from February to March 2010 [49; 52].