## 3 Results and Discussion  
### General Characteristics of PM\({}_{2.5}\) and Its Chemical Components  
The concentrations of the 24 h average PM\({}_{2.5}\), SO\({}_{4}\)\({}^{2-}\), NO\({}_{3}\), OC, EC, and WSOC during the summer and winter at the CNU and BKO sites are summarized in Table 1. In addition, their temporal profiles are presented in Figure 4. The temporal variations in the 24 h average ambient temperature, relative humidity (RH), and wind speed (WS), which were observed at Gwangju regional meteorological agency approximately 2 km away from our sampling sites, are illustrated in Figure 5. The 24 h average temperature, WS, and RH values were, respectively, 24 (14-28) \({}^{\circ}\)C, 1.4 (0.7-2.8) m/s, and 80 (58-96)% during the summer, and 4 (\(-2\)-11) \({}^{\circ}\)C, 1.3 (0.4-2.4) m/s, and 70 (40-96)% during the winter. The general meteorological conditions during the summer and winter included rather low winds and high RH, causing an accumulation of PM\({}_{2.5}\) and enhanced secondary formation, as shown in Figure 4. Due to the  
Figure 3: Diurnal patterns of aerosol absorption coefficients at 370 and 880 nm during the summer and winter at CNU.  
short distance between the two measurement sites (\(\sim\)0.5 km), the concentrations of PM\({}_{2.5}\) and its major chemical components showed similar trends, with higher concentrations in the winter, as shown in Figure 4. As shown in Table 1 and Figure 4, the PM\({}_{2.5}\), NO\({}_{3}\)\({}^{-}\), and OC concentrations were observed to be higher in the winter than in the summer at both sites, with enhanced concentrations of NO\({}_{3}\)\({}^{-}\) and OC in the winter owing to less favorable air dispersion and low ambient temperatures (Figure 5). Moreover, increased fossil fuel consumption for domestic heating and biomass burning (BB) emissions could greatly contribute to the high OC levels in winter [11; 12]. On the contrary, the SO\({}_{4}\)\({}^{2-}\) concentration was higher in the summer than in the winter due to the stronger photochemical activity and a greater influence of long-range transported SO\({}_{4}\)\({}^{2-}\), which is discussed further below. However, the average concentrations of the secondary inorganic species NO\({}_{3}\)\({}^{-}\), SO\({}_{4}\)\({}^{2-}\), and NH\({}_{4}\)\({}^{+}\) were similar between the two sites during the summer and winter. The difference in the major chemical composition of PM\({}_{2.5}\) between the two sites is attributable to organic carbon aerosols. A detailed discussion of this observation is provided in Section 3.3. As shown in Figure 4, the daily OC concentrations during the summer and winter were higher at BKO, where higher total traffic volumes were observed and the distance from the roadway was lower (\(\sim\)15 m) than at CNU, but there was no noticeable difference between the sites in winter. This may be due to a greater influence of other emission sources (e.g., biomass burning) during winter, along with traffic emissions from the roads. Contrary to our expectations, the EC concentration was higher in the summer than in the winter at both sites, but little difference in the EC concentration was found between the two sites. This may be due to an increased volume of diesel-powered vehicles travelling near the BKO and CNU sites in summer.