## 3 Results and Discussion
### Characteristics of Carbonaceous Components
In this study, the primary OC/EC ratio was calculated as the lowest 10% of OC/EC ratios from all the OC-EC data measured at the two sites for each season [2; 10; 11; 60]. The primary OC/EC ratio was 2.3 during the summer, and 3.0 and 5.3 for the case I and II winter periods. The higher primary OC/EC ratio during the case I winter period vs. the summer was likely due to the impact of the BB emissions during the winter period I. During summer, the average estimated POC and SOC concentrations were 3.7 (2.0-5.3) and 1.7 (0.4-4.6) \(\mu\)gC/m\({}^{3}\) at the BKO site, contributing 69% (47-90%) and 31% (10-53%) to the OC concentration, respectively, and 3.3 (1.6-6.0) and 0.7 (0.0-2.8) \(\mu\)gC/m\({}^{3}\) at the CNU site, contributing 84% (54-100%) and 16% (0-46%) to the OC concentration, respectively. The contribution of SOC to the total OC was approximately two-fold greater at BKO than at CNU. Typically, a higher SOC contribution was found for the periods when the PM\({}_{2.5}\) pollution episodes occurred at both sites, along with a low wind speed, high temperature, and strong photochemical oxidation. For the entire winter period (case I + case II), the POC and SOC concentrations were estimated to be 4.3 (1.2-9.7) and 1.2 (0.0-4.2) \(\mu\)gC/m\({}^{3}\) at the BKO site, accounting for 81% and 19% of the total OC, respectively, and 4.0 (1.3-9.1) and 1.0 (0.0-3.9) \(\mu\)gC/m\({}^{3}\) at the CNU site, accounting for 82% and 18% of the total OC, respectively. The results revealed that the primary combustion emissions (e.g., traffic in summer vs. traffic + BB emissions in winter) were major contributors to the OC near roadways during both the summer and winter. Similar to the summertime OC pattern, the WSOC concentrations during the summer were observed to be higher at BKO than at CNU, resulting in higher WSOC/OC and WSOC/EC ratios at BKO (0.42 and 1.82) than at CNU (0.38 and 1.36). In this study, however, a higher WSOC/OC was observed in the winter than in the summer, possibly due to the contribution of BB emissions to the wintertime WSOC, as discussed above. Previous studies have shown that the WSOC/OC in ambient air varies with the study location and season [61; 62; 63; 61; 60; 11]. For example, the WSOC/OC ratios measured at Mt. Abu, India [6]; Hong Kong, China [64]; and an urban site in Tokyo [62] were higher in  
Figure 7: Regression relationships between the organic and elemental carbon (OC and EC) concentrations during the summer and winter at the BKO and CNU sites. the winter than in the summer. On the other hand, higher ratios were found in the summer than in the winter at the same CNU site in different measurement periods [10; 52]. The WSOC/OC ratios measured during the summer and winter were comparable to those from our previous studies conducted at the CNU site [11; 40; 52]. WSOC in ambient air can be used as a proxy for SOA formation [65; 66]. In addition, biomass burning is an important primary source of WSOC [67; 68; 69; 70], as well as traffic emissions [10; 11; 13; 40; 71] and shipping activity [72; 73]. To examine the influence of sources on the WSOC observed during the summer and winter at the CNU site, we performed a regression analysis of the WSOC with the EC, K\({}^{+}\), POC, and SOC. During the summer, the WSOC showed moderate-to-good correlations with the EC (R\({}^{2}\) = 0.52), K\({}^{+}\) (R\({}^{2}\) = 0.38), POC (R\({}^{2}\) = 0.58), and SOC (R\({}^{2}\) = 0.37), suggesting that primary combustion emissions such as those from traffic were a main contributor to the summertime WSOC concentration. This also suggests some influence of SOA formation on the WSOC concentration during the summer. However, the WSOC during the winter was strongly correlated with the K\({}^{+}\) (R\({}^{2}\) = 0.86), POC (R\({}^{2}\) = 0.69), and SOC (R\({}^{2}\) = 0.71), with a weak correlation with the primary EC (R\({}^{2}\) = 0.18). These results reveal that the wintertime WSOC was strongly associated with primary traffic and biomass burning emissions and SOA formation.