## 3 Results and Discussion
### Air Analysis
#### 3.2.3 Analysis of the Effects of Emission Sources for Selection Cases (SCs)  
* Estimation of the effect of carbon emissions from transportation. EC is mainly generated by the combustion of fossil fuels. It has been used as a tracer of anthropogenic pollutants that are stable to chemical reaction. The OC/EC ratio is used to estimate the amount of secondary organic carbons (SOC) [47; 65; 66; 67], and is an important factor for investigating the emission sources [68; 69]. In this study, the correlations between OC and EC concentrations were analyzed, first for all cases, and then for SCs, as shown in Figure 10.  
The OC/EC ratio of all cases was 4.87 (r = 0.77); for SCs, it was 4.34 (r = 0.81). The correlation between OC and EC increased compared with all cases, but was still lower than that reported in previous studies (r = 0.79-0.99) [70; 71; 72]. This is because the diversity of cases increased as the study was conducted for an entire year. Figure 9: Annual mean concentrations of components in PM2.5 in Ulsan, obtained using the hourly data, for SCs. (**a**) Annual mean; (**b**) mean for winter months; (**c**) mean for spring months; (**d**) mean for summer months; (**e**) mean for fall months (unit: \(\upmu\)g/m\({}^{3}\) for pie-chart, ng/m\({}^{3}\) for vertical chart) (see Table S5 for more detail). Figure 11 shows that the PM2.5 concentration for all cases tended to decrease during the day, when the atmospheric boundary layer is higher, and increase during the night. However, the PM2.5 concentration of SCs did not show this circadian tendency. In ALL, the OC and EC concentrations showed the peak values (OC: 20 KST 3.29 \(\mathrm{\SIUnitSymbolMicro g}\)/m\({}^{3}\), EC: 09 KST 0.76 \(\mathrm{\SIUnitSymbolMicro g}\)/m\({}^{3}\)) and tended to become low during the daytime when the atmospheric boundary layer is high (OC: 14-17 KST 2.63 \(\mathrm{\SIUnitSymbolMicro g}\)/m\({}^{3}\), EC: 14 KST 0.55 \(\mathrm{\SIUnitSymbolMicro g}\)/m\({}^{3}\)). In SCs, the OC concentrations did not show circadian changes; however, in ALL, not only PM2.5 concentration but also OC and EC concentrations were higher, and the OC/EC ratio was lower than that in SCs. The EC concentrations showed circadian changes for both all cases and SCs; in other words, EC concentrations increased at rush hour, which is presumed to be due to the effect of vehicular emissions in traffic during the commuting time [73]. Furthermore, in SCs, the EC concentrations were higher and the OC/EC ratio was lower than those in ALL. However, the proportion of the sum of OC and EC in PM2.5 did not show a large difference for SCs (16.39-22.61%), and the trend of the sum of OC and EC in PM2.5 did not follow the diurnal changes of EC (Figures S1 and S2), such as during the rush hour. Thus, we can conclude that traffic does not increase the PM2.5 concentration in SCs, and does not appear to be a major contributor to PM2.5 emissions. Figure 11: Diurnal changes in the ratio of OC to EC, the concentrations of OC and EC, and the correlation between OC and EC (**left**: ALL, **right**: SCs). Figure 10: Correlation between organic carbon (OC) and elemental carbon (EC) concentrations in ALL and SCs (all cases: black, SCs: red). The r\({}^{2}\) of OC and EC became lower prior to the morning rush hour and following the afternoon rush hour in ALL; however, in SCs, it became lower outside of rush hours. The high correlation between OC and EC in SCs, excluding the daytime, indicates the consistency of the OC/EC ratio during the corresponding hours and appears to be an effect of specific carbon emission sources such as mobile sources. In contrast, during the daytime (11:00-16:00), the correlation between OC and EC decreased as the carbons emitted from various sources mixed with the SOCs. The temporal variations of the carbon component present important findings compared with the results of previous studies, which did not show temporal changes in OC and EC [74].