## 4 Summary and Conclusions  
The 24 h integrated filter-based measurements of the ambient PM\({}_{2.5}\) were simultaneously made at two sites near roadways during the summer (10 August through 13 September, 2015) and winter (7 December 2015 through 17 January 2016) to investigate differences in the major chemical composition of the PM\({}_{2.5}\) at the sites and to uncover important driving components and formation processes (local vs. regional) that lead to seasonal PM\({}_{2.5}\) pollution episodes. The two sampling sites were located at distances of 15 (BKO site) and 150 m (CNU site) from busy roads. Aerosol light absorption coefficients with a time resolution of 1 min were also observed using a multi-wavelength aethalometer at the CNU site to verify the influence of emission sources (traffic vs. biomass burning) on the concentrations of fine organic aerosols measured during the two seasons. The differences in the major chemical composition of the PM\({}_{2.5}\) between the two sites arose from organic carbon aerosols. The OC concentrations during the summer and winter were observed to be higher at the BKO site, which was subject to a higher traffic density (27% higher) and shorter distance from the roadway than the CNU site, but no noticeable difference was found between the sites during the winter, which is attributable to the significant contributions of other sources (e.g., BB emissions) to the organic aerosol concentration. During the summer, the high PM\({}_{2.5}\) episodes exceeding the 24 h Korean PM\({}_{2.5}\) standard were directly linked to the elevated OM and SO\({}_{4}\)\({}^{2-}\) concentrations, which contributed 35-41% and 26-30% of the PM\({}_{2.5}\) at the two sites. A tight correlation between the OC and EC, a negligible correlation between the brown carbon absorption at 370 nm (AbsBrC370 nm) and K\({}^{+}\) (R\({}^{2}\) = 0.00), and little difference between the AAE\({}_{370\text{-}950\text{nm}}\) (1.1) and AAE\({}_{370\text{-}520\text{nm}}\) (1.2) values suggest that the carbonaceous aerosols observed during the summer at the two sites were greatly affected by primary traffic emissions. The summertime WSOC concentration was attributable to primary combustion emissions (e.g., traffics), with some influence of SOA formation. However, the SOC concentration, as estimated using a primary EC tracer method, accounted for 31% of the OC concentration at the BKO site, closer to the roadway, and 16% at the CNU site, further from the roadway. The importance of SO\({}_{4}\)\({}^{2-}\) to the summertime PM\({}_{2.5}\) pollution was attributed to the long-range transportation of aerosols from polluted regions of China rather than local production, as evidenced by both the MODIS images and the transport pathways of the air masses that reached the sites. The wintertime PM\({}_{2.5}\) episodes, however, were associated with increases in the OM and NO\({}_{3}\)\({}^{-}\) concentrations, accounting for on average 26-28% (BKO site) and 22-23% (CNU site) of the PM\({}_{2.5}\) concentration. A good correlation between the gaseous NO\({}_{2}\) and NO\({}_{3}\)\({}^{-}\), along with weather conditions such as a low wind speed and high relative humidity, suggest that the wintertime particulate NO\({}_{3}\)- was caused by local production through the heterogeneous oxidation of NO\({}_{2}\) on moist particles at the sites rather than the regional transport of aerosol particles. One interesting feature that was observed was the existence of two distinct slopes in the plots of OC vs. EC concentration in winter; one is associated with a significant contribution of traffic emissions, providing a lower OC/EC ratio (\(\sim\)3.0), while the other is strongly linked to BB and traffic emissions, resulting in a higher OC/EC ratio (\(\sim\)6.5). This was supported by the strong correlations between the OC and EC, the enhanced AAE\({}_{370\text{-}520\text{nm}}\) values, and an increased contribution of light-absorbing BrC particles to the total aerosol absorption.