## 2 Experimental Methods  
### Measurement of 24 h Integrated PM\({}_{2.5}\) and Aerosol Light Absorptions  
The 24 h integrated PM\({}_{2.5}\) samples were simultaneously collected at two urban sites in Gwangju, Korea (Figure 1), both of which are close to busy roads, during intensive observation periods in the summer and winter. Gwangju is situated in the southern part of Korea and has a population of approximately 1.5 million people and an area of 501.4 km\({}^{2}\). It is reported that motor vehicle sources account for about 80% of the total air pollution emissions in the city, and both the local and continental air pollution could influence the Gwangju metropolitan area, depending on the airflow patterns [10; 11; 12; 13]. One site is located on the rooftop of a three-story building in the Buk-Ku District Office (BKO site, 35.17N, 126.91E, 54.3 m above mean sea level) and is approximately 15 m away from the nearest roads. Surrounding the BKO sampling site, there are four main roads with a high concentration of cars, trucks, and buses, especially during rush hour. The other site is on the rooftop of a three-story engineering building on Chonnam National University campus (CNU site, 35.18N, 126.91E, 54.3 m above mean sea level); this site is approximately 150 m away from a busy road and 0.7 km south of a highway. The distance between the two sampling sites is about 500 m. The Gwangju regional meteorological agency and an air pollution monitoring station are located at approximately 2.0 km west and east of the BKO site, respectively (see Figure 1). The hourly CO and NO\({}_{2}\) concentrations used in this study were observed at the height of approximately 10 m at the air monitoring station. The measurement periods covered 10 August-13 September 2015 (summer) and 7 December 2015-17 January 2016 (winter). The aerosol measurements started at 9:00 AM and lasted approximately 24 h.  
Figure 1 (lower part) shows the total traffic density around the BKO site at 08:00-09:00 and 18:00-19:00. The hourly traffic volume was investigated in October 2015 by Gwangju Metropolitan City [46]. The daily total traffic volume at the BKO site (# vehicles/day) was about 62,600. The hourly total traffic flow (# of vehicles/h) at the BKO site was 4327 at 08:00 and 4041 at 18:00. The CNU site had about 73% of the total traffic volume seen at the BKO site at the same times. Figure 2a illustrates the diurnal patterns of the total traffic density at the CNU and BKO sites. Additionally, the hourly traffic densities of passenger cars (gasoline vs. diesel) and diesel-powered vehicles at BKO are shown in Figure 2b,c. Light-duty gasoline and diesel-powered vehicles (<seven passengers) contributed 43% and 23% of the total traffic flow, respectively. Medium-duty diesel vehicles (with 8-15 passengers) and heavy-duty diesel buses accounted for 3.8% and 4.3% of the total flow. Diesel trucks, including light-duty (<2.5 tons), medium-duty (2.5-8.5 tons), and heavy-duty (>8.5 tons) trucks, accounted for 9.2% of the total vehicles. Altogether, the diesel-powered vehicles comprised approximately 40% of the total traffic flow. Aerosol samples were collected on a pre-baked (500 \({}^{\circ}\)C for 24 h) quartz-fiber filter (47 mm, Pall flex Tissue quartz 2500 QAT-UP) using a low-volume PM\({}_{2.5}\) cyclone inlet sampler (URG-2000-30 EH, URG Corporation) operated at 16.7 L/min. A carbon-impregnated diffusion denuder (Sunset Lab, OR) was placed downstream of the cyclone inlet system to minimize positive artifacts by the adsorption of semi-volatile organics onto the quartz-fiber filter during sampling. The collected filter samples were used to determine the amounts of organic and elemental carbon (OC and EC), water-soluble OC (WSOC), and eight water-soluble inorganic constituents. Field blank filters were also collected at each site to correct for background contamination.