## 3 Results and Discussion
Figure 6: (**a**) Source-based mass concentrations, (**b**) source-based extinction coefficients (\(\rm d_{g}\) = 0.1 \(\rm\mu m\), GSD = 1.5), (**c**) contribution of the source-based extinction coefficients (\(\rm d_{g}\) = 0.1 \(\rm\mu m\), GSD = 1.5), (**d**) source-based extinction coefficients (\(\rm d_{g}\) = 0.5 \(\rm\mu m\), GSD = 1.5), and (**e**) contribution of source-based extinction coefficients (\(\rm d_{g}\) = 0.5 \(\rm\mu m\), GSD = 1.5) during sampling periods. The mass contributions of each source are shown in Figure 7. Figure 7 shows contributions of 20.3%, 21.2%, 22.8%, 8.2%, 6.3%, 7.9%, 5.2%, and 8.2% by TS, LS, LN, G, D, TBB, LBB, and FSD, respectively. The contribution of \(b_{ext}\) depended on the size distribution. For example, for d\({}_{\rm g}\) of 0.1 \(\upmu\)m and a GSD of 1.5, the contributions made by D, TBB, and LBB to mass and to \(b_{ext}\) increased from 6.3%, 7.9%, and 5.2% for mass to 22.6%, 15.5%, and 7.3% for \(b_{ext}\), respectively. Meanwhile, the contributions of TS, LS, LN, G, and FSD decreased from 20.3% (TS), 21.2% (LS), 22.8% (LN), 8.2% (G), 8.2% (FSD) for mass to 13% (TS), 14% (LS), 17.8% (LN), 6.5% (G), and 3.4% (FSD) for \(b_{ext}\), respectively. Theoretically, the MEE for each source is the extinction coefficient to mass ratio in m\({}^{2}\)/g, which is shown in Figure 4. This means that sources with large MEE values (D, TBB) had higher contributions to the extinction coefficient compared with other sources. Under the condition of a GSD of 1.5 and d\({}_{\rm g}\) of 0.1 \(\upmu\)m, the ratio of total \(b_{ext}\) to PM2.5 mass concentration was 1.7 m\({}^{2}\)/g. Sources with MEE higher than 1.7 m\({}^{2}\)/g will make larger contributions to \(b_{ext}\) than to mass, and sources with MEE lower than 1.7 m\({}^{2}\)/g will make smaller contributions to \(b_{ext}\) than to mass. As shown in Figure 7, the contributions of \(b_{ext}\) for TS (MEE = 1.2 for a GSD of 1.5 and d\({}_{\rm g}\) of 0.1 \(\upmu\)m), LS (MEE = 1.234), LN (MEE = 1.463), G (MEE = 1.478), and FSD (MEE = 0.775) to total \(b_{ext}\) became smaller as compared to the mass contribution. In contrast, the contributions of \(b_{ext}\) for D (MEE = 6.735), TBB (MEE = 3.679), and LBB (MEE = 2.634) to total \(b_{ext}\) became larger as compared to the contribution based on chemical compound mass concentration. It should also be noted that sources containing EC increased the MEE, and this made the contribution of sources containing EC to \(b_{ext}\) increase compared with the contribution to mass. Figure 7 shows how the size distribution and refractive index influenced the source-based \(b_{ext}\) contribution. As discussed, the optical properties depend on chemical species mass concentration. It was important to validate the reliability of the obtained \(b_{ext}\). In this study, we compared our \(b_{ext}\) with AOD (Aerosol Optical Depth) from the Aerosol Robotic Network (AERONET) network. The AOD is the integration of \(b_{ext}\) over the height (z) of the atmospheric column between sensor and sun [26]:  
\[\text{AOD}=\int_{0}^{z}b_{ext}dz \tag{7}\]  
Figure 8 shows a comparison of the daily averaged AOD in Seoul from the AERONET Seoul-SNU station (37.458 N, 126.951 E) with the obtained extinction coefficient (\(b_{ext}\)) based on daily mass concentration data of species [27]. The comparison yielded R\({}^{2}\) of 0.60 to 0.68 for the d\({}_{\rm g}\) range of 0.1-2.5 \(\upmu\)m at a GSD of 1.5. A more direct measurement comparison, such as of in situ aerosol scattering  
Figure 7: Comparisons of the average source-based mass contribution with the source-based \(b_{ext}\) contributions (GSD = 1.5). coefficient values from nephelometer scattering, was not possible because of data unavailability. However, the comparison between AOD from AERONET and our calculated \(b_{ext}\) showed a comparable correlation and regression.