## 3 Results and Discussion
### Optical Properties of Brown Light Absorbing Carbon at the CNUI Site  
Generally, it is known that fresh BC particles from vehicle emissions have aerosol absorption Angstrom exponent (AAE) values close to 1.0, whereas light-absorbing organic aerosols (OAs) from fresh BB emissions have AAE values of approximately 2.0 or greater [7; 49; 70; 76; 77]. It has also been shown that these values vary with the biomass type and burning temperature [49; 77; 78]. It is well known that BC absorption exhibits a weak spectral dependence, but BrC shows a strong spectral dependence, with greater absorption in the near UV region [7]. Figure 9a shows the daily temporal profiles of absorption coefficients of BC (Abs\({}_{\text{BC,370 nm}}\)) and BrC (Abs\({}_{\text{BrC,370 nm}}\)) at 370 nm derived from the total aerosol absorption in the summer and winter at CNU. Figure 9b,c also represent the temporal variations in the AAE\({}_{\text{370-950 nm}}\) and AAE\({}_{\text{370-520 nm}}\) values and the BrC AAE values estimated in the wavelength ranges of 370-660 nm, respectively. AAE\({}_{\text{370-950 nm}}\) and AAE\({}_{\text{370-520 nm}}\) can be used to separate the aerosol absorption at IR wavelengths from that at near UV wavelengths. During the summer, the 24 h average Abs\({}_{\text{BC,370 nm}}\) values (32 \(\pm\) 9, 14-55 Mm\({}^{-1}\)) were greater than Abs\({}_{\text{BrC,370 nm}}\) (4\(\pm\) 2, 1-7 Mm\({}^{-1}\)). The very low contribution of Abs\({}_{\text{BrC,370 nm}}\) suggests that most aerosol absorption in the UV range during summer came from the BC absorption. However, the wintertime Abs\({}_{\text{BrC,370 nm}}\) values (19 \(\pm\) 9, 6-44 Mm\({}^{-1}\)) were significantly elevated compared to the summertime Abs\({}_{\text{BrC,370 nm}}\) values. Moreover, the Abs\({}_{\text{BC,370 nm}}\) (51 \(\pm\) 28, 12-118 Mm\({}^{-1}\)) was considerably enhanced. The contribution of light absorption by BrC to the total aerosol absorption at 370 nm was 11% \(\pm\) 3% (5-18%) during the summer and 29% \(\pm\) 5% (21-41%) during the winter. The significant difference in Abs\({}_{\text{BrC,370 nm}}\) between the summer and winter indicates an increased contribution of light-absorbing OA absorption to the total aerosol absorption during the winter. This enhanced light absorption by aerosols at shorter wavelengths (Abs\({}_{\text{BrC,370 nm}}\)) during the winter indicates the presence of light-absorbing organic aerosols. The average AAE\({}_{\text{370-950 nm}}\) and AAE\({}_{\text{370-520 nm}}\) values, which were calculated by performing a linear regression fit between \(ln(\text{Abs}_{\text{A}})\) and \(ln(\lambda)\) in the wavelength ranges of 370-950 nm and 370-520 nm, respectively, were 1.1 \(\pm\) 0.0 (1.0-1.2) and 1.2 \(\pm\) 0.1 (1.1-1.3) in the summer and 1.4 \(\pm\) 0.1 (1.2-1.6) and 1.7 \(\pm\) 0.1 (1.4-2.0) in the winter, respectively. As shown in Figure 9b, there was no significant difference between the aerosol AAE\({}_{\text{370-950 nm}}\) and AAE\({}_{\text{370-520 nm}}\) values during the summer, which also indicates that the light absorption by aerosol particles resulted mostly from BC aerosols. On the other hand, the enhanced AAE\({}_{\text{370-520 nm}}\) values observed during the winter support a significant contribution of brown carbon absorption to the total aerosol absorption, indicating that the contribution of BB emissions at the site is greater in the winter. Figure 9c shows the temporal change in AAE\({}_{\text{370-660}}\) values for light-absorbing BrC during the summer and winter. The BrC AAE\({}_{\text{370-660 nm}}\) values were calculated for the wavelengths of 370-660 nm by assuming BC AAE values of 1.0. Over the study period, a greater variation in the BrC AAE values was seen during the summer than during the winter. The average BrC AAE\({}_{\text{370-660}}\) values were 3.9 \(\pm\) 0.5 (3.1-5.1) during the summer and 4.4 \(\pm\) 0.3 (3.7-5.1) during the winter (Table 1).