## 3 Results and Discussion
### Optical Properties of Brown Light Absorbing Carbon at the CNUI Site
The lower AAE values for BrC during the summer were due to a greater contribution of water-insoluble OAs, probably emitted from vehicles, to total aerosol absorption, rather than from water-soluble OAs. Many previous studies indicated that BB emissions have typical BrC AAE values of 6-8 due to a greater impact of water-soluble OC fractions [49; 68; 69; 77; 79]. Generally, previous studies reported that the AAE values of BrC-containing aerosols were lower than those of water-extracted BrC due to the contribution of the water-insoluble OC (WIOC) fraction [52; 78; 71]. Moreover, the WIOC fractions contribute more absorption than WSOC to the total light absorption [71] and are likely made up of high-molecular-weight PAHs emitted from the combustion of biomass materials and fossil fuels [71; 78]. As shown in Figure 9d, the estimated BrC light absorption clearly exhibited a strong spectral dependence in the summer and winter pollution episodes, but there was stronger absorption by BrC aerosols at shorter wavelengths during the winter. The spectral dependence of BrC absorption showed an exponentially increasing tendency with decreasing wavelength. Figure 9: Absorption coefficients of BC and brown carbon (BrC) at 370 nm (**a**), absorption Angström exponent (AAE) values (**b**), BrC AAE values (**c**), and wavelength-dependent absorption coefficients (**d**) during the summer and winter for the CNU site. It is well known that BrC particles containing water-soluble and insoluble fractions are emitted from the incomplete combustion of biomass materials [7; 49; 68; 69; 77] and fossil fuels (e.g., vehicle and ship emissions) [71; 72; 73; 78; 51] and produced in the atmosphere through a variety of chemical reactions involving volatile organic gases [80; 81]. To elucidate the driving forces contributing to BrC light absorption, the relationships of AbsBrC,370 nm with K\({}^{+}\), OC, WSOC, WIOC, and SOC were examined. During summer, the highest correlation was obtained between the AbsBrC and WIOC (R\({}^{2}\) = 0.26, \(p<0.01\)), followed by the correlation between the AbsBrC and WSOC (R\({}^{2}\) = 0.16). However, no relationship was found between the AbsBrC, BB tracer (K\({}^{+}\), R\({}^{2}\) = 0.00), and SOC (R\({}^{2}\) = 0.10). Furthermore, a good correlation between the EC and WIOC was found (R\({}^{2}\) = 0.58), implying that the WIOC and EC had similar sources--e.g., traffic emissions [71; 71; 52; 11]. As described above, the moderate corrections of the WSOC with EC and POC and the good relationship between the AbsBrC,370 nm and WIOC support the notion that brown light absorbing carbon from traffic emissions was an important contributor to the aerosol light absorption during the summer. However, the AbsBrC during the winter had strong-to-good correlations with the WIOC (R\({}^{2}\) = 0.83), POC (R\({}^{2}\) = 0.79), WSOC (R\({}^{2}\) = 0.53), K\({}^{+}\) (R\({}^{2}\) = 0.38), and SOC (R\({}^{2}\) = 0.32). These correlations indicate that the water-insoluble organic compounds from traffic emissions were an important contributor to the BrC absorption during the summer, while during the winter, various sources such as primary traffic, BB emissions, and SOA contribute to the light absorption by BrC aerosols.