Seasonal variation of optical properties and source apportionment of black and brown carbon in Xi'an, China

To acquire the seasonal variation of black carbon (BC) and brown carbon (BrC) light absorption (Abs), PM_2.5 samples were obtained over an entire year of 2018 and analyzed for chemical composition and wavelength-dependent light absorption. Contributions of Abs_BC and Abs_BrC, the absorption Ångström exponent (AAE) and mass absorption cross section (MAC) are examined. The results show that while BC was the dominant light absorbing carbonaceous component at 780 nm–980 nm, BrC made a significant contribution to light absorption at shorter wavelengths (405 nm and 445 nm), especially during spring. At 405 nm, BrC experienced the dominant light absorption contribution in spring, autumn and winter, whereas BC had a greater contribution in summer. The annual average of BrC contributions to total absorption at 405 nm (56.3%) is higher than that of BC (43.7%), with lower BrC contributions at longer wavelengths. Average MACs for BC and BrC at 405 nm were 5.67 ± 10.4 and 1.74 ± 3.78 m²g^-1, respectively. The MAC_BC versus MAC_BrC were well correlated in autumn and winter implying that BC and BrC had similar primary sources (biomass burning and fossil fuel combustion). Source factors and source-specific seasonal contribution to light absorption were identified using positive matrix factorization (PMF) modeling. Traffic-related emissions had the largest contribution to light absorption in all seasons at all wavelengths. Biomass burning had a greater impact in spring and winter, and coal combustion contributed the highest levels in winter. Mineral dust had minor contributions to light absorption in all seasons.