Light absorption of brown carbon in PM_(2.5) in the Three Gorges Reservoir region, southwestern China: Implications of biomass burning and secondary formation

摘要

Brown carbon (BrC) is known as a light-absorbing organic aerosol which affects the visibility and radiative forcing budget in the troposphere. The optical properties were studied for filter-based PM2.5 samples collected from the winter of 2015 to the summer of 2016 at one rural and three urban sites in the Three Gorges Reservoir (TGR) region, China. The average light absorption coefficient for BrC (beta(abs,405, BrC)) at 405 nm and its contributions to total aerosol light absorption during winter were 12.1 +/- 7.0 Mm(-1) and 23.8 +/- 9.1% respectively, higher than those during summer (1.7 +/- 0.8 Mm(-1) and 11.2 +/- 4.1%). Spatially, the average beta(abs,405, BrC) was higher at the urban sites (13.4 +/- 7.3 Mm(-1)) than that at the rural site (7.8 +/- 3.2 Mm(-1)). The average mass absorption efficiency of BrC at 405 nm (MAE(405),(BrC)) was 0.8 +/- 0.4 m(2) g(-1) during winter which was 2.7 times higher than that during summer (0.3 +/- 0.1 m(2) g(-1)). Furthermore, the absorption Angstrom exponents (AAE) at 405-980 nm (AAE(405-980)) were 1.1 +/- 0.1 in summer and 1.3 +/- 0.2 in winter respectively. Correlation analysis suggests that biomass burning (BB) played an important role in beta(abs,405, BrC) during winter. Additionally, the relatively high AAE(405_980) during winter was mainly due to the BrC from both BB and secondary organic aerosol. The fractional contribution of solar energy absorption by BrC relative to BC in the wavelengths of 405-445 nm was 23.9 +/- 7.8% in summer and 63.7 +/- 14.2% in winter, significantly higher than that in the range of 405-980 nm (11.9 +/- 3.4% and 29.9 +/- 6.1% respectively). Overall, this study contributes to the understanding of sources of BrC in the climate-sensitive TGR region of southwestern China.