Impacts of biomass burning and photochemical processing on the lightabsorption of brown carbon in the southeastern Tibetan Plateau

Brown carbon (BrC) in the atmosphere can greatly influence aerosol's radiative forcing over the Tibetan Plateau (TP) because it has the non-negligible capacity of light absorption compared to black carbon (BC); however, our understanding of optical properties, sources, and atmospheric processes of BrC in this region remains limited. In this study, a multiple-wavelength Aethalometer coupled with a quadrupole aerosol chemical speciation monitor was deployed to investigate the highly time resolved BrC in the submicron aerosol at the southeastern edge of the TP during the pre-monsoon season. The results showed that BrC made substantial contributions (20.0 %-40.2 %) to the light absorption of submicron aerosol from 370 to 660 nm. Organic aerosol (OA), an alternative to BrC, was split into a biomass burning OA (BBOA) with aging processes and a photochemical-oxidation-processed oxygenated OA (po-OOA) by a hybrid environmental receptor model analysis. Combined with the light absorption coefficient of BrC (b(abs-BrC)), the source-specific mass absorption cross sections of BBOA (0.61-2.78 m(2) g(-1)) and po-OOA (0.30-1.43 m(2) g(-1)) at 370-660 nm were retrieved. On average, b(abs-BrC) from po-OOA (1.3-6.0 Mm(-1)) was comparable to that from BBOA (1.3-6.0 Mm(-1)) at all wavelengths. The concentration-weighted trajectory analysis showed that the most important potential source regions for b(abs-BrC) values from BBOA and po-OOA were located in northern Myanmar and along the China-Myanmar border, indicating the cross-border transport of BrC from Southeast Asia. A simple forcing efficiency evaluation further illustrated the importance of the BrC radiative effect with the high fractional radiative forcing by two OAs relative to BC (48.8 +/- 15.5 %). This study highlights a significant influence of BrC of biomass burning origin and secondary formation on climate change over the TP region during the pre-monsoon season.