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

10 Abstract. 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 as well as black carbon (BC); however, our understanding of optical properties, sources, atmospheric processes of BrC in this region remains limited. In this study, a multiple-wavelength Aethalometer coupled with a quadrupole aerosol chemical speciation monitor were deployed to investigate the highly time resolved BrC in the submicron aerosol in the southeastern edge of the TP during the pre- 15 monsoon season. The result showed that BrC had the 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 process and a photochemical-oxidation processed oxygenated OA (po-OOA) by a hybrid environmental receptor model analysis. Combined with light absorption coefficient of BrC ( b abs-BrC ), the source-specific mass absorption cross section of BBOA (0.50−1.75 m 2 g -1 ) and po-OOA (0.38−2.15 m 2 g -1 ) at 370−660 nm were 20 retrieved. On average, b abs-BrC from po-OOA (1.1−6.3 Mm -1 ) was higher than that from BBOA (0.7−2.3 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 the 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 BrC radiative effect with the high fractional radiative forcing by two OAs relative to BC 25 (50.6 ± 18.7 %). This study highlighted a significant influence of BrC from biomass burning emissions and secondary formation on climate change over the TP region during the pre-monsoon season. was 1.75 ± 0.48 and 2.15 ± 0.24 m 2 g -1 at 370 nm, and dropped to 0.50 ± 0.08 and 0.38 ± 0.04 m 2 g -1 at 660 nm, respectively. b abs-BrC from BBOA contributed 29.5−40.2 % of the reconstructed b abs-BrC at 370−660 nm, while the rest was from po-OOA. All the 72-h backward trajectories of the Gaomeigu site came from Myanmar. The spatial distributions of potential source regions for b abs-BrC showed the highest CWT values for BBOA and po-OOA were both in the northern Myanmar and along the China-Myanmar border, 335 demonstrating that biomass burning emissions and secondary formation from the cross-border transport of Southeast Asia were the major source of b abs-BrC at Gaomeigu. According to the integrated SFEs of BBOA, po-OOA, and BC over the solar spectra (370−660 nm) (17.4, 17.0, and 226.6 W g -1 , respectively), the fractional radiative forcing by BBOA (17.7 ± 6.6 %) and po-OOA (32.9 ± 18.0 %) relative to BC were obtained, highlighting the importance of BrC radiative effect. This study provides insights into light absorption properties of BrC and its potential impacts on climate change over the TP and surrounding areas.