Dual carbon isotope-based brown carbon aerosol characteristics at a high-altitude site in the northeastern Himalayas: Role of biomass burning.

PM samples (n = 34) were collected from January to April 2017 over Shillong (25.7°N, 91.9°E; 1064 m amsl), a high-altitude site situated in the northeastern Himalaya. The main aim was to understand the sources, characteristics, and optical properties of local vs long-range transported carbonaceous aerosols (CA) using chemical species and dual carbon isotopes (C and C). Percentage biomass burning (BB)/biogenic fraction (f calculated from C) varied from 67 to 92 % (78 ± 7) and correlated well with primary BB tracers like f, and K, suggesting BB as a considerable source. Rain events are shown to reduce the f fraction, indicating majority of BB-derived CA are transported. Further, δC (-26.6 ± 0.4) variability was very low over Shillong, suggesting it's limitations in source apportionment over the study region, if used alone. Average ratio of absorption coefficient of methanol-soluble BrC (BrC) to water-soluble BrC (BrC) at 365 nm was 1.8, indicating a significant part of BrC was water-insoluble. A good positive correlation between f and mass absorption efficiency of BrC and BrC at 365 nm with the higher slope for BrC suggests BB derived water-insoluble BrC was more absorbing. Relative radiative forcing (RRF, 300 to 2500 nm) of BrC and BrC with respect to EC were 11 ± 5 % and 23 ± 16 %, respectively. Further, the RRF of BrC was up to 60 %, and that of BrC was up to 22 % with respect to EC for the samples with f ≥ 0.85 (i.e., dominated by BB), reflecting the importance of BB in BrC RRF estimation.