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

PM<inf>2.5</inf> 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 (¹³C and ¹⁴C). Percentage biomass burning (BB)/biogenic fraction (f<inf>bio,</inf> calculated from ¹⁴C) varied from 67 to 92 % (78 ± 7) and correlated well with primary BB tracers like f<inf>60</inf>, and K⁺, suggesting BB as a considerable source. Rain events are shown to reduce the f<inf>bio</inf> fraction, indicating majority of BB-derived CA are transported. Further, δ¹³C (−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<inf>MS</inf>) to water-soluble BrC (BrC<inf>WS</inf>) at 365 nm was 1.8, indicating a significant part of BrC was water–insoluble. A good positive correlation between f<inf>bio</inf> and mass absorption efficiency of BrC<inf>WS</inf> and BrC<inf>MS</inf> at 365 nm with the higher slope for BrC<inf>MS</inf> suggests BB derived water-insoluble BrC was more absorbing. Relative radiative forcing (RRF, 300 to 2500 nm) of BrC<inf>WS</inf> and BrC<inf>MS</inf> with respect to EC were 11 ± 5 % and 23 ± 16 %, respectively. Further, the RRF of BrC<inf>MS</inf> was up to 60 %, and that of BrC<inf>WS</inf> was up to 22 % with respect to EC for the samples with f<inf>bio</inf> ≥ 0.85 (i.e., dominated by BB), reflecting the importance of BB in BrC RRF estimation.