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

M Devaprasad; N Rastogi; R Satish; A Patel; A Dabhi; A Shivam; R Bhushan; R Meena

doi:10.1016/j.scitotenv.2023.169451

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

Sci Total Environ. 2024 Feb 20:912:169451. doi: 10.1016/j.scitotenv.2023.169451. Epub 2023 Dec 22.

Authors

M Devaprasad¹, N Rastogi², R Satish³, A Patel³, A Dabhi³, A Shivam³, R Bhushan³, R Meena³

Affiliations

¹ Geosciences Division, Physical Research Laboratory, Ahmedabad 380009, India; Indian Institute of Technology, Gandhinagar, Gujarat 382355, India.
² Geosciences Division, Physical Research Laboratory, Ahmedabad 380009, India. Electronic address: nrastogi@prl.res.in.
³ Geosciences Division, Physical Research Laboratory, Ahmedabad 380009, India.

PMID: 38143007
DOI: 10.1016/j.scitotenv.2023.169451

Abstract

PM_2.5 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_bio, calculated from ¹⁴C) varied from 67 to 92 % (78 ± 7) and correlated well with primary BB tracers like f₆₀, and K⁺, suggesting BB as a considerable source. Rain events are shown to reduce the f_bio 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_MS) to water-soluble BrC (BrC_WS) at 365 nm was 1.8, indicating a significant part of BrC was water-insoluble. A good positive correlation between f_bio and mass absorption efficiency of BrC_WS and BrC_MS at 365 nm with the higher slope for BrC_MS suggests BB derived water-insoluble BrC was more absorbing. Relative radiative forcing (RRF, 300 to 2500 nm) of BrC_WS and BrC_MS with respect to EC were 11 ± 5 % and 23 ± 16 %, respectively. Further, the RRF of BrC_MS was up to 60 %, and that of BrC_WS was up to 22 % with respect to EC for the samples with f_bio ≥ 0.85 (i.e., dominated by BB), reflecting the importance of BB in BrC RRF estimation.

Keywords: Carbonaceous aerosols; Long-range transport; Optical properties; Radiocarbon; Relative radiative forcing.