Water-Soluble Brown Carbon in Atmospheric Aerosols from Godavari (Nepal), a Regional Representative of South Asia

Abstract

Brown carbon (BrC) has recently emerged as an important light-absorbing aerosol. This study provides interannual and seasonal variations in light absorption properties, chemical composition, and sources of water-soluble BrC (WS-BrC) based on PM₁₀ samples collected in Godavari, Nepal, from April 2012 to May 2014. The mass absorption efficiency of WS-BrC at 365 nm (MAE₃₆₅) shows a clear seasonal variability, with the highest MAE₃₆₅ of 1.05 ± 0.21 m² g^-1 in premonsoon season and the lowest in monsoon season (0.59 ± 0.16 m² g^-1). The higher MAE₃₆₅ values in nonmonsoon seasons are associated with fresh biomass burning emissions. This is further substantiated by a strong correlation ( r = 0.79, P < 0.01) between Abs₃₆₅ (light absorption coefficient at 365 nm) and levoglucosan. We found, using fluorescence techniques, that humic-like and protein-like substances are the main chromophores in WS-BrC and responsible for 80.2 ± 4.1% and 19.8 ± 4.1% of the total fluorescence intensity, respectively. BrC contributes to 8.78 ± 3.74% of total light absorption over the 300-700 nm wavelength range. Considering the dominant contribution of biomass burning to BrC over Godavari, this study suggests that reduction in biomass burning emission may be a practical method for climate change mitigation in South Asia.