Optical properties of aerosol brown carbon (BrC) in the eastern Indo-Gangetic Plain

We report here measurements of aerosol black carbon (BC) and aqueous and methanol-extractable brown carbon (BrC_aq and BrC_me) from a receptor location in the eastern Indo-Gangetic Plain (IGP) under two aerosol regimes: the photochemistry-dominated summer and biomass burning (BB) dominated post-monsoon. We couple time-resolved measurements of BC and aerosol light absorption coefficients (b_abs) with time-integrated analysis of BrC UV-Vis and fluorescence characteristics, along with measurements of total and water-soluble organic carbon (OC and WSOC), and ionic species (NH₄⁺, K⁺, NO₃^-). In the BB regime, BC and its BB-derived fraction (BC_BB) increased by factors of 3-4 over summertime values. In comparison, b_{abs_365_aq} and b_{abs_365_me} (absorption coefficients of BrC_aq and BrC_me at 365 nm) increased by a factor of 5 (9.7 ± 7.8 vs 2.1 ± 1.4 Mm^-1) and 2.5 (17.2 ± 9.0 vs 6.9 ± 2.9 Mm^-1), respectively, in the BB period over summer, and were highly correlated (r = 0.82-0.87; p < 0.01) with the BB-tracer nssK⁺. The wavelength dependence of b_{abs_BrC} (Ångstrom exponent: 5.9-6.2) and the presence of characteristic fluorescence peaks at 420-430 nm suggested presence of humic-like substances (HULIS) in the aged BB aerosol, while significant association between BrC_aq and NO₃^- (r = 0.73; p < 0.01) possibly indicated formation of water-soluble nitroaromatic compounds. BrC_aq contributed 55% to total BrC absorption at 300-400 nm while that for the water-insoluble component (WI-BrC) increased from 41% at 340 nm to ~60% at 550 nm, suggesting formation of water-insoluble polycyclic aromatic hydrocarbons (PAHs) and/or N-PAHs. Mass absorption efficiencies at 365 nm (MAE₃₆₅) of BrC_aq and BrC_me in the BB regime (0.95 ± 0.45 and 1.17 ± 0.78 m² g^-1, respectively) were in line with values expected from photobleaching of BB source emissions after transport to the eastern IGP. Overall, BrC_aq and BrC_me were significant components of light absorbing aerosol in the BB regime, with contributions of 9 ± 5% and 16 ± 7%, respectively, to radiative forcing vis-à-vis BC in the 300-400 nm range.