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 (BrCaq and BrCme) 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 (babs) 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 (BCBB) increased by factors of 3-4 over summertime values. In comparison, babs_365_aq and babs_365_me (absorption coefficients of BrCaq and BrCme 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 babs_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 BrCaq and NO3- (r = 0.73; p < 0.01) possibly indicated formation of water-soluble nitroaromatic compounds. BrCaq 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 BrCaq and BrCme 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, BrCaq and BrCme 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.