Light-absorbing organic aerosol (brown carbon (BrC)) can significantly affect Earth's radiation budget and hydrological cycle. Biomass burning (BB) is among the major sources of atmospheric BrC. In this study, day/night pair (10-h integrated) of ambient PM(2.5)were sampled every day before (defined as T1,n = 21), during (T2,n = 36), and after (T3,n = 8) a large-scale paddy-residue burning during October-November over Patiala (30.2 degrees N, 76.3 degrees E, 250 m amsl), a site located in the northwestern Indo-Gangetic Plain (IGP). PM(2.5)concentration varied from similar to 90 to 500 mu g m(-3)(average +/- 1 sigma standard deviation 230 +/- 114) with the average values of 154 +/- 57, 271 +/- 122, and 156 +/- 18 mu g m(-3)during T1, T2, and T3 periods, respectively, indicating the influence of BB emissions on ambient air quality. The absorption coefficient of BrC (b(abs)) is calculated from the high-resolution absorption spectra of water-soluble and methanol-soluble organic carbon measured at 300 to 700 nm, and that at 365 nm (b(abs_365)) is used as a general measure of BrC. The b(abs_365_Water)and b(abs_365_Methanol)ranged similar to 2 to 112 Mm(-1)(avg 37 +/- 27) and similar to 3 to 457 Mm(-1)(avg 121 +/- 108), respectively, suggesting a considerable presence of water-insoluble BrC. Contrasting differences were also observed in the daytime and nighttime values of b(abs_365_Water)and b(abs_365_Methanol). Further, the levoglucosan showed a strong correlation with K+(slope = 0.89 +/- 0.06,R = 0.92) during the T2 period. We propose that this slope (similar to 0.9) can be used as a typical characteristics of the emissions from paddy-residue burning over the IGP. Absorption angstrom ngstrom exponent (AAE) showed a clear day/night variability during the T2 period, and lower AAE(Methanol)compared to AAE(Water)throughout the sampling period. Further at 365 nm, average relative atmospheric radiative forcing (RRF) for BrC(Water)is estimated to be similar to 17%, whereas that of BrCMethanol similar to 62% with respect to elemental carbon, suggesting that BrC radiative forcing could be largely underestimated by studies those use BrC(Water)only as a surrogate of total BrC.

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 Imlo-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 (BCBB) increased by factors of 3-4 over summertime values. In comparison, b(abs_365_me) and b(abs_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 (172 +/- 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 nss-K+. The wavelength dependence of b(abs_BrC) (angstrom ngstrom exponent: 5.9-6.2) and the presence of characteristic fluorescence peaks at 420-430 nm suggested presence of humiclike 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 (PAH5) and/or N-PAHs. Mass absorption efficiencies at 365 nm (MAE 365 ) of BrC aq and BrCaq in the BB regime (0.95 +/- 0.45 and 1.17 +/- 0.78 m(2) g 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-a-vis BC in the 300-400 nm range. (C) 2020 Elsevier B.V. All rights reserved.