Brown carbon (BrC) is the ubiquitous part of the atmospheric organic carbon. It absorbs solar lights and greatly impacts the Earth's radiative balance. This study examines the spectral characteristics of BrC and its radiative effect in the Dhaka South (DS) site and Dhaka North (DN) site from July 2023 to January 2024 with a high-volume particulate matter sampler on quartz filters. Spectral characteristics such as absorption coefficient (babe,), mass absorption efficiency (MAE), absorption angstrom exponent (AAE), and refractive index (Kabs-x) were determined by using a UV -visible spectrophotometer, and fluorescence emission spectra were analyzed in different pH by the fluorescent spectrophotometer. The concentrations of BrC and black carbon (BC) were determined by an aethalometer. The mean concentrations of BrC and BC in Dhaka city were 18.63 +/- 3.84 mu g 111-3 and 17.93 +/- 3.82 pg M-3, respectively. The AAE values lie in the range of 3.20-4.01 (DN) and 3.27-4.53 (DS), and the radiative forcing efficiency of BrC was obtained at 4.43 +/- 1.02 W g-1 in DN and 3.93 +/- 0.74 W g-1 in DS, indicating the presence of highly light-absorbing BrC in these locations. Average MAE and Kabs_k values were 1.55 +/- 0.45 m2g1 and 0.044 + 0.013, respectively, in DS, alternatively 1.84 +/- 0.59 m2g1 and 0.052 +/- 0.016 in DN. The fluorescence excitation-emission spectra confirmed the presence of a polyconjugate cyclic ring with multifunctional groups in the structure of BrC. Light absorption properties and fluorescence emission spectra were varied with the change of pH. As the pH increased (2-8), the AAE value decreased and MAEB,c_365 increased due to protonation or deprotonation. This study highlights that the BrC has a significant impact on the air quality as well as the Earth's radiative balance, emphasizing its strong light-absorbing properties and variability with environmental factors.

Thermal-optical fractions of organic carbon (OC), elemental carbon (EC), delta C-13 and optical properties of PM(2.5 )from Vehicular Fuel Emissions (VFEs) and Biomass Mixed Fuel Emissions (BMFEs) in India were examined. Heterogeneities in these species across Bharat Stage (BS) emission standards, vehicle type and cooking processes were also captured. Results suggest that distributions of OC and EC sub-fractions and Mass Absorption Efficiency (MAE) are driven by the fuel type, operating, combustion conditions, and emissions control strategies. Variability in thermal-optical fractions of carbon was useful not only in delineating VFEs and BMFEs but also in differentiating compositionally similar sources like gasoline and diesel. The mean delta C-13 value for diesel exhaust (- 26.3 +/- 1.3 parts per thousand) was marginally higher than the value (-27.0 +/- 1.2 parts per thousand) for gasoline and BMFEs. The Brown Carbon (BrC) content in VFEs was <10% while it constituted similar to 60% of the BMFEs. The MAE of both EC and OC of all the sources were calculated at 7 wavelengths (405 nm, 445, 532, 632, 780,808, and 980 nm) and heterogeneity was observed across vehicle types (higher MAEs for MUVs), fuel type (lowest MAEoc values for gasoline-powered vehicles) and BS divisions (BSII category vehicles shown highest MAEs) along with light absorption by OC and EC emitted by these sources. The results of this study characterizing the chemical, optical and isotopic signatures of PM2.5 from three major combustion sources will be useful in enhancing source identification and resolution in source apportionment efforts and in radiative forcing calculations.

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.