Atmospheric brown carbon (BrC), a short-lived climate forcer, absorbs solar radiation and is a substantial contributor to the warming of the Earth ' s atmosphere. BrC composition, its absorption properties, and their evolution are poorly represented in climate models, especially during atmospheric aqueous events such as fog and clouds. These aqueous events, especially fog, are quite prevalent during wintertime in Indo-Gangetic Plain (IGP) and involve several stages (e.g., activation, formation, and dissipation, etc.), resulting in a large variation of relative humidity (RH) in the atmosphere. The huge RH variability allowed us to examine the evolution of water-soluble brown carbon (WS-BrC) diurnally and as a function of aerosol liquid water content (ALWC) and RH in this study. We explored links between the evolution of WS-BrC mass absorption efficiency at 365 nm (MAE WS- BrC-365 ) and chemical characteristics, viz., low-volatility organics and water-soluble organic nitrogen (WSON) to water-soluble organic carbon (WSOC) ratio (org-N/C), in the field (at Kanpur in central IGP) for the first time worldwide. We observed that WSON formation governed enhancement in MAE WS-BrC-365 diurnally (except during the afternoon) in the IGP. During the afternoon, the WS-BrC photochemical bleaching dwarfed the absorption enhancement caused by WSON formation. Further, both MAE WS-BrC-365 and org-N/C ratio increased with a decrease in ALWC and RH in this study, signifying that evaporation of fog droplets or bulk aerosol particles accelerated the formation of nitrogen-containing organic chromophores, resulting in the enhancement of WS-BrC absorptivity. The direct radiative forcing of WS-BrC relative to that of elemental carbon (EC) was -19 % during wintertime in Kanpur, and - 40 % of this contribution was in the UV -region. These findings highlight the importance of further examining the links between the evolution of BrC absorption behavior and chemical composition in the field and incorporating it in the BrC framework of climate models to constrain the predictions.

Carbonaceous aerosol, mainly comprising elemental carbon (EC) and organic carbon (OC), plays an important role in aerosol-climate interaction owing to its light-absorbing feature. The radiative forcing of carbonaceous aerosol is largely determined by its mass absorption efficiency (MAE). To depict the MAE of carbonaceous aerosol and its variations with aerosol size and chemical composition, light absorption coefficients and the main chemical composition of PM1 and PM2.5 were comparably measured in a winter field campaign in urban Beijing. On average, aerosol absorption by PM1 at 880 nm wavelength contributes to approximately half of aerosol absorption by PM2.5, mainly because aerosol absorption at this wavelength was dominated by EC and nearly half of the total EC mass in PM2.5 existed in PM1. The average MAE of EC at 880 nm (MAEEC,880nm) was 3.9 +/- 0.7 m2 g-1 for PM2.5, lower than that for PM1 (4.4 +/- 0.8 m2 g-1) likely because of the larger EC cores and lower coating degrees of EC particles in PM1-2.5. Variation in MAEEC,880nm was related to the bulk mass fractions of nitrate in PM1 and PM2.5, implying the important impact of secondary nitrate formation on the modification of EC mixing states and enhancing EC absorption efficiency in Beijing. Absorption by OC took up about 40% of the absorption coefficients for PM1 and PM2.5 at 370 nm. The mean MAE of OC at 370 nm (MAEOC,370nm) was 2.4 +/- 0.9 m2 g-1 for PM1, higher than that of for PM2.5 (1.9 +/- 0.6 m2 g-1). The high value of MAEOC,370nm might be associated with regionally transported aerosols during clean and polluted periods. Enlarged particle sizes might have considerably weakened MAEOC,370nm for PM2.5 while exerting negligible impact on MAEOC,370nm for PM1.

The study on the light absorption of carbonaceous components is a research hotspot in the field of aerosol optical. Size distribution and lensing effect have great influence on the absorption of elemental carbon (EC) and brown carbon (BrC). However, few studies were conducted on the spatial variations of BC and BrC absorption with different size ranges in different regions. In this study, the mass concentration, absorption coefficient and mass absorption efficiency (MAE) of size-resolved carbonaceous components in three functional areas of Nanjing were compared based on offline sampling and experimental analysis. Bimodal and unimodal size distributions were found for EC and BrC mass concentrations at the three sites, respectively. Affected by the emission of diesel vehicles and aggregation condensation, high EC concentration in the size range of 0.56-1 mu m was found at the suburban site. Due to the secondary formation from VOCs, high BrC concentration in the size range of 0.18-0.32 mu m was observed at the industrial site. High EC concentration in the size range of 0.18-0.32 mu m caused by diesel emission was the main reason for the high EC absorption in the regional area. Compared with vehicle emissions, the absorption capacity of secondary BrC was weaker. The variation of MAE values caused by various sources was the key factor leading to BrC absorption, which varied greatly in different regions. This study is helpful to understand the variation of light absorption of carbonaceous component and its source influence in a typical polluted city in the Yangtze River Delta, which provides important data support for the comprehensive evaluation of the role of aerosol light absorption in the change of radiative forcing in different regions.

The north-western Indo-Gangetic Plain (IGP) experiences massive crop-residue burning (viz. paddy and wheat) on an annual and seasonal basis. The long-range transport of the particulates emitted from paddyand wheat residue burning (expressed as PRB and WRB, respectively) degrades the air quality, perturb the radiative budget, and alter the atmospheric chemistry of downwind IGP locations. Therefore, chemical, absorption and radiative characteristics of carbonaceous aerosols (total carbon; TC) were explored in this study. The fraction of TC in ambient PM2.5 (particulates with aerodynamic diameter <= 2.5 mu m) was similar to 45% during PRB and similar to 24% during WRB. However, biomass burning emissions were the predominant source of TC during both PRB and WRB. The brown carbon (BrC) aerosols at Beas were similar to 2-3 times more abundant during PRB than in WRB. However, the absorption properties such as mass absorption efficiency and imaginary component of the refractive index for BrC at 405 nm (expressed as MAE(BrC-405), k(BrC-405), respectively) and radiative characteristics such as light absorption capacity were similar during both PRB and WRB. The similarity between these absorption and radiative characteristics indicate that BrC aerosols emitted during the burning of different biomass may depend only on their combustion condition. Further, the increased biomass burning emissions were linked with enhancement in the light absorption capacity of BrC during PRB. A similar light absorption capacity was observed (similar to 30 W/g) for water-soluble BrC (WS-BrC) and total BrC during PRB. Moreover, the % contribution of BrC and EC to their total direct radiative forcing (DRFTC) during PRB and WRB (similar to 40% and similar to 60% for BrC and EC, respectively) were also similar. The WS-BrC constitutes only similar to 15% of DRFTC during PRB. This difference signifies that non-WS-BrC aerosols were the predominant light-absorbing species during PRB (compared to WS-BrC), which needs to be factored into global climate models to mitigate uncertainties.

Under typical Chinese wintry rural conditions, dominating individual coal heating would emit lots of the fine fraction of ambient aerosol exclusively including carbonaceous particulate matter. In this study, a specified drop tube furnace system is employed to simulate experimentally particle matter emitted during individual coal combustion. Emphatically, the effects of coal types, oxygen concentration and combustion ambient on the formation characteristics of carbonaceous aerosols in the flue gas were discussed. It was found that the fraction of organic carbon (OC) and elemental carbon (EC) in the flue gas produced by bituminous coal combustion was lower than that of lignite. Meanwhile, with the increase of oxygen concentration, the production of OC and EC decreased, but the sensitivity of EC to oxygen concentration was higher than that of OC, which indicated that the formation mechanism of OC and EC is extremely different. Noticeable, the Absorption Angstrom Exponent (AAE) of methanol-soluble organic carbon (MSOC) is higher than that of water-soluble organic carbon (WSOC), which indicates that a large amount of methanol-soluble but water-insoluble brown carbon has strong light absorption capacity between 330 nm and 550 nm, and its light absorption capacity tends to be in the short-wave region. The mass absorption efficiency (MAE) of brown carbon produced by coal combustion (0.1-1 m(2)/gC) is similar to that of atmospheric aerosol (0.3-1.8 m(2)/gC), which indicates that the contribution of brown carbon emitted from coal combustion to the light absorption capacity of atmospheric aerosol should not be underestimated.

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.

Understanding of carbonaceous aerosols from different combustion sources and their optical properties are important to better understand atmospheric aerosol sources and estimate their radiative forcing. In this study, eight organic carbon (OC) and elemental carbon (EC) sub-fractions and light absorption properties of EC are investigated using thermal/optical method and compared among six typical solid and liquid fossil fuel combustion sources (e.g., coal combustion, industry, power plant, diesel and gasoline vehicle, and ship emissions) and within each source type, with consideration of different fuel types and combustion conditions. The results indicate that OC and EC sub-fraction distributions and mass absorption efficiency of EC (MAE(EC)) are sensitive and specific to sources, fuels, combustion and operating conditions. The differences in carbon fractions and AE(EC) between solid and fossil fuel source emissions are statistically significant (p < 0.05). The average MAE(EC) from liquid fossil fuel sources (7.9 +/- 3.5 m(2)/g) are around1.5-fold higher than those from solid fossil fuels (5.3 +/- 4.0 m(2)/g). Correlation analysis indicates that light attenuation of EC positively correlates with EC1 and EC2 fractions with correlation coefficients (r) around 0.6, while negatively correlates with the percentages of OC2 and OC3 in total carbon. Inter-comparisons of distributions of carbon sub-fractions and MAE(EC) from different coal samples indicate the tested new stoves and honeycomb-like shape may contribute to lower EC emission factors but with stronger light absorptivity of EC, suggesting curbing short-lived pollutants (e.g., EC) with improvement of coal stoves and clean coal at current stage might not always result in co-benefits of air quality and climate.

A major challenge in understanding radiative forcing of aerosols is accurately monitoring the light absorbing components and clarifying the main reasons of their spatial and temporal variations. In this study, the optical properties of light-absorbing carbon (LAC) in aerosols were measured over one year and the impacts from various emission sources and other influencing factors were analyzed at three sites (suburban (NJU), urban (PAES) and industrial (NUIST) in Nanjing, a typical polluted city in eastern China. With an improved method that combines online and offline techniques, we revised the multiple scattering correction factors and significantly reduced the uncertainty in measurement of absorption coefficients of black carbon (BC). The result reveals the necessity of developing the regional dependent factor for estimation of BC absorption. Relatively large mass absorption efficiency (MAE) of BC was found in summer and industrial region (NUIST), and the mixing state and coating relevant with secondary aerosol formation were the main reasons for such seasonal and site dependent variations. Distinct seasonal variations existed in the MAE of brown carbon (BrC) at NJU. In the winter, BrC from primary emissions such as diesel vehicles had a stronger absorption ability than that from secondary aerosol formation. The lowest MAE values of BrC appeared in summer, reflecting the formation of the non-absorbing biogenic secondary organic aerosol, and the effect of photobleaching. At the urban site PAES, BrC was expected to be mainly from gasoline vehicles and transport of biomass burning emissions, and had a stronger light absorbing ability than the other two sites. The results of simultaneous observations at NJU and PAES indicated that the formation of fresh secondary organic aerosol enhanced the optical absorption of BC but reduced that of BrC. The impacts of various influencing factors on LAC provided effective ways to alleviate their regional radiative forcing at the city scale.

Atmospheric PM1 (particulate matter with aerodynamic diameter <= 1 mu m) samples have been collected during foggy (n = 17) and non-foggy nights (n = 19) in wintertime at Kanpur in central Indo-Gangetic Plain (IGP) to assess light absorption characteristics and direct radiative forcing of water-extractable brown carbon (BrC). We have observed a significant enhancement (two-tailed t-test: t = 2.2; at significance level: p < 0.05) in the absorption coefficient of water-extractable BrC at 365 nm (b(abs-BrC-365)) from non-foggy (Avg.: 53.5 Mm(-1)) to foggy episodes (69.3 Mm(-1)). Enhancement in mass absorption efficiency (MAE) of BrC (1.8 m(2)/g C) during foggy episodes is consistent with that of b(abs-BrC-365). Absorption Angstrom exponent (AAE) remained similar (2.8) during foggy and non-foggy episodes. Significantly lower value of AAE (2.8) at Kanpur compared to other places in IGP ( similar to 5) highlights more light absorbing potential of atmospheric BrC over central IGP. Furthermore, MAE of EC at 660 nm during foggy period (8.5 m(2)/g) is relatively high as compared to that during the non-foggy episode (7.0 m(2)/g). The MAE of BrC and EC exhibited enhancement by similar to 15% and 20%, respectively during foggy events. These observations are also reflected by an increase (t = 11.1; p < 0.05) in direct radiative forcing of water-extractable BrC (relative to EC) in the atmosphere: from 23.7 +/- 10.8% during non-foggy to 54.3 +/- 16.5% during foggy episodes. Differences in chemical composition, loading, absorption properties and direct radiative forcing (DRF) of carbonaceous aerosols during non-foggy and foggy episodes indicate predominant influence of fog-processing.