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Biofuel combustion is an important source of particulate light absorbing organic carbon (OC), also known as brown carbon (BrC). We applied spectrophotometry to characterize methanol-extracted BrC from emission tests of 'real-world' biofuel combustion in India and Malawi, including wood stoves ('traditional', 'improved' and 'chimney') and artisanal charcoal kilns. Average mass absorption coefficient (MACbulk,λ) of extracted BrC was highest for 'traditional', followed by 'improved', 'chimney', and 'charcoal', at near-ultraviolet to blue wavelengths, with this order reversed for BrC absorption Angstrom exponent (AAE). MACbulk,λ in UV wavelengths was positively correlated with the elemental carbon to organic aerosol ratio (EC/OA), though the correlation was weaker than that observed in laboratory cookstove samples. BrC imaginary refractive indices (k) were anti-correlated with wavelength dependence (w), thus less wavelength dependent BrC had higher light absorptivity. MACbulk,λ correlated with the fraction of OC evolving at higher temperature steps in thermo-optical analysis, consistent with a link between BrC absorptivity and OC volatility, and suggesting that BrC absorption may be parameterized using existing OC data. Modeling analyses showed that BrC makes a strong contribution to overall absorption (average of 48% to 80% at 365 nm), and a strong negative correlation between EC/OA and the relative contribution of BrC to total aerosol (BrC + BC) light absorption; the latter trend is dominated by the quantity versus optical properties of BrC. The estimated direct radiative effect of BrC is approximately equal to that of BC for biofuel combustion emissions in India, highlighting the importance of BrC in the climate energy budget. Copyright © 2022 American Association for Aerosol Research [ABSTRACT FROM AUTHOR]

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 PM2.5 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° N, 76.3° E, 250 m amsl), a site located in the northwestern Indo-Gangetic Plain (IGP). PM2.5 concentration varied from ~ 90 to 500 μg m−3 (average ± 1σ standard deviation 230 ± 114) with the average values of 154 ± 57, 271 ± 122, and 156 ± 18 μg m−3 during T1, T2, and T3 periods, respectively, indicating the influence of BB emissions on ambient air quality. The absorption coefficient of BrC (babs) 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 (babs_365) is used as a general measure of BrC. The babs_365_Water and babs_365_Methanol ranged ~ 2 to 112 Mm−1 (avg 37 ± 27) and ~ 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 babs_365_Water and babs_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 (~ 0.9) can be used as a typical characteristics of the emissions from paddy-residue burning over the IGP. Absorption Ångström exponent (AAE) showed a clear day/night variability during the T2 period, and lower AAEMethanol compared to AAEWater throughout the sampling period. Further at 365 nm, average relative atmospheric radiative forcing (RRF) for BrCWater is estimated to be ~ 17%, whereas that of BrCMethanol ~ 62% with respect to elemental carbon, suggesting that BrC radiative forcing could be largely underestimated by studies those use BrCWater only as a surrogate of total BrC. [ABSTRACT FROM AUTHOR]

One of the extensively used methods for aerosol mass concentration measurement is the optical filter method used in Aethalometer. Due to the aerosol loading and multi scattering effect, there are uncertainties that can lead to underestimation or overestimation of aerosol concentration and measurement of the absorption coefficient. To overcome such uncertainties, a data correction method has been adopted in the present work. During the winter, pre-monsoon and post-monsoon seasons, Aethalometer underestimated black carbon (BC) concentration by 8 ± 1%, 11 ± 1%, and 10.5 ± 0.2%, due to the higher aerosol deposition. While in the monsoon season, underestimation of 5.5 ± 2% is observed because of the presence of bigger scattering nature aerosol. For 70% of days, found a positive correlation between the compensation parameter (k 880) and wavelengths, which indicated the dominance of the smaller aerosol particles. While 30% of days have a negative correlation due to the bigger aerosol particle. In the winter, pre-monsoon and monsoon seasons, the absorption coefficient at 370 nm was 86%, 90%, and 93% higher than that of 880 nm due to the higher biomass burning emissions. The absorption Ångström exponent (α 370-880) was 1.02 for fossil fuel burning and the corresponding k 880 was 0.0037. In the winter and post-monsoon seasons, BC emissions from fossil fuel combustion dominated over the BC emitted by biomass burning emissions with α 370-880 < 1.1, whereas α 370-880 is found to be more than 1.2 during the pre-monsoon and monsoon seasons due to the higher biomass burning. • Transported and local aerosols have an impact on the BC concentration at the site. • Transported and local aerosols affect the compensation parameter. • Absorption Ångström Exponent increases from 1 to 1.5 at the station. [ABSTRACT FROM AUTHOR]

The near-simultaneous measurements of wavelength dependent light absorption properties of black carbon (BC) aerosols employing the seven-channel, model AE-33 Aethalometers are carried out from the campuses of Ahmednagar College, Ahmednagar (AC-Ahmednagar) and Indian Institute of Tropical Meteorology, Pune (IITM-Pashan) from December 2015 to December 2016. Analysis brings out statistically significant higher values of BC absorption coefficient [σ abs,880 nm (BC), Mm−1 (hence BC mass concentration) at site AC-Ahmednagar (annual mean = 99.95 ± 79.8 Mm−1) as compared to those at site IITM-Pashan (annual mean = 47.19 ± 59.9 Mm−1). The seasonal mean values of σ abs,880 nm (BC) illustrate conspicuous intra-seasonal variability at both the locations. This variability seems to be influenced by the seasonal nature and strengths of BC emission sources, local meteorology, dynamics in atmospheric boundary layer (ABL) and modulation in advection pathways. The absorption Ångström exponent (AAE) analysis reveals that there exists a predominant contribution to the total BC by fossil fuel emission enriched mixed carbonaceous aerosols as well as freshly emitted fossil fuel burning BC aerosols at site AC-Ahmednagar while at site IITM-Pashan the major contribution to the total BC appears to come from fossil fuel/biomass emission enriched mixed carbonaceous aerosols. The source apportionment of BC mass depicts that the fractional contributions of BC_FF to BC mass dominate throughout the year at both observing locations. Seasonal variability of BC_BB fraction of BC at both locations is found to be complimentary to the seasonal variability of BC_FF fraction of BC at respective sites. • Seasonal mean σ abs,800 nm (BC) illustrates conspicuous intra-seasonal variability. • AAE variability depicts predominance of fraction BC_FF to BC throughout the year. • BC_FF and BC_BB seasonal variabilities are found to be complimentary each other. • Seasonally different advection pathways prevail at studied locations. [ABSTRACT FROM AUTHOR]

Aerosol scattering and absorption characteristics were investigated at an urban megacity Delhi in the western Indo-Gangetic Basin (IGB) during the period from October 2011 to September 2012 using different in-situ measurements. The scattering coefficient (σ sp at 550 nm) varied between 71 and 3014 Mm −1 (mean ~710 ± 615 Mm −1 ) during the entire study period, which was about ten times higher than the absorption coefficient (σ abs at 550 nm ~67 ± 40 Mm −1 ). Seasonally, σ sp and σ abs were substantially higher during the winter/post-monsoon periods, which also gave rise to single scattering albedo (SSA) by ~5%. The magnitude of SSA (at 550 nm) varied between 0.81 and 0.94 (mean: 0.89 ± 0.05). Further, the magnitude of scattering Ångström exponent (SAE) and back-scattering Ångström exponent (BAE) showed a wide range from −1.20 to 1.57 and −1.13 to 0.87, respectively which suggests large variability in aerosol sizes and emission sources. Relatively higher aerosol backscatter fraction ( b at 550 nm) during the monsoon (0.25 ± 0.10) suggests more inhomogeneous scattering, associated with the coarser dust particles. However, lower value of b during winter (0.13 ± 0.02) is associated with more isotropic scattering due to dominance of smaller size particles. This is further confirmed with the estimated asymmetry parameter (AP at 550 nm), which exhibits opposite trend with b . The aerosol optical parameters were used in a radiative transfer model to estimate aerosol radiative forcing. A mean radiative forcing of −61 ± 22 W m −2 (ranging from −111 to −40 W m −2 ) was observed at the surface and 42 ± 24 W m −2 (ranging from 18 to 87 W m −2 ) into the atmosphere, which can give rise to the mean atmospheric heating rate of 1.18 K day −1 . [ABSTRACT FROM AUTHOR]

We present here spectroscopic compositional analysis of brown carbon (BrC) and humic-like substances (HULIS) in the Indian context under varying conditions of source emissions and atmospheric processing. To this end, we study bulk water-soluble organic matter (WSOM), neutral- and acidic-HULIS (HULIS-n and HULIS-a), and high-polarity (HP)-WSOM collected in the eastern Indo-Gangetic Plain (IGP) with respect to UV–Vis, fluorescence, FT-IR, 1H NMR and 13C characteristics under three aerosol regimes: photochemistry-dominated summer, aged biomass burning (BB)-dominated post-monsoon, and fresh BB-dominated winter. Absorption coefficients (b abs_365 nm ; Mm−1) of WSOM and HULIS fractions increase by a factor of 2–9 during winter as compared to summer, with HULIS-n dominating total HULIS + HP-WSOM absorption (73–81%). Fluorophores in HULIS-n appear to contain near-similar levels of aromatic and unsaturated aliphatic conjugation across seasons, while HULIS-a exhibits distinctively smaller-chain structures in summer and post-monsoon. FT-IR spectra reveals, among others, strong signatures of aromatic phenols in winter WSOM suggesting a BB-related origin. 1H NMR-based source attribution coupled with back trajectory analysis indicate the presence of secondary and BB-related organic aerosol (SOA and BBOA) in the post-monsoon and winter, and marine-derived OA (MOA) in the summer, which is supported by 13C measurements. Overall, these observations uncover a complex interplay of emissions and atmospheric processing of carbonaceous aerosols in the IGP. Image 1 • Bulk BrC and HULIS fractions were enriched during biomass burning (BB) periods. • HULIS-n dominated (73–81%) the seasonal total HULIS + HP-WSOM absorption at 365 nm. • Polar functional groups from atmospheric photooxidation prevailed in summer. • Strong signatures of aromatic phenols were observed in BB-influenced samples. • Marine, secondary formation, and BB sources dominated seasonal organic aerosol. We use multiple spectroscopic approaches to characterize the sources and atmospheric processing of aerosol brown carbon and humic-like substances in the eastern Indo-Gangetic Plain, India. We find that secondary formation, marine and biomass burning sources constrain seasonal organic aerosol composition and its optical properties in this region. [ABSTRACT FROM AUTHOR]

The simultaneous near surface measurements of aerosol scattering and absorption coefficients over different environments (Ahmedabad, urban and Gurushikhar, a high altitude remote site) in western India were conducted to estimate SSA and investigate the importance of SSA in aerosol radiative forcing. The surface SSA (0.79, 0.85) is lower than the column SSA (0.92, 0.95) as emission sources for black carbon aerosols (absorbing in nature) are abundant near the surface (Ahmedabad, Gurushikhar). The atmospheric warming over the urban region estimated using column SSA is a factor of 3 lower (~ 18 W m − 2 ) than that of the warming (~ 52 W m − 2 ) estimated utilising surface SSA. The significant difference in atmospheric warming arises due to the differences in the SSA as aerosol optical depth (a measure of column concentration of aerosols) is the same. Surface and column SSA are comparatively higher over the high altitude remote site as the abundance of absorbing aerosols is less over a non-source region. In addition, the differences between surface and column SSA are less (< 9%) resulting in comparable aerosol radiative forcing estimates. This study highlights the differences that can arise in aerosol radiative effects due to the differences in SSA as a function of altitude (surface vs. column) and environment (urban vs. remote), thereby providing regional bounds on aerosol radiative forcing which can further be used in climate assessment studies. Image 1 • Surface SSA is lower than the columnar SSA as most of absorbing aerosols exist near surface. • SSA over Ahmedabad (urban) is lower than Gurushikhar (high alti-tude). • Heating rate estimated from surface and column SSA is comparable as SSA is comparable over Gurushikhar. • The differences in forcing are due to the differences in SSA as AOD remains same. [ABSTRACT FROM AUTHOR]

This work investigates the absorption properties of soluble brown carbon (BrC), extracted in methanol and water, from ambient aerosol (PM 10) samples, collected over an urban background site in Mumbai, India. The diurnal variability was investigated in samples collected in the morning (7–11 a.m.) and afternoon (12–4 p.m.) periods. Absorption properties of BrC (in the 300–600-nm wavelength range) were measured in filter extracts of water-soluble organic carbon (WSOC) and methanol-soluble organic carbon (MSOC). WSOC and MSOC accounted for on average 52% and 77%, respectively, of the measured OC, potentially indicating unextracted BrC and rendering these values the lower bound. Compared with afternoon samples, the morning samples of MSOC and WSOC had increased BrC concentrations and absorption coefficients (b abs 365; 40%–65%). The correlation between b abs 365 and EC, ns-K+, and NO 3 − in the morning samples indicated contributions from primary sources, including both biomass and vehicular sources. The decreased b abs 365 in the afternoon samples was partly explained by mixing layer dilution, accompanied by a reduction in the concentrations of primary aerosol constituents. Furthermore, in the afternoon samples, 1HNMR spectroscopy revealed the presence of more oxidized functional groups and significantly higher OC/EC and WSOC/OC ratios, indicating the greater aging of afternoon aerosol. The MAC 365 (m2gC−1) for both WSOC and MSOC extracts decreased significantly by 20%–34% in the afternoon samples compared with the morning samples, indicating degradation in the absorption properties of the particles and potentially a change in the constituent BrC chromophores. Image 1 • Enhanced BrC levels & absorption, morning vis-à-vis afternoon, at urban Indian site. • Correlation between b abs 365& EC/ns-K+ implies primary source contribution in morning. • More oxidized functional groups, more aging found via 1HNMR, in afternoon. Compared with morning, BrC absorption of ambient aerosols decreased in the afternoon in Mumbai, India, with boundary layer dilution and change in emission sources being the influencing factors. Relatively higher OC/EC, WSOC/OC ratios and 1HNMR spectroscopy suggest the presence of more oxidized functional groups indicating greater aerosol aging in the afternoon. [ABSTRACT FROM AUTHOR]