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Humic‐like substances (HULIS) are a ubiquitous reactive component of atmospheric aerosol. They participate in the formation of secondary organic aerosols via chemical reactions with atmospheric oxidants. Here, we assess the influence of transition metal ions (namely ferric iron, Fe(III)), and nitrate ions (NO3− ${{\text{NO}}_{3}}^{-}$) on the heterogeneous reaction of gaseous NO2 with an aqueous film containing gallic acid (GA) or tannic acid (TA) as proxies for HULIS. Using a vertical wetted wall flow tube technique, the uptake coefficients of gaseous NO2 on GA and TA increased nonlinearly with increasing [Fe(III)], in dark and under light irradiation. However, the combined effect of both ions, Fe(III) and NO3− ${{\text{NO}}_{3}}^{-}$, led to a substantial decrease in NO2 uptake in the dark and under simulated near‐UV sunlight irradiation (300 < λ < 400 nm). The lifetime of GA in dilute aqueous phase, which corresponds to cloud water, due to reaction with NO2 would be 6 hr during both nighttime and daytime. However, the lifetime of GA in aerosol particles which contain both ions, that is, Fe(III) and NO3− ${{\text{NO}}_{3}}^{-}$, would increase to 27 hr during nighttime and 11 days and 6 hr due to light‐induced reaction with NO2. Also, we observed, using Fourier transform ion cyclotron resonance mass spectrometry, the formation of nitrocatechols compounds (e.g., methyl‐nitrocatechol), which contribute to brown carbon. Compounds with reduced functional groups such as amines were also observed in the presence of iron and nitrate ions in the dark and under irradiation, indicating that Fe(III) and NO3− ${{\text{NO}}_{3}}^{-}$, can influence the kinetics and product distribution in deliquescent aerosol particles. Plain Language Summary: Biomass burning is one of the main sources of humic‐like substances (HULIS) in the aerosol particles covering a large fraction of the water‐soluble organic carbon. Here we assessed the impact of nitrate and iron ions on the uptake coefficients of NO2 on gallic acid (GA) and tannic acid (TA) as HULIS proxies. The uptake coefficients and consequently the atmospheric lifetimes were dramatically impacted by the presence of nitrate and iron ions. The analysis of the formed product compounds in the condensed phase by Fourier transform ion cyclotron resonance mass spectrometry indicates that the presence of nitrate and iron ions largely impact the observed nitrogen‐containing organic compounds formed during the reaction of NO2 with GA. Majority of the formed compounds contain reduced nitrogen‐functional groups such as amines, amides, and imines. We identified the formation of nitroaromatics with at least one nitro functional group attached directly to a benzene ring, like 5‐Nitroguaiacol, 4‐methyl‐5‐nitrocatechol. These compounds, can cause positive radiative forcing of aerosols due to their light‐absorbing properties. Key Points: Uptake coefficients of gas phase NO2 on humic‐like substances proxies in aerosol particles are largely impacted by nitrate and iron ionsFormation of nitrogen containing product compounds in aerosols are impacted by nitrate and iron ions compared to that in cloud waterFormation of nitrocatechols like methyl‐nitrocatechol, which contribute to brown carbon is observed upon influence of nitrate and iron ions [ABSTRACT FROM AUTHOR]

Humic‐like substances (HULIS) are present in every environmental reservoir, including the ocean and the atmosphere. Ocean‐derived HULIS can be transferred to the atmosphere in the form of sea spray aerosols (SSA). Little information exists on the factors controlling this transfer process or how HULIS alter SSA physicochemical properties, cloud‐forming ability, and atmospheric reactions. Here, using excitation‐emission matrix spectroscopy and isolated ocean‐atmosphere systems, we investigated how ocean biology affects HULIS sea‐to‐air transfer during multiple phytoplankton bloom experiments. We posit that bacterial enzymatic activity on phytoplankton‐derived organic matter control HULIS size, production, and chemical composition. We found that changes in fluorescence indices and shifts in the HULIS fluorescence emission spectra reveal changes in HULIS chemical composition induced by bacteria. High bacterial enzymatic activity on the proteinaceous, lipid, and phosphorus‐rich organic pools enhanced HULIS production and its transfer to SSA. Seawater HULIS consistently accumulated across all experiments. The majority of HULIS was smaller than 0.2 μm or 50 kDa. Our results suggest that enzymatic‐processing bacteria transform the composition of HULIS in seawater, degrading dissolved organic matter into diverse chemical structures that are more efficiently transferred to the atmosphere in SSA. Plain Language Summary: Humic‐like substances (HULIS) are complex molecules found in the ocean. When waves break, HULIS can be launched into the atmosphere in tiny sea spray aerosol particles (SSA). While it is well documented that bacteria modify molecules in the ocean, far less is known about how they modify the characteristics and ocean‐to‐atmosphere transfer of HULIS. Here, we use a laboratory ocean‐atmosphere simulator to study how ocean biology affects HULIS properties in air and ocean samples. We found associations between ocean enzyme activities, HULIS chemistry, and the relative abundance of HULIS in SSA. Therefore, this study shows that marine bacteria enzymes can modify the chemical structures of HULIS, which in turn impacts their ability to transfer to the air via SSA. This study provides insight into the complex role marine biology plays in affecting SSA composition which controls marine clouds and climate. Key Points: Seawater humic‐like substances (HULIS) concentrations consistently increase during phytoplankton blooms, but remain mostly in the <0.2 μm or <50 kDa size fractionHigh bacterial enzymatic activity in seawater enhances HULIS productionHULIS chemical changes may play a role in HULIS sea‐air transfer [ABSTRACT FROM AUTHOR]

Over the past 50 years, fossil fuel consumption has increased dramatically, rising approximately eight-fold since 1950 and doubling since 1980. This surge has led to increased emissions of brown carbon (BrC) into the atmosphere, which are subsequently deposited onto oceans and land through dry or wet deposition processes. However, the source-specific fluxes of atmospheric organic carbon (OC) and BrC into the ocean are not adequately represented in the global carbon cycle. For the first time, we calculated BrC concentration using the optical intensity of organic matter and determined the global wet depositional flux of fossil fuel-derived BrC. Using the ratio of humic-like substances to OC fluxes, we estimated the global wet deposition of fossil fuel-derived BrC to be 2.0 ± 0.6 Tg C yr−1. Of this amount, the flux into oceans (0.7 ± 0.2 Tg C yr−1) represents 1.6% of the production rate of refractory dissolved organic carbon (RDOC) in the ocean (43 Tg yr−1). Notably, an increase in the proportion of fossil fuel-derived BrC in atmospheric OC may change the composition of OC in precipitation, resulting in a more refractory composition, which deviates from previously established paradigms. Our findings indicate that the flux of fossil fuel-derived RDOC from the atmosphere into the ocean, which is inadequately represented in current global DOC cycling models, may play a significant role in oceanic carbon cycles. These findings necessitate reconsidering our understanding of oceanic carbon cycling and highlight the need to improve existing models to better account for these newly identified processes and their potential impacts on global carbon dynamics. [Display omitted] • First calculation of BrC concentration using optical intensity of organic matter. • Global wet deposition of fossil fuel-derived BrC estimated at 2.0 ± 0.6 Tg C yr−1. • Ocean flux of fossil fuel-derived BrC is 1.6% of refractory DOC production rate. • BrC from fossil fuels may shift precipitation OC towards more refractory composition. • Fossil fuel-derived refractory DOC flux may significantly impact oceanic carbon cycles. [ABSTRACT FROM AUTHOR]

Throughout the cold and the warm periods of 2020, chemical and toxicological characterization of the water-soluble fraction of size segregated particulate matter (PM) (<0.49, 0.49–0.95, 0.95–1.5, 1.5–3.0, 3.0–7.2 and >7.2 μm) was conducted in the urban agglomeration of Thessaloniki, northern Greece. Chemical analysis of the water-soluble PM fraction included water-soluble organic carbon (WSOC), humic-like substances (HULIS), and trace elements (V, Cr, Mn, Fe, Ni, Cu, Zn, As, Cd and Pb). The bulk (sum of all size fractions) concentrations of HULIS were 2.5 ± 0.5 and 1.2 ± 0.3 μg m−3, for the cold and warm sampling periods, respectively with highest values in the <0.49 μm particle size fraction. The total HULIS-C/WSOC ratio ranged from 17 to 26% for all sampling periods, confirming that HULIS are a significant part of WSOC. The most abundant water-soluble metals were Fe, Zn, Cu, and Mn. The oxidative PM activity was measured abiotically using the dithiothreitol (DTT) assay. In vitro cytotoxic responses were investigated using mitochondrial dehydrogenase (MTT). A significant positive correlation was found between OPmDTT, WSOC, HULIS and the MTT cytotoxicity of PM. Multiple Linear Regression (MLR) showed a good relationship between OPMDTT, HULIS and Cu. [ABSTRACT FROM AUTHOR]

Routine measurements of carbonaceous species in PM2.5 inculidng organic carbon (OC), elemental carbon (EC), water-soluble organic carbon (WSOC), and humic-like-substance carbon (HULIS-C) in PM2.5 were performed at Anmyeon Island (AI) to clarify the seasonal variation and carbonaceous aerosol concentrations at a background site in Korea between 2015 and 2016. The annual average OC and EC concentrations were 4.52±3.25 μg/m3 and 0.46±0.28 μg/m3, respectively, and there were no clear seasonal variations in OC and EC concentrations. The average concentrations of WSOC and water-insoluble organic carbon (WISOC) were 2.56±1.95 μg/m3 and 1.96±1.45 μg/m3, respectively, and their composition in OC showed high temporal variations. A low correlation between WISOC and EC was observed, while WSOC concentrations were highly correlated with secondary organic carbon concentrations, which were estimated using the EC tracer method. The results indicate that the formation of secondary organic aerosols is a major factor for the determination of WSOC concentrations in this region. HULIS-C was the major component of WSOC, accounting for 39-99% of WSOC and the average concentration was 1.88±1.52 μg/m3. Two distinct periods with high carbonaceous speciess in PM2.5 were observed and characterized by their OC/EC ratios. The high concentration of OC with high ratio of OC/EC was due to the influence of a mixture of emissions from biomass burning and secondary formation transported from outside AI. While, the high concentrations of OC and EC with low OC/EC ratio were related to local vehicular emissions.

Throughout the cold and the warm periods of 2020, chemical and toxicological characterization of the water-soluble fraction of size segregated particulate matter (PM) (7.2 μm) was conducted in the urban agglomeration of Thessaloniki, northern Greece. Chemical analysis of the water-soluble PM fraction included water-soluble organic carbon (WSOC), humic-like substances (HULIS), and trace elements (V, Cr, Mn, Fe, Ni, Cu, Zn, As, Cd and Pb). The bulk (sum of all size fractions) concentrations of HULIS were 2.5 ± 0.5 and 1.2 ± 0.3 μg m−3, for the cold and warm sampling periods, respectively with highest values in the mDTT, WSOC, HULIS and the MTT cytotoxicity of PM. Multiple Linear Regression (MLR) showed a good relationship between OPMDTT, HULIS and Cu.

We characterize fine particulate matter (PM2.5) emitted from Indonesian peatland fires using ground-based source-dominated samplings of PM2.5 near peatland fire sources at two Regencies in Riau, Sumatra, Indonesia (number of samples = 13). Organic carbon (OC), elemental carbon, water-soluble OC (WSOC), the carbon content of humic-like substances (HULIS-C), and biomass burning tracers are determined. The carbon mass ratios of WSOC to OC (0.085 ± 0.015) and HULIS-C to WSOC (0.55 ± 0.085) are fairly constant and independent of the peatland fire sampling sites. By comparing diagnostic ratios using OC, WSOC, and HULIS-C at the peatland fire source and the receptor site (Malaysia) during peatland fire-induced haze periods, secondary WSOC and HULIS-C formation during transport from the source to the receptor site is highly possible. Interestingly, the mass ratio of syringic acid to levoglucosan (0.045 ± 0.0075) is fairly constant at Indonesian peatland fire sources. Because syringic acid is less stable than levoglucosan, this ratio is an aging indicator for Indonesian peatland fires at receptor sites. By comparing the mass fraction of each organic compound in the present study and previous studies, it is evident that the source profile for the coburning of peat with surface vegetation is significantly different compared with the burning of peat alone. Further knowledge of peat burning emissions is needed, particularly with respect to burning conditions, peat composition, and the effects of vegetative burning on peatland. Improved knowledge of these factors would lead to more reliable speciated emission inventories of Indonesian peatland fires, advancing chemical transport and radiative forcing modeling, as well as health risk assessment.

Inhalation of particulate matter is hypothesized to contribute to health effects by overproducing reactive oxygen species (ROS) and inducing oxidative stress. Fe(II) has been shown to contribute to ROS generation in acellular simulated lung fluids. Atmospheric humic-like substances (HULIS) have been shown to chelate Fe(II) and significantly enhance this ROS generation. Here, we investigate Fe(II)-mediated . OH generation from the iron active proteins in lung fluid, albumin and transferrin, and fulvic acid, a surrogate for HULIS, in human bronchoalveolar lavage fluid (BALF). We find that albumin enhances . OH generation from inorganic Fe(II) and that transferrin attenuates this enhancement. We estimate the rate constants for albumin-Fe(II) and fulvic acid-Fe(II) mediated O 2 . - reduction (1.9 ± 0.3) M−1 s−1 and (2.7 ± 0.3) M−1s−1 (pH = 5.5, T = 37 °C), 17–25 times the rate for free iron, which we measured to be (110 ± 20) × 10−3 M−1s−1, in agreement with the literature. . OH generation measured from fulvic acid-Fe(II) in BALF from 8 individuals with added fulvic acid is successfully predicted rates of . OH generation by mixtures of Fe(II), albumin, transferrin, fulvic acid, and ascorbate in saline solution. This indicates that fulvic acid enhances . OH formation in BALF, and that albumin and transferrin in BALF moderate the effect. We propose that fulvic acid, and thereby HULIS, is capable of mobilizing Fe(II) away from albumin and transferrin and this increases the formation rate of O 2 . - and ultimately of . OH. Furthermore, we find that albumin and transferrin have significantly different impacts on Fe(II)-mediated . OH than citrate, a common component of simulated lung fluids, a factor that should be considered carefully in the interpretation of results obtained from solutions containing citrate. [Display omitted] • o Albumin complexes inorganic iron enhancing reactive oxygen species generation. • o Fulvic acid takes iron from proteins in lung fluids enhancing.OH generation. • o Citrate in simulated lung fluids does not accurately mimic lung proteins. • o Albumin and fulvic acid increase iron O 2 .- generation rates by factors of about 20. [ABSTRACT FROM AUTHOR]

Total carbon content of the atmospheric suspended particulate matters consisted of as high as 89-93% organic carbon (OC) in Ulaanbaatar aerosol without showing seasonal variation. However, limited aerosol measurements have been conducted on these OC rich aerosols particularly for high molecular weight constituents. In order to address the gap above, abundance of high molecular weight humic-like substances (HULIS) in total suspended particulates (TSP) from Ulaanbaatar atmospheric aerosol were determined for the first time. HULIS molecular structure was characterised for different seasons using carbon content and UV absorbance measurements coupled with solid phase extraction methods. Although, HULIS contributions to water soluble organic fraction of the winter and summer aerosols were similar HULIS carbon concentration was higher in winter samples (9-37 mg·L-1) than in summer (2-6 mg·L-1). Consequently quantity of aromatic moieties and degree of aromaticity varied between seasons.

Atmospheric humic-like substances (HULIS) could affect regional climate due to their strong light-absorbing capacity. Daily fine particulate matter (PM 2.5) samples were collected from December 18, 2016 to January 8, 2017 at an urban site in Chongqing, Southwest China. The mean concentration of HULIS in terms of carbon (HULIS-C) was 6.4 ± 3.4 μg m−3, accounting for 72% of water-soluble organic carbon. The mass absorption efficiency at 365 nm (MAE 365) and absorption Ångström index (AAE) of atmospheric HULIS were 2.8 ± 0.30 m2 g−1 C and 4.6 ± 0.37, respectively. Good correlations between the light absorption coefficients of HULIS at 365 nm (Abs 365) and the concentrations of K+, elemental carbon, NO 3 −, and NH 4 + were observed, with correlation coefficients higher than 0.83, indicating that biomass burning and secondary formation were potential sources of light-absorbing HULIS, as evidenced by abundant fluorescent components related to less-oxygenated HULIS. Comparing the changes in Abs 365 values, concentrations of major water-soluble inorganic ions and carbonaceous compounds in PM 2.5 , and environmental factors during the clean and pollution periods, we found that extensive biomass burning during the pollution period contributed significantly to the increase of Abs 365 values. Moreover, the aerosol pH during the pollution period was close to 4, and NO 2 concentration and aerosol water content were about 1.6 and 2.7 times higher than those during the clean period, respectively, which were favorable to form secondary HULIS through aqueous phase reactions in the presence of high NO x , resulting in an evident increase in its light absorption. Knowledge generated from this study is critical for evaluating the regional radiative forcing of brown carbon in southwest China. [Display omitted] • The evolution of the light absorption of HULIS in winter was investigated. • Increased biomass burning was responsible for the increased HULIS light absorption. • Aerosol pH close to 4 facilitated to form light-absorbing HULIS. • High aerosol water content and NO 2 concentrations favored to form secondary HULIS. [ABSTRACT FROM AUTHOR]

In this study, measurements of size-segregated particulate matter (PM) emitted from the combustion of rice straw, pine needles, and sesame stem were conducted in a laboratory chamber. The collected samples were used to analyze amounts of organic and elemental carbon (OC and EC), water-soluble organic carbon (WSOC), humic-like substances (HULIS), and ionic species. The light absorption properties of size-resolved water extracts were measured using ultraviolet-visible spectroscopy. A solid-phase extraction method was first used to separate the size-resolved HULIS fraction, which was then quantified by a total organic carbon analyzer. The results show that regardless of particle cut sizes, the contributions of size-resolved HULIS (=1.94 ×HULIS-C) to PM size fractions (PM0.32, PM0.55, PM1.0, and PM1.8) were similar, accounting for 25.2-27.6, 15.2-22.4 and 28.2-28.7% for rice straw, pine needle, and sesame stem smoke samples, respectively. The PM1.8 fraction revealed WSOC/OC and HULIS-C/WSOC ratios of 0.51 and 0.60, 0.44 and 0.40, and 0.50 and 0.60 for the rice straw, pine needle, and sesame stem burning emissions, respectively. Strong absorption with decreasing wavelength was found by the water extracts from size-resolved biomass burning aerosols. The absorption Ångström exponent values of the size-resolved water extracts fitted between 300 and 400 nm wavelengths for particle sizes of 0.32-1.0 μm were 6.6-7.7 for the rice straw burning samples, and 7.5-8.0 for the sesame stem burning samples. The average mass absorption efficiencies of size-resolved WSOC and HULIS-C at 365 nm were 1.09 (range: 0.89-1.61) and 1.82 (range: 1.33-2.06) m2/g·C for rice straw smoke aerosols, and 1.13 (range: 0.85-1.52) and 1.83 (range: 1.44-2.05) m2/g·C for sesame stem smoke aerosols, respectively. The light absorption of size-resolved water extracts measured at 365 nm showed strong correlations with WSOC and HULIS-C concentrations (R2=0.89-0.93), indicating significant contribution of HULIS component from biomass burning emissions to the light absorption of ambient aerosols.

Routine measurements of carbonaceous species in PM2.5 inculidng organic carbon (OC), elemental carbon (EC), water-soluble organic carbon (WSOC), and humiclike- substance carbon (HULIS-C) in PM2.5 were performed at Anmyeon Island (AI) to clarify the seasonal variation and carbonaceous aerosol concentrations at a background site in Korea between 2015 and 2016. The annual average OC and EC concentrations were 4.52±3.25 µg/m3 and 0.46±0.28 µg/m3, respectively, and there were no clear seasonal variations in OC and EC concentrations. The average concentrations of WSOC and waterinsoluble organic carbon (WISOC) were 2.56±1.95 µg/m3 and 1.96±1.45 µg/m3, respectively, and their composition in OC showed high temporal variations. A low correlation between WISOC and EC was observed, while WSOC concentrations were highly correlated with secondary organic carbon concentrations, which were estimated using the EC tracer method. The results indicate that the formation of secondary organic aerosols is a major factor for the determination of WSOC concentrations in this region. HULIS-C was the major component of WSOC, accounting for 39-99% of WSOC and the average concentration was 1.88±1.52 µg/m3. Two distinct periods with high carbonaceous speciess in PM2.5 were observed and characterized by their OC/EC ratios. The high concentration of OC with high ratio of OC/EC was due to the influence of a mixture of emissions from biomass burning and secondary formation transported from outside AI. While, the high concentrations of OC and EC with low OC/EC ratio were related to local vehicular emissions. [ABSTRACT FROM AUTHOR]

Recent work has identified the presence of humic-like substances (HULIS) in ambient fine particulate matter (PM 2.5) in Beijing, China and that residential coal combustion as well as biomass burning are significant contributors to its presence. These results were based on the characterization of emissions from representative stoves and modeling of the aerosol with the Community Multiscale Air Quality (CMAQ) chemical transport model. The CMAQ source apportionment estimated that residential coal and biofuel burning and secondary aerosol formation were important annual sources of ambient HULIS, contributing 47.1%, 15.1%, and 38.9%, respectively. In this study, chemical composition data including concentrations of water-soluble organic carbon and HULIS across four seasons during 2012–2013 were analyzed with positive matrix factorization (PMF) to provide a complementary source apportionment. The PMF results indicate that the identified sources were Traffic, Biomass Burning, Nitrate/Sulfate, Incineration, Sulfate, Coal Combustion/Ammonium Chloride, Residential Coal/Biofuel Combustion, and Road Dust/Soil with mass contributions (fractions) to PM 2.5 of 12.35 (10.4%), 8.70 (8.9%), 24.51 (22.4%), 5.64 (7.2%), 25.14 (24.5%), 7.10 (6.2%), 14.18 (15.4%), and 5.33 μg/m3 (5.0%), respectively. The contributions to the observed HULIS concentrations were 0.63 (10.9%), 0.38 (6.4%), 0.07 (1.7%), 0.00 (0%), 1.12 (28.8%), 0.00 (0%), 1.50 (52.2%), and 0.01 μg/m3 (0.3%), respectively. These PMF modeling results were in reasonable agreement with the CMAQ values supporting the attribution of significant amounts of primary HULIS to residential coal and biofuel combustion. Currently, efforts are underway in China to replace solid fuel combustion for heating and cooking with natural gas and electricity by 2020. Thus, future studies should be able to see substantial reductions in both PM 2.5 and HULIS in the near term future. Unlabelled Image • Sources of PM2.5 and HULIS were apportioned for 2012–2013 using additional variables. • Eight sources were resolved including residential coal/wood and incinerator emissions. • Adding gaseous pollutants provided better agreement between PMF and CMAQ results. • Residential combustion was a major source of PM2.5 and HULIS in Beijing. [ABSTRACT FROM AUTHOR]

Abstract The irradiation under simulated sunlight of some phenolic compounds typically emitted in ambient air by biomass burning, namely vanillin and acetosyringone, yielded intermediates with humic-like fluorescence properties that can be assimilated to humic-like substances (HULIS). Evidence was obtained by ultra-high-resolution mass spectrometry of the occurrence of oligomerization processes up to the formation of trimeric species. In contrast, the irradiation of other biomass-burning compounds such as vanillic acid, m-cresol and guaiacol did not yield either HULIS-type fluorescence or oligomers. We suggest that the photolysis of biomass-burning compounds is a potential HULIS source in the atmosphere, if the relevant substrates undergo photoinduced oligomerization reactions. Graphical abstract Image 1 Highlights • Humic-like fluorescence is produced by irradiation of biomass-burning phenols. • Vanillin and acetosyringone are involved in the process. • Evidence of oligomerization, up to formation of trimeric species. • Norrish-type and phenoxyl reactions are reasonable oligomerization pathways. • This is a potential route to atmospheric humic-like substances (HULIS). [ABSTRACT FROM AUTHOR]

The chemical and optical properties and sources of atmospheric PM2.5 humic-like substances (HULIS) were investigated from October to December 2016 in both industrial and suburban areas in Changzhou, China, during polluted and fair days. The average PM2.5 concentration in the industrial region was 113.06 (±64.3) μg m−3, higher than 85.27 (±41.56) μg m−3 at the suburban site. The frequency of polluted days was significantly higher in the industrial region. In contrast, the chemical compositions of PM2.5 at the two sampling sites exhibited no statistically significant differences. Rapidly increased secondary inorganic ions (SNA = NH4+ + SO42− + NO3−) concentrations suggested secondary formation played an important role in haze formation. The daily mean concentration of humic-like substance (HULIS) was 1.8–1.9 times that of HULIS-C (the carbon content of HULIS). Our results showed that HULIS accounted for a considerable fraction of PM2.5 (industrial region: 6.3% vs. suburban region: 9.4%). There were no large differences in the mass ratios of HULIS-C/WSOC at the two sites (46% in the industrial region and 52% in the suburban region). On average, suburban HULIS-C constituted 35.1% of organic carbon (OC), higher than that (21.1%) in the industrial region. Based on different MAE (mass absorption efficiency) values under different pollution levels, we can infer that the optical properties of HULIS varied with PM levels. Moreover, our results showed no distinct difference in E2/E3 (the ratio of light absorbance at 250 nm to that at 365 nm) and AAE300–400 (Absorption Angstrom Exponent at 300–400 nm) for HULIS and WSOC. the MAE365 (MAE at 365 nm) value of HULIS-C was different under three PM2.5 levels (low: PM2.5 < 75 μg m−3, moderate: PM2.5 = 75–150 μg m−3, high: PM2.5 > 150 μg m−3), with the highest MAE365 value on polluted days in the industrial region. Strong correlations between HULIS-C and SNA revealed that HULIS might be contributed from secondary formation at both sites. In addition, good correlations between HULIS-C with K+ in the industrial region implied the importance of biomass burning to PM2.5-bound HULIS. Three common sources of HULIS-C (i.e., vehicle emissions, biomass burning, and secondary aerosols) were identified by positive matrix factorization (PMF) for both sites, but the contributions were different, with the largest contribution from biomass burning in the industrial region and secondary sources in the suburban region, respectively. The findings presented here are important in understanding PM2.5 HULIS chemistry and are valuable for future air pollution control measures.

In the present study, PM 2.5 samples (n = 104) were collected during February 2020–March 2021 on diurnal basis in Delhi (a site of Indo-Gangetic Plain (IGP)), with a view to investigate seasonal and diurnal patterns of ambient brown carbon (BrC) with consideration of their potential sources. Given that the polarity of ambient aerosol influences their chemical and physical processing in the atmosphere, we compared the light absorption properties of brown carbon (BrC) in PM 2.5 samples extracted with three different solvents i.e., water, methanol, and n-hexane. Methanol extracts showed much stronger light-absorbing ability followed by water and hexane extracts in the wavelength range of 300–600 nm. Peak values of mass absorption coefficient (b abs-BrC-365nm) were observed during post-monsoon season in both polar- (water-and methanol-soluble) and non-polar (n-hexane soluble) fractions of BrC, mainly due to the influence of biomass-burning (BB) aerosols. This point is further supported by air mass back trajectories which are well correlated with fire-count data, and marker species such as EC, OC and non-sea-salt K+. Comparison of b abs-BrC-365nm , mass absorption efficiency (MAE 365nm), absorption Ångstorm exponent (AÅE) and simple forcing efficiency (SFE 300 – 700) from this study with that in published datasets reveal the significant contribution of biomass-burning activities to BrC absorption and abundance in the ambient atmosphere. Furthermore, the optical properties of HULIS (humic-like substances) (n = 40, during day- and nighttime) in post-monsoon (October – November 2020) and winter aerosols (January – February 2021) were also characterised. Based on UV–Vis analysis, similar average b abs-365nm of HULIS was observed for daytime and nighttime samples i.e., 9.96 ± 3.8 and 9.89 ± 3.3 Mm−1, respectively in post-monsoon samples. Opposingly in winter period, significantly high daytime value of b abs-365nm of HULIS (7.24 ± 2.7 M m−1) compared to that of nighttime (4.45 ± 2.8 M m−1) was found. These results along with indicative E 2 /E 3 (ratio of absorbance at 250 nm and 365 nm, respectively) and AÅE values in HULIS fraction suggest a more significant contribution from regional crop-residue burning emission in the formation of HULIS in post-monsoon. While in winter period local emissions and subsequent secondary formation of HULIS are important. To the best of our knowledge, this study is the first attempt to investigate light absorption properties of different polarity BrC extracts (methanol, water, n-hexane, HULIS) in aerosol samples collected throughout the year at a representative site in Indo-Gangetic Plain (IGP). [Display omitted] • Spectral properties of atmospheric BrC (polar- and non-polar) and HULIS are studied • Strong light absorption is observed during biomass burning period in methanol-soluble BrC. • Regional crop residue burning in post-monsoon, while local secondary formation in winter are major sources of HULIS [ABSTRACT FROM AUTHOR]

Atmospheric humic-like substances (HULIS) have a great impact on radiative forcing and atmospheric oxidative capacity due to their efficient light absorption properties. Knowledge on atmospheric HULIS abundance and light absorption properties in southwest China is still very limited. In this study, a total of 81 daily samples of fine particulate matter (PM 2.5) were collected at two sites in Chongqing during four one-month periods in 2015, each of which representing a typical season. Mass concentrations of HULIS in term of carbon (HULIS-C) were measured by a total organic carbon analyzer, while mass absorption efficiency at 365 nm (MAE 365) and absorption Ångström exponent (AAE) were estimated from light absorption of HULIS that was recorded by UV–visible spectrophotometry. The annual mean concentration of HULIS-C was around 3.4 ± 1.6 μg m−3, while the annual mean values of MAE 365 and AAE were approximately 2.7 ± 0.7 m2 g−1C and 4.0 ± 0.2, respectively. Seasonal variations were significant for MAE 365 but negligible for AAE at both sites. A direct comparison of MAE 365 of HULIS in this study with the median MAE 365 values from different sources in laboratory studies indicated that crop residues burning and/or wood burning were the primary sources of light-absorbing HULIS, as is also supported by the strong correlation between the light absorption coefficient at 365 nm (Abs 365) and K+ (r > 0.85, p < 0.01) throughout the year. Furthermore, the good correlation between Abs 365 and secondary organic carbon implied secondary sources also contributed to light-absorbing HULIS (e.g., nitrated aromatic compounds production), as evidenced by the good correlation between Abs 365 and NO 3 − and NH 4 +. The contribution of primary and secondary sources to light-absorbing HULIS was quantified using a machine learning algorithm, which showed >65% contribution from primary sources during the cold season, but only 35% during the warm seasons. [Display omitted] • Regional-scale HULIS pollution occurred in Chongqing in 2015. • HULIS were mainly from biomass burning and aging processes of aromatic compounds. • Good correlation between Abs 365 and SO 4 2− implied secondary HULIS formation. • Primary sources accounted for >65% of HULIS light absorption in winter. [ABSTRACT FROM AUTHOR]

This study reports for the first-time the ambient concentrations of HULIS mass (HULIS-OM, Humic-like substances) and HULIS-C (carbon) in PM10 (particulate matter with aerodynamic diameter ≤ 10 μm) from the Indo-Gangetic Plain (IGP at Kanpur, wintertime). HULIS extraction followed by purification and isolation protocol with methanol: acetonitrile (1:1 v/v) on HLB (Hydrophilic-Lipophilic Balanced) cartridge has been established. Quantification of HULIS-C was achieved on a total organic carbon (TOC) analyser whereas HULIS-OM was determined gravimetrically. Consistently high recovery (> 90%) of HULIS-C based on analysis of Humic standard (sodium salt of Humic acid) suggested suitability of our established analytical protocol involving solvent extraction, purification and accurate quantification of HULIS. HULIS-OM varied from 17.3-38 μg m−3 during daytime and from 19.8-40.6 μg m−3 during night in this study. During daytime the HULIS-OM constituted 20-30% mass fraction of OMTotal and 10-15% of PM10 mass. However, a relatively low contribution of HULIS-OM has been observed during the night. This observation has been attributed to higher concentrations of OM and PM10 in night owing to nighttime chemical reactivity and condensation of organics in conjunction with shallower planetary boundary layer height. Strong correlation of HULIS-C with K+BB (R2 > 0.80) and significant day-night variability of HULIS-C/WSOC ratio in conjunction with air-mass back trajectories (showing transport of pollutants from upwind IGP) suggest biomass burning emission and secondary transformations as important sources of HULIS over IGP. High-loading of atmospheric PM10 (as high as 440 μg m−3) with significant contribution of water-soluble organic aerosols (WSOC/OC: ~ 0.40-0.80) during wintertime highlights their plausible potential role in fog and haze formation and their impact on regional-scale atmospheric radiative forcing over the IGP. [ABSTRACT FROM AUTHOR]

Abstract Crop straw combustion is a significant source for humic-like substances (HULIS) in fine particles (PM 2.5), which have strong light absorption in the near ultraviolet and visible light region. In this study, the relative abundances, light absorption properties and fluorescence characteristics of HULIS produced by three crop straws (wheat, rice and corn straws) combusted under different moisture contents and stacking modes were comprehensively characterized. The results show that the moisture contents and stacking modes can affect the ratio of carbon content in HULIS to water-soluble organic carbon (HULIS-C/WSOC) and the ratio of HULIS to PM 2.5 (HULIS/PM 2.5). The absorption Ångström exponent (AAE) and absorptivity ratios between 250 and 365 nm (E2/E3) for HULIS increased either under high moisture or stacking conditions for all three crop straws, while mass absorption efficiencies at 365 nm (MAE 365) decreased under high moisture or stacking conditions. Four fluorescent components of HULIS were identified based on the characterization of the excitation-emission matrix combined with a parallel factor analysis model (EEM-PARAFAC): protein-like organic matter (C1 and C4), highly oxygenated humic-like substances (C3) and less-oxygenated humic-like substances (C2). A correlation analysis of the fluorescent components and light absorption indicators showed that the components C4, (C2, C3) and (C1, C2) determined the variations in the light absorption properties of HULIS derived from wheat, rice and corn straw burning, respectively. We also found that the flame burning of crop straw could reduce the contribution of light absorption to HULIS per unit mass of crop straw. Highlights • Effects of MC and stacking mode on biomass HULIS optical properties were studied. • HULIS have stronger light absorption abilities in low MC and tiling combustion. • Correlation of HULIS optical properties and fluorescent components was studied. [ABSTRACT FROM AUTHOR]

PM 10 source apportionment was performed by positive matrix factorization (PMF) using specific primary and secondary organic molecular markers on samples collected over a one year period (2013) at an urban station in Grenoble (France). The results provided a 9-factor optimum solution, including sources rarely apportioned in the literature, such as two types of primary biogenic organic aerosols (fungal spores and plant debris), as well as specific biogenic and anthropogenic secondary organic aerosols (SOA). These sources were identified thanks to the use of key organic markers, namely, polyols, odd number higher alkanes, and several SOA markers related to the oxidation of isoprene, α-pinene, toluene and polycyclic aromatic hydrocarbons (PAHs). Primary and secondary biogenic contributions together accounted for at least 68% of the total organic carbon (OC) in the summer, while anthropogenic primary and secondary sources represented at least 71% of OC during wintertime. A very significant contribution of anthropogenic SOA was estimated in the winter during an intense PM pollution event (PM 10 > 50 μg m − 3 for several days; 18% of PM 10 and 42% of OC). Specific meteorological conditions with a stagnation of pollutants over 10 days and possibly Fenton-like chemistry and self-amplification cycle of SOA formation could explain such high anthropogenic SOA concentrations during this period. Finally, PMF outputs were also used to investigate the origins of humic-like substances (HuLiS), which represented 16% of OC on an annual average basis. The results indicated that HuLiS were mainly associated with biomass burning (22%), secondary inorganic (22%), mineral dust (15%) and biogenic SOA (14%) factors. This study is probably the first to state that HuLiS are significantly associated with mineral dust. [ABSTRACT FROM AUTHOR]

This study investigates the absorption characteristics of brown carbon (BrC) obtained from water and methanol extracts of fine particulate matter measured at an urban site in Gwangju, Korea during the KOREA U.S. – Air Quality campaign (May 2–June 11, 2016). The measurement period was classified into two intervals: biomass burning (BB) and non-BB periods. During the non-BB period, water-soluble organic carbon (WSOC) and humic-like substances (HULIS) primarily resulted from secondary organic aerosol (SOA) formation and primary vehicle emissions. Water-soluble organic aerosols during the BB period, meanwhile, were closely related to SOA formation and regionally transported BB emissions. The light absorption coefficient measured at 365 nm (b abs,365 ) by methanol extracts was 2.6 and 6.1 times higher than the coefficients from the water and HULIS extracts, respectively, indicating the importance of BrC absorption by water-insoluble organic carbon. This was demonstrated by a good correlation between the water-insoluble BrC absorption and the elemental carbon concentration. A comparison of b abs,365 between the methanol- and water-extracted BrC indicated that water-insoluble BrC accounted for approximately 61% (33–86%) of the total BrC absorption. The contributions of SOA, primary BB emissions, and traffic emissions to the water extract b abs,365 were estimated using a stepwise multiple linear regression (MLR) analysis and found to be 1.17 ± 0.55, 0.65 ± 0.62, and 0.25 ± 0.09 Mm − 1 , respectively, accounting for 59.6, 26.1, and 14.3% of the absorption coefficient by the water-soluble BrC. Further, it was determined that the contribution of the BB emissions to the water-soluble BrC absorption was approximately two times higher in the BB period than in the non-BB period. The average absorption Ångstrӧm exponent was 4.8 ± 0.3, 5.3 ± 0.7, and 6.8 ± 0.8 for the methanol, water, and HULIS extracts, respectively. The average mass absorption efficiency (MAE 365 ) of methanol extracted BrC was 1.3 ± 0.4 m 2 /g·C. Water- and HULIS-extracted BrC had a MAE 365 of 1.0 ± 0.3 and 0.8 ± 0.3 m 2 /g·C, respectively. These results suggest that methanol-extracted BrC could provide a better estimation of BrC absorption compared to WSOC and HULIS. [ABSTRACT FROM AUTHOR]

The primary objective of this study was to investigate the molecular compositions of humic-like substances (HULISs) in size-resolved ambient aerosols, which were collected using an Anderson-type air sampler (eight size cuts between 0.43 and 11 μm) during the winter season (i.e., the heating period of 8−12 January 2018) in Songdo, South Korea. The aerosol samples collected during the pre- (preheating, 27 November−1 December 2017) and post-winter (postheating, 12−16 March 2018) periods were used as controls for the winter season samples. According to the concentrations of the chromophoric organics determined at an ultraviolet (UV) wavelength of 305 nm, most of the HULIS compounds were found to be predominantly enriched in particles less than 2.1 μm regardless of the sampling period, which shows that particulate matter (diameter less than 2.5 μm; PM2.5) aerosols were the dominant carriers of airborne organics. Ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry (UHR FT−ICR MS) analysis of the aerosol-carried organic substances revealed that as the aerosol size increased the proportions of CHO and nitrogen-containing CHO (CHON) compounds decreased, while the proportion of sulfur-containing CHO (CHOS) species increased. In particular, the ambient aerosols during the heating period seemed to present more CHO and CHON and less CHOS molecules compared to aerosols collected during the pre- and postheating periods. The aerosols collected during the heating period also exhibited more aromatic nitrogen-containing compounds, which may have originated from primary combustion processes. Overall, the particle size distribution was likely influenced by source origins; smaller particles are likely from local sources, such as traffic and industries, and larger particles (i.e., aged particles) are likely derived from long-range transport generating secondary organic aerosols (SOAs) in the atmosphere. The results of the size-segregated particles can be utilized to understand particle formation mechanisms and shed light on their toxicity to human health.

Long-term measurements of carbon in HUmic-LIke Substances (HULIS-C) of ambient size-segregated water soluble organic aerosols were performed using a ten-stage low-pressure Berner impactor from December 2014 to November 2015 at an urban background environment in Ljubljana, Slovenia. The mass size distribution patterns of measured species (PM - particulate matter, WSOC - water-soluble organic carbon and HULIS-C) for all seasons were generally tri-modal (primarily accumulation mode) but with significant seasonal variability. HULIS-C was found to have similar distributions as WSOC, with nearly the same mass median aerodynamic diameters (MMADs), except for winter when the HULIS-C size distribution was bimodal. In autumn and winter, the dominant accumulation mode with MMAD at ca. 0.65 μm contributed 83 and 97% to the total HULIS-C concentration, respectively. HULIS-C accounted for a large fraction of WSOC, averaging more than 50% in autumn and 40% in winter. Alternatively, during warmer periods the contributions of ultrafine (27% in summer) and coarse mode (27% in spring) were also substantial. Based on mass size distribution characteristics, HULIS-C was found to be of various sources. In colder seasons, wood burning was confirmed as the most important HULIS source; secondary formation in atmospheric liquid water also contributed significantly, as revealed by the MMADs of the accumulation mode shifting to larger sizes. The distinct difference between the spring and summer ratios of HULIS-C/WSOC in fine particles (ca. 50% in spring, but only 10% in summer) indicated different sources and chemical composition of WSOC in summer (e.g., SOA formation from biogenic volatile organic compounds (BVOCs) via photochemistry). The enlarged amount of HULIS-C in the ultrafine mode in summer suggests that the important contribution was most likely from new particle formation during higher emissions of BVOC due to the vicinity of a mixed deciduous forest; the higher contribution of HULIS-C in the coarse mode demonstrated that beside soil erosion other sources, such as pollen and plant fragments, could also be responsible. [ABSTRACT FROM AUTHOR]

Total carbon content of the atmospheric suspended particulate matters consisted of as high as 89-93% organic carbon (OC) in Ulaanbaatar aerosol without showing seasonal variation. However, limited aerosol measurements have been conducted on these OC rich aerosols particularly for high molecular weight constituents. In order to address the gap above, abundance of high molecular weight humic-like substances (HULIS) in total suspended particulates (TSP) from Ulaanbaatar atmospheric aerosol were determined for the first time. HULIS molecular structure was characterised for different seasons using carbon content and UV absorbance measurements coupled with solid phase extraction methods. Although, HULIS contributions to water soluble organic fraction of the winter and summer aerosols were similar HULIS carbon concentration was higher in winter samples (9-37 mg·L-1) than in summer (2-6 mg·L-1). Consequently quantity of aromatic moieties and degree of aromaticity varied between seasons. [ABSTRACT FROM AUTHOR]

Water-soluble organic carbon (WSOC) is a significant part of ambient aerosol and plays an active role in contributing to aerosol's effect on visibility degradation and radiation budget through its interactions with atmospheric water. Size-segregated aerosol samples in the range of 0.056–18 μm were collected using a ten-stage impactor sampler at an urban site in Hong Kong over one-year period. The WSOC samples were separated into hydrophilic (termed WSOC_h) and hydrophobic fractions (i.e., the humic-like substances (HULIS) fraction) through solid-phase extraction procedure. Carbon in HULIS accounted for 40 ± 14% of WSOC. The size distribution of HULIS was consistently characterized in all seasons with a dominant droplet mode (46–71%) and minor condensation (9.0–18%) and coarse modes (20–35%). The droplet mode had a mass median aerodynamic diameter in the range of 0.7–0.8 μm. This size mode showed the largest seasonal variation in abundance, lowest in the summer (0.41 μg/m 3 ) and highest in the winter (3.3 μg/m 3 ). WSOC_h also had a dominant droplet mode, but was more evenly distributed among different size modes. Inter-species correlations within the same size mode suggest that the condensation-mode HULIS was partly associated with combustion sources and the droplet-mode was strongly associated with secondary sulfate formation and biomass burning particle aging processes. There is evidence to suggest that the coarse-mode HULIS largely originated from coagulation of condensation-mode HULIS with coarse soil/sea salt particles. The formation process and possible sources of WSOC_h was more complicated and multiple than HULIS and need further investigation. Our measurements indicate that WSOC components contributed a dominant fraction of water-soluble aerosol mass in particles smaller than 0.32 μm while roughly 20–30% in the larger particles. [ABSTRACT FROM AUTHOR]

Although Humic-LIke Substances (HULIS) are important contributors to the mass of organic aerosol in airborne particulate matter (PM), little is known about their chemical composition, while, their size-resolved optical properties have not been studied yet. Here, HULIS fractions were isolated from size-resolved aerosol samples (≤0.49, 0.49–0.95, 0.95–3 and 3–10 μm) collected in urban and suburban environments of four European cities during wintertime. The bulk ( i.e. , sum of all size fractions) concentration of HULIS ranged between 1.29 and 2.80 μg m −3 across sites with highest values in the ≤0.49 μm particle size fraction. The contribution of the carbon mass of HULIS (HULIS-C) to the water-soluble organic carbon content (WSOC) of PM was 32–43%, which is typical for urban sites affected by biomass burning. The Mass Absorption Efficiency (MAE), which characterizes the efficiency of absorbing solar energy per carbon mass of HULIS decreased with particle size, suggesting that the finest size fractions contain more light-absorbing chromophores, which could affect the light-absorbing ability of organic aerosols. The good correlation of HULIS with effective biomass tracers such as K + , as well as with secondary inorganic aerosol components, proposed that HULIS had both primary ( i.e. , biomass burning) and secondary sources. The Fourier Transfer Infrared coupled to Attenuation Total Reflectance (FTIR-ATR) spectra demonstrated prevalence of aromatic over carboxylic functional groups in most HULIS fractions, indicating contribution from coal combustion emissions in addition to fresh biomass burning aerosol. The new findings add to better understanding the sources and chemical structure of HULIS in urban and suburban environments. [ABSTRACT FROM AUTHOR]

This study (first attempt) characterizes HULIS (Humic Like Substances) in wintertime aerosols (n = 12 during day and nighttime each) from Indo-Gangetic Plain (IGP, at Kanpur) by using various state-of-the art techniques such as UV-VIS, FTIR, 1 H NMR and XPS. Based on UV-Vis analysis the absorption coefficient at 365 nm (b abs-365 ) of HULIS was found to average at 13.6 and 28.8 Mm −1 during day and nighttime, respectively. Relatively high b abs-365 of HULIS during the nighttime is attributed to influence of fog-processing. However, the power fit of UV-Vis spectrum provided near similar AAE (absorption Angstrom exponent) value of HULIS centering at 4.9 ± 1.4 and 5.1 ± 1.3 during daytime and nighttime, respectively. FTIR spectra and its double derivative revealed the presence of various functional groups viz. alcohols, ketones aldehydes, carboxylic acids as well as unsaturated and saturated carbon bonds. 1 H NMR spectroscopy was applied to quantify relative percentage of various types of hydrogen atoms contained in HULIS, whereas XPS technique provided information on surface composition and oxidation states of various elements present. A significantly high abundance of H‒C‒O group has been observed in HULIS (based on 1 H NMR); 41.4± 2.7% and 30.9± 2.4% in day and nighttime, respectively. However, aromatic protons (Ar-H) were higher in nighttime samples (19.3± 1.8%) as compared to that in daytime samples (7.5 ± 1.9). XPS studies revealed presence of various species on the surface of HULIS samples. Carbon existed in 7 different chemical states while total nitrogen and sulfur exhibited 3 and 2 different oxidation states (respectively) on the surface of HULIS. This study reports structural information and absorption properties of HULIS which has implications to their role as cloud condensation nuclei and atmospheric direct radiative forcing. [ABSTRACT FROM AUTHOR]

Brown carbon (BrC) is a class of unidentified organic compounds that efficiently absorb solar radiation in the ultraviolet (UV) wavelengths, and its effects on climate are poorly understood. Measurement of the light absorption properties of BrC in liquid extracts is a commonly used BrC analytical method, but the optical characteristics of water-soluble BrC may be affected by pH and solvent. In this work, we investigated the effects of concentration, pH, and solvent on water-soluble BrC from ambient aerosols, biomass burning, diesel exhaust, and a humic substance standard. The results showed that pH can affect the light absorption properties of water-soluble BrC, whereas concentration had little effect, except low concentrations dissolved in methanol. Therefore, the pH of humic-like substances (HULIS) should be adjusted to the same value as water-soluble carbon (WSOC) for calculating the light-absorption contribution of HULIS to WSOC. The light absorptivity of water-soluble BrC dissolved in methanol was higher than that in water. Considering the pH and concentration effects, extraction of WSOC with a particle: water ratio of 0.25 mg/mL is proposed as well as to get a reference pH for light absorption analysis. [ABSTRACT FROM AUTHOR]

We characterised surface-active substances (SASs) in aqueous extract of atmospheric aerosols by using phase sensitive alternating current voltammetry. The electrochemical method has mainly been used for the quantification of surfactants in sea water but has not been applied to atmospheric aerosols yet. The advantage of the method is its simplicity and sensitivity that enables direct analysis of aerosol extracts without the need for sample concentration. Aerosol samples were collected at Middle Adriatic Martinska station influenced by different air masses as well as from urban (Zagreb, Croatia) and rural (K-puszta, Hungary) areas from late spring to early autumn in 2010. The highest SAS concentrations, expressed in equivalents of T-X-100, ranging from 0.34 to 0.91 µg m−3 were detected in urban samples. The SAS concentrations obtained for marine, regional and continental samples ranged from 0.14 to 0.31, 0.18 to 0.42 and 0.07 to 0.28 µg m−3, respectively. The SAS concentrations in K-puszta aerosols ranged from 0.13 to 0.46 µg m−3. Investigation of humic-like substances isolated from K-puszta samples (2008) confirmed their significant surfactant nature. Different SAS chemistry was noticed for urban and non-urban samples. Investigations at different pH revealed anionic character of SASs in aerosol samples.

The article presents the Dithiothreitol-based oxidative potential (OPDTT) of PM 2.5 from an urban site in São Paulo, Brazil, in relation to its chemical composition focused on organic carbon (WSOC), Humic-Like Substances (HULIS), water-soluble elements (Al, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Cd and Pb), ionic species, polycyclic aromatic hydrocarbons (PAHs), and oxy- and nitro-PAH derivatives (OPAHs and NPAHs). Seasonal variations of the intrinsic (mass-normalized) oxidative potential (OPmDTT) and the extrinsic (air volume-normalized) oxidative potential (OP v DTT), as well as of targeted chemical components were investigated. Furthermore, a series of statistical analyses, including correlation, and multi linear regression (MLR) were performed to identify the most significant species contributing to the measured OPDTT. Although explaining a relatively small percentage of the OP variability (R2 = 0.389), the MLR model indicated HULIS as the major OPmDTT carrier contributing 58% on average. Two receptor modelling approaches were further employed for the source apportionment of OP v DTT (a) direct Positive Matrix Factorization (PMF), and (b) a combination of PMF of PM 2.5 mass concentrations followed by Multiple Linear Regression (PMF - MLR). Both approaches yielded meaningful, although somewhat different, results with direct PMF indicating vehicular emissions as the major contributor (76.3 ± 58.3%) of OP v DTT followed by industrial emissions (16.9 ± 17.7%), and biomass burning (7.3 ± 8.6%), whereas according to the PMF-MLR approach, the main contributors to OP v DTT were biomass burning (40.7 ± 26.3%), industrial emissions (24.3 ± 22.5%), vehicular emissions (20.5 ± 15.2%), and secondary aerosol (14.5 ± 15.0%). The study provides new and additional insights on the oxidative activity of ambient PM highlighting the importance of identifying its linkage with chemical constituents and emission sources for developing air quality control strategies aiming to protect human health. [Display omitted] • Dithiothreitol oxidative potential (OPDTT) of PM 2.5 was investigated in São Paulo. • OPDTT was an intrinsic property of PM 2.5 related to its composition than mass. • HULIS appeared to be a significant contributor to OPmDTT. • Direct PMF indicated vehicular emissions as the major source of OPvDTT. • The PMF - MLR approach indicated biomass burning as the major source of OPvDTT. [ABSTRACT FROM AUTHOR]

Evaporative light scattering detection (ELSD) was applied to quantify HULIS (humic-like substances) for the first time in 2012 winter and 2013 summer at an urban site in Lanzhou. Water soluble organic carbon (WSOC), water soluble inorganic ions, and carbonaceous species (OC/EC) were also analyzed. The results show that OM (Organic Matter = OC × 1.6, constituting 45.8% to PM 2.5 ) was the most abundant species, followed by SNA (SO 4 2 − + NO 3 − + NH 4 + , constituting 23.4% to PM 2.5 ). The chemical species were in the order of: OC > EC > SO 4 2 − > NO 3 − > NH 4 + > Cl − > Ca 2 + > K + . The annual average concentration of HULIS was 4.70 μg/m − 3 and HULISc (carbon content of HULIS) contributed 6.19% to PM 2.5 and 45.6% to WSOC, indicating that HULIS was the most important components of WSOC. The concentration of HULIS was 2.14 ± 0.80 μg/m 3 in summer and 7.24 ± 2.77 μg/m 3 in winter, respectively. The concentrations of HULIS were relatively low and stable in summer, while high and varied dramatically in winter. The abundance of HULISc in WSOC shows a more concentrated distribution in Lanzhou, with a range between 0.28–0.57. The ratios of HULIS/K + were 6.25 ± 1.41 and 6.14 ± 1.96 in summer and winter, respectively, suggesting there were other significant sources in addition to biomass burning emissions. HULIS and WSOC exhibited similar seasonal variation and had a strong positive correlation. In addition to the good relationship (0.89) between HULIS and Cl − in winter, the great enhancement of HULIS with significantly high Cl − and relatively low K + in winter indicated that residential coal burning was probably an important HULIS source in winter. Correlation and back trajectory analysis suggested that biomass burning and secondary formation were also important HULIS sources and the contribution of HULIS from dust could be neglected. Adverse meteorological conditions were also important factors for the accumulation of HULIS in winter. [ABSTRACT FROM AUTHOR]

PM 2.5 samples from biomass burning (BB) emissions of three types - rice straw (RS), pine needles (PN), and sesame stems (SS) - were collected through laboratory-controlled combustion experiments and analyzed for the mass, organic and elemental carbon (OC and EC), water-soluble organic carbon (WSOC), humic-like substances (HULIS), and water soluble inorganic species (Na + , NH 4 + , K + , Ca 2+ , Mg 2+ , Cl − , NO 3 − , SO 4 2− , and oxalate). The combustion experiments were carried out at smoldering conditions. Water-soluble HULIS in BB samples was isolated using a one-step solid phase extraction method, followed by quantification with a total organic carbon analyzer. This study aims to explore chemical and light absorption characteristics of HULIS from BB emissions. The contributions of HULIS (=1.94 × HULIS-C) to PM 2.5 emissions were observed to be 29.5 ± 2.0, 15.3 ± 3.1, and 25.8 ± 4.0%, respectively, for RS, PN, and SS smoke samples. Contributions of HULIS-C to OC and WSOC for the RS, PN, and SS burning emissions were 0.26 ± 0.03 and 0.63 ± 0.05, 0.15 ± 0.04 and 0.36 ± 0.08, and 0.29 ± 0.08 and 0.51 ± 0.08, respectively. Light absorption by the water extracts from BB aerosols exhibited strong wavelength dependence, which is characteristic of brown carbon spectra with a sharply increasing absorption as wavelength decreases. The average absorption Ångström exponents (AAE) of the water extracts (WSOC) fitted between wavelengths of 300–400 nm were 8.3 (7.4–9.0), 7.4 (6.2–8.5), and 8.0 (7.1–9.3) for the RS, PN, and SS burning samples, which are comparable to the AAE values of BB samples reported in previous publications (e.g., field and laboratory chamber studies). The average mass absorption efficiencies of WSOC measured at 365 nm (MAE 365 ) were 1.37 ± 0.23, 0.86 ± 0.09, and 1.38 ± 0.21 m 2 /g⋅C for RS, PN, and SS burning aerosols, respectively. Correlations of total WSOC, hydrophilic WSOC (= total WSOC–HULIS-C), and HULIS-C concentrations in solution with the light absorption of WSOC at 365 nm indicate that the light absorption of WSOC from BB emissions was strongly associated with HULIS (R 2 = 0.92) - i.e., a hydrophobic component of WSOC, rather than with the hydrophilic WSOC (R 2 = 0.31). [ABSTRACT FROM AUTHOR]

Humic-Like Substances (HULIS) are important macromolecular compounds that are present in PM 2.5 and play significant roles in the atmospheric environment. In this study, 48 PM 2.5 samples were collected from February 2010 to January 2011 at an urban site in Guangzhou, southern China. The water soluble HULIS fractions in PM 2.5 were analyzed to explore the temporal variation of abundance and optical properties and to identify their possible sources. The HULIS concentrations were in the range of 0.4 to 8.2 μg C m − 3 , with a mean of 2.4 μg C m − 3 . HULIS are important components in organic aerosols, accounting for 17 ± 5% of the organic carbon (OC), and 49 ± 6 and 68 ± 5% of water soluble organic carbon (WSOC) as determined with a total organic carbon (TOC) analyzer and UV absorbance at 250 nm, respectively. The special UV absorbance (SUVA) at 254 nm and 280 nm and the E 250 /E 365 ratio of HULIS were 3.2 ± 0.5 L (m mg C) − 1 , 2.2 ± 0.4 L (m mg C) − 1 , and 5.9 ± 0.9, respectively. The HULIS fractions had higher concentrations, slightly higher SUVA values, and lower E 250 /E 365 ratios from November to January, indicating the important contribution of aromatic compounds to HULIS in the dry season. The concentrations of HULIS were positively correlated with water soluble K + , secondary organic carbon (SOC), and secondary inorganic ions (NH 4 + , NO 3 − , and SO 4 2 − ). These results suggest that biomass burning and secondary photochemical formation are both sources of HULIS in our study area. In addition, the SUVA 280 of HULIS was strongly correlated with K + and SOC, suggesting that HULIS properties were also influenced by their primary source of biomass burning and secondary atmospheric formation. [ABSTRACT FROM AUTHOR]

To obtain evidence for secondary organic aerosol formation during the long-range transport of air masses over the East China Sea, we conducted field measurements in March 2012 at the Fukue atmospheric monitoring station, Nagasaki, in western Japan. The relative abundance of m/z 44 in fine organic aerosol ( f 44 ) was measured by an Aerodyne aerosol chemical speciation monitor. The stable carbon isotope ratio (δ 13 C) of low-volatile water-soluble organic carbon (LV-WSOC) in the daily filter samples of total suspended particulate matter was also analyzed using an elemental-analyzer coupled with an isotope ratio mass spectrometer. Additionally, in situ measurements of NO x and NO y were performed using NO x and NO y analyzers. The measurements showed that, unlike the systematic trends observed in a previous field study, a scatter plot for δ 13 C of LV-WSOC versus f 44 indicated a random variation. Comparison of f 44 with the estimated photochemical age by the NO x /NO y ratio revealed that the random distribution of f 44 values near 0.2 is likely an indication of saturation already. Such f 44 values were significantly lower than the observed f 44 (∼0.3) at Hedo in the previous study. These findings imply that the saturation point of f 44 , and the use of f 44 as an oxidation indicator, is case dependent. [ABSTRACT FROM AUTHOR]

Humic-like substances (HULIS) comprise a significant fraction of the organic compounds in aerosol particles. In the present study we report the chemical properties of HULIS samples originating from urban (Copenhagen, Denmark), rural (Melpitz, Germany) and remote (Storm Peak Laboratory, CO, USA) environments. Suwannee River Fulvic Acid Standard (SRFA) was included in the study as a reference. Raman, Fourier transform infrared (FTIR) and ultraviolet-visible (UV-Vis) spectroscopy were used together with high performance liquid chromatography-mass spectrometry (HPLC-MS) for characterisation of the samples. The same main functional groups were present in all samples, but the relative abundance of functional groups varied among the studied samples, which was mainly evident from the FTIR spectra. The urban and rural samples were found to be very similar with respect to the relative abundance of functional groups. The remote sample contained relatively more C=O and COOH groups, which may be due to the remote environment. Organonitrates appeared to be present in the three atmospheric samples, while it did not appear to be present in the SRFA. The SRFA sample comprised significantly larger amounts of aromatic groups relative to the atmospheric samples in line with previous findings. All the obtained mass spectra showed clear periods of 14-16 Da in line with previous studies. The estimated average molecular weight (AMW) was comparable for the atmospheric samples within the errors - while the AMW of SRFA was higher. In general the atmospheric HULIS samples from different environments were rather similar with respect to the properties investigated. [ABSTRACT FROM AUTHOR]

Polycyclic aromatic hydrocarbons (PAH) are a ubiquitous atmospheric pollutant, comprising between 2% and 27% of PM 2.5 by mass. They undergo rapid photooxidation forming oxidized products (oxPAH), and humic-like substances (HULIS). HULIS has known climate-forcing potential, and oxPAH have been reported to be more toxic than their parent PAH. The hydroxyl radical (OH) is associated with the photolysis of PAH. Anthracene and a selection of oxPAH (1-naphthol, 2-naphthol, 1,4-naphthoquinone, 1,4-anthraquinone, and 9,10-anthraquinone) were exposed to simulated sunlight for 16 h and OH concentration was measured. Formation rates (R OH) were calculated for the first 30 min of the reaction. The R OH for 1,4-naphthoquinone was initially very high and declined logarithmically over the 30-min interval, while anthracene had a low initial R OH which then increased linearly over the 30-min interval. It was determined that the production of OH is not a sole predictor of HULIS formation: photolysis of both 1-naphthol and 2-naphthol result in HULIS formation, but 2-naphthol does not result in a substantial OH concentration. Conversely, neither 1,4-anthraquinone or 9,10-anthraquinone result in HULIS, but OH was seen with 9,10- anthraquinone photolysis. This suggests that, while OH is important, pathways resulting in HULIS from PAH which do not include OH are also significant. [Display omitted] • Photolysis of 1,4-naphthaquinone resulted in the largest concentration of OH. • OH formation is not necessary for HULIS formation - other ROS play a role. • HULIS developed from some starting materials even in the absence of OH formation. [ABSTRACT FROM AUTHOR]

Biomass and coal combustion are major sources of water-soluble organic matter (WSOM) in PM 2.5. This study investigated the carbon content, light absorption capacity, molecular structures, and oxidative activity of three types of WSOM (neutral humic-like substances (HULIS-n), acidic HULIS (HULIS-a), and high-polarity WSOM (HP-WSOM)) produced from biomass and coal combustion in Northwest China. The carbon emission factors of HULIS-n, HULIS-a, and HP-WSOM were 5.52 ± 4.29, 3.82 ± 2.83, and 3.24 ± 2.85 g/kg from biomass burning, and 1.77 ± 0.65, 4.41 ± 1.24, and 3.94 ± 1.78 g/kg from coal combustion, respectively. HULIS-n had the highest light absorption coefficient at 365 nm (b abs-365) and the mass absorption efficiency at 365 nm (MAE 365) both for biomass and coal burning, suggesting that HULIS-n contains more chromophores than the others. The Fourier transform infrared spectra results revealed that aromatic substances and polysaccharides were predominated in HULIS-n and HP-WSOM. Reactive oxygen species (ROS) in coal combustion followed a decreasing order of HULIS-n > HP-WSOM > HULIS-a. In terms of WSOM from biomass combustion, the highest ROS was in HULIS-n for corn straw and wheat straw in fire kangs but the highest ROS was in HP-WSOM for pepper rod and sesame rod in hanging stoves as well as wood block fire. HULIS-a produced by HS of biomass burning had higher ROS activity, and oxidative activity of HULIS-n produced by burning biomass by HB and SS did not differ greatly. High ROS and strong light absorption of HULIS-n in both biomass burning and coal burning may attribute to its molecular structures such as abundance of R–ONO 2 organic nitrate. Our results also highlighted that the carbon content of HULIS-n occupied only 17.5% to HULIS + HP-WSOM in coal combustion, whereas HULIS-n contributed up to 58.3% and 64.8% of the total light absorption and ROS. Overall, this study revealed the complex emission properties of WSOM, which would improve the assessment of their environmental and climate impacts at a regional scale. [Display omitted] • The carbon content of HULIS + HP-WSOM from biomass burning was nearly double than coal combustion. • HULIS-n from coal combustion showed the highest ROS but lowest carbon content. • High ROS and light absorption of HULIS-n should attribute to nitrogenous organic substances. [ABSTRACT FROM AUTHOR]

Abstract: Significant amounts of humic-like substances (HULIS) are present in marine submicrometer particles. The cloud condensation nuclei (CCN) activation was investigated for marine model particles comprised of Nordic Aquatic Fulvic Acid Reference (NAFA) and sodium chloride (NaCl) in mass ratios of 100:0, 80:20, 50:50, 20:80 and 0:100 respectively. The CCN activity of NAFA was found to be represented by a κ value of 0.028. The CCN activities of the mixed particles were overestimated by volume weighted addition of the κ values of the pure compounds, which indicates that synergistic effects of the mixtures tend to lower the CCN activity. Parameterizations of water activity (aw ) and surface tension (σ) versus solute concentration were obtained from measurements on aqueous solutions. The CCN activity was modeled on the basis of the parameterizations of aw and σ using Köhler theory. For the particles containing 50% or more NAFA the model overpredicted the CCN activity compared to observations. Reasonable model results were obtained by assuming a surface tension of pure water. [Copyright &y& Elsevier]

Abstract: The mass specific optical absorption coefficient (MAC) of Humic-Like Substances (HULIS), isolated from a fine aerosol fraction (PM1) collected at a typical Central European rural background site (Kpuszta) was measured in the re-dispersed aerosol phase at 1064, 532, 355 and 266 nm wavelengths by our recently developed four wavelength photoacoustic spectrometer. It is found to be practically negligible in the visible (0.03 m2 g−1 @532 nm), while in the ultraviolet (4.9 m2 g−1 @266 nm) it becomes comparable with that of black carbon (BC), a major absorbing fraction of the ambient aerosol. This type of wavelength dependency was already hypothesized for HULIS aerosol, but it was proved previously only by indirect measurements on HULIS samples dissolved in the aqueous phase. On the other hand, the other generally accepted hypothesis, that this wavelength dependency can be described by a single, wavelength independent absorption Angström-exponent (AAE) is not justified by the presented measurements. [Copyright &y& Elsevier]

Abstract: The atmosphere contains a substantial amount of water-soluble organic material in aerosols, clouds and fogs. Despite years of efforts, little is known on the structure, composition and properties of this organic matter with most studies focusing on individual species while the bulk of the organic matter remains poorly characterized. In this work high-performance size-exclusion chromatography coupled with inline organic carbon detection (SEC-DOC) is used to characterize organic matter in fogs, clouds and aerosols collected in Fresno (CA), Whistler (BC), Davis (CA) and Selinsgrove (PA). The molecular weight distributions showed a fractional overlap of atmospheric samples and terrestrial fulvic acids although for clouds and aerosols the smaller molecular weight (MW) material is dominant. This smaller MW material is clearly resolved. Cloud and fog samples showed a larger fraction of small molecular weight organic species compared to the water-soluble fraction of aerosols, consistent with the partitioning of small molecular weight volatile species into the atmospheric aqueous phase. There are overall little differences between different sites for a same type of sample. These results obtained by one analytical set-up were confirmed with a second size-exclusion chromatography set-up using a different column and detection system. Size distributions for the same sampling location showed little inter-event variability and water-soluble organic carbon (WSOC) samples were slightly shifted toward larger sizes compared to clouds and fogs, consistent with an important contribution of volatile species to the latter ones. Cloud and aerosol samples contributed to a significant fraction (up to 21% of dissolved organic carbon (DOC)) of the macromolecular scale material. [Copyright &y& Elsevier]

Abstract: Aerosol samples were collected with a micro-orifice uniform deposit impactor in an aerodynamic diameter (AD) range of 0.5–10μm in central Budapest, Hungary for 12 days. Aqueous aerosol extracts and atmospheric humic-like substances (HULIS) were obtained from the combined aerosol samples for each impactor stage. Water-soluble organic carbon (WSOC) and carbon in HULIS (HULIS-C) were measured in the samples with a total organic carbon analyzer. The analytical data were inverted into semi-smooth mass size distributions, and modal parameters were derived. The size distributions for both WSOC and HULIS-C consisted of three peaks: a coarse mode and two accumulation submodes. The geometric mean AD for the coarse mode was 6.4μm for both WSOC and HULIS-C. Contribution of the coarse mode to the total concentration of WSOC and HULIS-C were substantial and similar for both components, i.e., approximately 20%. The splitting of the accumulation mode into condensation and droplet submodes was explained by water processing of aerosol particles in the air. The geometric mean ADs of the condensation submode for the WSOC and HULIS-C were 0.37 and 0.31μm, respectively, and the droplet submode appeared at 1.72 and 1.22μm, respectively. The condensation submode was larger than the droplet submode by similar ratios of 1.7–1.8 for both WSOC and HULIS-C. The relative concentrations of the two submodes were likely influenced by local meteorology, in particular by relative humidity, pollutant gases, and water uptake properties and aging of fine particles. [Copyright &y& Elsevier]

Abstract: Here we show that fluorescent compounds that could be classified as “M-like” (marine-like) fulvic acids are formed upon phototransformation of phenol by a triplet sensitiser (anthraquinone-2-sulphonate, AQ2S). The relevant process most likely involves phenol oxidation to phenoxyl radical by triplet AQ2S, followed by dimerisation of phenoxyl radicals into phenoxyphenols and dihydroxybiphenyls. It might be the first step of an oligomerization process that bears resemblance with the expected formation pathways of humic-like substances (HULIS) in the atmosphere. Such a process could account for the formation in surface waters of compounds having similar fluorescence properties as “M-like” fulvic acids. Presently it is thought that such species are formed upon photo-fragmentation of larger humic and fulvic acids (“top-down” pathway), and we propose that an opposite, “bottom-up” pathway could also be operational. [Copyright &y& Elsevier]

Abstract: PM1–2.5, PM0.1–1, and PM<0.1 water-soluble organic carbon (WSOC) fractions of aerosol samples collected during summer and fall 2009 were analyzed by UV/VIS spectroscopy and electrospray ionization–mass spectrometry (ESI/MS). The focus of this study was to compare optical properties, chemical nature, and molecular weight distribution in samples extracted from different size fractions and collected during different seasons (fall vs. summer). Diurnal patterns were also investigated. An increase in absorptivity, aromaticity, and average molecular weight (AMW) in all size fractions found in the fall samples indicates different formation processes for the organic carbon between the summer and fall periods. The fall ultrafine fraction (PM<0.1) demonstrated characteristics different from the other two PM size fractions and more similar to aquatic fulvic acids. It had the highest HULIS/WSOC ratio, molar absorptivity, and AMWs up to about 700 Da when analyzed by the UV/VIS method and about 475 Da by the ESI/MS. Higher concentrations of organosulfate (OS) compounds and polycarboxylic acids were detected in the summer samples while organonitrate (ON) compounds and monocarboxylic acids were higher in the fall samples. [Copyright &y& Elsevier]

We characterised surface-active substances (SASs) in aqueous extract of atmospheric aerosols by using phase sensitive alternating current voltammetry. The electrochemical method has mainly been used for the quantification of surfactants in sea water but has not been applied to atmospheric aerosols yet. The advantage of the method is its simplicity and sensitivity that enables direct analysis of aerosol extracts without the need for sample concentration. Aerosol samples were collected at Middle Adriatic Martinska station influenced by different air masses as well as from urban (Zagreb, Croatia) and rural (K-puszta, Hungary) areas from late spring to early autumn in 2010. The highest SAS concentrations, expressed in equivalents of T-X-100, ranging from 0.34 to 0.91 μg m-3 were detected in urban samples. The SAS concentrations obtained for marine, regional and continental samples ranged from 0.14 to 0.31, 0.18 to 0.42 and 0.07 to 0.28 μg m-3, respectively. The SAS concentrations in K-puszta aerosols ranged from 0.13 to 0.46 μg m-3. Investigation of humic-like substances isolated from K-puszta samples (2008) confirmed their significant surfactant nature. Different SAS chemistry was noticed for urban and non-urban samples. Investigations at different pH revealed anionic character of SASs in aerosol samples. [ABSTRACT FROM AUTHOR]