ROS-generation potential of Humic-like substances (HULIS) in ambient PM2.5 in urban Shanghai: Association with HULIS concentration and light absorbance

Effects of chemical composition of ambient PM2.5 measured in Seoul on potential health risks and generation of reactive oxygen species

This study assesses the health effects associated with the chemical species of ambient particulate matter (PM) with an aerodynamic diameter ≤2.5 μm (PM2.5) in Seoul, focusing on identifying key chemical constituents and their sources. We employed two approaches to estimate health risks: (1) evaluating carcinogenic and noncarcinogenic risks using IRIS (Integrated Risk Information System) data from the US EPA (Environmental Protection Agency), and (2) quantifying the generation of hydroxyl radicals (·OH) following exposure to PM2.5 in surrogate lung fluid (SLF). Our results show a significant impact on human health from certain elements (Cr, Ni, As, and Cd) and polycyclic aromatic hydrocarbons (PAHs) (DaeP, DahA, and BaP for carcinogenic risks; BaP and BeP for noncarcinogenic risks). Notably, Cr and BaP, which are influential in both risk assessment and ·OH generation, highlight their significant roles in human health impacts. However, other components (e.g., CPP, BaP, BghiP, BaA, CHR, PYR, FLT, Ca, Mg, and Cu), though contributors to ·OH generation, were not included in the EPA's health risk assessment, suggesting a need for a broader PM2.5 compositional analysis to more accurately determine exposure concentrations and assess inhalation risks. These components predominantly originate from anthropogenic sources, such as combustion, vehicles, and industrial activities, underscoring the significant health implications of the pollutants emitted from these sources. The study concluded that focusing solely on the mass reduction of PM2.5 may not suffice; a dual approach that reduces both mass concentration and chemical-specific health risks is imperative for effective public health protection.

Size-segregated characteristics of water-soluble oxidative potential in urban Xiamen: Potential driving factors and implications for human health

Oxidative potential (OP), defined as the ability of particulate matter (PM) to generate reactive oxygen species (ROS), has been considered as a potential health-related metric for PM. Particles with different sizes have different OP and deposition efficiencies in the respiratory tract and pose different health risks. In this study, size-segregated PM samples were collected at a coastal urban site in Xiamen, a port city in southeastern China, between August 2020 and September 2021. The water-soluble constituents, including inorganic ions, elements and organic carbon, were determined. Total volume-normalized OP based on the dithiothreitol assay was highest in spring (0.241 ± 0.033 nmol min-1 m-3) and lowest in summer (0.073 ± 0.006 nmol min-1 m-3). OP had a biomodal distribution with peaks at 0.25-0.44 μm and 1.0-1.4 μm in spring, summer, and winter and a unimodal pattern with peak at 0.25-0.44 μm in fall, which were different from the patterns of redox-active species. Variations in the seasonality of fine and coarse mode OP and their correlations with water-soluble constituents showed that the size distribution patterns of OP could be attributed to the combined effects of the size distributions of transition metals and redox-active organics and the interactions between them which varied with emissions, meteorological conditions and atmospheric processes. Respiratory tract deposition model indicated that the deposited OP and the toxic elements accounted for 47.9 % and 36.8 % of their measured concentrations, respectively. The highest OP doses and the excess lifetime carcinogenic risk (ELCR) were found in the head airway (>70 %). However, the size distributions of OP deposition and ELCR in the respiratory tract were different, with 63.9 % and 49.4 % of deposited ELCR and OP, respectively, coming from PM2.5. Therefore, attention must be paid to coarse particles from non-exhaust emissions and road dust resuspension.

Research progress on the characteristics, sources, and environmental and potential health effects of water-soluble organic compounds in atmospheric particulate matter

Water-soluble organic compounds (WSOCs) have received extensive attention due to their indistinct chemical components, complex sources, negative environmental impact, and potential health effects. To the best of our knowledge, until now, there has been no comprehensive review focused on the research progress of WSOCs. This paper reviewed the studies on chemical constituent and characterization, distribution condition, sources, environmental impact, as well as the potential health effects of WSOCs in the past 13 years. Moreover, the main existing challenges and directions for the future research on WSOCs were discussed from several aspects. Because of the complex composition of WSOCs and many unknown individual components that have not been detected, there is still a need for the identification and quantification of WSOCs. As modern people spend more time in indoor environments, it is meaningful to fill the gaps in the component characteristics and sources of indoor WSOCs. In addition, although in vitro cell experiments have shown that WSOCs could induce cellular oxidative stress and trigger the inflammatory response, the corresponding mechanisms of action need to be further explored. The current population epidemiology research of WSOCs is missing. Prospectively, we propose to conduct a comprehensive and simultaneous analysis strategy for concentration screening, source apportionment, potential health effects, and action mechanisms of WSOCs based on high throughput omics coupled with machine learning simulation and prediction.

Humic-like Substances (HULIS) in Ship Engine Emissions: Molecular Composition Effected by Fuel Type, Engine Mode, and Wet Scrubber Usage

Humic-like substances (HULIS), known for their substantial impact on the atmosphere, are identified in marine diesel engine emissions obtained from five different fuels at two engine loads simulating real world scenarios as well as the application of wet sulfur scrubbers. The HULIS chemical composition is characterized by electrospray ionization (ESI) ultrahigh resolution mass spectrometry and shown to contain partially oxidized alkylated polycyclic aromatic compounds as well as partially oxidized aliphatic compounds, both including abundant nitrogen- and sulfur-containing species, and clearly different to HULIS emitted from biomass burning. Fuel properties such as sulfur content and aromaticity as well as the fuel combustion efficiency and engine mode are reflected in the observed HULIS composition. When the marine diesel engine is operated below the optimum engine settings, e.g., during maneuvering in harbors, HULIS-C emission factors are increased (262-893 mg kg-1), and a higher number of HULIS with a shift toward lower degree of oxidation and higher aromaticity is detected. Additionally, more aromatic and aliphatic CHOS compounds in HULIS were detected, especially for high-sulfur fuel combustion. The application of wet sulfur scrubbers decreased the HULIS-C emission factors by 4-49% but also led to the formation of new HULIS compounds. Overall, our results suggest the consideration of marine diesel engines as a relevant regional source of HULIS emissions.

Complex polycyclic aromatic compound mixtures in PM2.5 in a Chinese megacity: Spatio-temporal variations, toxicity, and source apportionment

Polycyclic aromatic compounds (PACs) are important toxic organic components in fine particulate matter (PM2.5), whereas the links between PM2.5 toxicity and associated PACs in ambient air are poorly understood. This study investigated the spatial-temporal variations of PACs in PM2.5 collected from 11 sampling sites across a Chinese megacity and characterized the reactive oxygen species (ROS) generation and cytotoxicity induced by organic extracts of PM2.5 based on cellular assays. The extra trees regression model based on machine learning and ridge regression were used to identify the key toxicants among complex PAC mixtures. The total concentrations of these PACs varied from 2.12 to 71.7 ng/m3 across the study city, and polycyclic aromatic hydrocarbons (PAHs) are the main PACs. The spatial variations of the toxicological indicators generally resembled those of the PAC concentrations, and the PM2.5 related to waste treatment facilities exhibited the strongest toxic potencies. The ROS generation was highly correlated with high molecular weight PAHs (MW302 PAHs), followed by PAHs with MW<302 amu and oxygenated PAHs, but not with nitrated PAHs and the plastics additives. The cell mortality showed weak correlations with these organic constituents. The associations between the biological endpoints and these PM2.5-bound contaminants were further confirmed by exposure to authentic chemicals. Four primary sources of PACs were identified, among which coal and biomass combustion sources (30.2% of the total PACs) and industrial sources (31.0%) were predominant. PACs emitted from industrial sources were highly associated with ROS generation in this city. Our findings highlight the potent ROS-generating potential of MW302 PAHs and the importance of industrial sources contributing to PM2.5 toxicity in this megacity, raising public concerns and further administration.

Exploring the mechanisms of humic acid mediated degradation of polystyrene microplastics under ultraviolet light conditions

Microplastics (MPs) are emerging pollutants that interact extensively with dissolved organic matter (DOM) and this influences the environmental behavior of MPs in aqueous ecosystems. However, the effect of DOM on the photodegradation of MPs in aqueous systems is still unclear. The photodegradation characteristics of polystyrene microplastics (PS-MPs) in an aqueous system in the presence of humic acid (HA, a signature compound of DOM) under ultraviolet light conditions were investigated in this study through Fourier transform infrared spectroscopy coupled with two-dimensional correlation analysis, electron paramagnetic resonance, and gas chromatography-mass spectrometry (GC/MS). HA was found to promote higher levels of reactive oxygen species (0.631 mM of ▪OH), which accelerated the photodegradation of PS-MPs, with a higher degree of weight loss (4.3%), higher level of oxygen-containing functional groups, and lower average particle size (89.5 μm). Likewise, GC/MS analysis showed that HA contributed to a higher content of oxygen-containing compounds (42.62%) in the photodegradation of PS-MPs. Moreover, the intermediates and final degradation products of PS-MPs with HA were significantly different in the absence of HA during 40 days of irradiation. These results provide an insight into the co-existing compounds on the degradation and migration processes of MP and also support further research toward the remediation of MPs pollution in aqueous ecosystems.

Seasonal variation of driving factors of ambient PM2.5 oxidative potential in Shenzhen, China

Reactive oxygen species (ROS) play a central role in health effects of ambient fine particulate matter (PM_2.5). In this work, we screened for efficient and complementary oxidative potential (OP) measurements by comparing the response values of multiple chemical probes (OP^DTT, OP^OH, OP^GSH) to ambient PM_2.5 in Shenzhen, China. Combined with meteorological condition and PM_2.5 chemical composition analysis, we explored the effects of different chemical components and emission sources on the ambient PM_2.5 OP and analyzed their seasonal variations. The results show that OP_m^DTT(mass-normalized) and OP_m^GSH-SLF were highly correlated (r = 0.77). OP^DTT was mainly influenced by organic carbon, while OP^OH was highly dominated by heavy metals. The combination of OP^DTT and OP^OH provides an efficient and comprehensive measurement of OP. Temporally, the OPs were substantially higher in winter than in summer (1.4 and 4 times higher for OP_m^DTT and OP_m^OH, respectively). The long-distance transported biomass burning sources from the north dominated the OP^DTT in winter, while the ship emissions mainly influenced the summer OP. The OP_m^DTT increased sharply with the decrease of PM_2.5 mass concentration, especially when the PM_2.5 concentration was lower than 30 μg/m³. The huge differences in wind fields between the winter and summer cause considerable variations in PM_2.5 concentrations, components, and OP. Our work emphasizes the necessity of long-term, multi-method, multi-component assessment of the OP of PM_2.5.

Insights into reactive oxygen species formation induced by water-soluble organic compounds and transition metals using spectroscopic method

Ambient particulate matter (PM) can cause adverse health effects via their ability to produce Reactive Oxygen Species (ROS). Water-Soluble Organic Compounds (WSOCs), a complex mixture of organic compounds which usually coexist with Transition Metals (TMs) in PM, have been found to contribute to ROS formation. However, the interaction between WSOCs and TMs and its effect on ROS generation are still unknown. In this study, we examined the ROS concentrations of V, Zn, Suwannee River Fulvic Acid (SRFA), Suwannee River Humic Acid (SRHA) and the mixtures of V/Zn and SRFA/SRHA by using a cell-free 2',7'-Dichlorodihydrofluorescein (DCFH) assay. The results showed that V or Zn synergistically promoted ROS generated by SRFA, but had an antagonistic effect on ROS generated by SRHA. Fluorescence quenching experiments indicated that V and Zn were more prone to form stable complexes with aromatic humic acid-like component (C1) and fulvic acid-like component (C3) in SRFA and SRHA. Results suggested that the underlying mechanism involving the fulvic acid-like component in SRFA more tending to complex with TMs to facilitate ROS generation through π electron transfer. Our work showed that the complexing ability and complexing stability of atmospheric PM organics with metals could significantly affect ROS generation. It is recommended that the research deploying multiple analytical methods to quantify the impact of PM components on public health and environment is needed in the future.

Size-Resolved Redox Activity and Cytotoxicity of Water-Soluble Urban Atmospheric Particulate Matter: Assessing Contributions from Chemical Components

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.

Oxidative stress response in pulmonary cells exposed to different fractions of PM2.5-0.3 from urban, traffic and industrial sites

The aim of this work was to study the relationship between oxidative stress damages and particulate matter (PM) chemical composition, sources, and PM fractions. PM_2.5-0.3 (PM with equivalent aerodynamic diameter between 2.5 and 0.3 μm) were collected at urban, road traffic and industrial sites in the North of France, and were characterized for major and minor chemical species. Four different fractions (whole PM_2.5-0.3, organic, water-soluble and non-extractable matter) were considered for each of the PM_2.5-0.3 samples from the three sites. After exposure of BEAS-2B cells to the four different fractions, oxidative stress was studied in cells by quantifying reactive oxygen species (ROS) accumulation, oxidative damage to proteins (carbonylated proteins), membrane alteration (8-isoprostane) and DNA damages (8-OHdG). Whole PM_2.5-0.3 was capable of inducing ROS overproduction and caused damage to proteins at higher levels than other fractions. Stronger cell membrane and DNA damages were found associated with PM and organic fractions from the urban site. ROS overproduction was correlated with level of expression of carbonylated proteins, DNA damages and membrane alteration markers. The PM_2.5-0.3 collected under industrial influence appears to be the less linked to cell damages and ROS production in comparison with the other influences.

Optical properties, molecular characterizations, and oxidative potentials of different polarity levels of water-soluble organic matters in winter PM2.5 in six China's megacities

Humic-like substances (HULIS) accounted for a great fraction of water-soluble organic matter (WSOM) in PM_2.5, which efficiently absorb ultraviolet (UV) radiation and pose climate and health impacts. In this study, the molecular structure, optical properties, and oxidative potential (OP) of acid- and neutral-HULIS (denoted as HULIS-a, and HULIS-n, respectively), and high-polarity WSOM (HP-WSOM) were investigated in winter PM_2.5 collected at six China's megacities. For both carbon levels and optical absorption coefficients (b_{abs_365}), HULIS-a/HULIS-n/HP-WSOM showed significant spatial differences. For each city, the carbon levels and b_{abs_365} follow a similar order of HULIS-n > HULIS-a > HP-WSOM. Besides, the b_{abs_365} of HULIS-n and HULIS-a showed the same order of Harbin > Beijing ≈ Wuhan > Xi'an > Guangzhou > Chengdu, while HP-WSOM exhibited an order of Wuhan > Chengdu > Xi'an > Harbin > Beijing > Guangzhou. Both HULIS-a and HULIS-n were abundant in aromatic and aliphatic compounds, whereas HP-WSOM was dominated by a carboxylic acid group. The OP (in unit of nmol H₂O₂ μg^-1C) followed the order of HP-WSOM > HULIS-a > HULIS-n in all the cities. The OPs of HULIS-a, HULIS-n, and HP-WSOM in Harbin and Beijing were much higher than those of other cities, attributing to the high contribution from biomass burning. Highly positive correlations between reactive oxygen species (ROS) of HULIS-a and MAE₃₆₅ were obtained in Chengdu, Wuhan, and Harbin, but ROS of HULIS-n had stronger correlation with MAE₃₆₅ in Harbin, Chengdu, and Xi'an.

pH modifies the oxidative potential and peroxide content of biomass burning HULIS under dark aging

Humic-like substances (HULIS) account for a major redox-active fraction of biomass burning organic aerosols (BBOA). During atmospheric transport, fresh acidic BB-HULIS in droplets and humid aerosols are subject to neutralization and pH-modified aging process. In this study, solutions containing HULIS isolated from wood smoldering emissions were first adjusted with NaOH and NH₃ to pH values in the range of 3.6-9.0 and then aged under oxic dark conditions. Evolution of HULIS oxidative potential (OP) and total peroxide content (equivalent H₂O₂ concentration, H₂O₂eq) were measured together with the changes in solution absorbance and chemical composition. Notable immediate responses such as peroxide generation, HULIS autoxidation, and an increase in OP and light absorption were observed under alkaline conditions. Initial H₂O₂eq, OP, and absorption increased exponentially with pH, regardless of the alkaline species added. Dark aging further oxidized the HULIS and led to pH-dependent toxic and chemical changes, exhibiting an alkaline-facilitated initial increase followed by a decrease of OP and H₂O₂eq. Although highly correlated with HULIS OP, the contributions of H₂O₂eq to OP are minor but increased both with solution pH and dark aging time. Alkalinity-assisted autoxidation of phenolic compounds and quinoids with concomitant formation of H₂O₂ and other alkalinity-favored peroxide oxidation reactions are proposed here for explaining the observed HULIS OP and chemical changes in the dark. Our findings suggest that alkaline neutralization of fresh BB-HULIS represents a previously overlooked peroxide source and pathway for modifying aerosol redox-activity and composition. Additionally, these findings imply that the lung fluid neutral environment can modify the OP and peroxide content of inhaled BB-HULIS. The results also suggest that common separation protocols of HULIS using base extraction methods should be treated with caution when evaluating and comparing their composition, absorption, and relative toxicity.

Quantitative Determination of Airborne Redox-Active Compounds Based on Heating-Induced Reduction of Gold Nanoparticles

Airborne redox-active compounds (ARC) account for a substantial fraction of atmospheric aerosols and play a vital role in chemical processes that influence global climate and human and ecological health. With the exception of the determination of total organic carbon by the expensive total organic carbon (TOC) analyzer, there is currently no easy-to-use method to quantify ARC. Here, we designed a method to detect the concentration of ARC by using the thermal-induced reduction and colorimetric behaviors of gold nanoparticles (AuNPs), in which the humic substances (HS) was used as a standard model of ARC to calculate the HS-equivalent concentration of ARC. Distinguished from the conventional complex methods, e.g., TOC analysis, the proposed approach measured localized surface plasmon resonance absorption of AuNPs and the target ARC concentration can be either directly quantified by the absorption spectrometer or qualitatively evaluated by the naked eyes. By using the absorption spectrometer, a limit of detection of 0.005 ppm by our AuNP sensor was achieved. To validate this sensing technique, aerosol samples collected from Basel (suburban), Bern (urban), and Rigi mountain (rural and high-altitude) sites in Switzerland were further investigated through the TOC combustion method. The results thereby substantiated that our plasmonic absorption-based AuNP sensor upholds a great promise for fast, cost-efficient total ARC detection and air quality assessment.

Approval Research for Carcinogen Humic-Like Substances (HULIS) Emitted from Residential Coal Combustion in High Lung Cancer Incidence Areas of China

The incidence and mortality rate of lung cancer is the highest in Xuanwei County, Yunnan Province, China. The mechanisms of the high lung incidence remain unclear, necessitating further study. However, the particle size distribution characteristics of HULIS emitted from residential coal combustion (RCC) have not been studied in Xuanwei. In this study, six kinds of residential coal were collected. Size-resolved particles emitted from the coal were sampled by using a burning system, which was simulated according to RCC made in our laboratory. Organic carbon (OC), elemental carbon (EC), water-soluble inorganic ion, water-soluble potentially toxic metals (WSPTMs), water-soluble organic carbon (WSOC), and HULIS-C (referred to as HULIS containing carbon contents) in the different size-segregated particulate matter (PM) samples were determined for health risk assessments by inhalation of PM. In our study, the ratio of HULIS-Cx to WSOCx values in RCC particles were 32.73–63.76% (average 53.85 ± 12.12%) for PM2.0 and 33.91–82.67% (average 57.06 ± 17.32%) for PM2.0~7.0, respectively. The carcinogenic risks of WSPTMs for both children and adults exceeded the acceptable level (1 × 10−6, indicating that we should pay more attention to these WSPTMs). Exploring the HULIS content and particle size distribution of the particulate matter produced by household coal combustion provides a new perspective and evidence for revealing the high incidence of lung cancer in Xuanwei, China.

Light absorption properties and molecular profiles of HULIS in PM2.5 emitted from biomass burning in traditional "Heated Kang" in Northwest China

Humic-like substances (HULIS) in PM_2.5 emitted from biomass burning (BB), including maize cob, wheat straw, maize straw, wood branch, and wood, in a traditional "Heated Kang" were investigated. The relative abundances, optical properties, chemical functional groups, and molecular components in HULIS were characterized using total organic carbon (TOC) analyzer, ultraviolet-visible spectroscopy (UV-vis), Fourier-transform infrared spectroscopy (FT-IR), and Fourier-transform ion cyclotron resonance mass spectrometer (FT-ICR/MS), respectively. The emission factors (EF) of HULIS-C (in term of carbon weight, EF_HULIS-C) from BB were in the range of 0.83 to 5.17 g/kg fuel, with a mean value of 1.93 ± 1.31 g/kg fuel. The HULIS-C accounted for 15.0-37.8% and 9.1-12.6% of fractions in organic carbon (OC) and PM_2.5, respectively, suggesting that BB is an important emission source of atmospheric HULIS. The FT-IR spectra showed BB HULIS mainly contain O-containing, aliphatic CH, and aromatic CC functional groups. The presences of carboxyl group and OH band demonstrated the uniqueness of maize straw and wood burning. Moreover, the higher ratio of CH₃ and -CH₂ groups could be used to distinguish the wood branches from the maize cob. CHO and CHON were much dominant in BB HULIS, which accounted for 44.6-47.6% and 50.1-54.2%, respectively, to the total molecular mass. The positive correlation between MAE₃₆₅ and AAE in term of number concentration of CHNO implied that the CHNO species could greatly influence on the light absorption properties of the BB HULIS. The CHO and S-containing compounds (i.e., CHNOS and CHOS, that is CHNOS+CHOS) showed weak light absorbances of the BB HULIS. The BB HULIS from maize straw had relatively high molecular weight in comparison to that in other BB emissions. The highest and lowest aromaticity were seen on the wood burning and maize cob, respectively.

Toxicity of Water- and Organic-Soluble Wood Tar Fractions from Biomass Burning in Lung Epithelial Cells

Widespread smoke from wildfires and biomass burning contributes to air pollution and the deterioration of air quality and human health. A common and major emission of biomass burning, often found in collected smoke particles, is spherical wood tar particles, also known as "tar balls". However, the toxicity of wood tar particles and the mechanisms that govern their health impacts and the impact of their complicated chemical matrix are not fully elucidated. To address these questions, we generated wood tar material from wood pyrolysis and isolated two main subfractions: water-soluble and organic-soluble fractions. The chemical characteristics as well as the cytotoxicity, oxidative damage, and DNA damage mechanisms were investigated after exposure of A549 and BEAS-2B lung epithelial cells to wood tar. Our results suggest that both wood tar subfractions reduce cell viability in exposed lung cells; however, these fractions have different modes of action that are related to their physicochemical properties. Exposure to the water-soluble wood tar fraction increased total reactive oxygen species production in the cells, decreased mitochondrial membrane potential (MMP), and induced oxidative damage and cell death, probably through apoptosis. Exposure to the organic-soluble fraction increased superoxide anion production, with a sharp decrease in MMP. DNA damage is a significant process that may explain the course of toxicity of the organic-soluble fraction. For both subfractions, exposure caused cell cycle alterations in the G2/M phase that were induced by upregulation of p21 and p16. Collectively, both subfractions of wood tar are toxic. The water-soluble fraction contains chemicals (such as phenolic compounds) that induce a strong oxidative stress response and penetrate living cells more easily. The organic-soluble fraction contained more polycyclic aromatic hydrocarbons (PAHs) and oxygenated PAHs and induced genotoxic processes, such as DNA damage.

Chemical and Optical Characteristics and Sources of PM2.5 Humic-Like Substances at Industrial and Suburban Sites in Changzhou, China

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.