Environmentally Persistent Free Radical (EPFR) Formation by Visible-Light Illumination of the Organic Matter in Atmospheric Particles

Phototransformation of tetrabromobisphenol A bis (allyl ether) in an aqueous solution: Role of environmentally persistent free radicals

Tetrabromobisphenol A bis (allyl ether) (TBBPA-BAE) represents an extensively used brominated flame retardant (BFRs) in the production of many fields and their phototransformation in natural water is still unclear. The environmentally persistent free radicals (EPFRs) with preserved activities could exist in the environment for a long time and involve in the phototransformation of many organic pollutants. Here, the photodegradation of TBBPA-BAE with the degradation rate constant (k= 0.060 h-1) under simulate sunlight and the promoting effect of EPFRs on TBBPA-BAE photodegradation (k = 0.135 h-1) were investigated. According to the detected photogenerated electrons (e-) and singlet oxygen (1O2) rather than hydroxyl radicals (•OH) by the electron paramagnetic resonance (EPR), the effect mechanism may not be related to the typical •OH induced by EPFRs. The possible transformation pathways of the ether cleavage, hydrolysis and hydroxylation of propenyl bond and the debromination were proposed by the primary byproducts identified by UPLC-Q-Exactive Orbitrap MS. EPFRs caused a further debromination and ether cleavage and probably be due to EPFRs directly providing electrons to TBBPA-BAE which promoted the photodegradation of TBBPA-BAE, and their reaction mechanism needed further attention. Overall, this study provided useful information to understand the role of EPFRs on phototransformation of TBBPA-BAE in water.

Soil Environmental Persistent Free Radicals in highly polluted soils and the association with polycyclic aromatic compounds

Environmental persistent free radicals (EPFRs) as emerging contaminants in environment can induce oxidative stress causing severe adverse health outcomes. The formation of EPFRs is thought to be associated with the transformation of aromatic compounds like polycyclic aromatic hydrocarbons (PAHs). Herein this study firstly evaluated EPFRs in industrial soils being highly polluted by PAHs, and explored its associated with PAHs, with the modification of soil organic matter content. Soil EPFRs from two industrial plants were 4.1×1016 and 4.5×1016 spins/g, respectively, that were significantly higher than the levels in the surrounding areas. Carbon-centered EPFRs account for approximately 80% inside the plant, but outside the plants, nearly 50∼70% of EPFRs were carbon-centered with adjacent heteroatoms. As one important precursor of EPFRs, PAHs exhibited a significantly positive correlation with EPFRs in this study (p<0.05), explaining 40%-60% of the variation in EPFRs concentration. The relationship between soil organic matter and EPFRs concentration normalized by PAHs forms an inverted V-shape, illustrating an inhibition effect of soil organic matter on the EPFR formation potentials from PAHs.

Residential Wood Burning and Vehicle Emissions as Major Sources of Environmentally Persistent Free Radicals in Fairbanks, Alaska

Environmentally persistent free radicals (EPFRs) play an important role in aerosol effects on air quality and public health, but their atmospheric abundance and sources are poorly understood. We measured EPFRs contained in PM2.5 collected in Fairbanks, Alaska, in winter 2022. We find that EPFR concentrations were enhanced during surface-based inversion and correlate strongly with incomplete combustion markers, including carbon monoxide and elemental carbon (R2 > 0.75). EPFRs exhibit moderately good correlations with PAHs, biomass burning organic aerosols, and potassium (R2 > 0.4). We also observe strong correlations of EPFRs with hydrocarbon-like organic aerosols, Fe and Ti (R2 > 0.6), and single-particle mass spectrometry measurements reveal internal mixing of PAHs, with potassium and iron. These results suggest that residential wood burning and vehicle tailpipes are major sources of EPFRs and nontailpipe emissions, such as brake wear and road dust, may contribute to the stabilization of EPFRs. Exposure to the observed EPFR concentrations (18 ± 12 pmol m-3) would be equivalent to smoking ∼0.4-1 cigarette daily. Very strong correlations (R2 > 0.8) of EPFR with hydroxyl radical formation in surrogate lung fluid indicate that exposure to EPFRs may induce oxidative stress in the human respiratory tract.

Can the concentration of environmentally persistent free radicals describe its toxicity to Caenorhabditis elegans? Evidence provided by neurotoxicity and oxidative stress

Environmentally persistent free radicals (EPFRs) are emerging pollutants stabilized on or inside particles. Although the toxicity of EPFR-containing particles has been confirmed, the conclusions are always ambiguous because of the presence of various compositions. A clear dose-response relationship was always challenged by the fact that the concentrations of these coexisted components simultaneously changed with EPFR concentrations. Without these solid dose-response pieces of evidence, we could not confidently conclude the toxicity of EPFRs and the description of potential EPFR risks. In this study, we established a particle system with a fixed catechol concentration but different reaction times to obtain particles with different EPFR concentrations. Caenorhabditis elegans (C. elegans) in response to different EPFR concentrations was systematically investigated at multiple biological levels, including behavior observations and biochemical and transcriptome analyses. Our results showed that exposure to EPFRs disrupted the development and locomotion of C. elegans. EPFRs cause concentration-dependent neurotoxicity and oxidative damage to C. elegans, which could be attributed to reactive oxygen species (ROS) promoted by EPFRs. Furthermore, the expression of key genes related to neurons was downregulated, whereas antioxidative genes were upregulated. Overall, our results confirmed the toxicity from EPFRs and EPFR concentration as a rational parameter to describe the extent of toxicity.

Distribution and bioavailability of mercury in size-fractioned atmospheric particles around an ultra-low emission power plant in Southwest China

Ultra-low emission (ULE) technology retrofits significantly impact the particulate-bound mercury (Hg) emissions from coal-fired power plants (CFPPs); however, the distribution and bioavailability of Hg in size-fractioned particulate matter (PM) around the ULE-retrofitted CFPPs are less understood. Here, total Hg and its chemical speciation in TSP (total suspended particles), PM10 (aerodynamic particle diameter ≤ 10 µm) and PM2.5 (aerodynamic particle diameter ≤ 2.5 µm) around a ULE-retrofitted CFPP in Guizhou Province were quantified. Atmospheric PM2.5 concentration was higher around this ULE-retrofitted CFPP than that in the intra-regional urban cities, and it had higher mass Hg concentration than other size-fractioned PM. Total Hg concentrations in PM had multifarious sources including CFPP, vehicle exhaust and biomass combustion, while they were significantly higher in autumn and winter than those in other seasons (P < 0.05). Regardless of particulate size, atmospheric PM-bound Hg had lower residual fractions (< 21%) while higher HCl-soluble fractions (> 40%). Mass concentrations of exchangeable, HCl-soluble, elemental, and residual Hg in PM2.5 were higher than those in other size-fractioned PM, and were markedly elevated in autumn and winter (P < 0.05). In PM2.5, HCl-soluble Hg presented a significantly positive relationship with elemental Hg (P < 0.05), while residual Hg showed the significantly positive relationships with HCl-soluble Hg and elemental Hg (P < 0.01). Overall, these results suggested that atmospheric PM-bound Hg around the ULE-retrofitted CFPP tends to accumulate in finer PM, and has higher bioavailable fractions, while has potential transformation between chemical speciation.

Role of Electronegativity in Environmentally Persistent Free Radicals (EPFRs) Formation on ZnO

Environmentally persistent free radicals (EPFRs), a group of emerging pollutants, have significantly longer lifetimes than typical free radicals. EPFRs form by the adsorption of organic precursors on a transition metal oxide (TMO) surface involving electron charge transfer between the organic and TMO. In this paper, dihalogenated benzenes were incorporated to study the role of electronegativity in the electron transfer process to obtain a fundamental knowledge of EPFR formation mechanism on ZnO. Upon chemisorption on ZnO nanoparticles at 250 °C, electron paramagnetic resonance (EPR) confirms the formation of oxygen adjacent carbon-centered organic free radicals with concentrations between 1016 and 1017 spins/g. The radical concentrations show a trend of 1,2-dibromobenzene (DBB) > 1,2-dichlorobenzene (DCB) > 1,2-difluorobenzene (DFB) illustrating the role of electronegativity on the amount of radical formation. X-ray absorption spectroscopy (XAS) confirms the reduction of the Zn2+ metal center, contrasting previous experimental evidence of an oxidative mechanism for ZnO single crystal EPFR formation. The extent of Zn reduction for the different organics (DBB > DCB > DFB) also correlates to their polarity. DFT calculations provide theoretical evidence of ZnO surface reduction and exhibit a similar trend of degree of reduction for different organics, further building on the experimental findings. The lifetimes of the EPFRs formed confirm a noteworthy persistency.

Molecular characterization of diverse quinone analogs for discrimination of aerosol-bound persistent pyrolytic and photolytic radicals

Aerosol-bound organic radicals, including environmentally persistent free radicals (EPFRs), are key components that affect climate, air quality, and human health. While putative structures have been proposed, the molecular characteristics of EPFRs remain unknown. Here, we report a surrogate method to characterize EPFRs in real ambient samples using mass spectrometry. The method identifies chemically relevant oxygenated polycyclic aromatic hydrocarbons (OxPAH) that interconvert with oxygen-centered EPFR (OC-EPFR). We found OxPAH compounds most relevant to OC-EPFRs are structurally rich and diverse quinones, whose diversity is strongly associated with OC-EPFR levels. Both atmospheric oxidation and combustion contributed to OC-EPFR formation. Redundancy analysis and photochemical aging model show pyrolytic sources generated more oxidized OC-EPFRs than photolytic sources. Our study reveals the detailed molecular characteristics of OC-EPFRs and shows that oxidation states can be used to identify the origins of OC-EPFRs, offering a way to track the development and evolution of aerosol particles in the environment.

Oxidative potential, environmentally persistent free radicals and reactive oxygen species of size-resolved ambient particles near highways

Particulate matter (PM) is a group of atmospheric pollutants with an uncertain toxicity, particularly when collected near highways. This study examined the oxidative potential (OP) of, as well as the environmentally persistent free radicals (EPFRs) and reactive oxygen species (ROS) present in PM samples collected near highways in Xiamen, China. Our findings revealed that PM had a relatively high OP, ranging from 3.8 to 18.5 nmol/min/μg, surpassing values reported in previous research. The oxidative potential of the water-insoluble fraction (OPWIS), which accounted for 68% of the total oxidative potential (OPTotal), demonstrated rapid toxicity, whereas the oxidative potential of the water-soluble fraction (OPWS) displayed a steadier toxicity release pattern. The primary free radicals detected in PM were oxygen-centered. The measured concentration of EPFRs was 6.073 × 1014 spins/m3, which is lower than that reported in previous studies, possibly because of the high relative humidity of the road environment in Xiamen. We also investigated the interaction between PM and water near highways and observed the generation of R and OH radicals. Additionally, we analysed the sample composition and evaluated the contributions of the different components to OPTotal. Transition metals (Fe, Cu, and Zn) were identified as the major contributors, accounting for 33.2% of the OPTotal. The positive correlation observed between EPFRs and ROS suggests that EPFRs may be involved in ROS generation. The correlation analysis indicated that the oxidative potential measured using the DTT method (OPDTT) could serve as an indicator of ROS generation. Finally, based on the relationship between OPDTT, EPFRs, and ROS, we propose that reducing the emission of transition metals, particularly Fe, represents an effective control measure for mitigating PM toxicity near highways.

Pollution characteristics and light-driven evolution of environmentally persistent free radicals in PM2.5 in two typical northern cities of China

Environmentally persistent free radicals (EPFRs) in PM_2.5 can pose significant health risks by generating reactive oxygen species (ROS). In this study, Beijing and Yuncheng were chosen as two representative northern cities of China that mainly relied on natural gas and coal respectively as the energy source for domestic heating in winter. The pollution characteristics and exposure risks of EPFRs in PM_2.5 around the heating season of 2020 were investigated and compared between the two cities. Through laboratory simulation experiments, the decay kinetics and secondary formation of EPFRs in PM_2.5 collected in both cities were also studied. EPFRs in PM_2.5 collected in Yuncheng in the heating period showed longer lifetime and lower reactivity, suggesting that EPFRs originated from coal combustion were more stable in the atmosphere. However, the generation rate of hydroxyl radical (·OH) by the newly formed EPFRs in PM_2.5 in Beijing under ambient conditions was 4.4 times of that in Yuncheng, suggesting higher oxidative potential of EPFRs from the atmospheric secondary processes. Accordingly, the control strategies of EPFRs and their health risks were raised for the two cities, which would also have direct implication for the control of EPFRs in other areas of similar atmospheric emission and reaction patterns.

Characterization of water-soluble brown carbon in atmospheric fine particles over Xi'an, China: Implication of aqueous brown carbon formation from biomass burning

Brown carbon (BrC) aerosols can affect not only the climate but also human health, however, the light absorption, chemical compositions, and formation mechanisms of BrC are still uncertain, which leads to uncertainties in the accurate estimation of its climate and health impacts. In this study, highly time - resolved brown carbon (BrC) in fine particles was investigated in Xi'an using offline aerosol mass spectrometer analysis. The light absorption coefficient (b_abs365) and mass absorption efficiency (MAE₃₆₅) at 365 nm of water-soluble organic aerosol (WSOA) generally increased with oxygen-to-carbon (O/C) ratios, indicating that oxidized OA could have more impacts on BrC light absorption. Meanwhile, the light absorption appeared to increase generally with the increases of nitrogen-to-carbon (N/C) ratios and water-soluble organic nitrogen; strong correlations (R of 0.76 for C_xH_yN_p⁺ and R of 0.78 for C_xH_yO_zN_p⁺) between b_abs365 and the N - containing organic ion families were observed, suggesting that the N - containing compounds are the effective BrC chromophores. b_abs365 correlated relatively well with BBOA (r of 0.74) and OOA (R of 0.57), but weakly correlated with CCOA (R of 0.33), indicating that BrC in Xi'an was likely to be associated with biomass burning and secondary sources. A multiple linear regression model was applied to apportion b_abs365 to contributions of different factors resolved from positive matrix factorization on water-soluble organic aerosols (OA) and obtained MAE₃₆₅ values of different OA factors. We found that biomass-burning organic aerosol (BBOA) dominated the b_abs365 (48.3 %), followed by oxidized organic aerosol (OOA, 33.6 %) and coal combustion organic aerosol (CCOA, 18.1 %). We further observed that nitrogen-containing organic matter (i.e., C_xH_yN_p⁺ and C_xH_yO_zN_p⁺) increased with the increase of OOA/WSOA and the decrease of BBOA/WSOA, especially under high ALWC conditions. Our work offered proper observation evidence that BBOA is oxidized through the aqueous formation to produce BrC in Xi'an, China.

Formation of Environmentally Persistent Free Radicals from the Irradiation of Polycyclic Aromatic Hydrocarbons

Polycyclic aromatic hydrocarbons (PAHs) provide a complex matrix for environmentally persistent free radicals (EPFRs) to stabilize in particulate matter, allowing them to be transported over long distances in the atmosphere while participating in light-driven reactions and causing various cardiopulmonary diseases. In this study, four PAHs ranging from three to five rings (anthracene, phenanthrene, pyrene, and benzo[e]pyrene) were investigated for EPFR formation upon photochemical and aqueous-phase aging. Through electron paramagnetic resonance (EPR) spectroscopy, it was found that approximately 10¹⁵ to 10¹⁶ spins g^-1 of EPFRs were formed from the PAH upon aging. EPR analysis also revealed that carbon-centered and monooxygen-centered radicals were predominantly formed by irradiation. However, oxidation and fused-ring matrices have added complexity to the chemical environment of these carbon-centered radicals, as observed by their g-values. This study showed that atmospheric aging results not only in the transformation of PAH-derived EPFR but also in an increase in EPFR concentrations of up to 10¹⁷ spins g^-1. Therefore, because of their stability and photosensitivity, PAH-derived EPFRs have a major impact on the environment.

Scientific and regulatory challenges of environmentally persistent free radicals: From formation theory to risk prevention strategies

EPFRs (Environmentally Persistent Free Radicals) are a class of pollutants that have been identified as potential environmental contaminants due to their persistence and ability to generate reactive oxygen species (ROS) that can cause oxidative stress in living organisms. However, no study has comprehensively summarized the production conditions, influencing factors and toxic mechanisms of EPFRs, impeding exposure toxicity assessments and risk prevention strategies. To bridge the gap between theoretical research and practical application, a thorough literature review to summarize the formation, environmental effects, and biotoxicity of EPFRs are conducted. A total of 470 relevant papers were screened in Web of Science Core collection databases. The transfer of electrons between interfaces and the cleavage of covalent bonds of persistent organic pollutants is crucial to the generation of EPFRs, which is induced by external sources of energy, including thermal energy, light energy, transition metal ions, and others. In the thermal system, the stable covalent bond of organic matter can be destroyed by heat energy at low temperature to form EPFRs, while the formed EPFRs can be destroyed at high temperature. Light can also accelerate the production of free radicals and promote the degradation of organic matter. The persistence and stability of EPFRs are synergistically influenced by individual environmental factors such as environmental humidity, oxygen content, organic matter content, and environmental pH. Studying the formation mechanism of EPFRs and their biotoxicity is essential for fully understanding the hazards posed by these emerging environmental contaminants.

Particle-size distributions of environmentally persistent free radicals and oxidative potential of soils from a former gasworks site

The formation of toxic by-products, such as environmentally persistent free radicals (EPFRs), is one of the causes for concern by polycyclic aromatic hydrocarbons (PAHs) in soils. However, the distribution of EPFRs in different soil fractions and their relative contribution to the oxidation potential (OP) have not been investigated. In the present paper, contaminated samples were obtained from the former gasworks sites and were fractionated into different size particles, which were analyzed for EPFRs, reactive oxygen species (ROS), and OP-assayed by dithiothreitol (DTT) (OP^DTT). The results showed the highest concentration of EPFRs in the soil particle size with diameters <0.15 mm due to co-existence of PAHs and transition metals. ROS generation is in accordance with the size-specific distribution of EPFRs. Using the DTT assays, the redox activity of various size soil particles was examined, and found it was approximately 4- to 8-folds higher than that of un-contaminated samples and strongly associated with EPFRs, ROS, and PAHs. The obtained results advanced our knowledge on the EPFRs distribution in soil fractions at former MGP sites and emphasized the significance of PAH-EPFRs as a class of compounds to be considered in risk assessment of contaminated sites.

Pollution characteristics of environmental persistent free radicals (EPFRs) and their contribution to oxidation potential in road dust in a large city in northwest China

Environmental persistent free radicals (EPFRs) are new environmental health risk substances in the atmosphere, and their oxidative toxicity (OT) has not been strongly confirmed. In this study, the fugitive characteristics of EPFRs in road dust in a metropolitan city located in northwest China, and their potential oxidative toxicity were investigated. The results showed that the road dust contains Carbon-centered EPFRs with the mean mass concentration of (6.6 ± 5.0) × 10¹⁷ spins/g. EPFRs in road dust are degradable and have a half-life of 4.5 years. The water insoluble (WIS) components contribute 71% to the oxidative toxicity of road dust and show a rapid toxicity generation process, while the oxidative toxicity generation rate of water-soluble dust is more stable. Based on the positive matrix factorization (PMF) model, the contribution of EPFRs-dominated factors to Total-OT and WIS-OT is 17.3% and 33.3%, respectively. The PMF model results indicated that different types of EPFRs contributed differently to the oxidative toxicity of road dust and Carbon-centered EPFRs are more likely to participate in reactive oxygen species generation. Our results highlight that the EPFRs are an important contributor to the oxidative toxicity of atmospheric particulate matter, and their oxidative toxicity is dependent on the types of free radicals. It also provides an important insight into the influence of other potentially toxic substances on the oxidative toxicity of atmospheric PM.

The overlooked formation of environmentally persistent free radicals on particulate matter collected from biomass burning under light irradiation

BACKGROUND

The illumination process may be an important contributor to environmentally persistent free radicals (EPFRs) in atmospheric particles, but the ability of light to generate EPFRs in combustion products remains unclear.

OBJECTIVE

This paper studies the characteristics and formation mechanism of EPFRs in combustion particles after photoexcitation.

METHOD

The secondary photochemical processes and the generation and decay capability of EPFRs in size-resolved (<10 µm) biomass combustion particles were analysed by electron paramagnetic resonance (EPR) spectroscopy.

RESULT

Our results indicated that secondary EPFRs can be generated after illumination and the produced EPFRs have a lifetime of approximately 1 day. The content of secondary EPFRs after light exposure increased by 20 %-30 % compared to that of the original EPFRs. Through the analysis of components of different polarities, it was found that non-extractable substances were the main contributors to secondary EPFRs (75 %), followed by extractable organics. This study showed that metal species and quinones are important precursors for the formation of secondary EPFRs from non-extractable and extractable PM components, respectively. We found that O₂ molecules are an important factor for the formation of secondary EPFRs from organic substances without oxygen functional groups.

CONCLUSIONS

This study presents information about the effects of light and O₂ on the generation of EPFRs, and the unstable nature of secondary EPFRs has important implications for assessing the health risks of atmospheric particles.

Effects of polycyclic aromatic hydrocarbons fluoranthene on the soil aggregate stability and the possible underlying mechanism

Soil contamination by polycyclic aromatic hydrocarbons (PAHs) is an increasing problem in many countries, impacting the ecological environment's sustainable development. This study investigated the effects of fluoranthene (Fla) on soil aggregate stability. A possible mechanism for the interaction of Fla with soil aggregates was proposed by characterizing the aggregate structure. The results showed that Fla significantly improved the aggregate stability in the concentration range of 0-30.0 mg/kg. The content of macro-aggregates reached the maximum value at 10 mg/kg of Fla, which increased by 24.25% compared with the control group, while the content of large-aggregates decreased by 12.11%. Meanwhile, the mean weight diameter (MWD) and geometric mean diameter (GMD) increased by 56.63% and 37.66%, respectively. However, the macro-aggregates zeta potential value and specific surface area (SSA) decreased by 12.68% and 13.61%, respectively. The cracks of macro-aggregates were also significantly reduced. In addition, Fla-based free radicals were detected on the macro-aggregates. The absorption peak of the C-O group significantly increased, indicating that Fla may be covalently bound to the aggregates by aromatic ether bonds, which is a possible mechanism for the interaction between Fla and aggregates. This study provides theoretical support for revealing the effects of PAHs on soil.

Toxic effects and primary source of the aged micro-sized artificial turf fragments and rubber particles: Comparative studies on laboratory photoaging and actual field sampling

Numerous micro-sized artificial turf fragments (MATF) and rubber particles (MRP) are generated and accumulated during the use of the artificial playing field. However, attention has rarely been paid to the potential toxic effects of MATF and MRP on sportsmen. In this study, the active components and chemical composition of aged MATF and MRP derived from laboratory photoaging and actual field sampling were detected, and their effects on cytotoxicity were examined correspondingly. Laboratory photoaging significantly increased environmental persistent free radicals (EPFRs), reactive oxygen species (ROS) abundances and oxidative potential (OP) levels on MATF and MRP, but they have limited cytotoxicity. Unfortunately, in the actual field, aged MATF and MRP with higher heavy metals and polycyclic aromatic hydrocarbons (PAHs) contents exhibited markedly higher cytotoxicity with the survival rate of cells of 78 % and 26 % (p < 0.05), although they had lower EPFRs and ROS yields. Correlation analysis revealed that the cell viability was closely linked to heavy metals of MATF (p < 0.05), and to organic hydroperoxide (OHP), PAHs and heavy metals of MRP (p < 0.05). By systematically considering the above results, heavy metals and PAHs enriched on MATF and MRP from the surrounding environment played the important role in the cytotoxicity, which was different from conventional perspectives. Our findings demonstrate that MATF and MRP associated with an artificial turf field contain potent mixtures of pollutants and can, therefore, be relevant yet underestimated factors contributing to the health risks.

Direct toxicity of environmentally persistent free radicals to nematode Caenorhabditis elegans after excluding the concomitant chemicals

Environmentally persistent free radicals (EPFRs) have attracted extensive attention due to their potential toxicity. However, EPFRs-containing particles always coexist with their parent organic contaminants and intermediate degradation products (IM), which may have hindered the toxicity assessment of EPFRs. In this study, the toxicity of EFFRs was specifically verified after comparing the systems without EPFRs, such as the immediate mixture of catechol (CT) and particles, solutions of CT only, IM extracted from the particles, as well as particles after EPFRs quenching. Caenorhabditis elegans (C. elegans) were used as model organisms. Our results showed that EPFRs-containing particles (Si-Al-CT) exhibited significant toxicity to C. elegans, but not for the parent chemical CT and IM on the particles. Higher levels of reactive oxygen species (ROS) and malondialdehyde (MDA) in the Si-Al-CT system were attributed to the mediated generation of ·O₂^- and ·OH via EPFRs. EPFRs could increase gene expressions related not only to oxidative stress and biotransformation in C. elegans, but also to indications of disturbances in energy homeostasis, survival, proliferation, cell and embryonic development. Overall, these results confirmed the direct toxicity of EPFRs and highlighted the key role of EPFRs which may be neglected in assessing the environmental risks of organic contaminants.

Distribution, influence factors, and biotoxicity of environmentally persistent free radical in soil at a typical coking plant

Environmentally persistent free radicals (EPFRs) are emerging pollutants in contaminated soils and have attracted significant attention. Chinese coke production making a great contribution to the globe is increasingly identified as the non-ignorable source of EPFRs. However, the distribution level, influence factors, and biotoxicity of EPFRs at coking sites remain poorly understood. Herein, a typical coking plant in Tangshan, China, featuring two functional regions (the reconstructed project (RP) and elimination engineering (EE)) was used to study the existence of EPFRs. The spin density of the EPFRs in coking soils was 3.20 × 10²⁰-3.11 × 10²¹ spins/g with g-factor values of 2.0020-2.0036. The EPFRs presented higher concentrations and g-factor values in RP region than in EE region, and a mixture of carbon-centered radicals and carbon-centered radicals with adjacent oxygen atoms as well as carbon-centered radical was ascertained in the former and the latter, respectively. Correlation analysis and FT-ICR-MS results indicated that polycyclic aromatic hydrocarbons (PAH) together with other unsaturated hydrocarbons and condensed aromatic contaminants, might contribute to the EPFRs formation in the soils of RP region, whereas PAHs were the main source of EPFRs in EE region. Soil components were determined to investigate the influence factors in EPFRs formation. Cu and Fe₂O₃ were recognized as the markedly positive influence factors, while TOC had a negative impact on EPFR formation. Visible light irradiation can induce the transformation and generation of EPFRs. As representative contaminants, both toluene and 2-chlorophenol can create EPFRs in coking soil under visible light irradiation. The potential biotoxicity tests of Photobacterium phosphoreum T3 spp. showed that EPFRs from the soils diminished bacterial luminescence. Such effect was proven to be induced by the OH based on the quenching experiment. Understanding the influence factors of EPFRs formation and their biotoxicity in coking soils is critical for developing risk assessments and prevention strategies.

Characteristics and sources of environmentally persistent free radicals in PM2.5 in Dalian, Northeast China: correlation with polycyclic aromatic hydrocarbons

Environmentally persistent free radicals (EPFRs) are an emerging class of environmental hazardous contaminants that extensively, stably exist in airborne particulate matter and pose harmful effects on human health. However, there was little research about the sources of EPFRs in actual atmospheric conditions. This study reported the occurrence, characteristics, and sources of EPFRs and polycyclic aromatic hydrocarbons (PAHs) in PM_2.5 collected in Dalian, China. The concentrations of PM_2.5-bound EPFRs ranged from 1.13 × 10¹³ to 8.97 × 10¹⁵ spins/m³ (mean value: 1.14 × 10¹⁵ spins/m³). Carbon-centered radicals and carbon-centered radicals with adjacent oxygen atoms were detected. The concentration of ∑PAHs ranged from 1.09 to 76.24 ng/m³, and PAHs with high molecular weight (HMW) were predominant species in PM_2.5. Correlation of EPFRs with SO₂, NO₂, O₃, and 12 kinds of PAHs indicated that both fuel (coal and biomass) combustion and photoreaction in atmosphere influenced the concentrations of EPFR. The positive matrix factorization (PMF) model results have shown that the primary sources contributed most of the EPFRs and those of secondary sources had a little proportion. Coal combustion (52.4%) was the primary contributor of EPFRs, followed by traffic emission (22.6%), industrial sources (9.6%), and secondary sources (9.2%) during the heating period, whereas industrial emission (39.2%) was the primary contributor, followed by coal combustion (38.1%), vehicular exhaust (23.5%), and secondary sources (9.6%) during the non-heating period. The finding of the present study provides an important evidence for further study on the formation mechanism of EPFRs in actual atmospheric to control the air pollution.

Photoformation of environmentally persistent free radicals on particulate organic matter in aqueous solution: Role of anthracene and formation mechanism

Environmentally persistent free radicals (EPFRs) generated under irradiation have been widely detected in soil particles, atmospheric particles and microplastic particles, but the formation of EPFRs in water is not well understood. This study investigated the formation of EPFRs on particulate organic matter (POM) in water contaminated by anthracene (Ant) under irradiation. The photoformation and decay progress of EPFRs was represented with the help of electron paramagnetic resonance (EPR) technique on both actual POM and Fe(III)-montmorillonite simulated samples. EPR signals at the range of 10¹⁶ to 10¹⁷ spin/g were detected and the half-life time of EPFRs stored in water was at around 16.62 h and 60.80 h, much shorter than those in the air. The g factors were all larger than 2.0040, which indicated the generation of oxygen centered EPFRs. The primary intermediates were identified by gas chromatography-mass spectrometer (GC-MS) and a possible EPFR formation pathway during Ant degradation was proposed. The interaction between Ant and POM, and the hydroxylation and carbonylation of the intermediates made contributions to the generation of EPFRs. Meanwhile, the indirect photodegradation of bisphenol A (BPA) has been demonstrated by analyzing the reactive oxygen species (ROS) and photogenerated electrons in the solution with POM containing EPFRs. It is found that hydroxyl radicals (•OH) and singlet oxygen (¹O₂) were induced and might promote the photodegration. Overall, our present study provided useful information to understand the photoformation of EPFRs on POM and their fate in aqueous environments.

The First Observation of the Formation of Persistent Aminoxyl Radicals and Reactive Nitrogen Species on Photoirradiated Nitrogen-Containing Microplastics

Nitrogen-containing microplastics (N-MPs) are widely present in the atmosphere, but their potential health risks have been overlooked. In this study, the formation of persistent aminoxyl radicals (PAORs) and reactive nitrogen species (RNSs) on the N-MPs under light irradiation was investigated. After photoaging, an anisotropic triplet with the g-factor of ∼2.0044, corresponding to PAORs, was detected on the nonaromatic polyamide (PA) rather than amino resin (AmR) by electron paramagnetic resonance and confirmed by density functional theory calculations. The generated amine oxide portions on the photoaged PA were identified using X-ray photoelectron spectroscopy and Raman spectroscopy, which were considered to be the main structural basis/precursors of a PAOR. Surprisingly, RNSs were also observed on the irradiated PA. The generated ·NO due to the aphotolysis of nitrone groups simultaneously reacted with peroxide radicals and O₂·^- to yield ·NO₂ and peroxynitrite, respectively, which were responsible for peroxyacyl nitrates (PAN) and CO₃·^- formation. Besides, a significantly higher oxidative potential and reductive potential were observed for the aged PA than AmR, which is assigned to the abundant RNSs, organic hydroperoxides and PANs, and a strong ability to transfer electrons from PAOR, respectively. This work provides important information for the potential risks of airborne N-MPs and may serve as a guide for future toxicological assessments.

Phytosampling-a supplementary tool for particulate matter (PM) speciation characterization

Ambient air particulate matter (PM) and PM-associated environmentally persistent free radicals (EPFRs) have been documented to contribute to pollution-related health effects. Studies of ambient air PM potentially bear artifacts stemming from the collection methods. We have investigated the applicability of PM phytosampling (PHS) as a supplementary tool to a classic PM sampler in respect of achieving better PM chemical composition assessment (primarily organic fraction). Phytosampling is a static PM collection method relying on the particle entrapment by the plant's leaf through electrostatic forces and surface trichomes. We have investigated the differences in the EPFR and polycyclic aromatic hydrocarbon (PAH) speciation and concentration on ambient air PM for PHS and high-volume PM sampler (HVS). The advantages of PHS are easy particle recovery from the matrix, collection under natural environmental conditions, and the ability to apply a dense collection network to accurately represent spatial pollutant distribution. The experimental results show that the PHS can provide valuable speciation information, sometimes different from that observed for HVS. For PM collected by PHS, we detected the larger contribution of oxygen-centered EPFRs, different decay behavior, and more consistent PAH distribution between different PM sizes compared to the PM from HVS. These results indicate that the isolation of samples from the ambient during HVS sampling and exposure to high-volume airflow may alter the chemical composition of the samples, while the PHS method could provide details on the original speciation and concentration and be more representative of the PM surface. However, PHS cannot evaluate an absolute air concentration of PM, so it serves as an excellent supplementary tool to work in conjunction with the standard PM collection method.

Formation of Environmentally Persistent Free Radicals during Thermochemical Processes and their Correlations with Unintentional Persistent Organic Pollutants

Attention is increasingly being paid to environmentally persistent free radicals (EPFRs), which are organic pollutants with the activities of free radicals and stabilities of organic pollutants. EPFRs readily form during thermal processes through the decomposition of organic precursors such as phenols, halogenated phenols, and quinone-type molecules, which are also important precursors of toxic unintentionally produced persistent organic pollutants (UPOPs). We have found that EPFRs are important intermediates for UPOP formation during thermal-related processes. However, interest in EPFRs is currently mostly focused on the toxicities and formation mechanisms of EPFRs themselves. Little information is available on the important roles EPFRs play in toxic UPOP formation during thermal processes. Here, we review the mechanisms involved in EPFR formation and transformation into UPOPs during thermal processes. The review is focused on typical EPFRs, including cyclopentadiene, phenoxy, and semiquinone radicals. The reaction temperature, metal species present, and oxygen concentration strongly affect EPFR and UPOP formation during thermal-related processes. Gaps in current knowledge and future directions for research into EPFR and UPOP formation, transformation, and control are presented. Understanding the relationships between EPFRs and UPOPs will allow synergistic control strategies to be developed for thermal-related industrial sources of EPFRs and UPOPs.

Surface-Radical Mobility Test by Self-Sorted Recombination: Symmetrical Product upon Recombination (SPR)

We describe here a study of the mobility of the alkoxy radical on a surface by detection of its recombination product. A novel method called symmetrical product recombination (SRP) uses an unsymmetrical peroxide that upon sensitized homolysis recombines to a symmetrical product [R'OOR → R'O^•↑ + ^•OR → ROOR]. This allows for self-sorting of the radical to enhance the recombination path to a symmetrical product, which has been used to deduce surface migratory aptitude. SPR also provides a new opportunity for mechanistic studies of interfacial radicals, including monitoring competition between radical recombination versus surface hydrogen abstraction. This is an approach that might work for other surface-borne radicals on natural and artificial particles.

Atmospheric conditions and composition that influence PM2.5 oxidative potential in Beijing, China

Epidemiological studies have consistently linked exposure to PM_2.5 with adverse health effects. The oxidative potential (OP) of aerosol particles has been widely suggested as a measure of their potential toxicity. Several acellular chemical assays are now readily employed to measure OP; however, uncertainty remains regarding the atmospheric conditions and specific chemical components of PM_2.5 that drive OP. A limited number of studies have simultaneously utilised multiple OP assays with a wide range of concurrent measurements and investigated the seasonality of PM_2.5 OP. In this work, filter samples were collected in winter 2016 and summer 2017 during the atmospheric pollution and human health in a Chinese megacity campaign (APHH-Beijing), and PM_2.5 OP was analysed using four acellular methods: ascorbic acid (AA), dithiothreitol (DTT), 2,7-dichlorofluorescin/hydrogen peroxidase (DCFH) and electron paramagnetic resonance spectroscopy (EPR). Each assay reflects different oxidising properties of PM_2.5, including particle-bound reactive oxygen species (DCFH), superoxide radical production (EPR) and catalytic redox chemistry (DTT/AA), and a combination of these four assays provided a detailed overall picture of the oxidising properties of PM_2.5 at a central site in Beijing. Positive correlations of OP (normalised per volume of air) of all four assays with overall PM_2.5 mass were observed, with stronger correlations in winter compared to summer. In contrast, when OP assay values were normalised for particle mass, days with higher PM_2.5 mass concentrations (μgm^-3) were found to have lower mass-normalised OP values as measured by AA and DTT. This finding supports that total PM_2.5 mass concentrations alone may not always be the best indicator for particle toxicity. Univariate analysis of OP values and an extensive range of additional measurements, 107 in total, including PM_2.5 composition, gas-phase composition and meteorological data, provided detailed insight into the chemical components and atmospheric processes that determine PM_2.5 OP variability. Multivariate statistical analyses highlighted associations of OP assay responses with varying chemical components in PM_2.5 for both mass- and volume-normalised data. AA and DTT assays were well predicted by a small set of measurements in multiple linear regression (MLR) models and indicated fossil fuel combustion, vehicle emissions and biogenic secondary organic aerosol (SOA) as influential particle sources in the assay response. Mass MLR models of OP associated with compositional source profiles predicted OP almost as well as volume MLR models, illustrating the influence of mass composition on both particle-level OP and total volume OP. Univariate and multivariate analysis showed that different assays cover different chemical spaces, and through comparison of mass- and volume-normalised data we demonstrate that mass-normalised OP provides a more nuanced picture of compositional drivers and sources of OP compared to volume-normalised analysis. This study constitutes one of the most extensive and comprehensive composition datasets currently available and provides a unique opportunity to explore chemical variations in PM_2.5 and how they affect both PM_2.5 OP and the concentrations of particle-bound reactive oxygen species.

Enhanced cytotoxicity of photoaged phenol-formaldehyde resins microplastics: Combined effects of environmentally persistent free radicals, reactive oxygen species, and conjugated carbonyls

Phenol-formaldehyde resin microplastic (PF-MP) is one of the major inhalable microplastics in environments released from the manufacture, processing and usage of PF materials. The associated toxicities of PF-MP might be affected by photoaging. In this study, the dynamic evolutions of the oxidative potential (OP) and redox-active species, including environmentally persistent free radicals (EPFRs), reactive oxygen species (ROS), peroxides and conjugated carbonyls, as well as the associated cytotoxicity of PF-MP were systematically investigated as a result of the simulated sunlight irradiation. As the photoaging time extended, the OP of PF-MP increased. The contents of the produced conjugated carbonyls, ROS and PF-bound EPFRs due to light irradiation increased as well, and displayed significant correlations with the OP (Spearman r > 0.6, p < 0.05). The photoaged PF-MP distinctly increased the cellular ROS and reduced the cell viability of human lung epithelial adenocarcinoma cells (A549). The cytotoxicity of PF-MP showed a similar trend with the OP level in PF-MP, suggesting that the produced active species induced the in vitro toxicities. The results not only highlight the adverse health effects of photoaged PF-MP, but also provide new perspectives for the environmental risks of airborne MPs.

Photoformation of persistent free radicals on a montmorillonite-humic acid complex simulated as particulate organic matter in an aqueous solution

This study investigates the formation of persistent free radicals (PFRs) on particulate organic matter (POM) under irradiation in water. A montmorillonite-humic acid complex (Mnt-HA complex) was prepared to simulate POM, and the generated PFRs were detected by the electron paramagnetic resonance (EPR) technique. EPR signals with the trend of an initial increase and then a decrease were observed under irradiation for 8 days, and the g factors were in the range of 2.0034-2.0039, which indicated the generation of carbon-centered radicals with electrophilic moieties. Different concentrations and types of halophenols and transition-metal ions were respectively adsorbed on the Mnt-HA complex to probe their influence on the formation of PFRs. The amount of PFRs generated in the Mnt-HA complexes was in the order: 2-bromophenol (2-BP) > 2,4-dibromophenol (2,4-DBP) > 2,4-dichlorophenol (2,4-DCP), which implied that halogen substitution and the number of substituents in the halophenols could affect the generation of PFRs. The effects of transition-metal ions that resulted in the reduction of PFRs when adsorbed on the Mnt-HA complex were as follows: Fe3+ > Zn2+ > Cu2+ > Mn2+, and this is in agreement with their redox capacity. Analyzing the induced generation of reactive oxygen species (ROS) and electrons on POM, it is found that halophenols and transition metal ions also affected this process under irradiation. These findings indicate that the photoformation of PFRs on POM could be a source of PFRs in aqueous environments and requires further attention.

Short-term exposure to ZnO/MCB persistent free radical particles causes mouse lung lesions via inflammatory reactions and apoptosis pathways

Environmentally persistent free radicals (EPFRs) are easily generated in the combustion processes of municipal solid waste (MSW) and can cause adverse effects on human health. This study focuses on understanding the toxicity of EPFR particles (ZnO/MCB containing EPFRs) to human bronchial epithelial cell lines BEAS-2B and 16HBE, murine macrophages Raw264.7, and the lung of BALB/c mice after a short exposure (7 days). Exposure of BEAS-2B, 16HBE, and Raw264.7 cells to ZnO/MCB particles significantly increased the reactive oxygen species (ROS) production and perturbed levels of intracellular redox conditions (decreased the intracellular GSH level and the activity of cytosolic SOD, and stimulated oxidative stress related proteins such as HO-1 and Nrf2). EPFR particles decreased the mitochondrial membrane potential (MMP) and induced cell apoptosis, including the activation of Caspase-3, Bax, and Bcl-2 apoptotic signalling pathways. A signature inflammatory condition was observed in both cell models and the mouse model for lung lesions. Our data suggest that EPFRs in particles have greater toxicity to lung cells and tissues that are potential health hazards to human lung.