Radical Formation by Fine Particulate Matter Associated with Highly Oxygenated Molecules

Seasonal characterization of chemical and optical properties of water-soluble organic aerosol in Beijing

Water-soluble organic aerosol (WSOA) plays a crucial role in altering radiative forcing and impacting human health. However, our understanding of the seasonal variations of WSOA in Chinese megacities after the three-year clean air action plan is limited. In this study, we analyzed PM2.5 filter samples collected over one year (2020-2021) in Beijing to characterize the seasonal changes in the chemical and optical properties of WSOA using an offline aerosol mass spectrometer along with spectroscopy techniques. The mean mass concentration of WSOA during the observation period was 8.84 ± 7.12 μg m-3, constituting approximately 64-67 % of OA. Our results indicate the contribution of secondary OA (SOA) increased by 13-28 % due to a substantial reduction in primary emissions after the clean air action plan. The composition of WSOA exhibited pronounced seasonal variations, with a predominant contribution from less oxidized SOA in summer (61 %) and primary OA originating from coal combustion and biomass burning during the heating season (34 %). The mass absorption efficiency of WSOA at 365 nm in winter was nearly twice that in summer, suggesting that WSOA from primary emissions possesses a stronger light-absorbing capability than SOA. On average, water-soluble brown carbon accounted for 33-48 % of total brown carbon absorption. Fluorescence analysis revealed humic-like substances as the most significant fluorescence component of WSOA, constituting 82 %. Furthermore, both absorption and fluorescence chromophores were associated with nitrogen-containing compounds, highlighting the role of nitrogen-containing species in influencing the optical properties of WSOA. The results are important for chemical transport models to accurately simulate the WSOA and its climate effects.

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.

Organic Peroxides in Aerosol: Key Reactive Intermediates for Multiphase Processes in the Atmosphere

Organic peroxides (POs) are organic molecules with one or more peroxide (-O-O-) functional groups. POs are commonly regarded as chemically labile termination products from gas-phase radical chemistry and therefore serve as temporary reservoirs for oxidative radicals (HOx and ROx) in the atmosphere. Owing to their ubiquity, active gas-particle partitioning behavior, and reactivity, POs are key reactive intermediates in atmospheric multiphase processes determining the life cycle (formation, growth, and aging), climate, and health impacts of aerosol. However, there remain substantial gaps in the origin, molecular diversity, and fate of POs due to their complex nature and dynamic behavior. Here, we summarize the current understanding on atmospheric POs, with a focus on their identification and quantification, state-of-the-art analytical developments, molecular-level formation mechanisms, multiphase chemical transformation pathways, as well as environmental and health impacts. We find that interactions with SO2 and transition metal ions are generally the fast PO transformation pathways in atmospheric liquid water, with lifetimes estimated to be minutes to hours, while hydrolysis is particularly important for α-substituted hydroperoxides. Meanwhile, photolysis and thermolysis are likely minor sinks for POs. These multiphase PO transformation pathways are distinctly different from their gas-phase fates, such as photolysis and reaction with OH radicals, which highlights the need to understand the multiphase partitioning of POs. By summarizing the current advances and remaining challenges for the investigation of POs, we propose future research priorities regarding their origin, fate, and impacts in the atmosphere.

Dynamic Wood Smoke Aerosol Toxicity during Oxidative Atmospheric Aging

Wildfires are a major source of biomass burning aerosol to the atmosphere, with their incidence and intensity expected to increase in a warmer future climate. However, the toxicity evolution of biomass burning organic aerosol (BBOA) during atmospheric aging remains poorly understood. In this study, we report a unique set of chemical and toxicological metrics of BBOA from pine wood smoldering during multiphase aging by gas-phase hydroxyl radicals (OH). Both the fresh and OH-aged BBOA show activity relevant to adverse health outcomes. The results from two acellular assays (DTT and DCFH) show significant oxidative potential (OP) and reactive oxygen species (ROS) formation in OH-aged BBOA. Also, radical concentrations in the aerosol assessed by electron paramagnetic resonance (EPR) spectroscopy increased by 50% following heterogeneous aging. This enhancement was accompanied by a transition from predominantly carbon-centered radicals (85%) in the fresh aerosol to predominantly oxygen-centered radicals (76%) following aging. Both the fresh and aged biomass burning aerosols trigger prominent antioxidant defense during the in vitro exposure, indicating the induction of oxidative stress by BBOA in the atmosphere. By connecting chemical composition and toxicity using an integrated approach, we show that short-term aging initiated by OH radicals can produce biomass burning particles with a higher particle-bound ROS generation capacity, which are therefore a more relevant exposure hazard for residents in large population centers close to wildfire regions than previously studied fresh biomass burning emissions.

Secondary organic aerosol formation from atmospheric reactions of anisole and associated health effects

Anisole (methoxybenzene) represents an important marker compound of lignin pyrolysis and a starting material for many chemical products. In this study, secondary organic aerosols (SOA) formed by anisole via various atmospheric processes, including homogeneous photooxidation with varying levels of OH• and NO_x and subsequent heterogeneous NO₃• dark reactions, were investigated. The yields of anisole SOA, particle-bound organoperoxides, particle-induced oxidative potential (OP), and cytotoxicity were characterized in view of the atmospheric fate of the anisole precursor. Anisole SOA yields ranged between 0.12 and 0.35, depending on the reaction pathways and aging degrees. Chemical analysis of the SOA suggests that cleavage of the benzene ring is the main reaction channel in the photooxidation of anisole to produce low-volatility, highly oxygenated small molecules. Fresh anisole SOA from OH• photooxidation are more light-absorbing and have higher OP and organoperoxide content. The high correlation between SOA OP and organoperoxide content decreases exponentially with the degree of OH• aging. However, the contribution of organoperoxides to OP is minor (<4%), suggesting that other, non-peroxide oxidizers play a central role in anisole SOA OP. The particle-induced OP and particulate organoperoxides yield both reach a maximum value after ∼2 days' of photooxidation, implicating the potential long impact of anisole during atmospheric transport. NO_x-involved photooxidation and nighttime NO₃• reactions facilitate organic nitrate formation and enhance particle light absorption. High NO_x levels suppress anisole SOA formation and organoperoxides yield in photooxidation, with decreased aerosol OP and cellular oxidative stress. In contrast, nighttime aging significantly increases the SOA toxicity and reactive oxygen species (ROS) generation in lung cells. These dynamic properties and the toxicity of anisole SOA advocate consideration of the complicated and consecutive aging processes in depicting the fate of VOCs and assessing the related effects in the atmosphere.

Effects of Nitrogen Oxides on the Production of Reactive Oxygen Species and Environmentally Persistent Free Radicals from α-Pinene and Naphthalene Secondary Organic Aerosols

Reactive oxygen species (ROS) and environmentally persistent free radicals (EPFR) play an important role in chemical transformation of atmospheric aerosols and adverse aerosol health effects. This study investigated the effects of nitrogen oxides (NO_x) during photooxidation of α-pinene and naphthalene on the EPFR content and ROS formation from secondary organic aerosols (SOA). Electron paramagnetic resonance (EPR) spectroscopy was applied to quantify EPFR content and ROS formation. While no EPFR were detected in α-pinene SOA, we found that naphthalene SOA contained about 0.7 pmol μg^-1 of EPFR, and NO_x has little influence on EPFR concentrations and oxidative potential. α-Pinene and naphthalene SOA generated under low NO_x conditions form OH radicals and superoxide in the aqueous phase, which was lowered substantially by 50-80% for SOA generated under high NO_x conditions. High-resolution mass spectrometry analysis showed the substantial formation of nitroaromatics and organic nitrates in a high NO_x environment. The modeling results using the GECKO-A model that simulates explicit gas-phase chemistry and the radical 2D-VBS model that treats autoxidation predicted reduced formation of hydroperoxides and enhanced formation of organic nitrates under high NO_x due to the reactions of peroxy radicals with NO_x instead of their reactions with HO₂. Consistently, the presence of NO_x resulted in the decrease of peroxide contents and oxidative potential of α-pinene SOA.

Characterization and Quantification of Particle-Bound Criegee Intermediates in Secondary Organic Aerosol

The ozonolysis of alkenes contributes substantially to the formation of secondary organic aerosol (SOA), which are important modulators of air quality and the Earth's climate. Criegee intermediates (CIs) are abundantly formed through this reaction. However, their contributions to aerosol particle chemistry remain highly uncertain. In this work, we present the first application of a novel methodology, using spin traps, which simultaneously quantifies CIs produced from the ozonolysis of volatile organic compounds in the gas and particle phases. Only the smallest CI with one carbon atom was detected in the gas phase of a β-caryophyllene ozonolysis reaction system. However, multiple particle-bound CIs were observed in β-caryophyllene SOA. The concentration of the most abundant CI isomer in the particle phase was estimated to constitute ∼0.013% of the SOA mass under atmospherically relevant conditions. We also demonstrate that the lifetime of CIs in highly viscous SOA particles is at least on the order of minutes, substantially greater than their gas-phase lifetime. The confirmation of substantial concentrations of large CIs with elongated lifetimes in SOA raises new questions regarding their influence on the chemical evolution of viscous SOA particles, where CIs may be a previously underestimated source of reactive species.

Seasonal variation of water-soluble brown carbon in Qingdao, China: Impacts from marine and terrestrial emissions

Brown carbon (BrC) has been attracting more and more attention owing to its significant effects on climate. However, the limited knowledge on its chemical composition and sources limits the precision of aerosol radiative forcing estimated by climate models. In this study, the chemical components of PM_2.5 and optical properties of water-soluble BrC (WS-BrC) were investigated from atmospheric particles collected in summer and winter in Qingdao, China. On the whole, though there were slight diurnal variations, seasonal differences were more obvious. Due to the influence of emission sources and meteorological conditions, the heavier pollution of carbonaceous aerosols occurred in winter. By comparison, the absorption Ångström exponent (AAE) and mass absorption efficiency of WS-BrC at 365 nm (MAE₃₆₅) showed that WS-BrC in winter had stronger wavelength dependence and light absorption capacity, which might be associated with biomass burning source contributions. This was further confirmed by a strong correlation between the light absorption coefficient at 365 nm (Abs₃₆₅) and non-sea salt K⁺, an indicator for biomass burning emissions. Four fluorescent components (C1∼C4) with high unsaturation in water-soluble organic carbon (WSOC) were identified by excitation-emission matrix fluorescence spectroscopy combined with parallel factor analysis method, which showed that WSOC in Qingdao was mainly related to humic-like chromophores. It is worth noting that C1 was similar to the water-soluble chromophore of simulated marine aerosols, which proved that marine emissions do have a certain impact on atmospheric particulate matter in coastal areas. In addition, the results of source analysis showed that WS-BrC originated from different terrestrial sources in different seasons. The current results may help to improve the knowledge of optical properties of WS-BrC in coastal cities, optimize the global climate model and formulate air management policies.

Characteristics of Environmentally Persistent Free Radicals in PM2.5 and the Influence of Air Pollutants in Shihezi, Northwestern China

Environmentally persistent free radicals (EPFRs) are a kind of hazardous substance that exist stably in the atmosphere for a long time. EPFRs combined with fine particulate matter (PM_2.5) can enter the human respiratory tract through respiration, causing oxidative stress and DNA damage, and they are also closely related to lung cancer. In this study, the inhalation risk for EPFRs in PM_2.5 and factors influencing this risk were assessed using the equivalent number of cigarette tar EPFRs. The daily inhalation exposure for EPFRs in PM_2.5 was estimated to be equivalent to 0.66–8.40 cigarette tar EPFRs per day. The concentration level and species characteristics were investigated using electron paramagnetic resonance spectroscopy. The concentration of EPFRs in the study ranged from 1.353–4.653 × 10¹³ spins/g, and the types of EPFRs were mainly oxygen- or carbon-centered semiquinone-type radicals. Our study showed that there is a strong correlation between the concentrations of EPFRs and conventional pollutants, except for sulfur dioxide. The major factors influencing EPFR concentration in the atmosphere were temperature and wind speed; the higher the temperature and wind speed, the lower the concentration of EPFRs. The findings of this study provide an important basis for further research on the formation mechanism and health effects of EPFRs.

Impact of COVID-19 Lockdown on Oxidative Potential of Particulate Matter: Case of Athens (Greece)

This work evaluates the aerosol oxidative potential (OP) and its changes from modified air pollution emissions during the COVID-19 lockdown period in 2020, with the intent of elucidating the contribution of aerosol sources and related components to aerosol OP. For this, daily particulate matter (PM) samples at an urban background site were collected and analyzed with a chemical (acellular) assay based on Dithiothreitol (DTT) during the COVID-19 restriction period in Athens (Greece). The obtained time-series of OP, PM2.5, organic matter (OM) and SO42− of the pre-, post- and lockdown periods were also compared to the data of the same time periods during the years 2017–2019. Even though all traffic-related emissions have been significantly reduced during the lockdown period (by 30%), there is no reduction in water-soluble OP, organics and sulfate concentrations of aerosol during 2020. The results reveal that the decrease in traffic was not sufficient to drive any measurable change on OP, suggesting that other sources—such as biomass burning and secondary aerosol from long-range transport, which remained unchanged during the COVID lockdown—are the main contributors to OP in Athens, Greece.

Isomer-Resolved Reactivity of Organic Peroxides in Monoterpene-Derived Secondary Organic Aerosol

Organic peroxides play a vital role in the formation, evolution, and health impacts of atmospheric aerosols, yet their molecular composition and fate in the particle phase remain poorly understood. Here, we identified, using iodometry-assisted liquid chromatography mass spectrometry, a large suite of isomer-resolved peroxide monomers (C_8-10H_12-18O_5-8) and dimers (C_15-20H_22-34O_5-14) in secondary organic aerosol formed from ozonolysis of the most abundant monoterpene (α-pinene). Combining aerosol isothermal evaporation experiments and multilayer kinetic modeling, bulk peroxides were found to undergo rapid particle-phase chemical transformation with an average lifetime of several hours under humid conditions, while the individual peroxides decompose on timescales of half an hour to a few days. Meanwhile, the majority of isomeric peroxides exhibit distinct particle-phase behaviors, highlighting the importance of the characterization of isomer-resolved peroxide reactivity. Furthermore, the reactivity of most peroxides increases with aerosol water content faster in a low relative humidity (RH) range than in a high RH range. Such non-uniform water effects imply a more important role of water as a plasticizer than as a reactant in influencing the peroxide reactivity. The high particle-phase reactivity of organic peroxides and its striking dependence on RH should be considered in atmospheric modeling of their fate and impacts on aerosol chemistry and health effects.

Ambient volatile organic compounds in tropical environments: Potential sources, composition and impacts - A review

Volatile organic compounds (VOCs) are widely recognized to affect the environment and human health. This review provides a comprehensive presentation of the types and levels of VOCs, their sources and potential effects on human health and the environment based on past and current observations made at tropical sites. Isoprene was found to be the dominant biogenic VOC in the tropics. Tropical broad leaf evergreen trees are the main emitters of isoprene, making up more than 70% of the total emissions. The VOCs found in the tropical remote marine atmosphere included isoprene (>100 ppt), dimethyl sulfide (≤100 ppt) and halocarbons, i.e. bromoform (≤8.4 ppt), dibromomethane (≤2.7 ppt) and dibromochloromethane (≤1.6 ppt). VOCs such as benzene, toluene, ethylbenzene and xylene (BTEX) are the most monitored anthropogenic VOCs and are present mainly due to motor vehicles emissions. Additionally, biomass burning contributes to anthropogenic VOCs, especially high molecular weight VOCs, e.g. methanol and acetonitrile. The relative contributions of VOC species to ozone are determined through the level of the Ozone Formation Potential (OFP) of different species. Emissions of VOCs (e.g. very short-lived halogenated gases) in the tropics are capable of contributing to stratospheric ozone depletion. BTEX has been identified as the main types of VOCs that are associated with the cancer risk in urban areas in tropical regions. Finally, future studies related to VOCs in the tropics and their associated health risks are needed to address these concerns.

Iron-Facilitated Organic Radical Formation from Secondary Organic Aerosols in Surrogate Lung Fluid

Respiratory deposition of secondary organic aerosols (SOA) and iron may lead to the generation of reactive oxygen species and free radicals in lung fluid to cause oxidative stress, but their underlying mechanism and formation kinetics are not well understood. Here we demonstrate substantial formation of organic radicals in surrogate lung fluid (SLF) by mixtures of Fe2+ and SOA generated from photooxidation of isoprene, α-terpineol, and toluene. The molar yields of organic radicals by SOA are measured to be 0.03-0.5% in SLF, which are 5-10 times higher than in water. We observe that Fe2+ enhances organic radical yields dramatically by a factor of 20-80, which can be attributed to Fe2+-facilitated decomposition of organic peroxides, in consistency with a positive correlation between peroxide contents and organic radical yields. Ascorbate mediates redox cycling of iron ions to sustain organic peroxide decomposition, as supported by kinetic modeling reproducing time- and concentration-dependence of organic radical formation as well as additional experiments observing the formation of Fe2+ and ascorbate radicals in mixtures of ascorbate and Fe3+. •OH and superoxide are found to be scavenged by antioxidants efficiently. These findings have implications on the role of organic radicals in oxidative damage and lipid peroxidation.

Water-soluble brown carbon in atmospheric aerosols along the transport pathway of Asian dust: Optical properties, chemical compositions, and potential sources

As an important type of light-absorbing aerosol, brown carbon (BrC) has the potential to affect the atmospheric photochemistry and Earth's energy budget. A comprehensive field campaign was carried out along the transport pathway of Asian dust during the spring of 2016, including a desert site (Erenhot), a rural site (Zhangbei), and an urban site (Jinan), in northern China. Optical properties, bulk chemical compositions, and potential sources of water-soluble brown carbon (WS-BrC) were investigated in atmospheric total suspended particulate (TSP) samples. Samples from Zhangbei had higher mass absorption efficiency at 365 nm (MAE₃₆₅, 1.32 ± 0.34 m² g^-1) than those from Jinan (1.00 ± 0.23 m² g^-1) and Erenhot (0.84 ± 0.30 m² g^-1). Compere to the non-dust samples, elevated water-soluble organic carbon (WSOC) concentrations and MAE₃₆₅ values of dust samples from Erenhot are related to the input of high molecular weight organic compounds and biogenic matter from the Gobi Desert, while lower values from Zhangbei and Jinan are attributed to the dilution effect caused by strong northwesterly winds. Based on fluorescence excitation-emission matrix spectra and parallel factor analysis, two humic-like (C1 and C2) and two protein-like (C3 and C4) substances were identified. Together, C1 and C2 accounted for ~64% of total fluorescence intensity at the highly polluted urban Jinan site; C3 represented ~45% at the rural Zhangbei site where local biomass burning affects; and C4 contributed ~24% in the desert region (Erenhot) due to dust-sourced biogenic substances. The relative absorptive forcing of WS-BrC compared to black carbon at 300-400 nm was about 31.3%, 13.9%, and 9.2% during non-dust periods at Erenhot, Zhangbei, and Jinan, respectively, highlighting that WS-BrC may significantly affect the radiative balance of Earth's climate system and should be included in radiative forcing models.

Fine Particulate Matter-Induced Oxidative Stress Mediated by UVA-Visible Light Leads to Keratinocyte Damage

The human skin is exposed to various environmental factors including solar radiation and ambient air pollutants. Although, due to its physical and biological properties, the skin efficiently protects the body against the harm of environmental factors, their excessive levels and possible synergistic action may lead to harmful effects. Among particulate matter present in ambient air pollutants, PM2.5 is of particular importance for it can penetrate both disrupted and intact skin, causing adverse effects to skin tissue. Although certain components of PM2.5 can exhibit photochemical activity, only a limited amount of data regarding the interaction of PM2.5 with light and its effect on skin tissue are available. This study focused on light-induced toxicity in cultured human keratinocytes, which was mediated by PM2.5 obtained in different seasons. Dynamic Light Scattering (DLS) and Atomic Force Microscopy (AFM) were employed to determine sizes of the particles. The ability of PM2.5 to photogenerate free radicals and singlet oxygen was studied using EPR spin-trapping and time-resolved singlet oxygen phosphorescence, respectively. Solar simulator with selected filters was used as light source for cell treatment to model environmental lightning conditions. Cytotoxicity of photoexcited PM2.5 was analyzed using MTT assay, PI staining and flow cytometry, and the apoptotic pathway was further examined using Caspase-3/7 assay and RT-PCR. Iodometric assay and JC-10 assay were used to investigate damage to cell lipids and mitochondria. Light-excited PM2.5 were found to generate free radicals and singlet oxygen in season-dependent manner. HaCaT cells containing PM2.5 and irradiated with UV-Vis exhibited oxidative stress features-increased peroxidation of intracellular lipids, decrease of mitochondrial membrane potential, enhanced expression of oxidative stress related genes and apoptotic cell death. The data indicate that sunlight can significantly increase PM2.5-mediated toxicity in skin cells.

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.

Superoxide Formation from Aqueous Reactions of Biogenic Secondary Organic Aerosols

Reactive oxygen species (ROS) play a central role in aqueous-phase processing and health effects of atmospheric aerosols. Although hydroxyl radical (^•OH) and hydrogen peroxide (H₂O₂) are regarded as major oxidants associated with secondary organic aerosols (SOA), the kinetics and reaction mechanisms of superoxide (O₂^•-) formation are rarely quantified and poorly understood. Here, we demonstrate a dominant formation of O₂^•- with molar yields of 0.01-0.03% from aqueous reactions of biogenic SOA generated by ^•OH photooxidation of isoprene, β-pinene, α-terpineol, and d-limonene. The temporal evolution of ^•OH and O₂^•- formation is elucidated by kinetic modeling with a cascade of aqueous reactions including the decomposition of organic hydroperoxides, ^•OH oxidation of primary or secondary alcohols, and unimolecular decomposition of α-hydroxyperoxyl radicals. Relative yields of various types of ROS reflect a relative abundance of organic hydroperoxides and alcohols contained in SOA. These findings and mechanistic understanding have important implications on the atmospheric fate of SOA and particle-phase reactions of highly oxygenated organic molecules as well as oxidative stress upon respiratory deposition.

New particle formation (NPF) events in China urban clusters given by sever composite pollution background

New Particle Formation (NPF) refers to transformation of gaseous precursors in the atmosphere due to nucleation and subsequent growth process through physicochemical interaction. It has generated a lot of interest due to its profound impact on global and regional environment, climate and human health. We reviewed the studies on NPF in three city clusters of China: the North China Plain, the Yangtze River Delta and the Pearl River Delta obtained through experiment simulations (e.g., chamber simulation, flow-tube simulation, etc.), field observations, and numerical simulations. Due to its atmospheric background pollution and strong oxidation capacities resulting in high source rate of precursors, China's atmosphere possesses challenges different from those evaluated in previous studies on cleaning sites and other developing countries. Hence, NPF events can simultaneously exhibit high condensable sink, formation rate and growth rate. In addition, the high intensity of anthropogenic emissions in urban China has led to greater diversity of pollutant species involved in NPF nucleation and subsequent growth, compared to the dominant role of biogenic precursors at cleaning sites. Differences in geographical location and industrial structure also lead to significant distinctions in NPF characteristics of the three city clusters. Consequently, the lack of understanding of nucleation mechanism of complexly polluted background sites makes the global and regional climate models with submodels based on clean background have enormous uncertainty when applied to urban China. The establishment of a mature research ecosystem including field observations, laboratory simulations and numerical simulations is the key to the breakthrough of NPF research in China.

Proteome-wide effects of naphthalene-derived secondary organic aerosol in BEAS-2B cells are caused by short-lived unsaturated carbonyls

Exposure to air pollution causes adverse health outcomes, but the toxicity mechanisms remain unclear. Here, we investigated the dynamic toxicities of naphthalene-derived secondary organic aerosol (NSOA) in a human bronchial epithelial cell line (BEAS-2B) and identified the chemical components responsible for toxicities. The chemical composition of NSOA was found to vary with six simulated atmospheric aging conditions (C₁-C₆), as characterized by high-resolution mass spectrometry and ion mobility mass spectrometry. Global proteome profiling reveals dynamic evolution in toxicity: Stronger proteome-wide impacts were detected in fresh NSOA, but the effects declined along with atmospheric aging. While Nrf2-regulated proteins (e.g., NQO1) were significantly up-regulated, the majority (78 to 97%) of proteins from inflammation and other pathways were down-regulated by NSOA exposure (e.g., Rho GTPases). This pattern is distinct from the reactive oxygen species (ROS)-mediated toxicity pathway, and an alternative cysteine reaction pathway was revealed by the decreased abundance of proteins (e.g., MT1X) prone to posttranslational thiol modification. This pathway was further validated by observing decreased Nrf2 response in reporter cells, after preincubating NSOA with cysteine. Ethynyl-naphthalene probe was employed to confirm the alkylation of cellular proteome thiols on the proteome-wide level by fresh NSOA via in-gel fluorescence imaging. Nontarget analysis identified several unsaturated carbonyls, including naphthoquinones and hydroxylated naphthoquinones, as the toxic components responsible for cysteine reactivity. Our study provides insights into the dynamic toxicities of NSOA during atmospheric aging and identifies short-lived unsaturated carbonyls as the predominant toxic components at the posttranslational level.

Seasonal variation characteristics of hydroxyl radical pollution and its potential formation mechanism during the daytime in Lanzhou

Hydroxyl free radicals (OH radicals) play the main role in atmospheric chemistry and their involving reactions are the dominant rate determining step in the formation of secondary fine particulate matter and in the removal of air pollutants from the atmosphere. In this paper, we studied the seasonal variation characteristics of OH radicals during the daytime in Lanzhou and explored the potential formation mechanism of high concentration OH radicals. We found that the OH radicals in four seasons was 2.7 × 10⁶, 2.6 × 10⁶, 3.1 × 10⁶, and 2.2 × 10⁶ cm^-3, respectively. Since the rainfall was concentrated in summer, the wet deposition had a significant effect on removing OH radicals. Among the four pollutants (including ozone (O₃), volatile organic compounds (VOCs), nitrogen dioxide (NO₂) and fine particulate matter (PM_2.5)), the variation of OH radicals were closely related to ozone concentration especially in spring and summer. In autumn, the correlation between PM_2.5 and OH radicals were the closest among the observing pollutants and its formation mechanism was different conventional regeneration pathway. In Event 1, high concentration of ozone was the main source of OH radicals; under the high humidity condition, except for ozone, the multiple factors including VOCs, NO₂ and PM_2.5 interplayed and leaded to the Event 2.

Aqueous-Phase Decomposition of Isoprene Hydroxy Hydroperoxide and Hydroxyl Radical Formation by Fenton-like Reactions with Iron Ions

Isoprene hydroxy hydroperoxides (ISOPOOH) formed by the photooxidation of isoprene under low-NO conditions play an important role in the formation and evolution of secondary organic aerosols, yet multiphase processes of ISOPOOH are poorly understood. By applying electron paramagnetic resonance spectroscopy, we observe that ISOPOOH undergoes aqueous-phase decomposition upon interacting with Fe(II) ions to form OH and organic radicals at room temperature. To reproduce the measured dependence of OH formation on the Fe concentrations by kinetic modeling, we postulate that Fe(II) ions react with ISOPOOH via Fenton-like reactions to form OH radicals with a rate constant of 7.3 × 10^-18 cm³ s^-1. At low concentrations, oxalate forms monocomplexes with Fe(II) ions, which can promote OH formation by ISOPOOH. However, at high concentrations, oxalate scavenges OH radicals, thereby lowering aqueous OH concentrations. These findings provide new insight for the atmospheric fate of ISOPOOH and reactive oxygen species generation in the aqueous phase.

Stability of Monoterpene-Derived α-Hydroxyalkyl-Hydroperoxides in Aqueous Organic Media: Relevance to the Fate of Hydroperoxides in Aerosol Particle Phases

The α-hydroxyalkyl-hydroperoxides [R-(H)C(-OH)(-OOH), α-HH] produced in the ozonolysis of unsaturated organic compounds may contribute to secondary organic aerosol (SOA) aging. α-HHs' inherent instability, however, hampers their detection and a positive assessment of their actual role. Here we report, for the first time, the rates and products of the decomposition of the α-HHs generated in the ozonolysis of atmospherically important monoterpenes α-pinene (α-P), d-limonene (d-L), γ-terpinene (γ-Tn), and α-terpineol (α-Tp) in water/acetonitrile (W/AN) mixtures. We detect α-HHs and multifunctional decomposition products as chloride adducts by online electrospray ionization mass spectrometry. Experiments involving D₂O and H₂¹⁸O, instead of H₂¹⁶O, and an OH-radical scavenger show that α-HHs decompose into gem-diols + H₂O₂ rather than free radicals. α-HHs decay mono- or biexponentially depending on molecular structure and solvent composition. e-Fold times, τ_1/e, in water-rich solvent mixtures range from τ_1/e = 15-45 min for monoterpene-derived α-HHs to τ_1/e > 10³ min for the α-Tp-derived α-HH. All τ_1/e's dramatically increase in <20% (v/v) water. Decay rates of the α-Tp-derived α-HH in pure water increase at lower pH (2.3 ≤ pH ≤ 3.3). The hydroperoxides detected in day-old SOA samples may reflect their increased stability in water-poor media and/or the slow decomposition of α-HHs from functionalized terpenes.

Oxygenated and Nitrated Polycyclic Aromatic Hydrocarbons in Ambient Air-Levels, Phase Partitioning, Mass Size Distributions, and Inhalation Bioaccessibility

Among the nitrated and oxygenated polycyclic aromatic hydrocarbons (NPAHs and OPAHs) are some of the most hazardous substances to public health, mainly because of their carcinogenicity and oxidative potential. Despite these concerns, the concentrations and fate of NPAHs and OPAHs in the atmospheric environment are largely unknown. Ambient air concentrations of 18 NPAHs, 5 quinones, and 5 other OPAHs were determined at two urban and one regional background sites in central Europe. At one of the urban sites, the total (gas and particulate) concentrations of Σ10OPAHs were 10.0 ± 9.2 ng/m3 in winter and 3.5 ± 1.6 ng/m3 in summer. The gradient to the regional background site exceeded 1 order of magnitude. Σ18NPAH concentrations were typically 1 order of magnitude lower than OPAHs. Among OPAHs, 9-fluorenone and (9,10)-anthraquinone were the most abundant species, accompanied by benzanthrone in winter. (9,10)-Anthraquinone represented two-thirds of quinones. We found that a large fraction of the target substance particulate mass was carried by submicrometer particles. The derived inhalation bioaccessibility in the PM10 size fraction is found to be ≈5% of the total ambient concentration of OPAHs and up to ≈2% for NPAHs. For 9-fluorenone and (9,10)-anthraquinone, up to 86 and 18%, respectively, were found at the rural site. Our results indicate that water solubility could function as a limiting factor for bioaccessibility of inhaled particulate NPAHs and OPAHs, without considerable effect of surfactant lipids and proteins in the lung lining fluid.

Connecting the Oxidative Potential of Secondary Organic Aerosols with Reactive Oxygen Species in Exposed Lung Cells

It has been hypothesized that the cytotoxicity of secondary organic aerosols (SOA) is mediated through the formation of reactive oxygen species (ROS) in the exposed cells. Here, lung epithelial cells (A549) residing at the air-liquid interface were exposed to proxies of anthropogenic and biogenic SOA that were photochemically aged under varying nitrogen oxide (NOx) concentrations in an oxidation flow reactor. The total organic peroxides and ROS radical content in the SOA were quantified by the iodometric spectrophotometric method and by continuous-wave electron paramagnetic resonance. The effect of the exposure was evaluated by measuring cell viability and cellular ROS production following the exposure. The results demonstrate that SOA that aged in the absence of NOx contained more ROS than fresh SOA and were more toxic toward the cells, while varying NOx conditions had no significant influence on levels of the ROS content in fresh SOA and their toxicity. Analysis of ROS in the exposed cells using flow cytometry showed a similar trend with the total ROS content in the SOA. This study provides a first and direct observation of such association.