Multiphase chemistry at the atmosphere-biosphere interface influencing climate and public health in the anthropocene

Computational chemistry of cluster: Understanding the mechanism of atmospheric new particle formation at the molecular level

New particle formation (NPF), which exerts significant influence over human health and global climate, has been a hot topic and rapidly expands field of research in the environmental and atmospheric chemistry recent years. Generally, NPF contains two processes: formation of critical nucleus and further growth of the nucleus. However, due to the complexity of the atmospheric nucleation, which is a multicomponent process, formation of critical clusters as well as their growth is still connected to large uncertainties. Detection limits of instruments in measuring specific gaseous aerosol precursors and chemical compositions at the molecular level call for computational studies. Computational chemistry could effectively compensate the deficiency of laboratory experiments as well as observations and predict the nucleation mechanisms. We review the present theoretical literatures that discuss nucleation mechanism of atmospheric clusters. Focus of this review is on different nucleation systems involving sulfur-containing species, nitrogen-containing species and iodine-containing species. We hope this review will provide a deep insight for the molecular interaction of nucleation precursors and reveal nucleation mechanism at the molecular level.

Acylperoxy Radicals as Key Intermediates in the Formation of Dimeric Compounds in α-Pinene Secondary Organic Aerosol

High molecular weight dimeric compounds constitute a significant fraction of secondary organic aerosol (SOA) and have profound impacts on the properties and lifecycle of particles in the atmosphere. Although different formation mechanisms involving reactive intermediates and/or closed-shell monomeric species have been proposed for the particle-phase dimers, their relative importance remains in debate. Here, we report unambiguous experimental evidence of the important role of acyl organic peroxy radicals (RO₂) and a small but non-negligible contribution from stabilized Criegee intermediates (SCIs) in the formation of particle-phase dimers during ozonolysis of α-pinene, one of the most important precursors for biogenic SOA. Specifically, we find that acyl RO₂-involved reactions explain 50-80% of total oxygenated dimer signals (C₁₅-C₂₀, O/C ≥ 0.4) and 20-30% of the total less oxygenated (O/C < 0.4) dimer signals. In particular, they contribute to 70% of C₁₅-C₁₉ dimer ester formation, likely mainly via the decarboxylation of diacyl peroxides arising from acyl RO₂ cross-reactions. In comparison, SCIs play a minor role in the formation of C₁₅-C₁₉ dimer esters but react noticeably with the most abundant C₉ and C₁₀ carboxylic acids and/or carbonyl products to form C₁₉ and C₂₀ dimeric peroxides, which are prone to particle-phase transformation to form more stable dimers without the peroxide functionality. This work provides a clearer view of the formation pathways of particle-phase dimers from α-pinene oxidation and would help reduce the uncertainties in future atmospheric modeling of the budget, properties, and health and climate impacts of SOA.

Strong impacts of biomass burning, nitrogen fertilization, and fine particles on gas-phase hydrogen peroxide (H2O2)

Gas-phase hydrogen peroxide (H₂O₂) plays an important role in atmospheric chemistry as an indicator of the atmospheric oxidizing capacity. It is also a vital oxidant of sulfur dioxide (SO₂) in the aqueous phase, resulting in the formation of acid precipitation and sulfate aerosol. However, sources of H₂O₂ are not fully understood especially in polluted areas affected by human activities. In this study, we reported some high H₂O₂ cases observed during one summer and two winter campaigns conducted at a polluted rural site in the North China Plain. Our results showed that agricultural fires led to high H₂O₂ concentrations up to 9 ppb, indicating biomass burning events contributed substantially to primary H₂O₂ emission. In addition, elevated H₂O₂ and O₃ concentrations were measured after fertilization as a consequence of the enhanced atmospheric oxidizing capacity by soil HONO emission. Furthermore, H₂O₂ exhibited unexpectedly high concentration under high NO_x conditions in winter, which are closely related to multiphase reactions in particles involving organic chromophores. Our findings suggest that these special factors (biomass burning, fertilization, and ambient particles), which are not well considered in current models, are significant contributors to H₂O₂ production, thereby affecting the regional atmospheric oxidizing capacity and the global sulfate aerosol formation.

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.

Does early life exposure to exogenous sources of reactive oxygen species (ROS) increase the risk of respiratory and allergic diseases in children? A longitudinal cohort study

BACKGROUND

Excess reactive oxygen species (ROS) can cause oxidative stress damaging cells and tissues, leading to adverse health effects in the respiratory tract. Yet, few human epidemiological studies have quantified the adverse effect of early life exposure to ROS on child health. Thus, this study aimed to examine the association of levels of ROS exposure at birth and the subsequent risk of developing common respiratory and allergic diseases in children.

METHODS

1,284 Toronto Child Health Evaluation Questionnaire (T-CHEQ) participants were followed from birth (born between 1996 and 2000) until outcome, March 31, 2016 or loss-to-follow-up. Using ROS data from air monitoring campaigns and land use data in Toronto, ROS concentrations generated in the human respiratory tract in response to inhaled pollutants were estimated using a kinetic multi-layer model. These ROS values were assigned to participants' postal codes at birth. Cox proportional hazards regression models, adjusted for confounders, were then used to estimate hazard ratios (HR) with 95% confidence intervals (CI) per unit increase in interquartile range (IQR).

RESULTS

After adjusting for confounders, iron (Fe) and copper (Cu) were not significantly associated with the risk of asthma, allergic rhinitis, nor eczema. However, ROS, a measure of the combined impacts of Fe and Cu in PM_2.5, was associated with an increased risk of asthma (HR = 1.11, 95% CI: 1.02-1.21, p < 0.02) per IQR. There were no statistically significant associations of ROS with allergic rhinitis (HR = 0.96, 95% CI: 0.88-1.04, p = 0.35) and eczema (HR = 1.03, 95% CI: 0.98-1.09, p = 0.24).

CONCLUSION

These findings showed that ROS exposure in early life significantly increased the childhood risk of asthma, but not allergic rhinitis and eczema.

Recent Advances in Vehicle Exhaust Treatment with Photocatalytic Technology

Vehicle exhaust has been acknowledged as an essential factor affecting human health due to the extensive use of cars. Its main components include volatile organic compounds (VOCs) and nitrogen oxides (NOx), which can cause acute irritation and chronic diseases, and significant research on the treatment of vehicle exhaust has received increasing attention in recent decades. Recently, photocatalytic technology has been considered a practical approach for eliminating vehicle emissions. This review highlights the crucial role of photocatalytic technology in eliminating vehicle emissions using semiconductor catalysts. A particular emphasis has been placed on various photocatalytic materials, such as TiO2-based materials, Bi-based materials, and Metal–Organic Frameworks (MOFs), and their recent advances in the performance of VOC and NOx photodegradation. In addition, the applications of photocatalytic technology for the elimination of vehicle exhaust are presented (including photocatalysts combined with pavement surfaces, making photocatalysts into architectural coatings and photoreactors), which will offer a promising strategy for photocatalytic technology to remove vehicle exhaust.

Direct Observation and Quantitative Measurement of OH Radical Desorption During the Ultraviolet Photolysis of Liquid Nonanoic Acid

Ultraviolet (UV) photolysis of fatty acid surfactants─which cover the surfaces of atmospheric liquid aerosols and are found in the oceans─such as nonanoic acid (NA) has recently been suggested as a source of hydroxyl (OH) radicals in the troposphere. We used laser-induced fluorescence to directly observe OH radicals desorbed from the surface of neat liquid NA as a primary photoproduct following 213 nm irradiation. The upper limit of photoreaction cross section for the OH radical desorption was estimated to be 9.0(4.1) × 10^-22 cm², which is only 1.2 ± 0.8% of the photoreaction cross section established for the photolysis of gas-phase acetic acid monomers. Vibrational sum-frequency generation spectroscopy for liquid NA revealed the hydrogen-bonded, cyclic, dimer structure of the NA molecules at the liquid surface. This dimerization can inhibit the formation of OH radicals and lead the present low photochemical reactivity of liquid NA.

The impact of engineered nanomaterials on the environment: Release mechanism, toxicity, transformation, and remediation

The presence and longevity of nanomaterials in the ecosystem, as well as their properties, account for environmental toxicity. When nanomaterials in terrestrial and aquatic systems are exposed to the prevailing environmental conditions, they undergo various transformations such as dissociation, dissolution, and aggregation, which affects the food chain. The toxicity of nanomaterials is influenced by a variety of factors, including environmental factors and its physico-chemical characteristics. Bioaccumulation, biotransformation, and biomagnification are the mechanisms that have been identified for determining the fate of nanomaterials. The route taken by nanomaterials to reach living cells provides us with information about their toxicity profile. This review discusses the recent advances in the transport, transformation, and fate of nanomaterials after they are released into the environment. The review also discusses how nanoparticles affect lower trophic organisms through direct contact, the impact of nanoparticles on higher trophic organisms, and the possible options for remediation.

Exposure to naphthalene and β-pinene-derived secondary organic aerosol induced divergent changes in transcript levels of BEAS-2B cells

The health effects of exposure to secondary organic aerosols (SOAs) are still limited. Here, we investigated and compared the toxicities of soot particles (SP) coated with β-pinene SOA (SOAβPin-SP) and SP coated with naphthalene SOA (SOANap-SP) in a human bronchial epithelial cell line (BEAS-2B) residing at the air-liquid interface. SOAβPin-SP mostly contained oxygenated aliphatic compounds from β-pinene photooxidation, whereas SOANap-SP contained a significant fraction of oxygenated aromatic products under similar conditions. Following exposure, genome-wide transcriptome responses showed an Nrf2 oxidative stress response, particularly for SOANap-SP. Other signaling pathways, such as redox signaling, inflammatory signaling, and the involvement of matrix metalloproteinase, were identified to have a stronger impact following exposure to SOANap-SP. SOANap-SP also induced a stronger genotoxicity response than that of SOAβPin-SP. This study elucidated the mechanisms that govern SOA toxicity and showed that, compared to SOAs derived from a typical biogenic precursor, SOAs from a typical anthropogenic precursor have higher toxicological potency, which was accompanied with the activation of varied cellular mechanisms, such as aryl hydrocarbon receptor. This can be attributed to the difference in chemical composition; specifically, the aromatic compounds in the naphthalene-derived SOA had higher cytotoxic potential than that of the β-pinene-derived SOA.

Coupled Interfacial and Bulk Kinetics Govern the Timescales of Multiphase Ozonolysis Reactions

Chemical transformations in aerosols impact the lifetime of particle phase species, the fate of atmospheric pollutants, and both climate- and health-relevant aerosol properties. Timescales for multiphase reactions of ozone in atmospheric aqueous phases are governed by coupled kinetic processes between the gas phase, the particle interface, and its bulk, which respond dynamically to reactive consumption of O₃. However, models of atmospheric aerosol reactivity often do not account for the coupled nature of multiphase processes. To examine these dynamics, we use new and prior experimental observations of aqueous droplet reaction kinetics, including three systems with a range of surface affinities and ozonolysis rate coefficients (trans-aconitic acid (C₆H₆O₆), maleic acid (C₄H₄O₄), and sodium nitrite (NaNO₂)). Using literature rate coefficients and thermodynamic properties, we constrain a simple two-compartment stochastic kinetic model which resolves the interface from the particle bulk and represents O₃ partitioning, diffusion, and reaction as a coupled kinetic system. Our kinetic model accurately predicts decay kinetics across all three systems, demonstrating that both the thermodynamic properties of O₃ and the coupled kinetic and diffusion processes are key to making accurate predictions. An enhanced concentration of adsorbed O₃, compared to gas and bulk phases is rapidly maintained and remains constant even as O₃ is consumed by reaction. Multiphase systems dynamically seek to achieve equilibrium in response to reactive O₃ loss, but this is hampered at solute concentrations relevant to aqueous aerosol by the rate of O₃ arrival in the bulk by diffusion. As a result, bulk-phase O₃ becomes depleted from its Henry's law solubility. This bulk-phase O₃ depletion limits reaction timescales for relatively slow-reacting organic solutes with low interfacial affinity (i.e., trans-aconitic and maleic acids, with k_rxn ≈ 10³-10⁴ M^-1 s^-1), which is in contrast to fast-reacting solutes with higher surface affinity (i.e., nitrite, with k_rxn ≈ 10⁵ M^-1 s^-1) where surface reactions strongly impact the observed decay kinetics.

Effects of Acidity on Reactive Oxygen Species Formation from Secondary Organic Aerosols

Reactive oxygen species (ROS) play a critical role in the chemical transformation of atmospheric secondary organic aerosols (SOA) and aerosol health effects by causing oxidative stress in vivo. Acidity is an important physicochemical property of atmospheric aerosols, but its effects on the ROS formation from SOA have been poorly characterized. By applying the electron paramagnetic resonance spin-trapping technique and the Diogenes chemiluminescence assay, we find highly distinct radical yields and composition at different pH values in the range of 1-7.4 from SOA generated by oxidation of isoprene, α-terpineol, α-pinene, β-pinene, toluene, and naphthalene. We observe that isoprene SOA has substantial hydroxyl radical (^•OH) and organic radical yields at neutral pH, which are 1.5-2 times higher compared to acidic conditions in total radical yields. Superoxide (O₂ ^•-) is found to be the dominant species generated by all types of SOAs at lower pH. At neutral pH, α-terpineol SOA exhibits a substantial yield of carbon-centered organic radicals, while no radical formation is observed by aromatic SOA. Further experiments with model compounds show that the decomposition of organic peroxide leading to radical formation may be suppressed at lower pH due to acid-catalyzed rearrangement of peroxides. We also observe 1.5-3 times higher molar yields of hydrogen peroxide (H₂O₂) in acidic conditions compared to neutral pH by biogenic and aromatic SOA, likely due to enhanced decomposition of α-hydroxyhydroperoxides and quinone redox cycling, respectively. These findings are critical to bridge the gap in understanding ROS formation mechanisms and kinetics in atmospheric and physiological environments.

A new method for measuring airborne elemental carbon using PUF disk passive samplers

Carbonaceous aerosol species, such as elemental carbon (EC), are important Short-Lived Climate Forcers (SLCFs), contributing to climate and health effects of air pollution. The quantification of carbonaceous aerosols has been conventionally carried out using active air sampling followed by various analytical techniques, such as thermal/thermal-optical analysis. Active sampling requires specific equipment and infrastructure with electricity and therefore may not be the best choice for studying carbonaceous aerosols at remote locations. Passive sampling on the other hand provides a simple and cost-effective alternative to study time-weighted temporal and spatial trends. For the first time in this study, we have developed a method to examine the viability of measuring EC using polyurethane foam passive air samplers (PUF-PAS) coupled with a thermal analysis, i.e., EnCan-Total-900 (ECT9). The method was found reproducible with coefficients of variation as low as 3% for EC measured in ambient passive samples. The method had relatively low background with EC levels in blanks being as low as 0.1% of those in deployed samples, allowing quantification within a wide range of concentrations. The results indicate a homogenous distribution of particles within the PUF-PAS substrate. EC concentrations measured with the passive method were not significantly different from those obtained from active samples at the study sites (p > 0.01). This proof of concept of the PUF-PAS method provides an opportunity to cost-effectively expand measurements of elemental carbon at the global scale, and could be further extended to include other carbonaceous aerosol species in the future. This helps address regional data gaps for improving uncertainties of SLCF impacts on global climate forcing and to inform policy decisions.

Photocatalytic Role of Atmospheric Soot Particles under Visible-Light Irradiation: Reactive Oxygen Species Generation, Self-Oxidation Process, and Induced Higher Oxidative Potential and Cytotoxicity

It is known that there are semiconductor oxides involved in mineral dust, which have photocatalytic properties. However, soot particles contained in carbonaceous aerosol and their photoactivity under sunlight are rarely realized. In this study, reactive oxygen species (ROS) such as superoxide anions and hydroxyl radicals were generated upon visible-light irradiation of soot particles, and the production activity was consistent with the carbonaceous core content, indicating that the atmospheric soot particles can serve as a potential photocatalyst. The increase of oxygen-containing functional groups, environmentally persistent free radicals, oxygenated polycyclic aromatic hydrocarbons, and the oxidative potential (OP) of soot after irradiation confirmed the occurrence of visible-light-triggered photocatalytic oxidation of the soot itself. The mechanism analyses suggested that the carbonaceous core caused the production of ROS, which subsequently oxidize the extractable organic species on the soot surface. It is oxidized organic extracts that are responsible for the enhancements of the OP, cell mortality, and intracellular ROS generation. These new findings shed light on both the photocatalytic role of the soot and the importance of ROS during the photochemical self-oxidation of soot triggered by visible light and will promote a more comprehensive understanding of both the atmospheric chemical behavior and health effects of soot particles.

Trace removal of benzene vapour using double-walled metal-dipyrazolate frameworks

In principle, porous physisorbents are attractive candidates for the removal of volatile organic compounds such as benzene by virtue of their low energy for the capture and release of this pollutant. Unfortunately, many physisorbents exhibit weak sorbate-sorbent interactions, resulting in poor selectivity and low uptake when volatile organic compounds are present at trace concentrations. Herein, we report that a family of double-walled metal-dipyrazolate frameworks, BUT-53 to BUT-58, exhibit benzene uptakes at 298 K of 2.47-3.28 mmol g^-1 at <10 Pa. Breakthrough experiments revealed that BUT-55, a supramolecular isomer of the metal-organic framework Co(BDP) (H₂BDP = 1,4-di(1H-pyrazol-4-yl)benzene), captures trace levels of benzene, producing an air stream with benzene content below acceptable limits. Furthermore, BUT-55 can be regenerated with mild heating. Insight into the performance of BUT-55 comes from the crystal structure of the benzene-loaded phase (C₆H₆@BUT-55) and density functional theory calculations, which reveal that C-H···X interactions drive the tight binding of benzene. Our results demonstrate that BUT-55 is a recyclable physisorbent that exhibits high affinity and adsorption capacity towards benzene, making it a candidate for environmental remediation of benzene-contaminated gas mixtures.

Photoinduced release of odorous volatile organic compounds from aqueous pollutants: The role of reactive oxygen species in increasing risk during cross-media transformation

Photoinduced volatile organic compounds (VOCs) release from fatty alcohols at the air-water interface, has attracted considerable attention. This paper comprehensively explores the release of odorous VOCs from aqueous micropollutants under photoirradiation, especially in terms of the important role of the reactive oxygen species (ROS) in increased risk by cross-media transformation. The formation and distribution of photoinduced VOCs produced by aqueous benzyl alcohol (BzOH), a common ingredient in personal care products, were monitored in situ by online gas chromatography equipped with mass spectrometry and flame ionization detector (GC-MS/FID). The photoreaction of BzOH followed first-order kinetics with a rate constant of 0.0158/min under air. After 180 min of ultraviolet irradiation, the accumulated output of the gas-phase products benzene and benzaldehyde (BA) reached 3.8 μmol and 2.6 μmol respectively, being approximately 10 times that under nitrogen. According to electron paramagnetic resonance measurements, singlet oxygen mainly promoted the oxidation of BzOH to BA, which was an important intermediate producing benzene via photocleavage. Odorous alicyclic hydrocarbons were also generated through photorearrangement under nitrogen. On the other hand, the Henry's law constants of the main products were much lower than those of BzOH, indicating that the photoproducts would volatilize from the aqueous phase into the gas phase. The odor threshold of gas-phase products decreased to varying degrees after photoirradiation. Especially for BA, one of the main products, its odor threshold decreased 130 times compared with BzOH. This study shows that the risk of cross-media pollution could significantly increase due to the transformation of aqueous pollutants into odorous VOCs under photoirradiation and provides new insight into its risk prevention.

Aerosol Proteinaceous Matter in Coastal Okinawa, Japan: Influence of Long-Range Transport and Photochemical Degradation

The characteristics, sources, and atmospheric oxidation processes of marine aerosol proteinaceous matter (APM), including total proteins and free amino acids (FAAs), were investigated using a set of 1 year total suspended particulate (TSP) samples collected in the coastal area of Okinawa Island in the western North Pacific rim. The concentrations of APM at this site (total proteins: 0.16 ± 0.10 μg m^-3 and total FAAs: 9.7 ± 5.6 ng m^-3, annual average) are comparable to those of marine APM. The major FAA species of APM are also similar to previously reported marine APM with glycine as the dominant species (31%). Based on the different seasonal trends and weak correlations of total proteins and FAAs, we found that they were contributed by different sources, especially with the influence of long-range transport from the Asian continent of northern China and Mongolia and the oceanic area of the Bohai Sea, Yellow Sea, and East China Sea. The photochemical oxidation processes of high-molecular-weight proteins releasing FAAs (especially glycine) were also considered as an important factor influencing the characteristics of APM at this site. In addition, we propose a degradation process based on the correlation with ozone and ultraviolet radiation, emphasizing their roles in the degradation of proteins. Our findings help to deepen the understanding of atmospheric photochemical reaction processes of organic aerosols.

Phase Behavior of Internal Mixtures of Hydrocarbon-like Primary Organic Aerosol and Secondary Aerosol Based on Their Differences in Oxygen-to-Carbon Ratios

The phase behavior, the number and type of phases, in atmospheric particles containing mixtures of hydrocarbon-like organic aerosol (HOA) and secondary organic aerosol (SOA) is important for predicting their impacts on air pollution, human health, and climate. Using a solvatochromic dye and fluorescence microscopy, we determined the phase behavior of 11 HOA proxies (O/C = 0-0.29) each mixed with 7 different SOA materials generated in environmental chambers (O/C 0.4-1.08), where O/C represents the average oxygen-to-carbon atomic ratio. Out of the 77 different HOA + SOA mixtures studied, we observed two phases in 88% of the cases. The phase behavior was independent of relative humidity over the range between 90% and <5%. A clear trend was observed between the number of phases and the difference between the average O/C ratios of the HOA and SOA components (ΔO/C). Using a threshold ΔO/C of 0.265, we were able to predict the phase behavior of 92% of the HOA + SOA mixtures studied here, with one-phase particles predicted for ΔO/C < 0.265 and two-phase particles predicted for ΔO/C ≥ 0.265. The threshold ΔO/C value provides a relatively simple and computationally inexpensive framework for predicting the number of phases in internal SOA and HOA mixtures in atmospheric models.

Uptake of N2O5 by aqueous aerosol unveiled using chemically accurate many-body potentials

The reactive uptake of N2O5 to aqueous aerosol is a major loss channel for nitrogen oxides in the troposphere. Despite its importance, a quantitative picture of the uptake mechanism is missing. Here we use molecular dynamics simulations with a data-driven many-body model of coupled-cluster accuracy to quantify thermodynamics and kinetics of solvation and adsorption of N2O5 in water. The free energy profile highlights that N2O5 is selectively adsorbed to the liquid-vapor interface and weakly solvated. Accommodation into bulk water occurs slowly, competing with evaporation upon adsorption from gas phase. Leveraging the quantitative accuracy of the model, we parameterize and solve a reaction-diffusion equation to determine hydrolysis rates consistent with experimental observations. We find a short reaction-diffusion length, indicating that the uptake is dominated by interfacial features. The parameters deduced here, including solubility, accommodation coefficient, and hydrolysis rate, afford a foundation for which to consider the reactive loss of N2O5 in more complex solutions.

Particle hygroscopicity inhomogeneity and its impact on reactive uptake

Atmospheric particles are important reaction vessels for multiphase chemistry. We conducted a meta-analysis of previous field observations in various environments (includes ocean, urban and rural regions), showing that particle hygroscopicity inhomogeneity (PHI) is ubiquitous for the continental atmospheric particles, in which a considerable part of the particulate matters is hydrophobic (10%-33% on average). However, the effects of PHI in quantifying the uptake process of reactive gases are still unclear. Here, taking N₂O₅ uptake as an example, we showed that using a laboratory-based parameterization scheme without considering the PHI might result in a misestimation of uptake rate coefficient, especially under low ambient relative humidity (RH). Such misestimation may be caused by the differences of the uptake coefficients, as well as the proportion of surface area concentration (SA) between hydrophilic and hydrophobic particles. We suggested that the PHI should be well-considered in establishing the reactive traces gases heterogeneous uptake parameterizations.

A New Type of Quartz Smog Chamber: Design and Characterization

Since the 1960s, many indoor and outdoor smog chambers have been developed worldwide. However, most of them are made of Teflon films, which have relatively high background contaminations due to the wall effect. We developed the world's first medium-size quartz chamber (10 m³), which is jointed with 32 pieces of 5 mm thick polished quartz glasses and a stainless-steel frame. Characterizations show that this chamber exhibits excellent performance in terms of relative humidity (RH) (2-80%) and temperature (15-30 ± 1 °C) control, mixing efficiency of the reactants (6-8 min), light transmittance (>90% above 290 nm), and wall loss of pollutants. The wall loss rates of the gas-phase pollutants are on the order of 10^-4 min^-1 at 298 K under dry conditions. It is 0.08 h^-1 for 100-500 nm particles, significantly lower than those of Teflon chambers. The photolysis rate of NO₂ (J_NO2) is automatically adjustable to simulate the diurnal variation of solar irradiation from 0 to 0.40 min^-1. The inner surface of the chamber can be repeatedly washed with deionized water, resulting in low background contaminations. Both experiments (toluene-NOx and α-pinene-ozone systems) and box model demonstrate that this new quartz chamber can provide high-quality data for investigating SOA and O₃ formation in the atmosphere.

Interface-Enhanced Oxygen Vacancies of CoCuOx Catalysts In Situ Grown on Monolithic Cu Foam for VOC Catalytic Oxidation

The development of highly efficient and stable monolithic catalysts is essential for the removal of volatile organic compounds (VOCs). Copper foam (CF) is a potential ideal carrier for monolithic catalysts, but its low surface area is not conducive to dispersion of active species, thus reducing the interface interaction with active species. Herein, a vertically oriented Cu(OH)₂ nanorod was in situ grown on the CF, which acted as the template and precursor to synthesize CoCu-MOF. The optimized catalyst (12CoCu-R) delivers excellent performance for acetone oxidation with a T₉₀ of 195 °C. Impressively, the catalyst demonstrated satisfactory stability in long-term, cycle, water resistance, and high airspeed tests. Therefore, the present study provides a novel strategy for rationally designing efficient monolithic catalysts for VOC oxidation and other environmental applications.

Air Quality Measurements in Kitchener, Ontario, Canada Using Multisensor Mini Monitoring Stations

The Region of Waterloo is the third fastest growing region in Southern Ontario in Canada with a population of 619,000 as of 2019. However, only one air quality monitoring station, located in a city park in Kitchener, Ontario, is currently being used to assess the air quality of the region. In September 2020, a network of AQMesh Multisensor Mini Monitoring Stations (pods) were installed near elementary schools in Kitchener located near different types of emission source. Data analysis using a custom-made long-distance scaling software showed that the levels of nitrogen oxides (NO and NO2), ground level ozone (O3), and fine particulate matter (PM2.5) were traffic related. These pollutants were used to calculate the Air Quality Health Index-Plus (AQHI+) at each location, highlighting the inability of the provincial air quality monitoring station to detect hotspot areas in the city. The case study presented here quantified the impact of the 2021 summer wildfires on the local air quality at a high time resolution (15-min). The findings in this article show that these multisensor pods are a viable alternative to expensive research-grade equipment. The results highlight the need for networks of local scale air quality measurements, particularly in fast-growing cities in Canada.

Air Pollutants Reduce the Physical Activity of Professional Soccer Players

The aim of the study was to determine the impact of air quality—analyzed on the basis of the model of integrating three types of air pollutants (ozone, O₃; particulate matter, PM; nitrogen dioxide, NO₂)—on the physical activity of soccer players. Study material consisted of 8927 individual match observations of 461 players competing in the German Bundesliga during the 2017/2018 and 2018/2019 domestic seasons. The measured indices included players’ physical activities: total distance (TD) and high-intensity effort (HIE). Statistical analysis showed that with increasing levels of air pollution, both TD (F = 13.900(3); p = 0.001) and HIE (F = 8.060(3); p = 0.001) decrease significantly. The worsening of just one parameter of air pollution results in a significant reduction in performance. This is important information as air pollution is currently a considerable problem for many countries. Improving air quality during training sessions and sports competitions will result in better well-being and sporting performance of athletes and will also help protect athletes from negative health effects caused by air pollution.

Metal-Organic Frameworks With Variable Valence Metal-Photoactive Components: Emerging Platform for Volatile Organic Compounds Photocatalytic Degradation

With their outstanding diversities in both structures and performances, newly emerging metal-organic frameworks (MOFs) materials are considered to be the most promising artificial catalysts to meet multiple challenges in the fields of energy and environment. Especially in absorption and conversion of solar energy, a variety of MOFs can be readily designed to cover and harvest the sun irradiation of ultraviolet (UV), visible and near-infrared region through tuning both organic linkers and metal nodes to create optimal photocatalytic efficiency. Nowadays, a variety of MOFs were successfully synthesized as powerful photocatalysts for important redox reactions such as water-splitting, CO₂ reduction and aqueous environmental pollutants detoxification. MOFs applications in indoor-air VOCs pollutants cleaning, however, are less concerned partially because of limited diffusion of both gaseous pollutant molecules and photo-induced active species in very porous MOFs structures. In this mini-review, we focus on the major breakthroughs of MOFs as photocatalysts for the effective removal of indoor-air VOCs such as aldehydes, aromatics and short-chain alcohols. According to their nature of photoactive centers, herein MOFs photocatalysts are divided into two categories to comment, that is, MOFs with variable valence metal nodes as direct photoactive centers and MOFs with non-variable valence metal nodes but after combining other photoactive variable valence metal centers as excellent concentrated and concerted electron-transfer materials. The mechanisms and current challenges of the photocatalytic degradation of indoor-air VOC pollutants by these MOFs will be discussed as deeply as possible.

A global observational analysis to understand changes in air quality during exceptionally low anthropogenic emission conditions

This global study, which has been coordinated by the World Meteorological Organization Global Atmospheric Watch (WMO/GAW) programme, aims to understand the behaviour of key air pollutant species during the COVID-19 pandemic period of exceptionally low emissions across the globe. We investigated the effects of the differences in both emissions and regional and local meteorology in 2020 compared with the period 2015-2019. By adopting a globally consistent approach, this comprehensive observational analysis focuses on changes in air quality in and around cities across the globe for the following air pollutants PM_2.5, PM₁₀, PMC (coarse fraction of PM), NO₂, SO₂, NOx, CO, O₃ and the total gaseous oxidant (OX = NO₂ + O₃) during the pre-lockdown, partial lockdown, full lockdown and two relaxation periods spanning from January to September 2020. The analysis is based on in situ ground-based air quality observations at over 540 traffic, background and rural stations, from 63 cities and covering 25 countries over seven geographical regions of the world. Anomalies in the air pollutant concentrations (increases or decreases during 2020 periods compared to equivalent 2015-2019 periods) were calculated and the possible effects of meteorological conditions were analysed by computing anomalies from ERA5 reanalyses and local observations for these periods. We observed a positive correlation between the reductions in NO₂ and NOx concentrations and peoples' mobility for most cities. A correlation between PMC and mobility changes was also seen for some Asian and South American cities. A clear signal was not observed for other pollutants, suggesting that sources besides vehicular emissions also substantially contributed to the change in air quality. As a global and regional overview of the changes in ambient concentrations of key air quality species, we observed decreases of up to about 70% in mean NO₂ and between 30% and 40% in mean PM_2.5 concentrations over 2020 full lockdown compared to the same period in 2015-2019. However, PM_2.5 exhibited complex signals, even within the same region, with increases in some Spanish cities, attributed mainly to the long-range transport of African dust and/or biomass burning (corroborated with the analysis of NO₂/CO ratio). Some Chinese cities showed similar increases in PM_2.5 during the lockdown periods, but in this case, it was likely due to secondary PM formation. Changes in O₃ concentrations were highly heterogeneous, with no overall change or small increases (as in the case of Europe), and positive anomalies of 25% and 30% in East Asia and South America, respectively, with Colombia showing the largest positive anomaly of ~70%. The SO₂ anomalies were negative for 2020 compared to 2015-2019 (between ~25 to 60%) for all regions. For CO, negative anomalies were observed for all regions with the largest decrease for South America of up to ~40%. The NO₂/CO ratio indicated that specific sites (such as those in Spanish cities) were affected by biomass burning plumes, which outweighed the NO₂ decrease due to the general reduction in mobility (ratio of ~60%). Analysis of the total oxidant (OX = NO₂ + O₃) showed that primary NO₂ emissions at urban locations were greater than the O₃ production, whereas at background sites, OX was mostly driven by the regional contributions rather than local NO₂ and O₃ concentrations. The present study clearly highlights the importance of meteorology and episodic contributions (e.g., from dust, domestic, agricultural biomass burning and crop fertilizing) when analysing air quality in and around cities even during large emissions reductions. There is still the need to better understand how the chemical responses of secondary pollutants to emission change under complex meteorological conditions, along with climate change and socio-economic drivers may affect future air quality. The implications for regional and global policies are also significant, as our study clearly indicates that PM_2.5 concentrations would not likely meet the World Health Organization guidelines in many parts of the world, despite the drastic reductions in mobility. Consequently, revisions of air quality regulation (e.g., the Gothenburg Protocol) with more ambitious targets that are specific to the different regions of the world may well be required.

Hypochlorous acid initiated lipid chlorination at air-water interface

There has been a surge of interest in interfacial hypochlorous acid (HOCl) chemistry for indoor air quality and public health. Here we combined nanoelectrospray mass spectrometry (nESI-MS) and acoustic levitation (AL) techniques to study the chlorination chemistry of three model lipids (DPPE, POPG, DOPG) mediated by HOCl at the air-water interface of levitated water droplet. For DPPE with no CC double bonds, HOCl was insensitive to the alkane chains, and showed considerable delay directing to head amino groups compared to that in aqueous environment. Chlorination chemistry, for POPG and DOPG with CC double bonds, preferentially reacted with double bonds of one chain. The mechanism was discussed in light of these observations, and it is concluded that the increased hydrophilicity of the chlorinated chain disturbed the lipid packing and attracted it toward the water phase. In addition, the reaction rate constant and reactive uptake coefficient suggested that the chlorination of lipids exposed to HOCl at the air-water interface is likely to occur rapidly. These results gain the knowledge of HOCl mediated lipid interface reaction at the molecule level, and would better understand the adverse health effects associated with elevated indoor pollutants.

Physical Activity in Polluted Air-Net Benefit or Harm to Cardiovascular Health? A Comprehensive Review

Both exposure to higher levels of polluted air and physical inactivity are crucial risk factors for the development and progression of major noncommunicable diseases and, in particular, of cardiovascular disease. In this context, the World Health Organization estimated 4.2 and 3.2 million global deaths per year in response to ambient air pollution and insufficient physical activity, respectively. While regular physical activity is well known to improve general health, it may also increase the uptake and deposit of air pollutants in the lungs/airways and circulation, due to increased breathing frequency and minute ventilation, thus increasing the risk of cardiovascular disease. Thus, determining the tradeoff between the health benefits of physical activity and the potential harmful effects of increased exposure to air pollution during physical activity has important public health consequences. In the present comprehensive review, we analyzed evidence from human and animal studies on the combined effects of physical activity and air pollution on cardiovascular and other health outcomes. We further report on pathophysiological mechanisms underlying air pollution exposure, as well as the protective effects of physical activity with a focus on oxidative stress and inflammation. Lastly, we provide mitigation strategies and practical recommendations for physical activity in areas with polluted air.

Characteristics and Potential Inhalation Exposure Risks of Environmentally Persistent Free Radicals in Atmospheric Particulate Matter and Solid Fuel Combustion Particles in High Lung Cancer Incidence Area, China

Environmentally persistent free radicals (EPFRs) were previously considered an unrecognized composition of air pollutants and might help explain the long-standing medical mystery of why non-smokers develop tobacco-related diseases such as lung cancer. However, there is no investigated on EPFRs in Xuanwei rural areas, especially in high prevalence of lung cancer areas. In this study, we selected six types of coal and three types of biomass in Xuanwei, then conducted simulated combustion, and six group of atmospheric particulate matters (APMs) to explore the content and particle size distribution pattern of EPFRs and a new health risk assessment method to evaluate the risk of EPFRs in PM for adults and children. Our results show that the contribution of EPFRs for biomass combustion, coal combustion and APMs were mainly distributed in the size range of <1.1 μm, which accounted for 76.15 ± 4.14%, 74.85 ± 10.76%, and 75.23 ± 8.18% of PM3.3. The mean g factors and ΔHp-p indicated that the EPFRs were mainly oxygen-centered radicals in PM in Xuanwei. The results suggest that the health risk of EPFRs is significantly increased when the particle size distribution of EPFRs is taken into account, and coal combustion particulate matter (174.70 ± 37.86 cigarettes for an adult, 66.39 ± 14.39 cigarettes per person per year for a child) is more hazardous to humans than biomass combustion particulate matter (69.41 ± 4.83 cigarettes for an adult, 26.37 ± 1.84 cigarettes per person per year for), followed by APMs (102.88 ± 39.99 cigarettes for an adult, 39.10 ±15.20 cigarettes per person per year for) in PM3.3. Our results provides a new perspective and evidence for revealing the reason for the high incidence of lung cancer in Xuanwei, China.

Exogenous electron transfer mediator enhancing gaseous toluene degradation in a microbial fuel cell: Performance and electron transfer mechanism

Effective electron transfer (ET) between microorganisms and electrodes is essential for the toluene degradation and power generation in a microbial fuel cell (MFC). In this work, the neutral red, with excellent electrochemical reversibility and compatible redox potential as NADH/NAD⁺, was selected as electron mediator to boost the performance of the MFC. Experimental results revealed that, with the 0.5 μM neutral red, the removal efficiency and coulombic efficiency of the gaseous toluene powered MFC was increased by ~19% and ~400%, respectively. However, further increase in neutral red concentration resulted in a decreased in removal efficiency and coulombic efficiency, which was attributed by the toxicity of neutral red to the microbes. The microbial community analysis indicated that, with the dosage of the neutral red, the dominated bacteria shifted from Geobacter to Ignavibacteriales, resulting in a high coulombic efficiency. With the further increase in the neutral red, the amount of Ignavibacteriales gradually decreased and thus the coulombic efficiency declined at a high neutral red concentration. Based on the cyclic voltammetry analysis, an electron transport pathway involving neutral red, cytochromes, and OMCs in neutral red mediated MFC was proposed. Overall, the dosage of neutral not only enhanced the electron transfer but also induced the growth of the exoelectrogens, and thus significantly improve the MFC performance.

Probing the Water Uptake and Phase State of Individual Sucrose Nanoparticles Using Atomic Force Microscopy

The effects of atmospheric aerosols on the climate and atmosphere of Earth can vary significantly depending upon their properties, including size, morphology, and phase state, all of which are influenced by varying relative humidity (RH) in the atmosphere. A significant fraction of atmospheric aerosols is below 100 nm in size. However, as a result of size limitations of conventional experimental techniques, how the particle-to-particle variability of the phase state of aerosols influences atmospheric processes is poorly understood. To address this issue, the atomic force microscopy (AFM) methodology that was previously established for sub-micrometer aerosols is extended to measure the water uptake and identify the phase state of individual sucrose nanoparticles. Quantified growth factors (GFs) of individual sucrose nanoparticles up to 60% RH were lower than expected values observed on the sub-micrometer sucrose particles. The effect could be attributed to the semisolid sucrose nanoparticle restructuring on a substrate. At RH > 60%, sucrose nanoparticles are liquid and GFs overlap well with the sub-micrometer particles and theoretical predictions. This suggests that quantification of GFs of nanoparticles may be inaccurate for the RH range where particles are semisolid but becomes accurate at elevated RH where particles are liquid. Despite this, however, the identified phase states of the nanoparticles were comparable to their sub-micrometer counterparts. The identified phase transitions between solid and semisolid and between semisolid and liquid for sucrose were at ∼18 and 60% RH, which are equivalent to viscosities of 10^11.2 and 10^2.5 Pa s, respectively. This work demonstrates that measurements of the phase state using AFM are applicable to nanosized particles, even when the substrate alters the shape of semisolid nanoparticles and alters the GF.

Hydroxyl Radical Production by Air Pollutants in Epithelial Lining Fluid Governed by Interconversion and Scavenging of Reactive Oxygen Species

Air pollution is a major risk factor for human health. Chemical reactions in the epithelial lining fluid (ELF) of the human respiratory tract result in the formation of reactive oxygen species (ROS), which can lead to oxidative stress and adverse health effects. We use kinetic modeling to quantify the effects of fine particulate matter (PM2.5), ozone (O₃), and nitrogen dioxide (NO₂) on ROS formation, interconversion, and reactivity, and discuss different chemical metrics for oxidative stress, such as cumulative production of ROS and hydrogen peroxide (H₂O₂) to hydroxyl radical (OH) conversion. All three air pollutants produce ROS that accumulate in the ELF as H₂O₂, which serves as reservoir for radical species. At low PM2.5 concentrations (<10 μg m^-3), we find that less than 4% of all produced H₂O₂ is converted into highly reactive OH, while the rest is intercepted by antioxidants and enzymes that serve as ROS buffering agents. At elevated PM2.5 concentrations (>10 μg m^-3), however, Fenton chemistry overwhelms the ROS buffering effect and leads to a tipping point in H₂O₂ fate, causing a strong nonlinear increase in OH production. This shift in ROS chemistry and the enhanced OH production provide a tentative mechanistic explanation for how the inhalation of PM2.5 induces oxidative stress and adverse health effects.

Interfacial interaction between benzo[a]pyrene and pulmonary surfactant: Adverse effects on lung health

Inhaled polycyclic aromatic hydrocarbons (PAHs) can directly interact with the lung surfactant (PS) lining of alveoli, thereby affecting the normal physiological functions of PS, which is a serious threat to lung health. In spite of the extensive study of benzo[a]pyrene (BaP, a representative of PAHs), its potential biophysical influence on the natural PS is still largely unknown. In this study, the interfacial interaction between PS (extracted from porcine lungs) and BaP is investigated in vitro. The results showed that the surface tension, phase behavior, and interfacial structure of the PS monolayers were obviously altered in the presence of BaP. A solubilization test manifested that PS and its major components (dipalmitoyl phosphatidylcholine, DPPC; bovine serum albumin, BSA) could in turn accelerate the dissolution of BaP, which followed the order: PS > DPPC > BSA, and mixed phospholipids were significantly responsible for the solubilization of BaP by PS. In addition, solubilization of BaP also enhanced the consumption of hydroxyl radicals (·OH) in the simulated lung fluid, which could disturb the balance between oxidation and antioxidation.

Environmental risk factors and cardiovascular diseases: a comprehensive review

Noncommunicable diseases (NCDs) are fatal for more than 38 million people each year and are thus the main contributors to the global burden of disease accounting for 70% of mortality. The majority of these deaths are caused by cardiovascular disease. The risk of NCDs is strongly associated with exposure to environmental stressors such as pollutants in the air, noise exposure, artificial light at night and climate change, including heat extremes, desert storms and wildfires. In addition to the traditional risk factors for cardiovascular disease such as diabetes, arterial hypertension, smoking, hypercholesterolemia and genetic predisposition, there is a growing body of evidence showing that physicochemical factors in the environment contribute significantly to the high NCD numbers. Furthermore, urbanization is associated with accumulation and intensification of these stressors. This comprehensive expert review will summarize the epidemiology and pathophysiology of environmental stressors with a focus on cardiovascular NCDs. We will also discuss solutions and mitigation measures to lower the impact of environmental risk factors with focus on cardiovascular disease.

Cytotoxicity and chemical composition of women's personal PM2.5 exposures from rural China

Personal exposure PM samples aid in determining the sources and chemical composition of real-world exposures, particularly in settings with household air pollution. However, their use in toxicological research is limited, despite uncertainty regarding health effects in these settings and evidence of differential toxicity among PM_2.5 sources and components. This study used women's PM_2.5 exposure samples collected using personal exposure monitoring in rural villages in three Chinese provinces (Beijing, Shanxi, and Sichuan) during summer and winter. Water-soluble organic carbon, ions, elements, and organic tracers (e.g. levoglucosan and polycyclic aromatic hydrocarbons [PAHs]) were quantified in water and organic PM_2.5 extracts. Human lung epithelial cells (A549) were exposed to the extracts. Cell death, reactive oxygen species (ROS), and gene expression were measured. Biomass burning contributions were higher in Sichuan samples than in Beijing or Shanxi. Some PM characteristics (total PAHs and coal combustion source contributions) and biological effects of organic extract exposures (cell death, ROS, and cytokine gene expression) shared a common trend of higher levels and effects in winter than in summer for Shanxi and Beijing but no seasonal differences in Sichuan. Modulation of phase I/AhR-related genes (cyp1a1 and cyp1b1) and phase II/oxidative stress-related genes (HO-1, SOD1/2, NQO-1, and catalase) was either low or insignificant, without clear trends between samples. No significant cell death or ROS production was observed for water extract treatments among all sites and seasons, even at possible higher concentrations tested. These results support organic components, particularly PAHs, as essential drivers of biological effects, which is consistent with some other evidence from ambient PM_2.5.

Direct measurement with personal samplers captures the chemical complexity of PM_2.5 exposures better than fixed monitors. To investigate biological effects, lung cells were exposed to extracts of exposure PM_2.5 samples.

Phase Behavior of Hydrocarbon-like Primary Organic Aerosol and Secondary Organic Aerosol Proxies Based on Their Elemental Oxygen-to-Carbon Ratio

A large fraction of atmospheric aerosols can be characterized as primary organic aerosol (POA) and secondary organic aerosol (SOA). Knowledge of the phase behavior, that is, the number and type of phases within internal POA + SOA mixtures, is crucial to predict their effect on climate and air quality. For example, if POA and SOA form a single phase, POA will enhance the formation of SOA by providing organic mass to absorb SOA precursors. Using microscopy, we studied the phase behavior of mixtures of SOA proxies and hydrocarbon-like POA proxies at relative humidity (RH) values of 90%, 45%, and below 5%. Internal mixtures of POA and SOA almost always formed two phases if the elemental oxygen-to-carbon ratio (O/C) of the POA was less than 0.11, which encompasses a large fraction of atmospheric hydrocarbon-like POA from fossil fuel combustion. SOA proxies mixed with POA proxies having 0.11 ≤ O/C ≤ 0.29 mostly resulted in particles with one liquid phase. However, two liquid phases were also observed, depending on the type of SOA and POA surrogates, and an increase in phase-separated particles was observed when increasing the RH in this O/C range. The results have implications for predicting atmospheric SOA formation and policy strategies to reduce SOA in urban environments.

Effects of reactive oxygen species generation induced by Wonju City particulate matter on mitochondrial dysfunction in human middle ear cell

Atmospheric particulate matter (PM) contains different components that can elicit varying adverse health effects in humans and animals. Studies on PM toxicity and its underlying mechanisms in the middle ear are limited, and they generally use a PM standard. However, as PM composition varies temporally and geographically, it is crucial to identify the toxic PM constituents according to season and region and investigate their associated health effects. Thus, we sought to determine whether PM induces cytotoxicity and inflammatory factor and reactive oxygen species (ROS) generation in human middle ear epithelial cells obtained from patients with otitis media. The cells were treated with both standard urban PM and PM directly captured from the atmosphere in Wonju City. The association between mitochondrial dysfunction and PM was investigated. PM exposure significantly increased COX-2 and TNF-α mRNA expression, increased ROS generation, induced inflammatory responses, and caused abnormalities in mitochondrial motility and function. Furthermore, PM induced cell apoptosis, which consequently reduced cell survival, particularly at the concentration of 100 μg/mL. Overall, our study provides new insights into the toxic effects of standard and atmospheric PM on middle ear cell line.

Rare-earth single atoms decorated 2D-TiO2 nanosheets for the photodegradation of gaseous O-xylene

In this work, rare-earth single atoms (La, Er) were decorated on the surface of 2D-TiO₂ nanosheets by an impregnation-calcination strategy. The formation of rare-earth single atoms was certified by AC HAADF-STEM and XAS. TiO₂ decorated with rare-earth single atoms (La₁-TiO₂ and Er₁-TiO₂) exhibited outstanding photocatalytic activity than pure 2D-TiO₂ nanosheets (2D-TiO₂) towards gas-phase degradation of O-xylene. Compared with 2D-TiO₂, the rare-earth single atoms greatly improved the adsorption capacity of O-xylene without increasing their specific surface area. This is because rare-earth single atoms provide additional adsorption sites and reduce the adsorption energy of O-xylene. In addition, the hybrid orbital formed by the combination of rare-earth single atom and oxygen atom is beneficial to the rapid transmission and separation of photo-induced electrons, thereby improving the performance of photocatalytic degradation. In addition, in-situ DRIFTS and GC-MS were used to reveal the photocatalytic oxidation mechanism. Interestingly, the results showed that the La₁-TiO₂ and Er₁-TiO₂ samples can reduce the types of intermediates and simplify the reaction route, implying that the single atoms play an important role in the modulation and thorough mineralization of intermediate products. This work shows that the rare-earth single atom decorated 2D-TiO₂ nanosheets have great potential in photocatalytic air pollution control.

The maximum carbonyl ratio (MCR) as a new index for the structural classification of secondary organic aerosol components

RATIONALE

Organic aerosols (OA) account for a large fraction of atmospheric fine particulate matter and thus are affecting climate and public health. Elucidation of the chemical composition of OA is the key for addressing the role of ambient fine particles at the atmosphere-biosphere interface and mass spectrometry is the main method to achieve this goal.

METHODS

High-resolution mass spectrometry (HRMS) is on its way to becoming one of the most prominent analytical techniques, also for the analysis of atmospheric aerosols. The combination of high mass resolution and accurate mass determination allows the elemental compositions of numerous compounds to be easily elucidated. Here a new parameter for the improved classification of OA is introduced - the maximum carbonyl ratio (MCR) - which is directly derived from the molecular composition and is particularly suitable for the identification and characterization of secondary organic aerosols (SOA).

RESULTS

The concept is exemplified by the analysis of ambient OA samples from two measurement sites (Hyytiälä, Finland; Beijing, China) and of laboratory-generated SOA based on ultrahigh-performance liquid chromatography (UHPLC) coupled to Orbitrap MS. To interpret the results, MCR-Van Krevelen (VK) diagrams are generated for the different OA samples and the individual compounds are categorized into specific areas in the diagrams. The results show that the MCR index is a valuable parameter for representing atmospheric SOA components in composition and structure-dependent visualization tools such as VK diagrams.

CONCLUSIONS

The MCR index is suggested as a tool for a better characterization of the sources and the processing of atmospheric OA components based on HRMS data. Since the MCR contains information on the concentration of highly electrophilic organic compounds in particulate matter (PM) as well as on the concentration of organic (hydro)peroxides, the MCR could be a promising metric for identifying health-related particulate matter parameters by HRMS.

Representation of Multiphase OH Oxidation of Amorphous Organic Aerosol for Tropospheric Conditions

Organic aerosol (OA) is ubiquitous in the atmosphere and, during transport, can experience chemical transformation with consequences for air quality and climate. Prediction of the chemical evolution of OA depends on its reactivity with atmospheric oxidants such as the OH radical. OA particles undergo amorphous phase transitions from liquid to solid (glassy) states in response to temperature changes, which, in turn, will impact its reactivity toward OH oxidation. To improve the predictability of OA reactivity toward OH oxidation, the reactive uptake coefficients (γ) of OH radicals reacting with triacontane and squalane serving as amorphous OA surrogates were measured at temperatures from 213-293 K. γ increases strongest with temperature when the organic species is in the liquid phase, compared to when being in the semisolid or solid phase. The resistor model is applied, accounting for the amorphous phase state changes using the organic species' glass transition temperature and fragility, to evaluate the physicochemical parameters of the temperature dependent OH uptake process. This allows for the derivation of a semiempirical formula, applicable to models, to predict the degree of oxidation and chemical lifetime of the condensed-phase organic species for typical tropospheric temperature and humidity when OA particle viscosity is known.

Important Oxidants and Their Impact on the Environmental Effects of Aerosols

Oxidants are central species in the atmosphere, where they not only determine secondary particle formation but also impact human health and climate change. In general, they are unstable, highly reactive, and recyclable and have been studied in field observations, laboratory studies, and model simulations. The most widely investigated oxidants, such as OH radicals, O₃, and Cl atom, HONO, NO₃, N₂O₅, and Criegee Intermediates (CIs) have attracted more attention recently. Furthermore, secondary particles formed in the oxidations processes impact the particle physicochemical properties, such as hygroscopicity and optical properties and therefore impact the atmospheric radiation balance. Therefore, the newest investigation results of important oxidants (HONO, NO₃, N₂O₅, and CIs) are reviewed in this manuscript, and the environmental effects of secondary particles formed through corresponding oxidation processes are also stated. Furthermore, some perspectives are further discussed in the article.

Direct observation and assessment of phase states of ambient and lab-generated sub-micron particles upon humidification

We present a new analytical platform that uses a tilted stage (60°) integrated to the Peltier cooling stage interfaced with an Environmental Scanning Electron Microscope (ESEM) to directly observe and assess the phase state of particles as a function of RH at a controlled temperature. Three types of organic particles have been studied: (a) Suwannee River Fulvic Acid (SRFA) particles, (b) lab generated soil organic particles, and (c) field-collected ambient particles. The chemical composition, morphology, and functional groups of individual particles were probed using computer-controlled scanning electron microscopy with energy-dispersive X-ray spectroscopy (CCSEM/EDX) and scanning transmission X-ray microscopy with near-edge X-ray absorption fine structure spectroscopy (STXM/NEXAFS). Results show that all three types of particles are organic-rich, but soil organic particles and ambient particles contain a considerable amount of inorganic species. The phase state can be determined based on the particle's aspect ratio (particle width/height), which we proposed for solid, semisolid, and liquid particles are 1.00–1.30, 1.30–1.85, and >1.85, respectively. We found that solid SRFA particles transition to a semisolid state at ∼90% RH and to the liquid state at ∼97% RH, in agreement with the literature. The solid soil organic particles transition to a semisolid state at ∼85% RH and to the liquid state at ∼97% RH. The solid ambient organic particles transition to a semisolid state at ∼65% RH and the liquid state at ∼97% RH. Our results indicate that this new platform can directly observe and quantitatively indicate the phase transition of field-collected particles under different ambient conditions.

We present a new analytical platform that uses a tilted and Peltier cooling stage interfaced with an environmental scanning electron microscope to directly observe and assess the phase state of individual particles as a function of relative humidity.

A large-scale outdoor atmospheric simulation smog chamber for studying atmospheric photochemical processes: Characterization and preliminary application

Understanding the formation mechanisms of secondary air pollution is very important for the formulation of air pollution control countermeasures in China. Thus, a large-scale outdoor atmospheric simulation smog chamber was constructed at Chinese Research Academy of Environmental Sciences (the CRAES Chamber), which was designed for simulating the atmospheric photochemical processes under the conditions close to the real atmospheric environment. The chamber consisted of a 56-m³ fluorinated ethylene propylene (FEP) Teflon film reactor, an electrically-driven stainless steel alloy shield, an auxiliary system, and multiple detection instrumentations. By performing a series of characterization experiments, we obtained basic parameters of the CRAES chamber, such as the mixing ability, the background reactivity, and the wall loss rates of gaseous compounds (propene, NO, NO₂, ozone) and aerosols (ammonium sulfate). Oxidation experiments were also performed to study the formation of ozone and secondary organic aerosol (SOA), including α-pinene ozonolysis, propene and 1,3,5-trimethylbenzene photooxidation. Temperature and seed effects on the vapor wall loss and SOA yields were obtained in this work: higher temperature and the presence of seed could reduce the vapor wall loss; SOA yield was found to depend inversely on temperature, and the presence of seed could increase SOA yield. The seed was suggested to be used in the chamber to reduce the interaction between the gas phase and chamber walls. The results above showed that the CRAES chamber was reliable and could meet the demands for investigating tropospheric chemistry.

Characterization of long-range transported bioaerosols in the Central Mediterranean

Airborne bacteria were characterized over a 2-y period via high-throughput massive sequencing of 16S rRNA gene in aerosol samples collected at a background mountain European Monitoring and Evaluation Programme (EMEP) Network site (Monte Martano, Italy) located in the Central Mediterranean area. The air mass origin of nineteen samples was identified by air mass modelling and a detailed chemical analysis was performed. Four main origins (Saharan, North-western, North-eastern, and Regional) were identified, and distinct microbial communities were associated with these air masses. Samples featured a great bacterial diversity with Protobacteria being the most abundant phylum, and Sphingomonas followed by Acidovorax, Acinetobacter and Stenotrophomonas the most abundant genera of the dataset. Bacterial genera including potential human and animal pathogens were more abundant in European and in Regional samples compared to Saharan samples; this stressed the relevance of anthropic impact on bacterial populations transported by air masses that cross densely populated areas. The principal aerosol chemical characteristics and the airborne bacterial communities were correlated by cluster analysis, similarity tests and non-metric multidimensional scaling analysis, explaining most of the variability observed. However, the strong correlation between bacterial community structure and air mass origin hampered the possibility to disentangle the effects of variations in bacterial populations and in dust provenance on variations in chemical variables.