Atmospheric photosensitized heterogeneous and multiphase reactions: from outdoors to indoors

Reaction kinetics and molecular characterization of the compounds formed by photosensitized degradation of the plastic additive bisphenol A in the atmospheric aqueous phase

Bisphenol A (BPA, 4,4'-(propane-2,2-diyl)diphenol) is a common plasticizer that is very widespread in the environment and is also found at significant concentrations in the global oceans, due to contamination by plastics. Here we show that triplet sensitization is an important degradation pathway for BPA in natural surface waters, which could prevail if the water dissolved organic carbon is above 2-3 mgC L-1. Bromide levels as per seawater conditions have the potential to slow down BPA photodegradation, a phenomenon that could not be offset by reaction of BPA with Br2•- (second-order reaction rate constant of (2.54 ± 0.09) × 108 M-1 s-1). Ultra-high resolution mass spectrometry revealed that the presence of inorganic salts (NaCl and NaBr) markedly increased the complexity of the observed CHO product compounds formed upon photosensitized degradation of BPA. The obtained results suggest that bisphenols can be efficiently removed by photosensitized reactions and generate higher number of oligomers and polyaromatic compounds in the sea surface and liquid water of marine aerosols compared to freshwaters and/or dilute cloud-water. Considering that polyaromatic compounds absorb solar actinic radiation, these results suggest that inorganic salts could significantly affect the photosensitized degradation of bisphenols and consequently influence the light-absorbing properties of marine aerosols and, ultimately, the Earth's radiative balance.

Photolytic degradation of commonly used pesticides adsorbed on silica particles

The currently used pesticides are mostly semi-volatile organic compounds. As a result, a fraction of them can be adsorbed on atmospheric aerosol surface. Their atmospheric photolysis is poorly documented, and gaps persist in understanding their reactivity in the particle phase. Laboratory experiments were conducted to determine the photolysis rates of eight commonly used pesticides (i.e., cyprodinil, deltamethrin, difenoconazole, fipronil, oxadiazon, pendimethalin, permethrin, and tetraconazole) using a flow reactor. These pesticides were individually adsorbed on hydrophobic silica particles and exposed to a filtered xenon lamp to mimic atmospheric aerosols and sunlight irradiation, respectively. The estimated photolysis rate constants ranged from less than (3.4 ± 0.3) × 10-7 s-1 (permethrin; >47.2 days) to (3.8 ± 0.2) × 10-5 s-1 (Fipronil; 0.4 days), depending on the considered compound. Moreover, this study assessed the influence of pesticide mixtures on their photolysis rates, revealing that certain pesticides can act as photosensitizers, thereby enhancing the reactivity of permethrin and tetraconazole. This study underscores the importance of considering photolysis degradation when evaluating pesticide fate and reactivity, as it can be a predominant degradation pathway for some pesticides. This contributes to an enhanced understanding of their behavior in the atmosphere and their impact on air quality.

Anthraquinones-based photocatalysis: A comprehensive review

In recent years, there has been significant interest in photocatalytic technologies utilizing semiconductors and photosensitizers responsive to solar light, owing to their potential for energy and environmental applications. Current efforts are focused on enhancing existing photocatalysts and developing new ones tailored for environmental uses. Anthraquinones (AQs) serve as redox-active electron transfer mediators and photochemically active organic photosensitizers, effectively addressing common issues such as low light utilization and carrier separation efficiency found in conventional semiconductors. AQs offer advantages such as abundant raw materials, controlled preparation, excellent electron transfer capabilities, and photosensitivity, with applications spanning the energy, medical, and environmental sectors. Despite their utility, comprehensive reviews on AQs-based photocatalytic systems in environmental contexts are lacking. In this review, we thoroughly describe the photochemical properties of AQs and their potential applications in photocatalysis, particularly in addressing key environmental challenges like clean energy production, antibacterial action, and pollutant degradation. However, AQs face limitations in practical photocatalytic applications due to their low electrical conductivity and solubility-related secondary contamination. To mitigate these issues, the design and synthesis of graphene-immobilized AQs are highlighted as a solution to enhance practical photocatalytic applications. Additionally, future research directions are proposed to deepen the understanding of AQs' theoretical mechanisms and to provide practical applications for wastewater treatment. This review aims to facilitate mechanistic studies and practical applications of AQs-based photocatalytic technologies and to improve understanding of these technologies.

Oxidant concentrations and photochemistry in a vehicle cabin

Indoor air quality (IAQ) in vehicles can be important to people's health, especially for those whose occupations require them to spend extensive time in vehicles. To date, research on vehicle IAQ has primarily focused on direct emissions as opposed to chemistry happening in vehicle cabins. In this work, we conducted time-resolved measurements of the oxidants and oxidant precursors ozone (O3), nitric oxide (NO), nitrogen dioxide (NO2), and nitrous acid (HONO) inside the cabin of a 2012 Toyota Rav4 under varying ventilation conditions (i.e., car off, car on with passive ventilation, car on with mechanical ventilation via the recirculating fan, and car on with mechanical ventilation via the direct fan). Ozone levels inside the vehicle were significantly lower than outdoors under most conditions, and were approximately half the outdoor levels when the direct fan was in operation. Nitric oxide and NO2 concentrations were very low both inside the vehicle and outdoors. Nitrous acid levels in the vehicle were lower than reported values in other indoor environments, though much higher than expected outdoor levels. We also investigated the potential for photochemical production of radicals in the vehicle. Time- and wavelength-resolved solar irradiance spectra were collected, and steady state hydroxyl radical (OH) and nitrate radical (NO3) concentrations were calculated. Steady state OH concentrations were predicted to be similar to those in air masses in residences illuminated by sunlight, suggesting the importance of HONO photolysis in vehicles. Conversely, nitrate radicals (NO3) were not considered significant indoor oxidants in our study due to rapid titration by NO. Overall, our findings emphasize the importance of both air exchange and photochemistry in shaping the composition of air inside vehicles.

A review of indoor nitrous acid (HONO) pollution: Measurement techniques, pollution characteristics, sources, and sinks

Indoor air quality is of major concern for human health and well-being. Nitrous acid (HONO) is an emerging indoor pollutant, and its indoor mixing ratios are usually higher than outdoor levels, ranging from a few to tens of parts per billion (ppb). HONO exhibits adverse effects to human health due to its respiratory toxicity and mutagenicity. Additionally, HONO can easily undergo photodissociation by ultraviolet light to produce hydroxyl radicals (OH•), which in turn trigger a series of further photochemical oxidation reactions of primary or secondary pollutants. The accumulation of indoor HONO can be attributed to both direct emissions from combustion sources, such as cooking, and secondary formation resulting from enhanced heterogeneous reactions of NOx on indoor surfaces. During the day, the primary sink of indoor HONO is photolysis to OH• and NO. Moreover, adsorption and/or reaction on indoor surfaces, and diffusion to the outside atmosphere contribute to HONO loss both during the day and at night. The level of indoor HONO is also affected by human occupancy, which can influence household factors such as temperature, humidity, light irradiation, and indoor surfaces. This comprehensive review article summarized the research progress on indoor HONO pollution based on indoor air measurements, laboratory studies, and model simulations. The environmental and health effects were highlighted, measurement techniques were summarized, pollution levels, sources and sinks, and household influencing factors were discussed, and the prospects in the future were proposed.

Organic peroxyl radicals from biacetyl accelerated the visible-light degradation of steroid estrogens in aqueous solution

Indirect photodegradation is an important pathway for the reduction of steroid estrogens in sunlit surface waters. Nevertheless, the kinetics and mechanisms governing the interaction between coexisting carbonyl compounds and estrogens under visible light (Vis) remain unexplored. This study systematically investigates the Vis-induced photodegradation of 17β-estradiol (E2) in the presence of five specific carbonyl compounds-biacetyl (BD), acetone, glyoxal, pyruvic acid, and benzoquinone. The results demonstrate that, among these compounds, only BD significantly enhanced the photodegradation of E2 under Vis irradiation (λ > 400 nm). The pseudo-first order photodegradation rate constants (k1) of E2 in the Vis/BD system were 0.025 min-1 and 0.076 min-1 in ultrapure water and river water, respectively. The enhancing effect of BD was found to be pH-dependent, increasing the pH from 3.0 to 11.0 resulted in a 76% reduction in the k1 value of E2 in the Vis/BD system. Furthermore, the presence of humic acid, NO3-, or HCO3- led to an increase of more than 35% in the k1 value of E2, while NO2- exerted a pronounced inhibitory effect, resulting in a 92% decrease. Peroxyacetyl and peroxymethyl radicals, derived from BD in a yield ratio of 9, played a crucial role in the degradation of E2. These peroxyl radicals primarily targeted electron-rich hydroxyl sites of E2, initiating hydroxylation and ring-opening reactions that culminated in the formation of acidic byproducts. Notably, toxicity evaluation indicates that these hydroxylated and acidic products exhibited lower toxicity than the parent compound E2. This study highlights the important role of peroxyl radicals in estrogen degradation within aquatic environment, and also helps to design efficient visible light-responsive photo-activators for the treatment of estrogen-contaminated waters.

Singlet fission initiating organic photosensitizations

The feasibility of singlet fission (SF) in organic photosensitizers is investigated through spin-flip long-range corrected time-dependent density functional theory. This study focuses on four major organic photosensitizer molecules: benzophenone, boron-dipyrromethene, methylene blue, and rose bengal. Calculations demonstrate that all these molecules possess moderate [Formula: see text]-stacking energies and closely-lying singlet (S) and quintet (triplet-triplet, TT) excitations, satisfying the essential conditions for SF: (1) Near-degenerate low-lying S and (TT) excitations with a significant S-T energy gap, and (2) Moderate [Formula: see text]-stacking energy of chromophores, slightly higher than solvation energy, enabling dissociation for triplet-state chromophore generation. Moreover, based on the El-Sayed rule, intersystem crossing is found to simultaneously proceed at very slow rates in all these photosensitizers. This is attributed to the fact that the lowest singlet excitation of the monomers partly involves [Formula: see text] transitions alongside the main [Formula: see text] transitions. The proposed mechanisms are strongly substantiated by comparisons with experimental studies.

Source apportionment for indoor air pollution: Current challenges and future directions

Source apportionment (SA) for indoor air pollution is challenging due to the multiplicity and high variability of indoor sources, the complex physical and chemical processes that act as primary sources, sinks and sources of precursors that lead to secondary formation, and the interconnection with the outdoor environment. While the major indoor sources have been recognized, there is still a need for understanding the contribution of indoor versus outdoor-generated pollutants penetrating indoors, and how SA is influenced by the complex processes that occur in indoor environments. This paper reviews our current understanding of SA, through reviewing information on the SA techniques used, the targeted pollutants that have been studied to date, and their source apportionment, along with limitations or knowledge gaps in this research field. The majority (78 %) of SA studies to date focused on PM chemical composition/size distribution, with fewer studies covering organic compounds such as ketones, carbonyls and aldehydes. Regarding the SA method used, the majority of studies have used Positive Matrix Factorization (31 %), Principal Component Analysis (26 %) and Chemical Mass Balance (7 %) receptor models. The indoor PM sources identified to date include building materials and furniture emissions, indoor combustion-related sources, cooking-related sources, resuspension, cleaning and consumer products emissions, secondary-generated pollutants indoors and other products and activity-related emissions. The outdoor environment contribution to the measured pollutant indoors varies considerably (<10 %- 90 %) among the studies. Future challenges for this research area include the need for optimization of indoor air quality monitoring and data selection as well as the incorporation of physical and chemical processes in indoor air into source apportionment methodology.

Targeted NO Oxidation and Synchronous NO2 Inhibition via Oriented 1O2 Formation Based on Lewis Acid Site Adjustment

An appealing strategy for ensuring environmental benefits of the photocatalytic NO oxidation reaction is to convert NO into NO3- instead of NO2, yet the selectivity of products remains challenging. Here, such a scenario could be realized by tailoring the exposure of Lewis acid sites on the surface of ZrO2, aiming to precisely regulate the ROS evolution process for the selective oxidation of NO into NO3-. As evidenced by highly combined experimental characterizations and density functional theory (DFT) simulations, Lewis acid sites serving as electron acceptors could induce itinerant electron redistribution, charge-carrier transfer, and further oxidation of •O2-, which promotes the oriented formation of 1O2. As a result, monoclinic ZrO2 with more Lewis acid sites exhibited an outstanding NO conversion efficiency (56.33%) and extremely low NO2 selectivity (5.04%). The ROS-based reaction process and promotion mechanism of photocatalytic performance have been revealed on the basis of ESR analysis, ROS-quenching experiments, and in situ ROS-quenching DRIFTS. This work could provide a critical view toward oriented ROS formation and advance a unique mechanism of selective NO oxidation into NO3-.

Sulfate Formation by Photosensitization in Mixed Incense Burning-Sodium Chloride Particles: Effects of RH, Light Intensity, and Aerosol Aging

Elevated particulate sulfate concentrations have been frequently observed in coastal areas when air masses are influenced by continental emissions, especially combustion sources like biomass burning. We studied the SO2 uptake by laboratory-generated droplets containing incense smoke extracts and sodium chloride (IS-NaCl) under irradiation and found enhanced sulfate production over pure NaCl droplets, attributable to photosensitization induced by constituents in IS. Low relative humidity and high light intensity facilitated sulfate formation and increased the SO2 uptake coefficient by IS-NaCl particles. Aging of the IS particles further enhanced sulfate production, attributable to the enhanced secondary oxidant production promoted by increased proportions of nitrogen-containing CHN and oxygen- and nitrogen-containing CHON species under light and air. Experiments using model compounds of syringaldehyde, pyrazine, and 4-nitroguaiacol verified the enhancements of CHN and CHON species in sulfate formation. This work provides experimental evidence of enhanced sulfate production in laboratory-generated IS-NaCl droplets via enhanced secondary oxidant production triggered by photosensitization in multiphase oxidation processes under light and air. Our results can shed light on the possible interactions between sea salt and biomass burning aerosols in enhancing sulfate production.

Recent Progress of Carbon Dots for Air Pollutants Detection and Photocatalytic Removal: Synthesis, Modifications, and Applications

Rapid industrialization has inevitably led to serious air pollution problems, thus it is urgent to develop detection and treatment technologies for qualitative and quantitative analysis and efficient removal of harmful pollutants. Notably, the employment of functional nanomaterials, in sensing and photocatalytic technologies, is promising to achieve efficient in situ detection and removal of gaseous pollutants. Among them, carbon dots (CDs) have shown significant potential due to their superior properties, such as controllable structures, easy surface modification, adjustable energy band, and excellent electron-transfer capacities. Moreover, their environmentally friendly preparation and efficient capture of solar energy provide a green option for sustainably addressing environmental problems. Here, recent advances in the rational design of CDs-based sensors and photocatalysts are highlighted. An overview of their applications in air pollutants detection and photocatalytic removal is presented, especially the diverse sensing and photocatalytic mechanisms of CDs are discussed. Finally, the challenges and perspectives are also provided, emphasizing the importance of synthetic mechanism investigation and rational design of structures.

Advances in Titanium Carbide (Ti3C2T x ) MXenes and Their Metal-Organic Framework (MOF)-Based Nanotextures for Solar Energy Applications: A Review

Introducing new materials with low cost and superior solar harvesting efficiency requires urgent attention to solve energy and environmental challenges. Titanium carbide (Ti3C2T x ) MXene, a 2D layered material, is a promising solution to solve the issues of existing materials due to their promising conductivity with low cost to function as a cocatalyst/support. On the other hand, metal-organic frameworks (MOFs) are emerging materials due to their high surface area and semiconducting characteristics. Therefore, coupling them would be promising to form composites with higher solar harvesting efficiency. Thus, the main objective of this work to disclose recent development in Ti3C2T x -based MOF nanocomposites for energy conversion applications to produce renewable fuels. MOFs can generate photoinduced electron/hole pairs, followed by transfer of electrons to MXenes through Schottky junctions for photoredox reactions. Currently, the principles, fundamentals, and mechanism of photocatalytic systems with construction of Schottky junctions are critically discussed. Then the basics of MOFs are discussed thoroughly in terms of their physical properties, morphologies, optical properties, and derivatives. The synthesis of Ti3C2T x MXenes and their composites with the formation of surface functionals is systematically illustrated. Next, critical discussions are conducted on design considerations and strategies to engineer the morphology of Ti3C2T x MXenes and MOFs. The interfacial/heterojunction modification strategies of Ti3C2T x MXenes and MOFs are then deeply discussed to understand the roles of both materials. Following that, the applications of MXene-mediated MOF nanotextures in view of CO2 reduction and water splitting for solar fuel production are critically analyzed. Finally, the challenges and a perspective toward the future research of MXene-based MOF composites are disclosed.

Photosensitization mechanisms at the air-water interface of aqueous aerosols

Photosensitization reactions are believed to provide a key contribution to the overall oxidation chemistry of the Earth's atmosphere. Generally, these processes take place on the surface of aqueous aerosols, where organic surfactants accumulate and react, either directly or indirectly, with the activated photosensitizer. However, the mechanisms involved in these important interfacial phenomena are still poorly known. This work sheds light on the reaction mechanisms of the photosensitizer imidazole-2-carboxaldehyde through ab initio (QM/MM) molecular dynamics simulations and high-level ab initio calculations. The nature of the lowest excited states of the system (singlets and triplets) is described in detail for the first time in the gas phase, in bulk water, and at the air-water interface, and possible intersystem crossing mechanisms leading to the reactive triplet state are analyzed. Moreover, the reactive triplet state is shown to be unstable at the air-water surface in a pure water aerosol. The combination of this finding with the results obtained for simple surfactant-photosensitizer models, together with experimental data from the literature, suggests that photosensitization reactions assisted by imidazole-2-carboxaldehyde at the surface of aqueous droplets can only occur in the presence of surfactant species, such as fatty acids, that stabilize the photoactivated triplet at the interface. These findings should help the interpretation of field measurements and the design of new laboratory experiments to better understand atmospheric photosensitization processes.

Enhanced Nitrite Production from the Aqueous Photolysis of Nitrate in the Presence of Vanillic Acid and Implications for the Roles of Light-Absorbing Organics

A prominent source of hydroxyl radicals (•OH), nitrous acid (HONO) plays a key role in tropospheric chemistry. Apart from direct emission, HONO (or its conjugate base nitrite, NO2-) can be formed secondarily in the atmosphere. Yet, how secondary HONO forms requires elucidation, especially for heterogeneous processes involving numerous organic compounds in atmospheric aerosols. We investigated nitrite production from aqueous photolysis of nitrate for a range of conditions (pH, organic compound, nitrate concentration, and cation). Upon adding small oxygenates such as ethanol, n-butanol, or formate as •OH scavengers, the average intrinsic quantum yield of nitrite [Φ(NO2-)] was 0.75 ± 0.15%. With near-UV-light-absorbing vanillic acid (VA), however, the effective Φ(NO2-) was strongly pH-dependent, reaching 8.0 ± 2.1% at a pH of 8 and 1.5 ± 0.39% at a more atmospherically relevant pH of 5. Our results suggest that brown carbon (BrC) may greatly enhance the nitrite production from the aqueous nitrate photolysis through photosensitizing reactions, where the triplet excited state of BrC may generate solvated electrons, which reduce nitrate to NO2 for further conversion to nitrite. This photosensitization process by BrC chromophores during nitrate photolysis under mildly acidic conditions may partly explain the missing HONO in urban environments.

Ambient Air Purification by Nanotechnologies: From Theory to Application

Air pollution has been a recurring problem in northern Chinese cities, and high concentrations of PM2.5 in winter have been a particular cause for concern. Secondary aerosols converted from precursor gases (i.e., nitrogen oxides and volatile organic compounds) evidently account for a large fraction of the PM2.5. Conventional control methods, such as dust removal, desulfurization, and denitrification, help reduce emissions from stationary combustion sources, but these measures have not led to decreases in haze events. Recent advances in nanomaterials and nanotechnology provide new opportunities for removing fine particles and gaseous pollutants from ambient air and reducing the impacts on human health. This review begins with overviews of air pollution and traditional abatement technologies, and then advances in ambient air purification by nanotechnologies, including filtration, adsorption, photocatalysis, and ambient-temperature catalysis are presented—from fundamental principles to applications. Current state-of-the-art developments in the use of nanomaterials for particle removal, gas adsorption, and catalysis are summarized, and practical applications of catalysis-based techniques for air purification by nanomaterials in indoor, semi-enclosed, and open spaces are highlighted. Finally, we propose future directions for the development of novel disinfectant nanomaterials and the construction of advanced air purification devices.

Tight electrostatic regulation of the OH production rate from the photolysis of hydrogen peroxide adsorbed on surfaces

Recently, experimental and theoretical works have reported evidence indicating that photochemical processes may significantly be accelerated at heterogeneous interfaces, although a complete understanding of the phenomenon is still lacking. We have carried out a theoretical study of interface and surface effects on the photochemistry of hydrogen peroxide (H2O2) using high-level ab initio methods and a variety of models. Hydrogen peroxide is an important oxidant that decomposes in the presence of light, forming two OH radicals. This elementary photochemical process has broad interest and is used in many practical applications. Our calculations show that it can drastically be affected by heterogeneous interfaces. Thus, compared to gas phase, the photochemistry of H2O2 appears to be slowed on the surface of apolar or low-polar surfaces and, in contrast, hugely accelerated on ionic surfaces or the surface of aqueous electrolytes. We give particular attention to the case of the neat air-water interface. The calculated photolysis rate is similar to the gas phase, which stems from the compensation of two opposite effects, the blue shift of the n→σ* absorption band and the increase of the absorption intensity. Nevertheless, due to the high affinity of H2O2 for the air-water interface, the predicted OH production rate is up to five to six orders of magnitude larger. Overall, our results show that the photochemistry of H2O2 in heterogeneous environments is greatly modulated by the nature of the surface, and this finding opens interesting new perspectives for technological and biomedical applications, and possibly in various atmospheres.

Screening of imidazoles in atmospheric aerosol particles using a hybrid targeted and untargeted method based on ultra-performance liquid chromatography-quadrupole time-of-flight mass spectrometry

The method for identification and quantification of imidazoles in atmospheric aerosol particles with an aerodynamic diameter up to 2.5 μm (PM_2.5) is scarce, and the existing method focus on only a few imidazoles. With the goal of measuring more imidazoles, especially some previously unidentified ones, we developed a screening workflow based on data-dependent acquisition (DDA) auto MS/MS with a preferred targeted list containing 421 imidazoles using ultra-performance liquid chromatography-quadrupole time-of-flight mass spectrometry (UPLC-QTOF-MS). To enable our method to effectively and accurately detect as many imidazoles as possible, we optimized and validated the method based on specificity, limit of detection (LOD), limit of quantification (LOQ), linearity, accuracy, precision and matrix effects using 20 imidazole standards with different functional groups. The method exhibited excellent performance with LOD and LOQ of 0.5-2 ng/mL and 1.5-6 ng/mL, respectively, and spiked recoveries ranging from 64.7 to 98.7% with standard deviations less than 16.0%, and with relatively shorter analytical time. The established method was then used to screen imidazoles in 37 ambient PM_2.5 samples. Ten targeted imidazoles were identified and quantified using imidazole standards, while five suspected imidazoles were identified without standards, and three imidazoles have not been reported before. Concentrations of the 10 targeted imidazoles ranged from 0.13 to 0.42 ng/m³. The established method enabled us to identify a wide range of imidazoles in ambient aerosol particles with and without using standards.

Indoor heterogeneous photochemistry of furfural drives emissions of nitrous acid

People spend approximately 80% of their time indoor, making the understanding of the indoor chemistry an important task for safety. The high surface-area-to-volume ratio characteristic of indoor environments leads the semi-volatile organic compounds (sVOCs) to deposit on the surfaces. Using a long path absorption photometer (LOPAP), this work investigates the formation of nitrous acid (HONO) through the photochemistry of adsorbed nitrate anions and its enhancement by the presence of furfural. Using a high-resolution proton-transfer-reaction time-of-flight mass spectrometer (PTR-TOF-MS), this work also investigates the surface emissions of VOCs from irradiated films of furfural and a mix of furfural and nitrate anions. Among the emitted VOCs, 2(5H)-furanone/2-Butenedial was observed at high concentrations, leading to maleic anhydride formation after UV irradiation. Moreover, the addition of potassium nitrate to the film formed NO_x and HONO concentrations up to 10 ppb, which scales to ca. 4 ppb for realistic indoor conditions. This work helps to understand the high levels of HONO and NO_x measured indoors.

Heterojunction interface of zinc oxide and zinc sulfide promoting reactive molecules activation and carrier separation toward efficient photocatalysis

Heterojunction photocatalysts, which can alleviate the low carrier separation efficiency and insufficient light absorption capacity of a single catalyst, have received widespread attention. However, the specific interfacial structure of the heterojunction and its effect on the photocatalytic reaction is still unclear. Herein, a battery of zinc oxide/zinc sulfide (ZnO@ZnS) heterojunction microspheres with different degrees of sulfuration were successfully constructed via a facile hydrothermal method. The as-prepared photocatalysts shown decent aerobic nitric oxide (NO) oxidation performance under visible light irradiation, and the results of various characterization techniques illustrated that the superior photoactivity could be ascribed to the spatial separation of photoinduced electron-hole pairs due to the synergy of the internal electric field and the band offset. More importantly, density functional theory (DFT) calculations revealed that the heterojunction interface can significantly promote the generation of reactive oxygen species (ROS) and NO⁺ reaction intermediates and thus accelerate the photocatalytic reaction. Finally, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) technology was used to time-dependently monitor the NO oxidation process, revealing the photocatalytic mechanism. This work investigated the role of the heterojunction interface in the gas-phase catalytic reaction, broadening the practical application of the ZnO@ZnS heterojunction.

Light-Enhanced Heterogeneous Conversion of NO2 to HONO on Solid Films Consisting of Fluorene and Fluorene/Na2SO4: An Impact on Urban and Indoor Atmosphere

Polycyclic aromatic hydrocarbons (PAHs) as constituents of urban grime and indoor surfaces can impact the photochemical conversion of nitrogen dioxide (NO₂) to nitrous acid (HONO) thereby impacting the oxidation capacity of the atmosphere. In this study we investigate the effect of relative humidity (RH%), light intensity, and NO₂ concentrations on uptake coefficients (γ) of NO₂ on solid film consisting of fluorene (FL) and a mixture of FL and Na₂SO₄ as a proxy for urban and indoor grime at ambient pressure and temperature. γ(NO₂) on solid FL increased markedly from (5.7 ± 1.7) × 10^-7 at 0% RH to (4.6 ± 1.0) × 10^-6 at 90% RH. The NO₂ to HONO conversion yield, (ΔHONO/ΔNO₂)%, increases with RH from 40% at 0% RH up to 80% at 60-90% RH, indicating that the water molecules favor the formation of HONO up to 60% RH. These results suggest that the heterogeneous photochemical reaction of NO₂ on FL and FL/Na₂SO₄ can be an important source of HONO in the urban environment and indoor atmosphere and should be considered in photochemical models.

Photoinduced Oxidation Reactions at the Air-Water Interface

Chemistry on water is a fascinating area of research. The surface of water and the interfaces between water and air or hydrophobic media represent asymmetric environments with unique properties that lead to unexpected solvation effects on chemical and photochemical processes. Indeed, the features of interfacial reactions differ, often drastically, from those of bulk-phase reactions. In this Perspective, we focus on photoinduced oxidation reactions, which have attracted enormous interest in recent years because of their implications in many areas of chemistry, including atmospheric and environmental chemistry, biology, electrochemistry, and solar energy conversion. We have chosen a few representative examples of photoinduced oxidation reactions to focus on in this Perspective. Although most of these examples are taken from the field of atmospheric chemistry, they were selected because of their broad relevance to other areas. First, we outline a series of processes whose photochemistry generates hydroxyl radicals. These OH precursors include reactive oxygen species, reactive nitrogen species, and sulfur dioxide. Second, we discuss processes involving the photooxidation of organic species, either directly or via photosensitization. The photochemistry of pyruvic acid and fatty acid, two examples that demonstrate the complexity and versatility of this kind of chemistry, is described. Finally, we discuss the physicochemical factors that can be invoked to explain the kinetics and thermodynamics of photoinduced oxidation reactions at aqueous interfaces and analyze a number of challenges that need to be addressed in future studies.

Does the ubiquitous use of essential oil-based products promote indoor air quality? A critical literature review

Essential oils are frequently used as natural fragrances in housecleaning products and air fresheners marketed as green and healthy. However, these substances are volatile and reactive chemical species. This review focuses on the impact of essential oil-based household products on indoor air quality. First, housecleaning products containing essential oils are explored in terms of composition and existing regulations. Specific insight is provided regarding terpenes in fragranced housecleaning products, air fresheners, and pure essential oils. Second, experimental methodologies for terpene monitoring, from sampling to experimental chambers and analytical methods, are addressed, emphasizing the experimental issues in monitoring terpenes in indoor air. Third, the temporal dynamics of terpene emissions reported in the literature are discussed. Despite experimental discrepancies, essential oil-based products are significant sources of terpenes in indoor air, inducing a high exposure of occupants to terpenes. Finally, the fate of terpenes is explored from sorptive and reactive points of view. In addition to terpene deposition on surfaces, indoor oxidants may induce homogeneous and heterogeneous reactions, resulting in secondary pollutants, such as formaldehyde and secondary organic aerosols. Overall, essential oil-based products can negatively impact indoor air quality; therefore, standard protocols and real-scale approaches are needed to explore the indoor physics and chemistry of terpenes, from emissions to reactivity.

Oxygen Vacancies Promoted the Selective Photocatalytic Removal of NO with Blue TiO2 via Simultaneous Molecular Oxygen Activation and Photogenerated Hole Annihilation

Semiconductor photocatalytic technology has great potential for the removal of dilute gaseous NO in indoor and outdoor atmospheres but suffers from unsatisfactory NO-removal selectivity due to undesirable NO₂ byproduct generation. In this study, we demonstrate that the 99% selectivity of photocatalytic NO oxidation toward nitrate can be achieved over blue TiO₂ bearing oxygen vacancies (OVs) under visible-light irradiation. First-principles density functional theory calculation and experimental results suggested that the OVs of blue TiO₂ with localized electrons could facilitate the molecular oxygen activation through single-electron pathways to generate ·O₂^- and simultaneously promote the photogenerated hole annihilation. The generated ·O₂^- directly converted NO to nitrate, while the hole annihilation inhibited the side-reaction between holes and NO to avoid toxic NO₂ byproduct formation, resulting in the highly selective removal of NO. This study reveals the dual functions of OVs in defective photocatalysts and also provides fundamental guidance for the selective purification of NO with photocatalytic technology.

Ionic-Strength Effects on the Reactive Uptake of Ozone on Aqueous Pyruvic Acid: Implications for Air-Sea Ozone Deposition

A vertical wetted-wall flow-tube technique was used to explore the ionic strength effects at the air-water interface in mediating the sea-surface reaction between ozone (O₃) and pyruvic acid (PA). The uptake coefficients of ozone on aqueous PA increase substantially with the concentrations of bromide (Br^-) ions, clearly indicating that the dry deposition of ozone could be significantly enhanced due to the presence of carbonyl compounds such as PA at the bromide-rich sea surface. Based on the observed uptake coefficients, the estimated deposition velocity of ozone (100 ppb) for a nanomolar range of PA concentrations is ∼1 × 10^-3 m s^-1, which represents a significant contribution to the known deposition velocity of ozone at the sea surface. The analysis of reaction products by ultra-high-resolution Fourier transform-ion cyclotron resonance mass spectrometry suggests the formation of oligomers during both the dark and light-induced heterogeneous reactions between gaseous O₃ and PA occurring at the surface of a dilute aqueous phase (representative of cloud droplets). The detected high-molecular-weight compounds are much more complex than the oligomeric species identified during the photolytic degradation of bulk aqueous PA alone.

Assessment of functional nanomaterials in medical applications: can time mend public and occupational health risks related to the products' fate?

Surface coatings are one promising option to prevent bacterial adhesion and biofilm formation given the prevalence of antibiotic resistant bacterial strains. Titanium dioxide (TiO₂) is presently considered to be the only photocatalytic material suitable for commercial use, although the toxicity risks of TiO₂, particularly in its nanoparticulate form, have not been fully addressed. The aim of this study was to determine release of nanoparticles (NPs) from functional materials for medical applications and their aerosol formation. Further, the fate of the material with respect to its product lifetime was investigated. The present study examined the risk of NP exposure since released submicronic and inhalable manufactured nano-objects, their agglomeraates or aggregates containing Ti were detected. The coating of the material magnifies its emission levels when comparing the obtained product properties to those of an uncoated sample. The evolution of release tendecy with the material's time of use shows that release does not vanish upon continuous material losses induced by the release, thus the risk does not diminish with time. Consequently, this nanomaterial TiO₂ needs to be avoided in healthcare settings, or, alternatively, new TiO₂-deposition techniques are required to be developed.

Enhanced photochemical conversion of NO2 to HONO on humic acids in the presence of benzophenone

The photochemical conversion of NO₂ to HONO on humic acids (HA) in the presence of benzophenone (BP) was investigated using a flow tube reactor coupled to a NO_x analyzer at ambient pressure. BP significantly enhanced the reduction of NO₂ to HONO on HA under simulated sunlight, as shown by the increase of NO₂ uptake coefficient (γ) and HONO yield with the mass ratio of BP to HA. The γ and HONO yield on the mixtures of HA and BP obviously depended on the environmental conditions. Both γ and HONO yield increased with the increase of irradiation intensity and temperature, whereas they decreased with pH. The γ exhibited a negative dependence on the NO₂ concentration, which had slight influences on the HONO yield. There were maximum values for the γ and HONO yield at relative humidity (RH) of 22%. Finally, atmospheric implications about the photochemical reaction of NO₂ and HA in the presence of photosensitive species were discussed.

Environment-Friendly Carbon Quantum Dots/ZnFe2O4 Photocatalysts: Characterization, Biocompatibility, and Mechanisms for NO Removal

A highly efficient and environmentally-friendly oxidation process is always desirable for air purification. This study reported a novel carbon quantum dots (CQDs)/ZnFe₂O₄ composite photocatalyst for the first time through a facile hydrothermal process. The CQDs/ZnFe₂O₄ (15 vol %) composite demonstrates stronger transient photocurrent response, approximately 8 times higher than that of ZnFe₂O₄, indicating superior transfer efficiency of photogenerated electrons and separation efficiency of photogenerated electron-hole pairs. Compared with pristine ZnFe₂O₄ nanoparticles, CQDs/ZnFe₂O₄ displayed enhanced photocatalytic activities on gaseous NO_x removal and high selectivity for nitrate formation under visible light (λ > 420 nm) irradiation. Electron spin resonance analysis and a series of radical-trapping experiments showed that the reactive species contributing to NO elimination were ·O₂^- and ·OH radicals. The possible mechanisms were proposed regarding how CQDs improve the photocatalytic performance of ZnFe₂O₄. The CQDs are believed to act as an electron reservoir and transporter as well as a powerful energy-transfer component during the photocatalysis processes over CQDs/ZnFe₂O₄ samples. Furthermore, the toxicity assessment authenticated good biocompatibility and low cytotoxity of CQDs/ZnFe₂O₄. The results of this study indicate that CQDs/ZnFe₂O₄ is a promising photocatalyst for air purification.

Visible-Light-Active Plasmonic Ag-SrTiO3 Nanocomposites for the Degradation of NO in Air with High Selectivity

Harnessing inexhaustible solar energy for photocatalytic disposal of nitrogen oxides is of great significance nowadays. In this study, Ag-SrTiO3 nanocomposites (Ag-STO) were synthesized via one-pot solvothermal method for the first time. The deposition of Ag nanoparticles incurs a broad plasmonic resonance absorption in the visible light range, resulting in enhanced visible light driven activity on NO removal in comparison with pristine SrTiO3. The Ag loading amount has a significant influence on light absorption properties of Ag-STO, which further affects the photocatalytic efficiency. It was shown that 0.5% Ag loading onto SrTiO3 (in mass ratio) could remove 30% of NO in a single reaction path under visible light irradiation, which is twice higher than that achieved on pristine SrTiO3. Most importantly, the generation of harmful intermediate (NO2) is largely inhibited over SrTiO3 and Ag-STO nanocomposites, which can be ascribed to the basic surface property of strontium sites. As identified by electron spin resonance (ESR) spectra,·O2(-) and ·OH radicals are the major reactive species for NO oxidation. Essentially speaking, the abundance of reactive oxygen radicals produced over Ag-STO nanocomposites are responsible for the improved photocatalytic activity. This work provides a facile and controllable route to fabricate plasmonic Ag-SrTiO3 nanocomposite photocatalyst featuring high visible light activity and selectivity for NO abatement.

First Quantification of Imidazoles in Ambient Aerosol Particles: Potential Photosensitizers, Brown Carbon Constituents, and Hazardous Components

Imidazoles are widely discussed in recent literature. They have been studied as a secondary product of the reaction of dicarbonyls with nitrogen containing compounds in a number of laboratory studies, potentially acting as photosensitizers triggering secondary organic aerosol growth and are forming constituents of light absorbing brown carbon. Despite the knowledge from laboratory studies, no quantitative information about imidazoles in ambient aerosol particles is available. Within the present study, five imidazoles (1-butylimidazole, 1-ethylimidazole, 2-ethylimidazole, imidazol-2-carboxaldehyde, and 4(5)-methylimidazole) were successfully identified and quantified for the first time in ambient aerosol samples from different environments in Europe and China. Their concentrations range between 0.2 and 14 ng/m(3). 4(5)-Methylimidazole was found to be the most abundant imidazole. The occurrence of imidazoles seems to be favored at sites with strong biomass burning influence or connected to more polluted air masses. No connection was found between aerosol particle pH and imidazole concentration. Our work corroborates the laboratory studies by showing that imidazoles are present in ambient aerosol samples in measurable amounts. Moreover, it further motivates to explore the potential photosensitizing properties of small alkyl-substituted imidazoles.

Photosensitized Production of Atmospherically Reactive Organic Compounds at the Air/Aqueous Interface

We report on experiments that probe photosensitized chemistry at the air/water interface, a region that does not just connect the two phases but displays its own specific chemistry. Here, we follow reactions of octanol, a proxy for environmentally relevant soluble surfactants, initiated by an attack by triplet-state carbonyl compounds, which are themselves concentrated at the interface by the presence of this surfactant. Gas-phase products are determined using PTR-ToF-MS, and those remaining in the organic layer are determined by ATR-FTIR spectroscopy and HPLC-HRMS. We observe the photosensitized production of carboxylic acids as well as unsaturated and branched-chain oxygenated products, compounds that act as organic aerosol precursors and had been thought to be produced solely by biological activity. A mechanism that is consistent with the observations is detailed here, and the energetics of several key reactions are calculated using quantum chemical methods. The results suggest that the concentrating nature of the interface leads to its being a favorable venue for radical reactions yielding complex and functionalized products that themselves could initiate further secondary chemistry and new particle formation in the atmospheric environment.

Review of the bulk and surface chemistry of iron in atmospherically relevant systems containing humic-like substances

The current state of knowledge and future research directions of the bulk and surface chemistry of iron relevant to atmospheric surfaces are reviewed.

Formation of indoor nitrous acid (HONO) by light-induced NO2 heterogeneous reactions with white wall paint

Gaseous nitrogen dioxide (NO2) represents an oxidant that is present in relatively high concentrations in various indoor settings. Remarkably increased NO2 levels up to 1.5 ppm are associated with homes using gas stoves. The heterogeneous reactions of NO2 with adsorbed water on surfaces lead to the generation of nitrous acid (HONO). Here, we present a HONO source induced by heterogeneous reactions of NO2 with selected indoor paint surfaces in the presence of light (300 nm<λ<400 nm). We demonstrate that the formation of HONO is much more pronounced at elevated relative humidity. In the presence of light (5.5 W m(-2)), an increase of HONO production rate of up to 8.6·10(9) molecules cm(-2) s(-1) was observed at [NO2]=60 ppb and 50% relative humidity (RH). At higher light intensity of 10.6 (W m(-2)), the HONO production rate increased to 2.1·10(10) molecules cm(-2) s(-1). A high NO2 to HONO conversion yield of up to 84% was observed. This result strongly suggests that a light-driven process of indoor HONO production is operational. This work highlights the potential of paint surfaces to generate HONO within indoor environments by light-induced NO2 heterogeneous reactions.

Average chemical properties and potential formation pathways of highly oxidized organic aerosol

Measurements of ambient organic aerosol indicate that a substantial fraction is highly oxidized and low in volatility, but this fraction is generally not reproduced well in either laboratory studies or models. Here we describe a new approach for constraining the viable precursors and formation pathways of highly oxidized organic aerosol, by starting with the oxidized product and considering the possible reverse reactions, using a set of simple chemical rules. The focus of this work is low-volatility oxidized organic aerosol (LV-OOA), determined from factor analysis of aerosol mass spectrometer data. The elemental composition and volatility of the aerosol enable the determination of its position in a three-dimensional chemical space (defined by H/C, O/C, and carbon number) and thus its average chemical formula. Consideration of possible back-reactions then defines the movement taken through this chemical space, constraining potential reaction pathways and precursors. This approach is taken for two highly oxidized aerosol types, an average of LV-OOA factors from ten field campaigns (average formula C10.5H13.4O7.3), and extremely oxidized LV-OOA (from Mexico City, average formula C10H12.1O8.4). Results suggest that potential formation pathways include functionalization reactions that add multiple functional groups per oxidation step, oligomerization of highly oxidized precursors, and, in some cases, fragmentation reactions that involve the loss of small, reduced fragments.

Unexpectedly high indoor hydroxyl radical concentrations associated with nitrous acid

The hydroxyl (OH) radical is the most important oxidant in the atmosphere since it controls its self-oxidizing capacity. The main sources of OH radicals are the photolysis of ozone and the photolysis of nitrous acid (HONO). Due to the attenuation of solar radiation in the indoor environment, the possibility of OH formation through photolytic pathways indoors has been ignored up to now. In the indoor air, the ozonolysis of alkenes has been suggested as an alternative route of OH formation. Models and indirect measurements performed up to now according to this hypothesis suggest concentrations of OH radicals on the order of 10(4)-10(5) molecules per cubic centimeter. Here, we present direct measurements of significant amounts of OH radicals of up to 1.8⋅10(6) molecules per cubic centimeter during an experimental campaign carried out in a school classroom in Marseille. This concentration is on the same order of magnitude of outdoor OH levels in the urban scenario. We also show that photolysis of HONO is an important source of OH radicals indoors under certain conditions (i.e., direct solar irradiation inside the room). Additionally, the OH concentrations were found to follow a linear dependence with the product J(HONO)⋅[HONO]. This was also supported by using a simple quasiphotostationary state model on the OH radical budget. These findings force a change in our understanding of indoor air quality because the reactivity linked to OH would involve formation of secondary species through chemical reactions that are potentially more hazardous than the primary pollutants in the indoor air.

Emission characteristics of air pollutants from incense and candle burning in indoor atmospheres

Volatile organic compounds (VOCs) and particles emitted by incense sticks and candles combustion in an experimental room have been monitored on-line and continuously with a high time resolution using a state-of-the-art high sensitivity-proton transfer reaction-mass spectrometer (HS-PTR-MS) and a condensation particle counter (CPC), respectively. The VOC concentration-time profiles, i.e., an increase up to a maximum concentration immediately after the burning period followed by a decrease which returns to the initial concentration levels, were strongly influenced by the ventilation and surface interactions. The obtained kinetic data set allows establishing a qualitative correlation between the elimination rate constants of VOCs and their physicochemical properties such as vapor pressure and molecular weight. The emission of particles increased dramatically during the combustion, up to 9.1(±0.2) × 10(4) and 22.0(±0.2) × 10(4) part cm(-3) for incenses and candles, respectively. The performed kinetic measurements highlight the temporal evolution of the exposure level and reveal the importance of ventilation and deposition to remove the particles in a few hours in indoor environments.

Global sensitivity analysis of the regional atmospheric chemical mechanism: an application of random sampling-high dimensional model representation to urban oxidation chemistry

Chemical mechanisms play a crucial part for the air quality modeling and pollution control decision-making. Parameters in a chemical mechanism have uncertainties, leading to the uncertainties of model predictions. A recently developed global sensitivity analysis (SA) method based on Random Sampling-High Dimensional Model Representation (RS-HDMR) was applied to the Regional Atmospheric Chemical Mechanism (RACM) within a zero-dimensional photochemical model to highlight the main uncertainty sources of atmospheric hydroxyl (OH) and hydroperoxyl (HO(2)) radicals. This global SA approach can be applied as a routine in zero-dimensional photochemical modeling to comprehensively assess model uncertainty and sensitivity under different conditions. It also highlights the parameters to which the model is most sensitive during periods when the model/measurement OH and HO(2) discrepancies are greatest. Uncertainties in 584 model parameters were assigned for measured constituents used to constrain the model, for photolysis and kinetic rate coefficients, and for product yields of the reactions. With simulations performed for the hourly field data of two typical days, modeled and measured OH and HO(2) generally agree better for polluted conditions than for cleaner conditions, except during morning rush hour. Sensitivity analysis shows that the modeled OH and HO(2) depend most critically on the reactions of xylenes and isoprene with OH, NO(2) with OH, NO with HO(2), and internal alkenes with O(3) and suggests that model/measurement discrepancies in OH and HO(2) would benefit from a closer examination of these reactions.