A Review of Laboratory Studies on the Heterogeneous Chemistry of NO2: Mechanisms and Uptake Kinetics

NO2 is a significant primary atmospheric pollutant that plays a key role in atmospheric chemistry. It serves as a crucial precursor to photochemical smog, acid rain, and secondary particulate matter and is instrumental in determining the atmospheric oxidation capacity. In this review, we focus on the heterogeneous chemistry of NO2, which has been demonstrated to significantly influence the sources and sinks of various nitrogen-containing species through field measurements and model simulations. We provide a comprehensive summary of laboratory studies investigating the reaction mechanisms and uptake kinetics of NO2 in heterogeneous reactions. NO2 can undergo disproportionation reactions on atmospheric particles. For instance, it may hydrolyze on wetted surfaces to form HONO and HNO3, produce nitrate and NO on mineral dust, or generate nitrate and NOCl on sea salt. Additionally, NO2 can be reduced to HONO on soot and Fe-bearing minerals or photocatalytically reduced to HONO and NO on photosensitive components. Furthermore, NO2 can be photo-oxidized to nitrate or N2O5 on illuminated TiO2. In addition, the synergistic effect of the heterogeneous reactions between NO2 and SO2 is discussed. The uptake coefficients of NO2 on typical particles and the factors influencing these coefficients are also summarized. Finally, based on the current insufficient understanding of heterogeneous reactions of NO2, we propose prospects for future research.

Structural and Electronic Evolution of Ethanolamine upon Microhydration: Insights from Hyperfine Resolved Rotational Spectroscopy

Ethanolamine hydrates containing from one to seven water molecules were identified via rotational spectroscopy with the aid of accurate quantum chemical methods considering anharmonic vibrational corrections. Ethanolamine undergoes significant conformational changes upon hydration to form energetically favorable hydrogen bond networks. The final structures strongly resemble the pure (H2O)3-9 complexes reported before when replacing two water molecules by ethanolamine. The 14N nuclear quadrupole coupling constants of all the ethanolamine hydrates have been determined and show a remarkable correlation with the strength of hydrogen bonds involving the amino group. After addition of the seventh water molecule, both hydrogen atoms of the amino group actively contribute to hydrogen bond formation, reinforcing the network and introducing approximately 21-27 % ionicity towards the formation of protonated amine. These findings highlight the critical role of microhydration in altering the electronic environment of ethanolamine, enhancing our understanding of amine hydration dynamics.

Selective molecular characterization of organic aerosols using in situ laser desorption ionization mass spectrometry

RATIONALE

The sources and chemical compositions of organic aerosol (OA) exert a significant influence on both regional and global atmospheric conditions, thereby having far-reaching implications on environmental chemistry. However, existing mass spectrometry (MS) methods have limitations in characterizing the detailed composition of OA due to selective ionization as well as fractionation during cold-water extraction and solid-phase extraction (SPE).

METHODS

A comprehensive MS study was conducted using aerosol samples collected on dusty, clean, and polluted days. To supplement the data obtained from electrospray ionization (ESI), a strategy for analyzing OAs collected using the quartz fiber filter directly utilizing laser desorption ionization (LDI) was employed. Additionally, the ESI method was conducted to explore suitable approaches for determining various OA compositions from samples collected on dusty, clean, and polluted days.

RESULTS

In situ LDI has the advantages of significantly reducing the sample volume, simplifying sample preparation, and overcoming the problem of overestimating sulfur-containing compounds usually encountered in ESI. It is suitable for the characterization of highly unsaturated and hydrophobic aerosols, such as brown carbon-type compounds with low volatility and high stability, which is supplementary to ESI.

CONCLUSIONS

Compared with other ionization methods, in situ LDI helps provide a complementary description of the molecular compositions of OAs, especially for analyzing OAs in polluted day samples. This method may contribute to a more comprehensive MS analysis of the elusive compositions and sources of OA in the atmosphere.

Compositional evolution for mixed aerosols containing gluconic acid and typical nitrate and the effect of multiply factors on hygroscopicity

The aging process of atmospheric aerosols usually leads to a mixture of inorganic salts and organic compounds of anthropogenic origin. In organic compounds, polyhydroxy organic acids are important components, however, the study on composition and hygroscopic properties of the mixture containing inorganics and polyhydroxy organic acids is scanty. In this study, gluconic acid, the proxy of polyhydroxy organic acids, is mixed with the representative nitrate (Mg(NO3)2, Ca(NO3)2) to form aerosols. ATR-FTIR and optical microscopy are employed to study the component changes and hygroscopicity as a function of relative humidity. As relative humidity fluctuates, the FTIR-ATR spectra display that the internal mixed gluconic acid (CH2(CH)4(OH)5COOH) and nitrate can react to release acidic gases, forming relevant gluconate and further affecting the hygroscopicity. The specific presentation is particles cannot be recovered to their original size after the dehydration-hydration process and there will be some disparities in GF for mixed particles. For the gluconic acid-Ca(NO3)2/Mg(NO3)2 mixtures with molar ratios of 1:1, higher degree of reaction resulting in the production of large amounts of gluconate should be responsible to the lower hygroscopicity compared to ZSR model. For 1:2 gluconic acid-nitrate mixed systems (with higher nitrate content), the hygroscopicity of mixtures are higher than the ZSR prediction. A possible reason could be 'salt-promoting effect' on the organic fractions of the surplus inorganic salt in the mixture. These data can improve the chemical composition list evaluation, in turn hygroscopic properties and phase state of atmospheric aerosol, and then the climate effect.

Acidity and oxidative potential of atmospheric aerosols over a remote mangrove ecosystem during the advection of anthropogenic plumes

To investigate the acidity and the water-soluble oxidative potential of PM10, during the continental biomass-burning plume transport, a three-year (2018-2020) winter-time campaign was conducted over a pristine island (21.35°N, 88.32°E) of Sundarban mangrove ecosystem situated at the shore of Bay of Bengal. The average PM10 concentration over Sundarban was found to be 98.3 ± 22.2 μg m-3 for the entire study period with a high fraction of non-sea-salt- SO42- and water-soluble organic carbons (WSOC) that originated from the regional solid fuel burning. The thermodynamic E-AIM(IV) model had estimated that the winter-time aerosols over Sundarban were acidic (pH:2.4 ± 0.6) and mainly governed by non-sea-salt-SO42-. The volume and mass normalized oxidative potential of PM10 was found to be 1.81 ± 0.40 nmol DTT min-1 m-3 and 18.4 ± 6.1 pmol DTT min-1 μg-1 respectively which are surprisingly higher than several urban atmospheres across the world including IGP. The acid-digested water-soluble transition metals (Cu, Mn) show higher influences in the oxidative potential (under high aerosol acidity) compared to the WSOC. The study revealed that the advection of regional solid fuel burning plume and associated non-sea-salt-SO42- is enhancing aerosol acidity and oxidative stress that in turn alters the intrinsic properties of aerosols over such marine ecosystems rich in ecology and bio-geochemistry.

Hygroscopic Growth of Adsorbed Water Films on Smectite Clay Particles

Hygroscopic growth of adsorbed water films on clay particles underlies a number of environmental science questions, from the air quality and climate impacts of mineral dust aerosols to the hydrology and mechanics of unsaturated soils and sedimentary rocks. Here, we use molecular dynamics (MD) simulations to establish the relation between adsorbed water film thickness (h) and relative humidity (RH) or disjoining pressure (Π), which has long been uncertain due to factors including sensitivity to particle shape, surface roughness, and aqueous chemistry. We present a new MD simulation approach that enables precise quantification of Π in films up to six water monolayers thick. We find that the hygroscopicity of phyllosilicate mineral surfaces increases in the order mica < K-smectite < Na-smectite. The relationship between Π and h on clay surfaces follows a double exponential decay with e-folding lengths of 2.3 and 7.5 Å. The two decay length scales are attributed to hydration repulsion and osmotic phenomena in the electrical double layer (EDL) at the clay-water interface.

Significant roles of aged dust aerosols on rapid nitrate formation under dry conditions in a semi-arid city

Mineral dust can accelerate secondary aerosol formation under humid conditions. However, it is unclear whether it can promote secondary aerosol formation under dry conditions. To investigate this issue, two years of comprehensive observations was conducted at a semi-arid site, near the dust source regions. Three types of episodes were selected: dust, anthropogenic-dominated, and mixed (mixed with dust and anthropogenic aerosols). Compared to anthropogenic-dominated episodes under humid conditions, rapid nitrate formation was still observed in mixed episodes under dry conditions, suggesting that active metallic oxides in dust, such as titanium dioxide, could promote photochemical reactions of nitrogen dioxide. The detailed evolutionary processes are further illustrated by a typical dust-to-mixed episode. After the arrival of the dust, titanium sharply increased ten-fold and rapid nitrate formation was observed, together with a rapid increase in the two most important photochemical pollutants, ozone and peroxyacetyl nitrate. The increased secondary organic carbon further illustrated that the suspended dust particles accelerated the atmospheric oxidative capacity, thereby enhancing secondary aerosol formation and eventually leading to haze pollution. These results differ from those in humid regions and therefore expand the scientific understanding of the impact of dust aerosols on haze pollution under dry conditions.

Enhanced secondary organic aerosol formation during dust episodes by photochemical reactions in the winter in Wuhan

To investigate the effect of frequently occurring mineral dust on the formation of secondary organic aerosol (SOA), 106 volatile organic compounds (VOCs), trace gas pollutants and chemical components of PM_2.5 were measured continuously in January 2021 in Wuhan, Central China. The observation period was divided into two stages that included a haze period and a following dust period, based on the ratio of PM_2.5 and PM₁₀ concentrations. The average ratio of secondary organic carbon (SOC) to elemental carbon (EC) was 1.98 during the dust period, which was higher than that during the haze period (0.69). The contribution of SOA to PM_2.5 also increased from 2.75% to 8.64%. The analysis of the relationships between the SOA and relative humidity (RH) and the odd oxygen (e.g., O_X = O₃ + NO₂) levels suggested that photochemical reactions played a more important role in the enhancement of SOA production during the dust period than the aqueous-phase reactions. The heterogeneous photochemical production of OH radicals in the presence of metal oxides during the dust period was believed to be enhanced. Meanwhile, the ratios of trans-2-butene to cis-2-butene and m-/p-xylene to ethylbenzene (X/E) dropped significantly, confirming that stronger photochemical reactions occurred and SOA precursors formed efficiently. These results verified the laboratory findings that metal oxides in mineral dust could catalyse the oxidation of VOCs and induce higher SOA production.

Spectroscopy of cluster aerosol models: IR and UV spectra of hydrated glyoxylate with and without sea salt

Glyoxylic acid is formed in the troposphere by oxidation of organic molecules. In sea salt aerosols, it is expected to be present as glyoxylate, integrated into the salt environment and strongly interacting with water molecules. In water, glyoxylate is in equilibrium with its gem-diol form. To understand the influence of water and salt on the photophysics and photochemistry of glyoxylate, we generate small model clusters containing glyoxylate by electrospray ionization and study them by Fourier-Transform Ion Cyclotron Resonance (FT-ICR) mass spectrometry. We used infrared multiple photon dissociation spectroscopy and UV/vis photodissociation spectroscopy for structural characterization as well as quantum chemical calculations to model the spectra and dissociation patterns. Resonant absorption of infrared radiation leads to water evaporation, which indicates that water and glyoxylate are separate molecular entities in a significant fraction of the clusters, in line with the observed absorption of UV light in the actinic region. Hydration of glyoxylate leads to a change of the dihedral angle in the CHOCOO-·H2O complex, causing a slight redshift of the S1 ← S0 transition. However, the barriers for internal rotation are below 5 kJ mol-1, which explains the broad S1 ← S0 absorption extending from about 320 to 380 nm. Most importantly, hydration hinders dissociation in the S1 state, thus enhancing the quantum yield of fluorescence combined with water evaporation. No C-C bond photolysis is observed, but due to the limited signal-to-noise ratio, it cannot be ruled out. The quantum yield, however, will be relatively low. Fluorescence dominates the photophysics of glyoxylate embedded in the dry salt cluster, but the quantum yield shifts towards internal conversion upon addition of one or two water molecules.

Effect of photothermal conversion on ozone uptake over deposited mineral dust

The deposited dust provides a large surface for heterogeneous ozone uptake reactions in urban regions. Prior studies have barely considered the effect of the photothermal conversion of deposited dust and underlying surface on ozone uptake. In this study, Fe₂O₃, TiO₂, α-Al₂O₃, and SiO₂ were selected as model mineral dusts (MDs) to evaluate the photothermal effect. With an irradiation intensity of 100 mW/cm², the uptake coefficients of ozone by Fe₂O₃, TiO₂, α-Al₂O₃, and SiO₂ were 2.4, 30, 2.72, and 2.83 times higher than those in a dark condition. For SiO₂ and α-Al₂O₃, the increase in the uptake coefficient was due to the temperature increase induced by photothermal conversion. For Fe₂O₃ and TiO₂, photoelectric and photothermal conversion simultaneously participated in ozone uptake reactions. At 70 °C, the contribution of thermal catalysis to ozone uptake over Fe₂O₃ and TiO₂ was approximately 55.4 % and 55.0 %, respectively. The temperature increase induced by photothermal conversion also promoted MDs' activity for ozone uptake after removing the light source (after sunset). This work proves that the ozone uptake induced by the photothermal effect of deposited MDs and the underlying surface was the primary ozone elimination pathway in urban atmospheres.

Molecular investigation of the multi-phase photochemistry of Fe(III)-citrate in aqueous solution

Iron (Fe) is ubiquitous in nature and found as Fe^II or Fe^III in minerals or as dissolved ions Fe²⁺ or Fe³⁺ in aqueous systems. The interactions of soluble Fe have important implications for fresh water and marine biogeochemical cycles, which have impacts on global terrestrial and atmospheric environments. Upon dissolution of Fe^III into natural aquatic systems, organic carboxylic acids efficiently chelate Fe^III to form [Fe^III-carboxylate]²⁺ complexes that undergo a wide range of photochemistry-induced radical reactions. The chemical composition and photochemical transformations of these mixtures are largely unknown, making it challenging to estimate their environmental impact. To investigate the photochemical process of Fe^III-carboxylates at the molecular level, we conduct a comprehensive experimental study employing UV-visible spectroscopy, liquid chromatography coupled to photodiode array and high-resolution mass spectrometry detection, and oil immersion flow microscopy. In this study, aqueous solutions of Fe^III-citrate were photolyzed under 365 nm light in an experimental setup with an apparent quantum yield of (φ) ∼0.02, followed by chemical analyses of reacted mixtures withdrawn at increment time intervals of the experiment. The apparent photochemical reaction kinetics of Fe³⁺-citrates (aq) were expressed as two generalized consecutive reactions of with the experimental rate constants of j₁ ∼ 0.12 min^-1 and j₂ ∼ 0.05 min^-1, respectively. Molecular characterization results indicate that R and I consist of both water-soluble organic and Fe-organic species, while P compounds are a mixture of water-soluble and colloidal materials. The latter were identified as Fe-carbonaceous colloids formed at long photolysis times. The carbonaceous content of these colloids was identified as unsaturated organic species with low oxygen content and carbon with a reduced oxidation state, indicative of their plausible radical recombination mechanism under oxygen-deprived conditions typical for the extensively photolyzed mixtures. Based on the molecular characterization results, we discuss the comprehensive reaction mechanism of Fe^III-citrate photochemistry and report on the formation of previously unexplored colloidal reaction products, which may contribute to atmospheric and terrestrial light-absorbing materials in aquatic environments.

Investigation of a haze-to-dust and dust swing process at a coastal city in northern China part I: Chemical composition and contributions of anthropogenic and natural sources

The long retention of dust air masses in polluted areas, especially in winter, may efficiently change the physicochemical properties of aerosols, causing additional health and ecological effects. A large-scale haze-to-dust weather event occurred in the North China Plain (NCP) region during the autumn-to-winter transition period in 2018, affecting the coastal city Qingdao several times between Nov. 27th and Dec. 1st. To study the evolution of the pollution process, we analyzed the chemical characteristics of PM_2.5 and PM_10-2.5 and source apportionments of PM_2.5 and PM_10, The dust stagnated around NCP and moved out and back to the site, noted as dust swing process, promoting SO₄^2- formation in PM_2.5 and NO₃^- formation in PM_10-2.5. Source apportionments were analyzed using the Positive Matrix Factorization (PMF) receptor model and weighted potential source contribution function (WPSCF). Before the dust invasion, Qingdao was influenced by severe haze; waste incineration and coal burning were the major contributors (~80 %) to PM_2.5, and the source region was in the southwest of Shandong Province. During the initial dust event, mineral dust and the mixed factor of dust and sea salt were the major contributors (46.0 % of PM_2.5 and 86.5 % of PM₁₀). During the polluted dust period, the contributions of regional transported biomass burning (22.3 %), vehicle emissions (20.8 %), and secondary aerosols (33.8 %) to PM_2.5 from the Beijing-Tianjin-Hebei region significantly increased. The secondary aerosols source was more regional than that of vehicle emissions and biomass burning and contributed considerably to PM₁₀ (30.8 %) during the dust swing process. Our findings demonstrate that environmental managers should consider the possible adverse effects of winter dust on regional and local pollution.

A novel pathway of atmospheric sulfate formation through carbonate radicals

Abstract. Carbon dioxide is considered an inert gas that rarely participates in atmospheric chemical reactions. Nonetheless, we show here that CO2 is involved in some important photo-oxidation reactions in the atmosphere through the formation of carbonate radicals (CO3⚫-). This potentially active intermediate CO3⚫- is routinely overlooked in atmospheric chemistry concerning its effect on sulfate formation. The present work demonstrates that the SO2 uptake coefficient is enhanced by 17 times on mineral dust particles driven by CO3⚫-. Importantly, upon irradiation, mineral dust particles are speculated to produce gas-phase carbonate radical ions when the atmospherically relevant concentration of CO2 presents, thereby potentially promoting external sulfate aerosol formation and oxidative potential in the atmosphere. Employing a suite of laboratory investigations of sulfate formation in the presence of carbonate radicals on the model and authentic dust particles, ground-based field measurements of sulfate and (bi)carbonate ions within ambient PM, together with density functional theory (DFT) calculations for single electron transfer processes in terms of CO3⚫--initiated S(IV) oxidation, a novel role of carbonate radical in atmospheric chemistry is elucidated.

Atmospheric deposition of inorganic nutrients to the Western North Pacific Ocean

We evaluated the potential impacts of atmospheric deposition on marine productivity and inorganic carbon chemistry in the northwestern Pacific Ocean (8-39°N, 125-157°E). The nutrient concentration in atmospheric total suspended particles decreased exponentially with increasing distance from the closest land-mass (Asia), clearly revealing anthropogenic and terrestrial contributions. The predicted mean depositional fluxes of inorganic nitrogen were approximately 34 and 15 μmol m^-2 d^-1 to the west and east of 140°E, respectively, which were at least two orders of magnitude greater than the inorganic phosphorus flux. On average, atmospheric particulate deposition would support 3-4% of the net primary production along the surveyed tracks, which is equivalent to ~2% of the dissolved carbon increment caused by the penetration of anthropogenic CO₂. Our observations generally fell within the ranges observed over the past 18 years, despite an increasing trend of atmospheric pollution in the source regions during the same period, which implies high temporal and spatial variabilities of atmospheric nutrient concentration in the study area. Continued atmospheric anthropogenic nitrogen deposition may alter the relative abundances of nitrogen and phosphorus.

Water-Air Interfaces as Environments to Address the Water Paradox in Prebiotic Chemistry: A Physical Chemistry Perspective

The asymmetric water-air interface provides a dynamic aqueous environment with properties that are often very different than bulk aqueous or gaseous phases and promotes reactions that are thermodynamically, kinetically, or otherwise unfavorable in bulk water. Prebiotic chemistry faces a key challenge: water is necessary for life yet reduces the efficiency of many biomolecular synthesis reactions. This perspective considers water-air interfaces as auspicious reaction environments for abiotic synthesis. We discuss recent evidence that (1) water-air interfaces promote condensation reactions including peptide synthesis, phosphorylation, and oligomerization; (2) photochemistry at water-air interfaces may have been a significant source of prebiotic molecular complexity, given the lack of oxygen and increased availability of near-ultraviolet radiation on early Earth; and (3) water-air interfaces can promote spontaneous reduction and oxidation reactions, potentially providing protometabolic pathways. Life likely began within a relatively short time frame, and water-air interfaces offer promising environments for simultaneous and efficient biomolecule production.

In Situ PM-IRRAS at the Air/Electrolyte/Solid Interface Reveals Oxidation of Iron to Distinct Minerals

Iron interfaces undergo redox and catalytic processes in various environments, on the surface of soils, dust, minerals, and materials that comprise industrial infrastructure. Measuring reactions at interfaces in complex environments is challenging, where adsorption of gases and interaction of aqueous species occur at the surface. This is due to the presence of several ionic species in solutions that catalyze surface oxidation and undergo ion exchange between the solution and the surface and from the influx of oxygen and other gases. Corrosion is an electrochemical redox reaction that is affected by the presence of oxygen and water, but accelerated by dissolved ions. Polarized modulated-infrared reflection absorption spectroscopy was used to measure in situ surface oxidation at the air/electrolyte/iron interface in semineutral NaCl(aq) and acidic HCl(aq) solutions using the meniscus method under ambient conditions. The iron interface was exposed to air, primarily oxygen, allowing for surface oxidation, where metallic iron was found to transform to siderite in NaCl(aq) and lepidocrocite in HCl(aq). Mechanisms are suggested for the transformation of iron to these corrosion products, which significantly impact our understanding of redox processes in the water cycle, material degradation, and energy applications.

Soluble Fe release from iron-bearing clay mineral particles in acid environment and their oxidative potential

Soluble iron from atmospheric aerosol particles has toxicological effects on ambient environment due to their oxidative potential. However, the dissolution process and factors affecting this process are poorly understood. In this study, by solid phase characterization and aqueous dissolution experiments, we investigated the influence of acids, including HCl, H₂SO₄ and HNO₃, and H⁺ concentration on iron dissolution rate, solubility and speciation of iron in chlorite, illite, kaolinite and pyrite. The dissolution of iron-bearing clay minerals, i.e. chlorite, illite and kaolinite, was a multi-stage process with a rapid rate in the initial stage and then decreasing rate in the following stages. In contrast, the regularly crystallized pyrite proceeded with an extremely rapid dissolution rate at very beginning and then remained almost constant. In all acid solutions, the dissolution rate was in the order of pyrite > illite > chlorite > kaolinite. H₂SO₄ was stronger than HCl and HNO₃ in the destruction of mineral structures to release iron, while HNO₃ dissolved more iron in pyrite (FeS₂). High H⁺ concentration easily destroyed the mineral structures to release the structural or interlayer iron, whereas low H⁺ concentration increased the proportion of Fe (II) in clay minerals. Non-linear fitting of continuous dissolution models showed that the iron dissolution rates and iron redox speciation as functions of time were well predicted, with r² > 0.99 for chlorite and illite, and r² > 0.96 for kaolinite. Oxidative potential analysis proved that the dissolved iron possessed a considerable potential to generate reactive oxygen species.

Setting benchmarks for modelling gas-surface interactions using coherent control of rotational orientation states

The coherent evolution of a molecular quantum state during a molecule-surface collision is a detailed descriptor of the interaction potential which was so far inaccessible to measurements. Here we use a magnetically controlled molecular beam technique to study the collision of rotationally oriented ground state hydrogen molecules with a lithium fluoride surface. The coherent control nature of the technique allows us to measure the changes in the complex amplitudes of the rotational projection quantum states, and express them using a scattering matrix formalism. The quantum state-to-state transition probabilities we extract reveal a strong dependency of the molecule-surface interaction on the rotational orientation of the molecules, and a remarkably high probability of the collision flipping the rotational orientation. The scattering matrix we obtain from the experimental data delivers an ultra-sensitive benchmark for theory to reproduce, guiding the development of accurate theoretical models for the interaction of H₂ with a solid surface.

A fundamental and predictive understanding of molecule-surface interactions is challenging to obtain. Here the authors report an experimental technique allowing direct measurement of the scattering matrix, which reports on the coherent evolution of quantum states of a molecule scattering from a surface.

Investigation of N2 adsorption on Fe3O4(001) using ambient pressure X-ray photoelectron spectroscopy and density functional theory

Reactions on iron oxide surfaces are prevalent in various chemical processes from heterogeneous catalysts to minerals. Nitrogen (N₂) is known to dissociate on iron surfaces, a precursor for ammonia production in the Haber-Bosch process, where the dissociation of N₂ is the limiting step in the reaction under equilibrium conditions. However, little is known about N₂ adsorption on other iron-based materials, such as iron oxide surfaces that are ubiquitous in soils, steel pipelines, and other industrial materials. An atomistic description is reported for the binding of N₂ on the Fe₃O₄(001) surface using first principles calculations with ambient pressure X-ray photoelectron spectroscopy. Two primary adsorption sites are experimentally identified from N₂ dissociation on Fe₃O₄(001). The electronic signatures associated with the valence band region unambiguously show how the electronic structure of magnetite transforms near ambient pressures due to the binding of atomic nitrogen to different surface sites. Overall, the experimental and theoretical results of our study bridge the gap between ultra-high vacuum studies and reaction conditions to provide insight into other nitrogen-based chemistry on iron oxide surfaces that impact the agriculture and energy industries.

Spontaneous selective deposition of iron oxide nanoparticles on graphite as model catalysts

Iron oxide nanomaterials participate in redox processes that give them ideal properties for their use as earth-abundant catalysts. Fabricating nanocatalysts for such applications requires detailed knowledge of the deposition and growth. We report the spontaneous deposition of iron oxide nanoparticles on HOPG in defect areas and on step edges from a metal precursor solution. To study the nucleation and growth of iron oxide nanoparticles, tailored defects were created on the surface of HOPG using various ion sources that serve as the target sites for iron oxide nucleation. After solution deposition and annealing, the iron oxide nanoparticles were found to nucleate and coalesce at 400 °C. AFM revealed that the particles on the sp³ carbon sites enabled the nanoparticles to aggregate into larger particles. The iron oxide nanoparticles were characterized as having an Fe³⁺ oxidation state and two different oxygen species, Fe-O and Fe-OH/Fe-OOH, as determined by XPS. STEM imaging and EDS mapping confirmed that the majority of the nanoparticles grown were converted to hematite after annealing at 400 °C. A mechanism of spontaneous and selective deposition on the HOPG surface and transformation of the iron oxide nanoparticles is proposed. These results suggest a simple method for growing nanoparticles as a model catalyst.

Transformation of gaseous 2-bromophenol on clay mineral dust and the potential health effect

Iron-bearing clays are ubiquitously distributed as mineral dusts in the atmosphere. Bromophenols were reported as the major products from thermal decomposition of the widely used brominated flame retardants (BFRs). However, little information is available for the reactivity of iron associated with mineral dusts to interact with the atmospheric bromophenols and the subsequent toxic effects. Herein, three common clay minerals (montmorillonite, illite and kaolinite) were used to simulate mineral dusts, and the reactions with gaseous 2-bromophenol were systematically investigated under environmentally relevant atmospheric conditions. Our results demonstrate that structural Fe(III) in montmorillonite and Fe(III) from iron oxide in illite mediated the dimerization of 2-bromophenol to form hydroxylated polybrominated biphenyl and hydroxylated polybrominated diphenyl ether. The surface reaction is favored to occur at moisture environment, since water molecules formed complex with 2-bromophenol and the reaction intermediates via hydrogen bond to significantly lower the reaction energy and promote the dimerization reaction. More importantly, the formed dioxin-like products on clay mineral dust increased the toxicity of the particles to A549 lung cell by decreasing cell survival and damaging cellular membrane and proteins. The results of this study indicate that not only mineral dust itself but also the associated surface reaction should be fully considered to accurately evaluate the toxic effect of mineral dust on human health.

Water Vapor Binding on Organic Matter-Coated Minerals

Atmospheric water vapor binding to soils is a key process driving water availability in unsaturated terrestrial environments. Using a representative hydrophilic iron oxyhydroxide, this study highlights key mechanisms through which water vapor (i) adsorbs and (ii) condenses at mineral surfaces coated with Leonardite humic acid (LHA). Microgravimetry and vibrational spectroscopy showed that liquid-like water forms in the three-dimensional array of mineral-bound LHA when present at total C/Fe ratios well exceeding ∼73 mg C per g Fe (26 C atoms/nm²). Below these loadings, minerals become even less hydrophilic than in the absence of LHA. This lowering in hydrophilicity is caused by the complexation of LHA water-binding sites to mineral surfaces, and possibly by conformational changes in LHA structure removing available condensation environments for water. An empirical relationship predicting the dependence of water adsorption densities on LHA loadings was developed from these results. Together with the molecular-level description provided in this work, this relationship should guide efforts in predicting water availability, and thereby occurrences of water-driven geochemical processes in terrestrial environments.

Reactive oxygen species formed in aqueous mixtures of secondary organic aerosols and mineral dust influencing cloud chemistry and public health in the Anthropocene

Mineral dust and secondary organic aerosols (SOA) account for a major fraction of atmospheric particulate matter, affecting climate, air quality and public health. How mineral dust interacts with SOA to influence cloud chemistry and public health, however, is not well understood. Here, we investigated the formation of reactive oxygen species (ROS), which are key species of atmospheric and physiological chemistry, in aqueous mixtures of SOA and mineral dust by applying electron paramagnetic resonance (EPR) spectrometry in combination with a spin-trapping technique, liquid chromatography-tandem mass spectrometry (LC-MS/MS), and a kinetic model. We found that substantial amounts of ROS including OH, superoxide as well as carbon- and oxygen-centred organic radicals can be formed in aqueous mixtures of isoprene, α-pinene, naphthalene SOA and various kinds of mineral dust (ripidolite, montmorillonite, kaolinite, palygorskite, and Saharan dust). The molar yields of total radicals were ∼0.02-0.5% at 295 K, which showed higher values at 310 K, upon 254 nm UV exposure, and under low pH (<3) conditions. ROS formation can be explained by the decomposition of organic hydroperoxides, which are a prominent fraction of SOA, through interactions with water and Fenton-like reactions with dissolved transition metal ions. Our findings imply that the chemical reactivity and aging of SOA particles can be enhanced upon interaction with mineral dust in deliquesced particles or cloud/fog droplets. SOA decomposition could be comparably important to the classical Fenton reaction of H₂O₂ with Fe²⁺ and that SOA can be the main source of OH radicals in aqueous droplets at low concentrations of H₂O₂ and Fe²⁺. In the human respiratory tract, the inhalation and deposition of SOA and mineral dust can also lead to the release of ROS, which may contribute to oxidative stress and play an important role in the adverse health effects of atmospheric aerosols in the Anthropocene.

The Hydropathy Scale as a Gauge of Hygroscopicity in Sub-Micron Sodium Chloride-Amino Acid Aerosols

Sodium chloride, NaCl, is commonly used as a proxy for sea spray aerosols. However, field work has demonstrated that sea spray aerosols also often contain a significant organic component. In this work, we examine the effect of amino acids on the hygroscopic properties of NaCl aerosols using a Fourier transform infrared spectrometer coupled to a flow-cell apparatus. It is found that the effect can be drastically different depending on the nature of the amino acid. More hydrophilic amino acids such as glycine lead to continuous hygroscopic growth of internally mixed NaCl-amino acid aerosols generated from an equimolar precursor solution. However, more hydrophobic amino acids such as alanine do not significantly alter the hygroscopicity of NaCl aerosols. The hydropathy scale is found to be a good qualitative diagnostic for the effect that an amino acid will have on the hygroscopicity of NaCl.

Water Structure Controls Carbonic Acid Formation in Adsorbed Water Films

Reaction pathways and kinetics in highly structured H₂O adsorbed as Ångstrom to nanometer thick layers on mineral surfaces are distinct from those facilitated by bulk liquid water. We investigate the role of the interfacial H₂O structure in the reaction of H₂O and CO₂ to form carbonic acid (H₂CO₃) in thin H₂O films condensed onto silica nanoparticles from humidified supercritical CO₂. Rates of carbonic acid formation are correlated with spectroscopic signatures of H₂O structure using oxygen isotopic tracers and infrared spectroscopy. While carbonic acid virtually does not form in the supercritical phase, the silica surface catalyzes this reaction by concentrating H₂O through adsorption at hydrophilic silanol groups. Within measurement uncertainty, we found no evidence that carbonic acid forms when exclusively ice-like structured H₂O is detected at the silica surface. Instead, formation of H₂C¹⁸O¹⁶O₂ from H₂¹⁸O and C¹⁶O₂ was found to be linearly correlated with liquid-like structured H₂O that formed on the ice-like layer.

Increased Acid Dissociation at the Quartz/Water Interface

As shown by a quite significant amount of literature, acids at the water surface tend to be "less" acid, meaning that their associated form is favored over the conjugated base. What happens at the solid/liquid interface? In the case of the silica/water interface, we show how the acidity of adsorbed molecules can instead increase. Using a free energy perturbation approach in combination with electronic structure-based molecular dynamics simulations, we show how the acidity of pyruvic acid at the quartz/water interface is increased by almost two units. Such increased acidity is the result of the specific microsolvation at the interface and, in particular, of the stabilization of the deprotonated form by the silanols on the quartz surface and the special interfacial water layer.

Interactions of Gaseous 2-Chlorophenol with Fe3+-Saturated Montmorillonite and Their Toxicity to Human Lung Cells

The interactions of gaseous 2-chlorophenol with Fe³⁺-saturated montmorillonite particles in a gas-solid system were investigated to simulate the reactions of mineral dusts with volatile organic pollutants in the atmosphere. Results suggested that Fe³⁺-saturated montmorillonite mediated the dimerization of gaseous 2-chlorophenol to form hydroxylated polychlorinated biphenyl, hydroxylated polychlorinated diphenyl ether, and hydroxylated polychlorinated dibenzofuran. The toxicity of Fe³⁺-montmorillonite particles to A549 human lung epithelial cells before and after interaction with 2-chlorophenol was examined to explore their adverse impact on human health. Based on cell morphological analysis, cytotoxicity tests, and Fourier-transform infrared imaging spectra, surface-catalyzed reactions of Fe³⁺-montmorillonite with 2-chlorophenol increased the toxicity of montmorillonite particle on A549 cells. This was supported by increased cellular membrane permeability, the release of extracellular lactate dehydrogenase, and cell damages on cellular DNA, proteins, and lipids. Since mineral dusts are important components of particulate matter, our results help to understand the interactions of volatile organic pollutants with particulate matter in the atmosphere and their adverse impacts on human health.

Delayed Nrf2-regulated antioxidant gene induction in response to silica nanoparticles

Silica nanoparticles with iron on their surface cause the production of oxidants and stimulate an inflammatory response in macrophages. Nuclear factor erythroid-derived 2 - like factor 2 (Nrf2) signaling and its regulated antioxidant genes play critical roles in maintaining redox homeostasis. In this study we investigated the regulation of four representative Nrf2-regulated antioxidant genes; i.e., glutamate cysteine ligase (GCL) catalytic subunit (GCLC), GCL modifier subunit (GCLM), heme oxygenase 1 (HO-1), and NAD(P)H:quinone oxidoreductase-1 (NQO-1), by iron-coated silica nanoparticles (SiO2-Fe) in human THP-1 macrophages. We found that the expression of these four antioxidant genes was modified by SiO2-Fe in a time-dependent manner. At 6h, their expression was unchanged except for GCLC, which was reduced compared with controls. At 18h, the expression of these antioxidant genes was significantly increased compared with controls. In contrast, the Nrf2 activator sulforaphane induced all antioxidant genes at as early as 3h. The nuclear translocation of Nrf2 occurred later than that for NF-κB p65 protein and the induction of proinflammatory cytokines (TNFα and IL-1β). NF-κB inhibitor SN50 prevented the reduction of GCLC at 6h and abolished the induction of antioxidant genes at 18h by SiO2-Fe, but did not affect the basal and sulforaphane-induced expression of antioxidant genes, suggesting that NF-κB signaling plays a key role in the induction of Nrf2-mediated genes in response to SiO2-Fe. Consistently, SN50 inhibited the nuclear translocation of Nrf2 caused by SiO2-Fe. In addition, Nrf2 silencing decreased the basal and SiO2-induced expression of the four reprehensive antioxidant genes. Taken together, these data indicated that SiO2-Fe induced a delayed response of Nrf2-regulated antioxidant genes, likely through NF-κB-Nrf2 interactions.

Case study of dust event sources from the Gobi and Taklamakan deserts: An investigation of the horizontal evolution and topographical effect using numerical modeling and remote sensing

A severe dust event occurred from April 23 to April 27, 2014, in East Asia. A state-of-the-art online atmospheric chemistry model, WRF/Chem, was combined with a dust model, GOCART, to better understand the entire process of this event. The natural color images and aerosol optical depth (AOD) over the dust source region are derived from datasets of moderate resolution imaging spectroradiometer (MODIS) loaded on a NASA Aqua satellite to trace the dust variation and to verify the model results. Several meteorological conditions, such as pressure, temperature, wind vectors and relative humidity, are used to analyze meteorological dynamic. The results suggest that the dust emission occurred only on April 23 and 24, although this event lasted for 5days. The Gobi Desert was the main source for this event, and the Taklamakan Desert played no important role. This study also suggested that the landform of the source region could remarkably interfere with a dust event. The Tarim Basin has a topographical effect as a "dust reservoir" and can store unsettled dust, which can be released again as a second source, making a dust event longer and heavier.

Synergistic effect of nitrate-doped TiO2 aerosols on the fast photochemical oxidation of formaldehyde

The uptake of formaldehyde (HCHO) on mineral dust affects its budget as well as particle properties, yet the process has not yet been fully investigate. Here, TiO₂ and nitrate-doped TiO₂ aerosols were used as proxies for mineral dust, and the uptake of HCHO was explored in a chamber under both dark and illuminated conditions. The uptake loss of HCHO on UV-illuminated aerosols is 2–9 times faster than its gaseous photolysis in our experimental system. The uptake coefficient in the range of 0.43–1.68 × 10⁻⁷ is 1–2 orders of magnitude higher than previous reports on model mineral dust particles. The reaction rate exhibits a Langmuir-Hinshelwood-type dependence on nitrate content and relative humidity, suggesting the competitive role of nitrate salts, water vapor and HCHO on the TiO₂ surface. The reaction produces carbon dioxide as the main product and gaseous formic acid as an important intermediate. The hydroxyl radical produced on illuminated TiO₂ primarily drives the fast oxidation of HCHO. The nitrate radical arising from the TiO₂-catalyzed photoreaction of nitrate synergistically promotes the oxidation process. This study suggests a novel oxidation route for HCHO in the atmosphere, taking into account high abundance of both mineral dust and anthropogenic TiO₂ aerosols.

SO2 Emissions in China - Their Network and Hierarchical Structures

SO₂ emissions lead to various harmful effects on environment and human health. The SO₂ emission in China has significant contribution to the global SO₂ emission, so it is necessary to employ various methods to study SO₂ emissions in China with great details in order to lay the foundation for policymaking to improve environmental conditions in China. Network analysis is used to analyze the SO₂ emissions from power generation, industrial, residential and transportation sectors in China for 2008 and 2010, which are recently available from 1744 ground surface monitoring stations. The results show that the SO₂ emissions from power generation sector were highly individualized as small-sized clusters, the SO₂ emissions from industrial sector underwent an integration process with a large cluster contained 1674 places covering all industrial areas in China, the SO₂ emissions from residential sector was not impacted by time, and the SO₂ emissions from transportation sector underwent significant integration. Hierarchical structure is obtained by further combining SO₂ emissions from all four sectors and is potentially useful to find out similar patterns of SO₂ emissions, which can provide information on understanding the mechanisms of SO₂ pollution and on designing different environmental measure to combat SO₂ emissions.

Temperature dependence of the heterogeneous uptake of acrylic acid on Arizona test dust

In this study, the temperature dependence of the heterogeneous uptake of acrylic acid on Arizona test dust (ATD) has been investigated within a temperature range of 255-315K using a Knudsen cell reactor. Combined with diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) experiment, it was found that acrylic acid could adsorb on ATD via surface OH groups and convert to carboxylate on the particle surface. The kinetics study suggests that the initial true uptake coefficient (γ_t) of acrylic acid on ATD decreases from (4.02±0.12)×10^-5 to (1.73±0.05)×10^-5 with a temperature increase from 255 to 315K. According to the temperature dependence of uptake coefficients, the enthalpy (ΔH_obs) and entropy (ΔS_obs) of uptake processes were determined to be -(9.60±0.38) KJ/mol and -(121.55±1.33) J·K/mol, respectively. The activation energy for desorption (E_des) was calculated to be (14.57±0.60) KJ/mol. These results indicated that the heterogeneous uptake of acrylic acid on ATD surface was sensitive to temperature. The heterogeneous uptake on ATD could affect the concentration of acrylic acid in the atmosphere, especially at low temperature.

Introductory lecture: atmospheric chemistry in the Anthropocene

The term “Anthropocene” was coined by Professor Paul Crutzen in 2000 to describe an unprecedented era in which anthropogenic activities are impacting planet Earth on a global scale. Greatly increased emissions into the atmosphere, reflecting the advent of the Industrial Revolution, have caused significant changes in both the lower and upper atmosphere. Atmospheric reactions of the anthropogenic emissions and of those with biogenic compounds have significant impacts on human health, visibility, climate and weather. Two activities that have had particularly large impacts on the troposphere are fossil fuel combustion and agriculture, both associated with a burgeoning population. Emissions are also changing due to alterations in land use. This paper describes some of the tropospheric chemistry associated with the Anthropocene, with emphasis on areas having large uncertainties. These include heterogeneous chemistry such as those of oxides of nitrogen and the neonicotinoid pesticides, reactions at liquid interfaces, organic oxidations and particle formation, the role of sulfur compounds in the Anthropocene and biogenic–anthropogenic interactions. A clear and quantitative understanding of the connections between emissions, reactions, deposition and atmospheric composition is central to developing appropriate cost-effective strategies for minimizing the impacts of anthropogenic activities. The evolving nature of emissions in the Anthropocene places atmospheric chemistry at the fulcrum of determining human health and welfare in the future.

Atmospheric chemistry of bioaerosols: heterogeneous and multiphase reactions with atmospheric oxidants and other trace gases

Once airborne, biologically-derived aerosol particles are prone to reaction with various atmospheric oxidants such as OH, NO₃, and O₃.

Advances in analytical techniques and instrumentation have now established methods for detecting, quantifying, and identifying the chemical and microbial constituents of particulate matter in the atmosphere. For example, recent cryo-TEM studies of sea spray have identified whole bacteria and viruses ejected from ocean seawater into air. A focal point of this perspective is directed towards the reactivity of aerosol particles of biological origin with oxidants (OH, NO₃, and O₃) present in the atmosphere. Complementary information on the reactivity of aerosol particles is obtained from field investigations and laboratory studies. Laboratory studies of different types of biologically-derived particles offer important information related to their impacts on the local and global environment. These studies can also unravel a range of different chemistries and reactivity afforded by the complexity and diversity of the chemical make-up of these particles. Laboratory experiments as the ones reviewed herein can elucidate the chemistry of biological aerosols.

Multiple sulfur-isotope signatures in Archean sulfates and their implications for the chemistry and dynamics of the early atmosphere

Sulfur isotopic anomalies (∆(33)S and ∆(36)S) have been used to trace the redox evolution of the Precambrian atmosphere and to document the photochemistry and transport properties of the modern atmosphere. Recently, it was shown that modern sulfate aerosols formed in an oxidizing atmosphere can display important isotopic anomalies, thus questioning the significance of Archean sulfate deposits. Here, we performed in situ 4S-isotope measurements of 3.2- and 3.5-billion-year (Ga)-old sulfates. This in situ approach allows us to investigate the diversity of Archean sulfate texture and mineralogy with unprecedented resolution and from then on to deconvolute the ocean and atmosphere Archean sulfur cycle. A striking feature of our data is a bimodal distribution of δ(34)S values at ∼+5‰ and +9‰, which is matched by modern sulfate aerosols. The peak at +5‰ represents barite of different ages and host-rock lithology showing a wide range of ∆(33)S between -1.77‰ and +0.24‰. These barites are interpreted as primary volcanic emissions formed by SO2 photochemical processes with variable contribution of carbonyl sulfide (OCS) shielding in an evolving volcanic plume. The δ(34)S peak at +9‰ is associated with non-(33)S-anomalous barites displaying negative ∆(36)S values, which are best interpreted as volcanic sulfate aerosols formed from OCS photolysis. Our findings confirm the occurrence of a volcanic photochemical pathway specific to the early reduced atmosphere but identify variability within the Archean sulfate isotope record that suggests persistence throughout Earth history of photochemical reactions characteristic of the present-day stratosphere.

Mineral dust aerosols promote the formation of toxic nitropolycyclic aromatic compounds

Atmospheric nitrated polycyclic aromatic hydrocarbons (NPAHs), which have been shown to have adverse health effects such as carcinogenicity, are formed in part through nitration reactions of their parent polycyclic aromatic hydrocarbons (PAHs) in the atmosphere. However, little is known about heterogeneous nitration rates of PAHs by gaseous NO2 on natural mineral substrates, such as desert dust aerosols. Herein by employing kinetic experiments using a flow reactor and surface analysis by Fourier transform infrared spectroscopy with pyridine adsorption, we demonstrate that the reaction is accelerated on acidic surfaces of mineral dust, particularly on those of clay minerals. In support of this finding, we show that levels of ambient particle-associated NPAHs in Beijing, China, significantly increased during heavy dust storms. These results suggest that mineral dust surface reactions are an unrecognized source of toxic organic chemicals in the atmosphere and that they enhance the toxicity of mineral dust aerosols in urban environments.

Sunlight as an energetic driver in the synthesis of molecules necessary for life

This review considers how photochemistry and sunlight-driven reactions can abiotically generate prebiotic molecules necessary for the evolution of life.