Fate, transformation and toxicological implications of environmental diclofenac: Role of mineralogy and solar flux.

Diclofenac is an emerging surface water contaminant, yet the environmental impact of its degradation products remains elusive. The current study focuses on mineralogy-controlled diclofenac photo-degradation and its potential health impacts. Under irradiated conditions, we studied the effects of kaolinite, hematite, and anatase on diclofenac degradation. Our results showed that kaolinite doubled the diclofenac degradation rate, which can be attributed to the high catalytic effect, mediated via increased surface area and pore size of mineral surface in the low pH. Conversely, anatase, a crystal phase of titanium dioxide (TiO2), diminished the diclofenac degradation compared to treatments without TiO2. Hematite, on the other hand, showed no effect on diclofenac degradation. Photo-degradation products also varied with the mineral surface. We further assessed in vitro toxicological effects of photo-degraded products on two human cell lines, HEK293T and HepG2. Biological assays confirmed that photo-degraded compound 6 (1-(2,6-dichlorophenyl)indolin-2-one) decreased HEK293T cell survival significantly (p < 0.05) when compared to diclofenac in all concentrations. At lower concentrations, inhibition of HEK293T cells caused by compounds 4 (2-(8-chloro-9H-carbazol-1-yl)acetic acid), and 5 (2-(9H-carbazol-1-yl)acetic acid) was greater than diclofenac. Compound 7 (1-phenylindolin-2-one) was toxic only at 250 µM. Additionally, compound 6 decreased HepG2 cell viability significantly when compared to diclofenac. Overall, our data highlighted that mineralogy plays a vital role in environmental diclofenac transformation and its photo-degraded products. Some photo-degraded compounds can be more cytotoxic than the parent compound, diclofenac.

Heterogeneous Photocatalysis of Amoxicillin under Natural Conditions and High-Intensity Light: Fate, Transformation, and Mineralogical Impacts.

The ß-Lactam antibiotic amoxicillin is among the most widely used antibiotics in human and veterinary medicine. Consequently, amoxicillin is abundant in natural waters and can undergo diverse abiotic reactions to form degradation compounds under environmental conditions. Yet, little is known about these decay pathways and mineralogical impacts on environmental amoxicillin degradation. The current study focuses on understanding the mineralogical influences of amoxicillin degradation under ecological conditions. We studied the role of anatase and kaolinite on amoxicillin degradation under irradiated and non-irradiated conditions. Anatase increases amoxicillin degradation by 4.5-fold in the presence of light compared to just being exposed to sunlight. Interestingly, anatase also showed a higher degradation rate under dark than light controls. Conversely, kaolinite diminishes the amoxicillin degradation under irradiation. The formation of degradation compounds was mineralogy-controlled, while no mineralization was observed. Further, we irradiated amoxicillin with a high-intensity light to evaluate its removal from wastewater. The formation of varying amoxicillin degradation products with high-intensity light will limit its removal from wastewater. Our study emphasizes that the mineralogical impact on amoxicillin degradation is diverse, and the role of anatase is significant. Consequently, the increased addition of manufactured titanium nanoparticles to the environment can further enhance these effects.

The fate of inhaled uranium-containing particles upon clearance to gastrointestinal tract.

Uranium-bearing respirable dust can cause various health problems, such as cardiovascular and neurological disorders, cancers, immunosuppression, and autoimmunity. Exposure to elevated levels of uranium is linked to many such health conditions in Navajo Nation residents in northwestern New Mexico. Most studies have focused on the fate of inhaled dust particles (<4 µm) in the lungs. However, larger-sized inhaled particles (10-20 µm) can be cleared to the human gastrointestinal tract (GIT), thereby enabling them to interact with stomach and intestinal fluids. Despite the vital importance of understanding the fate of uranium-bearing solids entering the human GIT and their impact on body tissues, cells, and gut microbiota, our understanding remains limited. This study investigated uranium solubility from dust and sediment samples collected near two uranium mines in the Grants Mining District in New Mexico in two simulated gastrointestinal fluids representing fasting conditions in the GIT: Simulated Gastric Fluid (SGF) and Simulated Intestinal Fluid (SIF). The dissolution of uranium from dust depends on its mineralogy, fluid pH, and composition. The dust samples from the Jackpile mine favored higher solubility in the SIF solution, whereas the sediment samples from the St. Anthony Mine favored higher solubility in the SGF solution. Further, geochemical calculations performed with the PHREEQC modeling program suggested that samples rich in the minerals andersonite, tyuyamunite, and/or autunite have higher uranium dissolution in the SIF solution than in the SGF solution. We also tested the effect of added kaolinite and microcline, which are both present in some samples. The ratio of dissolved uranium in SGF relative to SIF decreases with the addition of kaolinite for all mineral phases but andersonite. With the addition of microcline, the ratio of dissolved uranium in SGF relative to SIF decreases for all the tested uranium minerals. The most prevalent oxidation state of dissolved uranium was computationally determined as +6, U(VI). The geochemical calculations made with PHREEQC agree with the experimentally observed results. Therefore, this study gives insight into the mineralogy-controlled toxicological assessment of uranium-containing inhaled dust cleared to the gastrointestinal tract.

Mechanochemistry of Group 4 Element-Based Metal-Organic Frameworks.

Mechanochemical synthesis is emerging as an environmentally friendly yet efficient approach to preparing metal-organic frameworks (MOFs). Herein, we report our systematic investigation on the mechanochemical syntheses of Group 4 element-based MOFs. The developed mechanochemistry allows us to synthesize a family of Hf4O4(OH)4(OOC)12-based MOFs. Integrating [Zr6O4(OH)4(OAc)12]2 and [Hf6O4(OH)4(OAc)12]2 under the mechanochemical conditions leads to a unique family of cluster-precise multimetallic MOFs that cannot be accessed by the conventional solvothermal synthesis. Extensive efforts have not yielded an effective pathway for preparing TiIV-derived MOFs, tentatively because of the relatively low Ti-O bond dissociation energy.

Atmospheric Processing of Iron-Bearing Mineral Dust Aerosol and Its Effect on Growth of a Marine Diatom, Cyclotella meneghiniana.

Iron (Fe) is a growth-limiting micronutrient for phytoplankton in major areas of oceans and deposited wind-blown desert dust is a primary Fe source to these regions. Simulated atmospheric processing of four mineral dust proxies and two natural dust samples followed by subsequent growth studies of the marine planktic diatom Cyclotella meneghiniana in artificial sea-water (ASW) demonstrated higher growth response to ilmenite (FeTiO3) and hematite (α-Fe2O3) mixed with TiO2 than hematite alone. The processed dust treatment enhanced diatom growth owing to dissolved Fe (DFe) content. The fresh dust-treated cultures demonstrated growth enhancements without adding such dissolved Fe. These significant growth enhancements and dissolved Fe measurements indicated that diatoms acquire Fe from solid particles. When diatoms were physically separated from mineral dust particles, the growth responses become smaller. The post-mineralogy analysis of mineral dust proxies added to ASW showed a diatom-induced increased formation of goethite, where the amount of goethite formed correlated with observed enhanced growth. The current work suggests that ocean primary productivity may not only depend on dissolved Fe but also on suspended solid Fe particles and their mineralogy. Further, the diatom C. meneghiniana benefits more from mineral dust particles in direct contact with cells than from physically impeded particles, suggesting the possibility for alternate Fe-acquisition mechanism/s.

Charge-Separated and Lewis Paired Metal-Organic Framework for Anion Exchange and CO2 Chemical Fixation.

Charge-separated metal-organic frameworks (MOFs) are a unique class of MOFs that can possess added properties originating from the exposed ionic species. A new charge-separated MOF, namely, UNM-6 synthesized from a tetrahedral borate ligand and Co2+ cation is reported herein. UNM-6 crystalizes into the highly symmetric P43n space group with fourfold interpenetration, despite the stoichiometric imbalance between the B and Co atoms, which also leads to loosely bound NO3 - anions within the crystal structure. These NO3 - ions can be quantitatively exchanged with various other anions, leading to Lewis acid (Co2+ ) and Lewis base (anions) pairs within the pores and potentially cooperative catalytic activities. For example, UNM-6-Br, the MOF after anion exchange with Br- anions, displays high catalytic activity and stability in reactions of CO2 chemical fixation into cyclic carbonates.

A toxicological study on photo-degradation products of environmental ibuprofen: Ecological and human health implications.

Increasing quantities of pharmaceutical waste in the environment have disrupted the balance of ecosystems, and may have subsequent effects on human health. Although a handful of previous studies have shown the impacts of pharmaceutically active compounds on the environment, the toxicological effects of their degradation products remain largely unknown. In the current study, the photo-degradation products of environmental ibuprofen were assessed for both ecotoxicological and human health effects using a series of in vitro assays. Here, six of the major degradation products are synthesized with high purity (>98%) and characterized with 1HNMR, 13CNMR, FT-IR and HRMS. To evaluate human health effects, three gut microbiota species, Lactobacillus acidophilus, Enterococcus faecalis and Escherichia coli, and two human cell lines, HEK293T and HepG2, are exposed to various concentrations of ibuprofen and its degradation products. On L. acidophilus, the ibuprofen degradation product (±)-(2R,3R)-2-(4-isobutylphenyl)-5-methylhexan-3-ol shows a greater toxic effect while ibuprofen enhances its growth at lower concentrations. At higher concentrations, ibuprofen shows at least a 2-fold higher toxicity compared to that of its degradation products. However, E. faecalis shows little or no effect upon exposure to these compounds. An induction of the SOS response in E. coli is observed but limited to only ibuprofen and 4-acetylbenzoic acid. In human cell line studies, survival of both HEK293T and HepG2 cell lines is profoundly impaired by the photo-degradation products of (±)- (2R,3R)-2-(4-isobutylphenyl)-5-methylhexan-3-ol, (±)-(2R,3S)-2-(4-isobutylphenyl)-5-methylhexan-3-ol, and (±)-1-(4-(1-hydroxy-2methylpropyl)phenyl)ethan-1-one. In this work, the bioluminescence bacterium, Aliivibrio fischeri, is used as a model to assess environmental impact. Both ibuprofen and its degradation products inhibit the growth of this gram-negative bacteria with the primary compound showing the most significant impact. Overall, our results highlight that some of the degradation products of ibuprofen can be more toxic to human kidney cell line and liver cell line than the parent compound while ibuprofen can be more toxic to human gut microbiota and A. fischeri than ibuprofen degradation products.

Mineralogy Controlled Dissolution of Uranium from Airborne Dust in Simulated Lung Fluids (SLFs) and Possible Health Implications.

The recent increase in cardiovascular and metabolic disease in the Navajo population residing close to the Grants Mining District (GMD) in New Mexico is suggested to be due to exposure to environmental contaminants, in particular uranium in respirable dusts. However, the chemistry of uranium-containing-dust dissolution in lung fluids and the role of mineralogy are poorly understood, as is their impact on toxic effects. The current study is focused on the dissolution of xcontaining-dust, collected from several sites near Jackpile and St. Anthony mines in the GMD, in two simulated lung fluids (SLFs): Gamble's solution (GS) and Artificial Lysosomal Fluid (ALF). We observe that the respirable dust includes uranium minerals that yield the uranyl cation, UO2 2+, as the primary dissolved species in these fluids. Dust rich in uraninite and carnotite is more soluble in GS, which mimics interstitial conditions of the lungs. In contrast, dust with low uraninite and high kaolinite is more soluble in ALF, which simulates the alveolar macrophage environment during phagocytosis. Moreover, geochemical modeling, performed using PHREEQC, is in good agreement with our experimental results. Thus, the current study highlights the importance of site-specific toxicological assessments across mining districts with the focus on their mineralogical differences.

A charge-separated diamondoid metal-organic framework.

We report the synthesis, characterization, and gas adsorption analyses of a new charge-separated metal-organic framework (MOF), UNM-1 (C52H16BCuF16N4), possessing diamondoid structures, assembled from an anionic tetrahedral borate ligand and cationic Cu(i) metal ion. The resulting MOF structure displays four-fold interpenetration, resulting in high environmental stability, and at the same time possesses relatively large surface area (SABET = 621 m2 g-1) due to the absence of free ions. Gas adsorption measurements revealed temperature-dependent CO2 adsorption/desorption hysteresis and large CO2/N2 ideal selectivities up to ca. 99 at 313 K and 1 bar, suggesting potential applications of this type of charge-separated MOFs in flue gas treatment and CO2 sequestration.

Fate, Transformation, and Toxicological Impacts of Pharmaceutical and Personal Care Products in Surface Waters.

With the growth of the human population, a greater quantity of pharmaceutical and personal care products (PPCPs) have been released into the environment. Although research has addressed the levels and the impact of PPCPs in the environment, the fate of these compounds in surface waters is neither well known nor characterized. In the environment, PPCPs can undergo various transformations that are critically dependent on environmental factors such as solar radiation and the presence of soil particles. Given that the degradation products of PPCPs are poorly characterized, these "secondary residues" can be a significant environmental health hazard due to their drastically different toxicologic effects when compared with the parent compounds. To better understand the fate of PPCPs, we studied the degradation of selected PPCPs, including ibuprofen and clofibric acid, in aqueous solutions that contained kaolinite clay and were irradiated with a solar simulator. The most abundant degradation products were identified and assessed for their toxicologic impact on selected microorganisms. The degraded mixtures showed lower toxicity than the starting compounds; however, as these degradation products are capable of further transformation and interaction with other PPCPs in natural waters, our work highlights the importance of additionally characterizing the PPCP degradation products.

Atmospheric Processing and Iron Mobilization of Ilmenite: Iron-Containing Ternary Oxide in Mineral Dust Aerosol.

Over the last several decades, iron has been identified as a limiting nutrient in about half of the world's oceans. Its most significant source is identified as deposited iron-containing mineral dust that has been processed during atmospheric transportation. The current work focuses on chemical and photochemical processing of iron-containing mineral dust particles in the presence of nitric acid, and an organic pollutant dimethyl sulfide under atmospherically relevant conditions. More importantly, ilmenite (FeTiO3) is evaluated as a proxy for the iron-containing mineral dust. The presence of titanium in its lattice structure provides higher complexity to mimic mineral dust, yet it is simple enough to study reaction pathways and mechanisms. Here, spectroscopic methods are combined with dissolution measurements to investigate atmospheric processing of iron in mineral dust, with specific focus on particle mineralogy, particle size, and their environmental conditions (i.e., pH and solar flux). Our results indicate that the presence of titanium elemental composition enhances iron dissolution from mineral dust, at least by 2-fold comparison with its nontitanium-containing counterparts. The extent of iron dissolution and speciation is further influenced by the above factors. Thus, our work highlights these important, yet unconsidered, factors in the atmospheric processing of iron-containing mineral dust aerosol.

Abiotic degradation and environmental toxicity of ibuprofen: Roles of mineral particles and solar radiation.

The growing medical and personal needs of human populations have escalated release of pharmaceuticals and personal care products into our natural environment. This work investigates abiotic degradation pathways of a particular PPCP, ibuprofen, in the presence of a major mineral component of soil (kaolinite clay), as well as the health effects of the primary compound and its degradation products. Results from these studies showed that the rate and extent of ibuprofen degradation is greatly influenced by the presence of clay particles and solar radiation. In the absence of solar radiation, the dominant reaction mechanism was observed to be the adsorption of ibuprofen onto clay surface where surface silanol groups play a key role. In contrast, under solar radiation and in the presence of clay particles, ibuprofen breaks down to several fractions. The decay rates were at least 6-fold higher for irradiated samples compared to those of dark conditions. Toxicity of primary ibuprofen and its secondary residues were tested on three microorganisms: Bacillus megaterium, Pseudoaltermonas atlantica; and algae from the Chlorella genus. The results from the biological assays show that primary PPCP is more toxic than the mixture of secondary products. Overall, however, biological assays carried out using only 4-acetylbenzoic acid, the most abundant secondary product, show a higher toxic effect on algae compared to its parent compound.

Iron oxide nanoparticles induce Pseudomonas aeruginosa growth, induce biofilm formation, and inhibit antimicrobial peptide function.

Given the increased use of iron-containing nanoparticles in a number of applications, it is important to understand any effects that iron-containing nanoparticles can have on the environment and human health. Since iron concentrations are extremely low in body fluids, there is potential that iron-containing nanoparticles may influence the ability of bacteria to scavenge iron for growth, affect virulence and inhibit antimicrobial peptide (AMP) function. In this study, Pseudomonas aeruginosa (PA01) and AMPs were exposed to iron oxide nanoparticles, hematite (α-Fe2O3), of different sizes ranging from 2 to 540 nm (2 ± 1, 43 ± 6, 85 ± 25 and 540 ± 90 nm) in diameter. Here we show that the greatest effect on bacterial growth, biofilm formation, and AMP function impairment is found when exposed to the smallest particles. These results are attributed in large part to enhanced dissolution observed for the smallest particles and an increase in the amount of bioavailable iron. Furthermore, AMP function can be additionally impaired by adsorption onto nanoparticle surfaces. In particular, lysozyme readily adsorbs onto the nanoparticle surface which can lead to loss of peptide activity. Thus, this current study shows that co-exposure of nanoparticles and known pathogens can impact host innate immunity. Therefore, it is important that future studies be designed to further understand these types of impacts.

Surface photochemistry of adsorbed nitrate: the role of adsorbed water in the formation of reduced nitrogen species on α-Fe2O3 particle surfaces.

The surface photochemistry of nitrate, formed from nitric acid adsorption, on hematite (α-Fe2O3) particle surfaces under different environmental conditions is investigated using X-ray photoelectron spectroscopy (XPS). Following exposure of α-Fe2O3 particle surfaces to gas-phase nitric acid, a peak in the N1s region is seen at 407.4 eV; this binding energy is indicative of adsorbed nitrate. Upon broadband irradiation with light (λ > 300 nm), the nitrate peak decreases in intensity as a result of a decrease in adsorbed nitrate on the surface. Concomitant with this decrease in the nitrate coverage, there is the appearance of two lower binding energy peaks in the N1s region at 401.7 and 400.3 eV, due to reduced nitrogen species. The formation as well as the stability of these reduced nitrogen species, identified as NO(-) and N(-), are further investigated as a function of water vapor pressure. Additionally, irradiation of adsorbed nitrate on α-Fe2O3 generates three nitrogen gas-phase products including NO2, NO, and N2O. As shown here, different environmental conditions of water vapor pressure and the presence of molecular oxygen greatly influence the relative photoproduct distribution from nitrate surface photochemistry. The atmospheric implications of these results are discussed.

Heterogeneous uptake and adsorption of gas-phase formic acid on oxide and clay particle surfaces: the roles of surface hydroxyl groups and adsorbed water in formic acid adsorption and the impact of formic acid adsorption on water uptake.

Organic acids in the atmosphere are ubiquitous and are often correlated with mineral dust aerosol. Heterogeneous chemistry and the uptake of organic acids on mineral dust particles can potentially alter the properties of the particle. In this study, heterogeneous uptake and reaction of formic acid, HCOOH, the most abundant carboxylic acid present in the atmosphere, on oxide and clays of the most abundant elements, Si and Al, present in the Earth's crust are investigated under dry and humid conditions. In particular, quantitative adsorption measurements using a Quartz Crystal Microbalance (QCM) coupled with spectroscopic studies using Attenuated Total Reflection Fourier Transform Infrared (ATR-FTIR) spectroscopy are combined to allow for both quantification of the amount of uptake and identification of distinct adsorbed species formed on silica, alumina, and kaolinite particle surfaces at 298 K. These oxides and clay particles show significant differences in the extent and speciation of adsorbed HCOOH due to inherent differences in surface -OH group reactivity. Adsorbed water, controlled by relative humidity, can increase the irreversible uptake of formic acid. Interestingly, the resulting layer of adsorbed formate on the particle surface decreases the particle hydrophilicity thereby decreasing the amount of water taken up by the surface as measured by QCM. Atmospheric implications of this study are discussed.

Role(s) of adsorbed water in the surface chemistry of environmental interfaces.

The chemistry of environmental interfaces such as oxide and carbonate surfaces under ambient conditions of temperature and relative humidity is of great interest from many perspectives including heterogeneous atmospheric chemistry, heterogeneous catalysis, photocatalysis, sensor technology, corrosion science, and cultural heritage science. As discussed here, adsorbed water plays important roles in the reaction chemistry of oxide and carbonate surfaces with indoor and outdoor pollutant molecules including nitrogen oxides, sulfur dioxide, carbon dioxide, ozone and organic acids. Mechanisms of these reactions are just beginning to be unraveled and found to depend on the details of the reaction mechanism as well as the coverage of water on the surface. As discussed here, adsorbed water can: (i) alter reaction pathways and surface speciation relative to the dry surface; (ii) hydrolyze reactants, intermediates and products; (iii) enhance surface reactivity by providing a medium for ionic dissociation; (iv) inhibit surface reactivity by blocking sites; (v) solvate ions; (vi) enhance ion mobility on surfaces and (vii) alter the stability of surface adsorbed species. In this feature article, drawing on research that has been going on for over a decade on the reaction chemistry of oxide and carbonate surfaces under ambient conditions of temperature and relative humidity, a number of specific examples showing the multi-faceted roles of adsorbed water are presented.

Preservation of York Minster historic limestone by hydrophobic surface coatings.

Magnesian limestone is a key construction component of many historic buildings that is under constant attack from environmental pollutants notably by oxides of sulfur via acid rain, particulate matter sulfate and gaseous SO(2) emissions. Hydrophobic surface coatings offer a potential route to protect existing stonework in cultural heritage sites, however, many available coatings act by blocking the stone microstructure, preventing it from 'breathing' and promoting mould growth and salt efflorescence. Here we report on a conformal surface modification method using self-assembled monolayers of naturally sourced free fatty acids combined with sub-monolayer fluorinated alkyl silanes to generate hydrophobic (HP) and super hydrophobic (SHP) coatings on calcite. We demonstrate the efficacy of these HP and SHP surface coatings for increasing limestone resistance to sulfation, and thus retarding gypsum formation under SO(2)/H(2)O and model acid rain environments. SHP treatment of 19th century stone from York Minster suppresses sulfuric acid permeation.

Heterogeneous atmospheric chemistry of lead oxide particles with nitrogen dioxide increases lead solubility: environmental and health implications.

Heterogeneous chemistry of nitrogen dioxide with lead-containing particles is investigated to better understand lead metal mobilization in the environment. In particular, PbO particles, a model lead-containing compound due to its widespread presence as a component of lead paint and as naturally occurring minerals, massicot, and litharge, are exposed to nitrogen dioxide at different relative humidity. X-ray photoelectron spectroscopy (XPS) shows that upon exposure to nitrogen dioxide the surface of PbO particles reacts to form adsorbed nitrates and lead nitrate thin films with the extent of nitrate formation relative humidity dependent. NO(2)-exposed PbO particles are found to have an increase in the amount of lead that dissolves in aqueous suspensions at circumneutral pH compared to particles not exposed. These results point to the potential importance and impact that heterogeneous chemistry with trace atmospheric gases can have on increasing solubility and therefore the mobilization of heavy metals, such as lead, in the environment. This study also shows that surface intermediates that form, such as adsorbed lead nitrates, can yield higher concentrations of lead in water systems. These water systems can include drinking water, groundwater, estuaries, and lakes.

Proton-promoted dissolution of α-FeOOH nanorods and microrods: size dependence, anion effects (carbonate and phosphate), aggregation and surface adsorption.

Iron-containing oxide nanoparticles are of great interest from a number of technological perspectives and they are also present in the natural environment. Although recent evidence suggests that particle size plays an important role in the dissolution of metal oxides, a detailed fundamental understanding of the influence of particle size is just beginning to emerge. In the current study, we investigate whether nanoscale size-effects are observed for the dissolution of iron oxyhydroxide under different conditions. The dissolution of two particle sizes of goethite, α-FeOOH in the nanoscale and microscale size regimes (herein referred to as nanorods and microrods), in aqueous suspensions at pH 2 is investigated. It is shown here that in the presence of nitrate, nanorods shows greater dissolution on both a per mass and per surface area basis relative to microrods, in agreement with earlier studies. In the presence of carbonate and phosphate, however, dissolution of α-FeOOH nanorods at pH 2 is significantly inhibited, despite the fact that these anions result in a three- to fivefold enhancement of the dissolution of microrods relative to the nitrate anion. Light scattering techniques and electron microscopy show that nanorod suspensions are less stable compared to microrod suspensions resulting in nanorod aggregation under conditions where microrods stay more dispersed. Furthermore, spectroscopic studies using ATR-FTIR spectroscopy show distinct differences in phosphate and carbonate adsorption on nanorods compared to microrods. These results demonstrate that aggregation and the details of surface adsorption are important in the dissolution behavior of nanoscale materials.

Surface-catalyzed chlorine and nitrogen activation: mechanisms for the heterogeneous formation of ClNO, NO, NO2, HONO, and N2O from HNO3 and HCl on aluminum oxide particle surfaces.

It is well-known that chlorine active species (e.g., Cl(2), ClONO(2), ClONO) can form from heterogeneous reactions between nitrogen oxides and hydrogen chloride on aerosol particle surfaces in the stratosphere. However, less is known about these reactions in the troposphere. In this study, a potential new heterogeneous pathway involving reaction of gaseous HCl and HNO(3) on aluminum oxide particle surfaces, a proxy for mineral dust in the troposphere, is proposed. We combine transmission Fourier transform infrared spectroscopy with X-ray photoelectron spectroscopy to investigate changes in the composition of both gas-phase and surface-bound species during the reaction under different environmental conditions of relative humidity and simulated solar radiation. Exposure of surface nitrate-coated aluminum oxide particles, from prereaction with nitric acid, to gaseous HCl yields several gas-phase products, including ClNO, NO(2), and HNO(3), under dry (RH < 1%) conditions. Under humid more conditions (RH > 20%), NO and N(2)O are the only gas products observed. The experimental data suggest that, in the presence of adsorbed water, ClNO is hydrolyzed on the particle surface to yield NO and NO(2), potentially via a HONO intermediate. NO(2) undergoes further hydrolysis via a surface-mediated process, resulting in N(2)O as an additional nitrogen-containing product. In the presence of broad-band irradiation (λ > 300 nm) gas-phase products can undergo photochemistry, e.g., ClNO photodissociates to NO and chlorine atoms. The gas-phase product distribution also depends on particle mineralogy (Al(2)O(3) vs CaCO(3)) and the presence of other coadsorbed gases (e.g., NH(3)). These newly identified reaction pathways discussed here involve continuous production of active ozone-depleting chlorine and nitrogen species from stable sinks such as gas-phase HCl and HNO(3) as a result of heterogeneous surface reactions. Given that aluminosilicates represent a major fraction of mineral dust aerosol, aluminum oxide can be used as a model system to begin to understand various aspects of possible reactions on mineral dust aerosol surfaces.