Although recent evidence suggests that particle size plays an important role in the dissolution of iron from mineral dust aerosol, a fundamental understanding of how particle size influences the rate and extent of iron oxide dissolution processes remains unclear. In this study, surface spectroscopic methods are combined with solution phase measurements to explore ligand-promoted dissolution and photochemical reductive dissolution of goethite (α-FeOOH) of different particle sizes in the presence of oxalate at pH 3 and 298 K. Both X-ray photoelectron spectroscopy and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) revealed differences between α-FeOOH particles in the nanometer-size range as compared to α-FeOOH particles in the micrometer-size range (nanorods and microrods, respectively). ATR-FTIR spectra showed a significant presence of surface hydroxyl groups as well as differences in surface complexes formed on nanorod surfaces. Furthermore, the saturation coverage of oxalate adsorbed on nanorods relative to microrods is ∼30% less as determined from solution phase batch adsorption isotherms. Despite less oxalate uptake per unit surface area, the surface-area-normalized rate of oxalate-promoted dissolution was ∼4 times greater in nanorod suspensions, suggesting this process is particle-size-dependent. Photochemical dissolution experiments revealed only a moderate increase in the rate of oxalate oxidation per gram of α-FeOOH with decreasing particle size. However, concentration profiles of photochemically generated Fe(II) and Fe(III) suggest differences in the dominant mechanisms controlling nanorod and microrod dissolution. Although loss of reactive surface area arising from oxalate-induced particle aggregation can contribute to size-dependent reactivity trends toward oxalate, our data, taken collectively, suggest unique surface chemistry of nanorods as compared to larger microrods. Results from ligand-promoted and photochemical dissolution experiments also highlight the important, and sometimes dominant, role that iron oxides on the nanoscale may play in iron mobilization relative to the larger oxide phases present in mineral dust aerosol.

Understanding complex chemical changes that take place at nano–bio interfaces is of great concern for being able to sustainably implement nanomaterials in key applications such as drug delivery, imaging, and environmental remediation. Typical in vitro assays use cell viability as a proxy to understanding nanotoxicity but often neglect how the nanomaterial surface can be altered by adsorption of solution-phase components in the medium. Protein coronas form on the nanomaterial surface when incubated in proteinaceous solutions. Herein, we apply a broad array of techniques to characterize and quantify protein corona formation on silica nanoparticle surfaces. The porosity and surface chemistry of the silica nanoparticles have been systematically varied. Using spectroscopic tools such as FTIR and circular dichroism, structural changes and kinetic processes involved in protein adsorption were evaluated. Additionally, by implementing thermogravimetric analysis, quantitative protein adsorption measurements allowed for the direct comparison between samples. Taken together, these measurements enabled the extraction of useful chemical information on protein binding onto nanoparticles in solution. Overall, we demonstrate that small alkylamines can increase protein adsorption and that even large polymeric molecules such as poly(ethylene glycol) (PEG) cannot prevent protein adsorption in these systems. The implications of these results as they relate to further understanding nano–bio interactions are discussed.

Nanotechnology is no longer in its infancy and has made significant advances since the implementation of the National Nanotechnology Initiative (NNI) in 2000. Incorporation of nanotechnology in many fields including information technology, medicine, materials, energy, catalysis and cosmetics has led to an increase in engineered nanomaterial (ENM) production, and consequently, increased nanomaterial use. In comparison, the generation of concrete and consistent evidence related to the environmental health and safety of nanomaterials (NanoEHS) is lacking. The main factors contributing to the slower progress in NanoEHS versus conventional EHS are related to the complexity, property transformations, life cycles and behavior of nanomaterials even in carefully controlled environments. Therefore, new systematic, integrated research approaches in NanoEHS are needed for overcoming this complexity and bridging current knowledge gaps. A workshop on “NanoEHS: Fundamental Science Needs” brought together scientists and engineers to identify current fundamental science challenges and opportunities within NanoEHS. Detailed discussions were conducted on identifying the fundamental properties that are critical in NanoEHS, differentiating between conventional and NanoEHS studies as well as understanding, the effect of dynamic transformations on nanometrology, role of dosimetry and mechanistic data gaps in nanotoxicology. An important realization that even simple nanoscale materials can be complex when considering NanoEHS implications was noted several times during the workshop. Despite this fact, a number of fundamental research areas to further the scientific foundation to address NanoEHS needs are suggested.

Correction for ‘Humidity-dependent surface tension measurements of individual inorganic and organic submicrometre liquid particles’ by Holly S. Morris et al., Chem. Sci., 2015, 6, 3242–3247.

Surface tension, an important property of liquids, is easily measured for bulk samples. However, for droplets smaller than one micron in size, there are currently no reported measurements. In this study, atomic force microscopy (AFM) and force spectroscopy have been utilized to measure surface tension of individual submicron sized droplets at ambient pressure and controlled relative humidity (RH). Since the surface tension of atmospheric aerosols is a key factor in understanding aerosol climate effects, three atmospherically relevant systems (NaCl, malonic and glutaric acids) were studied. Single particle AFM measurements were successfully implemented in measuring the surface tension of deliquesced particles on the order of 200 to 500 nm in diameter. Deliquesced particles continuously uptake water at high RH, which changes the concentration and surface tension of the droplets. Therefore, surface tension as a function of RH was measured. AFM based surface tension measurements are close to predicted values based on bulk measurements and activities of these three chemical systems. Non-ideal behaviour in concentrated organic acid droplets is thought to be important and the reason for differences observed between bulk solution predictions and AFM data. Consequently, these measurements are crucial in order to improve atmospheric climate models as direct measurements hitherto have been previously inaccessible due to instrument limitations.

Atmospheric aerosols are often collected on substrates and analyzed weeks or months after the initial collection. We investigated how the selection of substrate and microscopy method influence the measured size, phase, and morphology of sea spray aerosol (SSA) particles and how sample storage conditions affect individual particles using three common microscopy techniques: optical microscopy, atomic force microscopy, and scanning electron microscopy. Micro-Raman spectroscopy was used to determine changes in the water content of stored particles. The results show that microscopy techniques operating under ambient conditions provide the most relevant and robust measurement of particle size. Samples stored in a desiccator and at ambient conditions leads to similar sizes and morphologies, while storage that involves freezing and thawing leads to irreversible changes due to phase changes and water condensation. Typically, SSA particles are deposited wet and, if possible, samples used for single-particle analysis should be stored at or near conditions at which they were collected in order to avoid dehydration. However, if samples need to be dry, as is often the case, then this study found that storing SSA particles at ambient laboratory conditions (17–23% RH and 19–21 °C) was effective at preserving them and reducing changes that would alter samples and subsequent data interpretation.

Chemistry, a field focused on the molecular scale, provides important contributions in understanding global-scale phenomena, including climate and climate change. This editorial focuses on chemistry’s contributions to our understanding of atmospheric science and climate from both research and chemical education perspectives.

Understanding the interactions of water with atmospheric aerosols is crucial for determining the size, physical state, reactivity, and climate impacts of this important component of the Earth’s atmosphere. Here we show that water uptake and hygroscopic growth of multicomponent, atmospherically relevant particles can be size dependent when comparing 100 nm versus ca. 6 μm sized particles. It was determined that particles composed of ammonium sulfate with succinic acid and of a mixture of chlorides typical of the marine environment show size-dependent hygroscopic behavior. Microscopic analysis of the distribution of components within the aerosol particles show that the size dependence is due to differences in the mixing state, that is, whether particles are homogeneously mixed or phase separated, for different sized particles. This morphology-dependent hygroscopicity has consequences for heterogeneous atmospheric chemistry as well as aerosol interactions with electromagnetic radiation and clouds.

Current practices of initial nanoparticle characterization with respect to particle size, shape, surface and bulk composition prior to experiments to test, for example, cellular interaction or toxicity, will not accurately describe nanomaterials in a given medium. The use of initial characterization data in subsequent analyses inherently assumes that nanoparticles are static entities. However, nanoparticle characterization, which is crucial in all studies related to their applications and implications, should also include information about the dynamics of the interfacial region between the nanomaterial surface and the surrounding medium. The objective of this tutorial review is to highlight the importance of in situ characterization of metal oxide nanoparticle surfaces in complex media. In particular, several examples of TiO2 (5 nm) and α-Fe2O3 (2 nm) nanoparticles, in different environmental and biological media, are presented so as to show the importance of the milieu to oxide surface composition. The surface composition is shown to be controlled by the adsorption of biological components (proteins and amino acids), inorganic oxyanions (phosphates and carbonates) and environmental ligands (humic acid). The extent of surface adsorption depends on the solution phase composition and the affinity of different components to adsorb to the nanoparticle surface. The examples presented here show that there is a range of possible surface interactions, adsorption energetics and adsorption modes including reversible adsorption, irreversible adsorption and co-adsorption.

Given the importance of nanoparticle surface composition in nanotoxicology, analytical tools that can probe nanoparticle surfaces in aqueous media are crucial but remain limited. Attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy is a technique capable of in situ characterization of the liquid–solid interface to probe surface adsorption on nanoparticle surfaces in environmentally and biologically relevant media. Furthermore, given that the interfacial region in these media is dynamic, ATR-FTIR spectroscopy facilitates monitoring these dynamics by interrogating a layer of immobilized nanoparticles coated on the ATR element while changing the overlying aqueous phase. The molecular information acquired from this technique allows for the determination of the adsorption mode, including conformational and structural changes of the coordinating ligand, and can directly measure ligand displacement reactions. Furthermore, in some cases, ATR-FTIR spectroscopy can be used as a quantitative surface analytical tool. In this article, we briefly review the fundamentals of the technique and then provide several examples of using ATR-FTIR spectroscopy to probe nanoparticle surfaces in general with respect to: (i) the adsorption of different environmentally and biologically relevant coordinating ligands; (ii) competitive ligand adsorption and; (iii) the determination of kinetic and thermodynamic parameters. We have also investigated surface adsorption of TiO2 nanoparticles in different biological media typically used for toxicity studies and show that the surface composition of TiO2 nanoparticles depends to a large extent on the composition of the medium due to surface adsorption. This result has important implications for the interpretation of toxicity data as well as inter-comparisons between toxicity studies.

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.

Current climate and atmospheric chemistry models assume that all sea spray particles react as if they are pure NaCl. However, recent studies of sea spray aerosol particles have shown that distinct particle types exist (including sea salt, organic carbon, and biological particles) as well as mixtures of these and, within each particle type, there is a range of single-particle chemical compositions. Because of these differences, individual particles should display a range of reactivities with trace atmospheric gases. Herein, to address this, we study the composition of individual sea spray aerosol particles after heterogeneous reaction with nitric acid. As expected, a replacement reaction of chloride with nitrate is observed; however, there is a large range of reactivities spanning from no reaction to complete reaction between and within individual sea spray aerosol particles. These data clearly support the need for laboratory studies of individual, environmentally relevant particles to improve our fundamental understanding as to the properties that determine reactivity.Keywords: atmospheric aerosol; heterogeneous reactions; micro-Raman spectroscopy; sea spray aerosol; single-particle analysis; surface chemistry; X-ray photoelectron spectroscopy;

The reactivity of O–H groups on titanium dioxide nanoparticle surfaces with gas-phase carbon dioxide, sulfur dioxide, and nitrogen dioxide is compared. Carbon dioxide, sulfur dioxide, and nitrogen dioxide react with ca. 5, 50, and nearly 100%, respectively, of all hydroxyl groups on the surface at 298 K. As shown here, the surface reactivity of O–H groups with these three triatomic gases differs considerably due to different reaction mechanisms for adsorption and surface chemistry. In addition to investigating O–H group reactivity, the role of adsorbed water in the stability of different surface species that form from adsorption of carbon dioxide, nitrogen dioxide, and sulfur dioxide on hydroxylated TiO2 nanoparticles is probed as a function of relative humidity as is quantitative measurements of water uptake on TiO2 nanoparticles before and after surface reaction. These water uptake studies provide insights into the stability of adsorbed species on oxide surfaces under atmospherically relevant conditions as well as changes in particle hygroscopicity and adsorbed water dynamics following reaction with these three atmospheric gases.

Formic acid adsorption and photooxidation on TiO2 nanoparticle surfaces at 296 K have been investigated using transmission FTIR spectroscopy. In particular, the role of adsorbed water in surface coordination, adsorption kinetics, and photoproduct formation is examined. Gas-phase formic acid adsorbs on the surface at low exposures to yield adsorbed bridged bidentate formate and, at higher exposures, molecularly adsorbed formic acid as well. Upon exposure to water vapor, adsorbed formate becomes solvated by coadsorbed water molecules, and the coordinatin mode changes as indicated by shifts in the vibrational frequencies. Adsorbed water also impacts the adsorption kinetics for formic acid on TiO2 and increases the adsorption rate, potentially by providing a medium for facile ionic dissociation. Ultraviolet irradiation of adsorbed formate on TiO2 in the presence of molecular oxygen results in the formation of gas-phase carbon dioxide, which increases in yield in the presence of adsorbed water on the surface. Additionally, the dispersion of TiO2 nanoparticles in water suspensions is found to change if first exposed to gas-phase formic acid before dispersion. The environmental implications of these results are discussed.

Nitrate ion adsorbed on the surface of mineral dust particles from heterogeneous reaction of nitric acid, nitrogen pentoxide, and nitrogen dioxide is thought to be a sink for nitrogen oxides. However, it has the potential to release gas-phase nitrogen oxides back into the atmosphere when irradiated with UV light. In this study, the wavelength dependence of nitrate ion photochemistry when adsorbed onto model laboratory proxies of mineral dust aerosol including Al2O3, TiO2, and NaY zeolite was investigated using FTIR spectroscopy. These proxies represent non-photoactive oxides, photoactive semiconductor oxides, and porous aluminosilicate materials, respectively, present in mineral dust aerosol. Nitrate photochemistry on mineral dust particles is governed by the wavelength of light, physicochemical properties of the dust particles, and the adsorption mode of the nitrate ion. Most interestingly, in some cases, nitrate ion adsorbed on oxide particles can undergo photochemistry over a broader wavelength region of the solar spectrum compared to nitrate ion in solution. As shown here, gas-phase NO2 is the major photolysis product formed from nitrate adsorbed on the surface of oxide particles under dry conditions. The NO2 yield and the initial rate of production is highest on TiO2, indicating that nitrate photochemistry is more efficient on photoactive oxides present in mineral dust. Nitrite ion complexed to Na+ sites in aluminosilicate zeolite pores is the major photolysis product found for zeolites. Mechanisms for the formation of gas-phase and surface-adsorbed products and a discussion of the wavelength dependence of nitrate ion photochemistry are presented, as is a discussion of the atmospheric implications.

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.

Single particle analysis of individual sea spray aerosol particles shows that cations (Na+, K+, Mg2+, and Ca2+) within individual particles undergo a spatial redistribution after heterogeneous reaction with nitric acid, along with the development of a more concentrated layer of organic matter at the surface of the particle. These data suggest that specific ion and aerosol pH effects play an important role in aerosol particle structure in ways that have not been previously recognized.

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.

A great deal of uncertainty exists regarding the chemical diversity of particles in sea spray aerosol (SSA), as well as the degree of mixing between inorganic and organic species in individual SSA particles. Therefore, in this study, single particle analysis was performed on SSA particles, integrating transmission electron microscopy with energy dispersive X-ray analysis and scanning transmission X-ray microscopy with near edge X-ray absorption fine structure spectroscopy, with a focus on quantifying the relative fractions of different particle types from 30 nm to 1 μm. SSA particles were produced from seawater in a unique ocean-atmosphere facility equipped with breaking waves. Changes to the SSA composition and properties after the addition of biological (bacteria and phytoplankton) and organic material (ZoBell growth media) were probed. Submicrometer SSA particles could be separated into two distinct populations: one with a characteristic sea salt core composed primarily of NaCl and an organic carbon and Mg2+ coating (SS-OC), and a second type consisting of organic carbon (OC) species which are more homogeneously mixed with cations and anions, but not chloride. SS-OC particles exhibit a wide range of sizes, compositions, morphologies, and distributions of elements within each particle. After addition of biological and organic material to the seawater, a change occurs in particle morphology and crystallization behavior associated with increasing organic content for SS-OC particles. The fraction of OC-type particles, which are mainly present below 180 nm, becomes dramatically enhanced with increased biological activity. These changes with size and seawater composition have important implications for atmospheric processes such as cloud droplet activation and heterogeneous reactivity.

Atmospheric organic acids potentially display different capacities in iron (Fe) mobilization from atmospheric dust compared with inorganic acids, but few measurements have been made on this comparison. We report here a laboratory investigation of Fe mobilization of coal fly ash, a representative Fe-containing anthropogenic aerosol, and Arizona test dust, a reference source material for mineral dust, in pH 2 sulfuric acid, acetic acid, and oxalic acid, respectively. The effects of pH and solar radiation on Fe dissolution have also been explored. The relative capacities of these three acids in Fe dissolution are in the order of oxalic acid > sulfuric acid > acetic acid. Oxalate forms mononuclear bidentate ligand with surface Fe and promotes Fe dissolution to the greatest extent. Photolysis of Fe–oxalate complexes further enhances Fe dissolution with the concomitant degradation of oxalate. These results suggest that ligand-promoted dissolution of Fe may play a more significant role in mobilizing Fe from atmospheric dust compared with proton-assisted processing. The role of atmospheric organic acids should be taken into account in global-biogeochemical modeling to better access dissolved atmospheric Fe deposition flux at the ocean surface.

Sea spray aerosol (SSA) represents one of the largest aerosol components in our atmosphere. SSA plays a major role in influencing climate; however the overall impacts remain poorly understood due to the overall chemical complexity. SSA is comprised of a mixture of inorganic and organic components in varying proportions that change as a function of particle size and seawater composition. In this study, nascent SSA particles were produced using breaking waves, resulting in compositions and sizes representative of the open ocean. The composition of individual SSA particles ranging in size from ca. 0.15 to 10 μm is measured using Raman microspectroscopy, while the interfacial composition of collections of size-resolved particles is probed by sum frequency generation (SFG). Raman spectra of single particles have bands in the 980 to 1030 cm−1 region associated with the symmetric stretch of the sulfate anion, the 2800 to 3000 cm−1 region associated with carbon–hydrogen stretches, and from 3200–3700 cm−1 associated with the oxygen–hydrogen stretches of water. The relative intensities of these features showed a strong dependence on particle size. In particular, submicrometer particles exhibited a larger amount of organic matter compared to supermicrometer particles. However, for external surfaces of homogeneous SSA particles (i.e. particles without a solid inclusion), and also the interfaces of mixed-phase particles, there was a strong SFG response in the aliphatic C–H stretching region for both sub- and supermicrometer particles. This finding suggests that organic material present in supermicrometer particles primarily resides at the interface. The presence of methylene contributions in the SFG spectra indicated disordered alkyl chains, in contrast to what one might expect for a surfactant layer on a sea salt particle. Changes in peak frequencies and relative intensities in the C–H stretching region are seen for some particles after the addition of bacteria, phytoplankton, and growth medium to the seawater. This study provides new insights into the bulk and surface composition of SSA particles and represents a step forward in our understanding of this globally abundant aerosol. It also provides insights into the development of model systems for SSA that may more accurately represent the organic layer at the surface.

We present vibrational sum frequency generation (SFG) spectra of the external surfaces and the internal interfaces of size-selected sea spray aerosol (SSA) particles generated at the wave flume of the Scripps Hydraulics Laboratory. Our findings support SSA particle models that invoke the presence of surfactants in the topmost particle layer and indicate that the alkyl chains of surfactant-rich SSA particles are likely to be disordered. Specifically, the SFG spectra suggest that across the range of sizes studied, surfactant-rich SSA particles contain CH oscillators that are subject to molecular orientation distributions that are broader than the narrow molecular distribution functions associated with well-ordered and well-aligned alkyl chains. This result is consistent with the interpretation that the permeability of organic layers at SSA particle surfaces to small reactive and nonreactive molecules may be substantial, allowing for much more exchange between reactive and nonreactive species in the gas or the condensed phase than previously thought. The SFG data also suggest that a one-component model is likely to be insufficient for describing the SFG responses of the SSA particles. Finally, the similarity of the SFG spectra obtained from the wave flume microlayer and 150 nm-sized SSA particles suggests that the SFG active CH oscillators in the topmost layer of the wave flume and the particle accumulation mode may be in similar chemical environments. Needs for additional research activities are discussed in the context of the results presented.

In the atmosphere, mineral dust particles are often associated with adsorbed nitrate from heterogeneous reactions with nitrogen oxides (N2O5, HNO3, NO3, and NO2). Nitrate ions associated with mineral dust particles can undergo further reactions including those initiated by solar radiation. Although nitrate photochemistry in aqueous media is fairly well studied, much less is known about the photochemistry of nitrate adsorbed on mineral dust particles. In this study, the photochemistry of nitrate from HNO3 adsorption in NaY zeolite under different environmental conditions has been investigated using transmission FTIR spectroscopy. NaY zeolite is used as a model zeolite for studying reactions that can occur in confined space such as those found in porous materials including naturally occurring zeolites and clays. Upon nitrate photolysis under dry conditions (relative humidity, RH, < 1%), surface nitrite is formed as the major adsorbed product. Although nitrite has been proposed as a product in the photochemistry of nitrate adsorbed on metal oxide particle surfaces, such as on alumina, it has not been previously detected. The stability of adsorbed nitrite in NaY is attributed to the confined three-dimensional structure of the porous zeolite, which contains a charge compensating cation that can stabilize the nitrite ion product. Besides adsorbed nitrite, small amounts of gas phase nitrogen-containing products are observed as well including NO2, NO, and N2O at long irradiation times. The amount of nitrite formed via nitrate photochemistry decreases with increasing relative humidity, whereas gas phase NO and N2O become the only detectable products. Gas-phase NO2 does not observe at RH > 1%. In the presence of gas phase ammonia, ammonium nitrate is formed in NaY zeolite. Photochemistry of ammonium nitrate yields gas phase N2O as the sole gas phase product. Evidence for an NH2 intermediate in the formation of N2O is identified with FTIR spectroscopy for HNO3 adsorption and photochemistry in NH4Y zeolite. Here, we discuss mechanisms for the formation of these intermediates from nitrate photochemistry as well as possible atmospheric implications.

Anthropogenic coal fly ash (FA) aerosol may represent a significant source of bioavailable iron in the open ocean. Few measurements have been made that compare the solubility of atmospheric iron from anthropogenic aerosols and other sources. We report here an investigation of iron dissolution for three FA samples in acidic aqueous solutions and compare the solubilities with that of Arizona test dust (AZTD), a reference material for mineral dust. The effects of pH, simulated cloud processing, and solar radiation on iron solubility have been explored. Similar to previously reported results on mineral dust, iron in aluminosilicate phases provides the predominant component of dissolved iron. Iron solubility of FA is substantially higher than of the crystalline minerals comprising AZTD. Simulated atmospheric processing elevates iron solubility due to significant changes in the morphology of aluminosilicate glass, a dominant material in FA particles. Iron is continuously released into the aqueous solution as FA particles break up into smaller fragments. These results suggest that the assessment of dissolved atmospheric iron deposition fluxes and their effect on the biogeochemistry at the ocean surface should be constrained by the source, environmental pH, iron speciation, and solar radiation.

The physicochemical properties of coarse-mode, iron-containing particles and their temporal and spatial distributions are poorly understood. Single-particle analysis combining X-ray elemental mapping and computer-controlled scanning electron microscopy (CCSEM-EDX) of passively collected particles was used to investigate the physicochemical properties of iron-containing particles in Cleveland, OH, in summer 2008 (Aug–Sept), summer 2009 (July–Aug), and winter 2010 (Feb–March). The most abundant classes of iron-containing particles were iron oxide fly ash, mineral dust, NaCl-containing agglomerates (likely from road salt), and Ca–S containing agglomerates (likely from slag, a byproduct of steel production, or gypsum in road salt). The mass concentrations of anthropogenic fly ash particles were highest in the Flats region (downtown) and decreased with distance away from this region. The concentrations of fly ash in the Flats region were consistent with interannual changes in steel production. These particles were observed to be highly spherical in the Flats region, but less so after transport away from downtown. This change in morphology may be attributed to atmospheric processing. Overall, this work demonstrates that the method of passive collection with single-particle analysis by electron microscopy is a powerful tool to study spatial and temporal gradients in components of coarse particles. These gradients may correlate with human health effects associated with exposure to coarse-mode particulate matter.

Copper nanomaterials are being used in a large number of commercial products because these materials exhibit unique optical, magnetic, and electronic properties. Metallic copper nanoparticles, which often have a thin surface oxide layer, can age in the ambient environment and become even more oxidized over time. These aged nanoparticles will then have different properties compared to the original nanoparticles. In this study, we have characterized three different types of copper-based nanoparticle (NP) samples designated as Cu(new) NPs, Cu(aged) NPs, and CuO NPs that differ in the level of oxidation. The solution phase behavior of these three copper-based nanoparticle samples is investigated as a function of pH and in the presence and absence of two common, complexing organic acids, citric and oxalic acid. The behavior of these three copper-based NP types shows interesting differences. In particular, Cu(aged) NPs exhibit unique chemistry including oxide phases that form and surface adsorption properties. Overall, the current study provides some insights into the impacts of nanoparticle aging and how the physicochemical characteristics and reactivity of nanomaterials can change upon aging.

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. NO2-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.

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.Graphical abstractHighlights► Enhance dissolution from nanorods compared to microrods, in the absence of added oxyanions. ► Oxyanions enhance dissolution from microrods whereas they quench nanorod dissolution. ► These differences between the effect of carbonate and phosphate are due to particle aggregation. ► Surface adsorption behavior of oxyanions on nanorods compared to microrods shows important differences.

In this study, heterogeneous interactions of H2O and HNO3 on goethite, α-FeOOH, a component of mineral dust aerosol, are investigated with simultaneous QCM measurements and ATR-FTIR spectroscopy. Laboratory synthesized α-FeOOH of varying sizes (microrods and nanorods) when exposed to gas phase H2O and HNO3 results in the uptake of these gases. This combined approach of QCM measurements and ATR-FTIR spectroscopy allows for both quantification of the amount of uptake and spectroscopic data that provides information on speciation of adsorbed products. The results show that, in the case of H2O, both microrods and nanorods take up water and that the total amounts of water, when normalized to surface area, are similar. However, for HNO3 uptake, the saturation coverage of total and irreversibly bound HNO3 on microrods was observed to be higher than that on nanorods, a size effect which is attributed to surface structural changes that occur as a function of particle size. Furthermore, an investigation of the behavior of nitric acid reacted with α-FeOOH in aqueous media was carried out such as to better understand the effects of atmospheric processing upon dispersal within the hydrosphere.

In this study, alternating current (AC) mode atomic force microscopy (AFM) combined with phase imaging and X-ray photoelectron spectroscopy (XPS) were used to investigate the effect of nitrogen dioxide (NO2) adsorption on calcium carbonate (CaCO3) (101̅4) surfaces at 296 K in the presence of relative humidity (RH). At 70% RH, CaCO3 (101̅4) surfaces undergo rapid formation of a metastable amorphous calcium carbonate layer, which in turn serves as a substrate for recrystallization of a nonhydrated calcite phase, presumably vaterite. The adsorption of nitrogen dioxide changes the surface properties of CaCO3 (101̅4) and the mechanism for formation of new phases. In particular, the first calcite nucleation layer serves as a source of material for further island growth; when it is depleted, there is no change in total volume of nitrocalcite, Ca(NO3)2, particles formed whereas the total number of particles decreases. This indicates that these particles are mobile and coalesce. Phase imaging combined with force curve measurements reveals areas of inhomogeneous energy dissipation during the process of water adsorption in relative humidity experiments, as well as during nitrocalcite particle formation. Potential origins of the different energy dissipation modes within the sample are discussed. Finally, XPS analysis confirms that NO2 adsorbs on CaCO3 (101̅4) in the form of nitrate (NO3–) regardless of environmental conditions or the pretreatment of the calcite surface at different relative humidity.

Nanoparticles and nanostructured aggregates of paratacamite are prepared in acidic solutions through the conversion of copper-based nanoparticles. Aged and oxidized copper nanoparticles with an average primary particle size of ∼15 nm, when combined with hydrochloric acid solutions in the range of 0.025 to 0.1 M, show interesting behavior yielding both a change in nanoparticle primary size, as measured by an electrospray scanning mobility particle sizer, and in chemical composition to produce a copper chloride hydroxide mineral identified as paratacamite (γ-Cu2(OH)3Cl) by powder X-ray diffraction of the dehydrated solid sample. Taken together, these data suggest that paratacamite nanoparticles in solution can aggregate to yield microporous paratacamite materials. Microporous paratacamite was characterized by several techniques including X-ray diffraction, transmission electron microscopy, energy dispersive X-ray analysis, electron energy loss spectroscopy, X-ray photoelectron spectroscopy and surface area measurements. Oxidation of these copper-based nanoparticles with molecular oxygen and the role of the oxidized layer in the formation of paratacamite have been investigated. Comparison to microscale copper particles showed there is unique oxidation behavior of nanoscale copper particles that results in unique reaction chemistry of oxidized nanoscale copper particles with hydrochloric acid solutions to form paratacamite. This study provides a new route for the formation of paratacamite nanomaterials that can be used in a wide range of chemically interesting applications including hydrogen storage materials and as a heterogeneous catalyst for the synthesis of green solvents such as dimethyl and diethyl carbonates. Additionally, this study suggests a potentially new pathway for the degradation of art objects and ancient artifacts as well as other cultural heritage materials containing small copper particles that has not been previously considered.

Nitrous oxide (N2O) is an important greenhouse gas and a primary cause of stratospheric ozone destruction. Despite its importance, there remain missing sources in the N2O budget. Here we report the formation of atmospheric nitrous oxide from the decomposition of ammonium nitrate via an abiotic mechanism that is favorable in the presence of light, relative humidity and a surface. This source of N2O is not currently accounted for in the global N2O budget. Annual production of N2O from atmospheric aerosols and surface fertilizer application over the continental United States from this abiotic pathway is estimated from results of an annual chemical transport simulation with the Community Multiscale Air Quality model (CMAQ). This pathway is projected to produce 9.3+0.7/−5.3 Gg N2O annually over North America. N2O production by this mechanism is expected globally from both megacities and agricultural areas and may become more important under future projected changes in anthropogenic emissions.

The adsorption of sulfur dioxide (SO2) on titanium dioxide (TiO2) nanoparticle surfaces at 296 K under a wide range of conditions has been investigated. X-ray photoelectron spectroscopy is used to investigate the surface speciation and surface coverage of sulfur-containing products on ca. 4 nm TiO2 anatase particles that remain on the surface following adsorption of SO2. The effects of various environmental conditions of relative humidity, molecular oxygen, and broadband UV/vis irradiation as well as sample pretreatment were found to impact the speciation of adsorbed SO2 as well as the saturation coverage. In particular, in the absence of light, the majority surface species upon SO2 adsorption is found to be adsorbed sulfite. Broadband UV/vis irradiation during sulfur dioxide adsorption leads to an increase (nearly 2-fold) in the amount of adsorbed sulfur species, as compared to experiments with no light, and results in the formation of adsorbed sulfate. The formation of sulfate was quantitative in the presence of molecular oxygen. New surface species including chemisorbed molecular SO2 were observed on samples that have been reduced in vacuum through argon ion sputtering. The total amount of adsorbed sulfur was impacted by surface hydroxyl group coverage and molecularly adsorbed water layer. Additionally, comparison of sulfur dioxide adsorption on 4 versus 32 nm sized anatase nanoparticles showed that surface saturation coverages of adsorbed sulfite on the 4 nm particles was almost twice that of 32 nm particles as measured by the S2p:Ti2p peak area ratios, thus showing an increase in the inherent adsorption capacity of the smaller particles. Proposed adsorption sites and mechanisms to account for the observed experimental data are discussed.

Mineral dust aerosol is known to provide a reactive surface in the troposphere for heterogeneous chemistry to occur. Certain components of mineral dust aerosol, such as semiconductor metal oxides, can act as chromophores that initiate chemical reactions, while adsorbed organic and inorganic species may also be photoactive. However, relatively little is known about the impact of heterogeneous photochemistry of mineral dust aerosol in the atmosphere. In this study, we investigate the heterogeneous photochemistry of trace atmospheric gases including HNO3 and O3 with components of mineral dust aerosol using an environmental aerosol chamber that incorporates a solar simulator. For reaction of HNO3 with aluminum oxide, broadband irradiation initiates photoreactions to form gaseous NO and NO2. A complex dynamic balance between surface adsorbed nitrate and gaseous nitrogen oxide products including NO and NO2 is observed. For heterogeneous photoreactions of O3, iron oxide shows catalytic decompositions toward O3 while aluminum oxide is deactivated by ozone exposure. Furthermore, the role of relative humidity, and, thus, adsorbed water, on heterogeneous photochemistry has been explored. The atmospheric implications of these results are discussed.

The heterogeneous chemistry and photochemistry of ozone on oxide components of mineral dust aerosol, including α-Fe2O3, TiO2, and α-Al2O3, at different relative humidities have been investigated using an environmental aerosol chamber. The rate and extent of ozone decomposition on these oxide surfaces are found to be a function of the nature of the surface as well as the presence of light and relative humidity. Under dark and dry conditions, only α-Fe2O3 exhibits catalytic decomposition toward ozone, whereas the reactivity of TiO2 and α-Al2O3 is rapidly quenched upon ozone exposure. However, upon irradiation, TiO2 is active toward O3 decomposition and α-Al2O3 remains inactive. In the presence of relative humidity, ozone decay on α-Fe2O3 subject to irradiation or under dark conditions is found to decrease. In contrast, ozone decomposition is enhanced for irradiated TiO2 as relative humidity initially increases but then begins to decrease at higher relative humidity levels. A kinetic model was used to obtain heterogeneous reaction rates for different homogeneous and heterogeneous reaction pathways taking place in the environmental aerosol chamber. The atmospheric implications of these results are discussed.

Metal and metal oxide nanomaterials are found in many consumer products for use in a wide range of applications including catalysis, sensors and contaminant remediation. Because of the extensive use of metal-based nanomaterials, there are concerns that these materials have the potential to get into the environment sometime during production, distribution, use and/or disposal. In particular, there exists the potential that they will make their way into water systems, e.g. drinking water systems, ground water systems, estuaries and lakes. In this review, some of the uncertainties in understanding nanoparticle behavior, which is often due to a lack of fundamental knowledge of the surface structure and surface energetics for very small particles, are discussed. Although classical models may provide guidance for understanding dissolution and aggregation of nanoparticles in water, it is the detailed surface structure and surface chemistry that are needed to accurately describe the surface free energy, a large component of the total free energy, in order to fully understand these processes. Without this information, it is difficult to develop a conceptual framework for understanding the fate, transport and potential toxicity of nanomaterials. Needed research areas to fill this void are discussed.

Citric acid plays an important role as a stabilizer in several nanomaterial syntheses and is a common organic acid found in nature. Here, the adsorption of citric acid onto TiO2 anatase nanoparticles with a particle diameter of ca. 4 nm is investigated at circumneutral and acidic pHs. This study focuses on both the details of the surface chemistry of citric acid on TiO2, including measurements of surface coverage and speciation, and its impact on nanoparticle behavior. Using macroscopic and molecular-based probes, citric acid adsorption and nanoparticle interactions are measured with quantitative solution phase adsorption measurements, attenuated total reflection-FTIR spectroscopy, dynamic light scattering techniques, and zeta-potential measurements as a function of solution pH. The results show that surface coverage is a function of pH and decreases with increasing pH. Surface speciation differs from the bulk solution and is time dependent. After equilibration, the fully deprotonated citrate ion is present on the surface regardless of the highly acidic solution pH indicating pKa values of surface adsorbed species are lower than those in solution. Nanoparticle interactions are also probed through measurements of aggregation and the data show that these interactions are complex and depend on the detailed interplay between bulk solution pH and surface chemistry.

Alkaline earth-based oxides are important materials in the storage of carbon dioxide. Here we present a template-free synthesis method for mesoporous magnesium oxide (MgO) via the thermal decomposition of anhydrous magnesium acetate. Characterization of the crystalline phase, particle size, pore size and surface area for mesoporous MgO samples was accomplished using a variety of techniques and methods including scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), powder X-ray diffraction (XRD), and nitrogen adsorption analysis. The results showed that mesoporous MgO prepared from anhydrous magnesium acetate had a high surface area in the range of 120–136 m2 g−1 and a narrow pore size distribution in the range of 3–4 nm. The pore structure was composed of small primary MgO nanoparticle aggregates with interparticle connections. In situ transmission FTIR spectroscopy was used to investigate CO2 adsorption on mesoporous MgO. This spectroscopic investigation showed that mesoporous MgO exhibited enhanced CO2 adsorption capacity relative to commercially available MgO nanoparticles. This difference was attributed mainly to an increase in surface area. Differences in surface carbonate/bicarbonate speciation were observed between the mesoporous and commercial MgO samples and were related to structural differences for the smaller nanoparticles.

Density functional and ab initio calculations have been performed on CO2−nH2O and Al(OH)3−CO2−nH2O (where n = 1, 2, 3) cluster models to elucidate the catalytic effect of a hydroxylated metal center on the formation of carbonic acid (H2CO3). B3LYP/6-311++G(d,p)-calculated geometries and RI-SCS-MP2/aug-cc-pVTZ//B3LYP/6-311++G(d,p)-calculated energies with respect to isolated gas-phase molecules and various H2O, CO2, and H2CO3−Al(OH)3 complexes are presented. It is shown here that H2CO3 formation proceeds via direct CO2 and nH2O reaction with very high activation barriers in the gas phase, 51.40, 29.64, and 19.84 kcal/mol for CO2−H2O, CO2−2H2O, and CO2−3H2O clusters, respectively, decreasing in magnitude with an increase in the number of H2O molecules. The energetics as well as the reaction mechanism and energy landscape change significantly when carbonic acid is formed from CO2 and nH2O in the presence of Al(OH)3, a hydroxylated metal center. Results presented here show important details of the influence of the coordinating metal center in the formation of H2CO3.

A number of recent studies have shown that iron dissolution in Fe-containing dust aerosol can be linked to source material (mineral or anthropogenic), mineralogy, and iron speciation. All of these factors need to be incorporated into atmospheric chemistry models if these models are to accurately predict the impact of Fe-containing dusts into open ocean waters. In this report, we combine dissolution measurements along with spectroscopy and microscopy to focus on nanoscale size effects in the dissolution of Fe-containing minerals in low-pH environments and the importance of acid type, including HNO3, H2SO4, and HCl, on dissolution. All of these acids are present in the atmosphere, and dust particles have been shown to be associated with nitrate, sulfate, and/or chloride. These measurements are done under light and dark conditions so as to simulate and distinguish between daytime and nighttime atmospheric chemical processing. Both size (nano- versus micron-sized particles) and anion (nitrate, sulfate, and chloride) are found to play significant roles in the dissolution of α-FeOOH under both light and dark conditions. The current study highlights these important, yet unconsidered, factors in the atmospheric processing of iron-containing mineral dust aerosol.

In this study, the dissolution of copper nanoparticles in aqueous low-pH suspensions is examined. The dissolution phenomenon is examined using both bulk measurements of copper ion production, as detected by inductively coupled plasma-optical emission spectroscopy (ICP/OES), and a decrease in nanoparticle size using particle-sizing instruments. For size measurements, an electrospray atomizer coupled to a scanning mobility particle sizer (ES-SMPS) was used to monitor changes in the particle size distribution (PSD) of the copper nanoparticles as they dissolved in hydrochloric acid solution in real time. Measured PSDs show interesting changes during the dissolution process, including a change in modality (mono to multi) with time. Although there may be several causes for the observed modality changes upon dissolution, it is clear that only through direct measurements of nanoparticles and nanoparticle PSDs can these dynamic details be captured as these particles change size, thus providing important insights into nanoscale processes.

In this study, CO2 adsorption in the presence and absence of co-adsorbed H2O was investigated in zeolite Y. Several different zeolite Y materials were investigated including commercial NaY, commercial NaY ion-exchanged with Ba2+ and nanocrystalline NaY; herein referred to as NaY, BaY and nano-NaY. Following heating of these zeolites to 573 K and cooling to room temperature, CO2 was adsorbed as a function of pressure. FTIR spectra show that a majority of CO2 adsorbs in the pores of these three zeolites (NaY, BaY and nano-NaY) in a linear complex with the exchangeable cation, as indicated by the intense absorption band near 2350 cm−1, assigned to the ν3 asymmetric stretch of adsorbed CO2. Most interestingly is the formation of carbonate and bicarbonate on the external surface of nano-NaY zeolites as indicated by the presence of several broad absorption bands in the 1200–1800 cm−1 region, suggesting unique sites for CO2 adsorption on the surface of the nanomaterial. For the other two zeolite materials investigated, bicarbonate formation is only evident in BaY zeolite in the presence of co-adsorbed water. Adsorption of 18O-labeled carbon dioxide and theoretical quantum chemical calculations confirm these assignments and conclusions.

In this study, alternating current (AC) mode Atomic Force Microscopy (AFM) height images combined with force measurements and phase imaging were used to investigate the surface reconstruction and chemistry of the lowest energy surface, (101¯4) plane, of calcite, a stable form of calcium carbonate (CaCO3), in the presence of relative humidity at different temperatures. At 296 K and 70% RH, calcite (101¯4) undergoes rapid restructuring during hydration forming regions on the surface that are most likely characterized as an amorphous hydrate layer similar to what forms in solution under high [Ca2+] supersaturation conditions. This hydrate layer in turn serves as a substrate for the crystallization of another layer that possesses structural properties which differ from hydrate layer. Phase imaging reveals that these different layer structures formed in the process of water adsorption and surface reconstruction have very different energy dissipation modes. The origin of the different dissipation modes are likely due to differences in water content and hydrophobicity of these regions. The newly formed layer on top of the hydration layer is proposed to be vaterite, another polymorph of calcium carbonate. At 278 K the formation mechanism of the vaterite layer changes due to nucleation of a more crystalline hydrate layer, similar to calcium carbonate hexahydrate, instead of the amorphous hydrate layer that forms at 296 K. Force measurements corroborate the assignment of the speciation of different regions on the surface. Importantly, the AFM data show that the surface of calcite is highly inhomogeneous with regions that vary in water content. This suggests that the reactivity of calcite in humid environments will be highly spatially dependent.

Nitrogen oxides, including nitrogen dioxide and nitric acid, react with mineral dust particles in the atmosphere to yield adsorbed nitrate. Although nitrate ion is a well-known chromophore in natural waters, little is known about the surface photochemistry of nitrate adsorbed on mineral particles. In this study, nitrate adsorbed on aluminum oxide, a model system for mineral dust aerosol, is irradiated with broadband light (λ > 300 nm) as a function of relative humidity (RH) in the presence of molecular oxygen. Upon irradiation, the nitrate ion readily undergoes photolysis to yield nitrogen-containing gas-phase products including NO2, NO, and N2O, with NO being the major product. The relative ratio and product yields of these gas-phase products change with RH, with N2O production being highest at the higher relative humidities. Furthermore, an efficient dark reaction readily converts the major NO product into NO2 during post-irradiation. Photochemical processes on mineral dust aerosol surfaces have the potential to impact the chemical balance of the atmosphere, yet little is known about these processes. In this study, the impact that adsorbed nitrate photochemistry may have on the renoxification of the atmosphere is discussed.

Surface scientists are dealing more and more with complex systems that are challenging to investigate from both experimental and theoretical perspectives. The surface science of complex interfaces, such as environmental interfaces under ambient conditions of temperature and relative humidity, requires both advances in experimental and theoretical methods in order for conceptual insights to emerge. In this prospective, several aspects of environmental interfaces and the field of environmental surface science are discussed. These include: (i) adsorbed water on oxide and carbonate interfaces; (ii) surface chemistry of oxide and carbonate interfaces in the presence of co-adsorbed water; (iii) solvation of ions by co-adsorbed water on environmental interfaces; and (iv) research needs and challenges in environmental surface science.

In this study, the adsorption of two organic acids, oxalic acid and adipic acid, on TiO2 nanoparticles was investigated at room temperature, 298 K. Solution-phase measurements were used to quantify the extent and reversibility of oxalic acid and adipic acid adsorption on anatase nanoparticles with primary particle sizes of 5 and 32 nm. At all pH values considered, there were minimal differences in measured Langmuir adsorption constants, Kads, or surface-area-normalized maximum adsorbate−surface coverages, Γmax, between 5 and 32 nm particles. Although macroscopic differences in the reactivity of these organic acids as a function of nanoparticle size were not observed, ATR−FTIR spectroscopy showed some distinct differences in the absorption bands present for oxalic acid adsorbed on 5 nm particles compared to 32 nm particles, suggesting different adsorption sites or a different distribution of adsorption sites for oxalic acid on the 5 nm particles. These results illustrate that molecular-level differences in nanoparticle reactivity can still exist even when macroscopic differences are not observed from solution phase measurements. Our results also allowed the impact of nanoparticle aggregation on acid uptake to be assessed. It is clear that particle aggregation occurs at all pH values and that organic acids can destabilize nanoparticle suspensions. Furthermore, 5 nm particles can form larger aggregates compared to 32 nm particles under the same conditions of pH and solid concentrations. The relative reactivity of 5 and 32 nm particles as determined from Langmuir adsorption parameters did not appear to vary greatly despite differences that occur in nanoparticle aggregation for these two different size nanoparticles. Although this potentially suggests that aggregation does not impact organic acid uptake on anatase particles, these data clearly show that challenges remain in assessing the available surface area for adsorption in nanoparticle aqueous suspensions because of aggregation.

In this study, the selective catalytic reduction (SCR) of NO2 to N2 and O2 with ammonia at 298 K on nanocrystalline NaY, Aldrich NaY and nanocrystalline CuY was investigated using in situ Fourier transform infrared (FTIR) spectroscopy. It was determined that the kinetics of SCR were 30% faster on nanocrystalline NaY compared to Aldrich NaY. The superior performance of the nanocrystalline zeolite was attributed to an increase in external surface reactivity. External surface sites, which include silanol groups and extra framework alumina (EFAL), gave rise to differences in the adsorption of NO2 and NH3 on nanocrystalline NaY compared to commercial NaY. Copper cation-exchanged nanocrystalline Y resulted in an additional increase in the rate of SCR as well as distinct NO2 and NH3 adsorption sites associated with the copper cation. This is the first study of a transition metal cation-exchanged nanocrystalline zeolite and its potential use as a catalyst in the SCR of nitrogen oxides.Nanocrystalline NaY and CuY zeolites were determined to be better catalysts in the selective catalytic reduction (SCR) of NO2 to N2 and O2 with ammonia at 298 K compared to commercial NaY zeolite composed of larger crystallites. This is the first study of a transition metal cation-exchanged nanocrystalline zeolite and its potential use as a catalyst for SCR-NOx.

It is clear that mineral dust particles can impact a number of global processes including the Earth's climate through direct and indirect climate forcing, the chemical composition of the atmosphere through heterogeneous reactions, and the biogeochemistry of the oceans through dust deposition. Thus, mineral dust aerosol links land, air, and oceans in unique ways unlike any other type of atmospheric aerosol. Quantitative knowledge of how mineral dust aerosol impacts the Earth's climate, the chemical balance of the atmosphere, and the biogeochemistry of the oceans will provide a better understanding of these links and connections and the overall impact on the Earth system. Advances in the applications of analytical laboratory techniques have been critical for providing valuable information regarding these global processes. In this mini review article, we discuss examples of current and emerging techniques used in laboratory studies of mineral dust chemistry and climate and potential future directions.

Nanocrystalline zeolites, such as silicalite-1 and zeolite Y, were synthesized in high yield by periodically removing nanocrystals from the synthesis solution and recycling the unused reagents, including the template and T-atom sources.

Calcium carbonate is a ubiquitous mineral and its reactivity with indoor and outdoor air pollutants will contribute to the deterioration of these materials through the formation of salts that deliquesce at low relative humidity (RH). As shown here for calcium nitrate thin films, deliquescence occurs at even lower relative humidity than expected from bulk thermodynamics and lower than the recommended humidity for the preservation of artifacts and antiques.

Environmental molecular surface science is an expanding area of current research. This review focuses on advances in the molecular level understanding of oxide surfaces as they play an important role in several environmental chemical processes. Oxide surfaces are often used as catalysts and adsorbents in environmental remediation. The surface of oxide particles in the troposphere can adsorb and catalyze reactions of trace gases and thus change the chemical balance of the atmosphere. Mineral oxide surfaces in contact with ground water can adsorb and catalyze reactions of pollutant molecules. Surface science studies of these environmental chemical processes provide the basis for delineating molecular-level information about surface structure under environmentally relevant conditions, adsorbate–surface interactions, surface reaction mechanisms, structure–reactivity relationships and an overall understanding of these processes on the molecular level. In order to glean insights into environmental processes, these studies need to be done under environmentally relevant conditions of temperature, pressure and relative humidity. Thus the need for the further development and use of techniques that can be employed under ambient conditions are discussed.

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.

Nanoparticles and nanostructured aggregates of paratacamite are prepared in acidic solutions through the conversion of copper-based nanoparticles. Aged and oxidized copper nanoparticles with an average primary particle size of ∼15 nm, when combined with hydrochloric acid solutions in the range of 0.025 to 0.1 M, show interesting behavior yielding both a change in nanoparticle primary size, as measured by an electrospray scanning mobility particle sizer, and in chemical composition to produce a copper chloride hydroxide mineral identified as paratacamite (γ-Cu2(OH)3Cl) by powder X-ray diffraction of the dehydrated solid sample. Taken together, these data suggest that paratacamite nanoparticles in solution can aggregate to yield microporous paratacamite materials. Microporous paratacamite was characterized by several techniques including X-ray diffraction, transmission electron microscopy, energy dispersive X-ray analysis, electron energy loss spectroscopy, X-ray photoelectron spectroscopy and surface area measurements. Oxidation of these copper-based nanoparticles with molecular oxygen and the role of the oxidized layer in the formation of paratacamite have been investigated. Comparison to microscale copper particles showed there is unique oxidation behavior of nanoscale copper particles that results in unique reaction chemistry of oxidized nanoscale copper particles with hydrochloric acid solutions to form paratacamite. This study provides a new route for the formation of paratacamite nanomaterials that can be used in a wide range of chemically interesting applications including hydrogen storage materials and as a heterogeneous catalyst for the synthesis of green solvents such as dimethyl and diethyl carbonates. Additionally, this study suggests a potentially new pathway for the degradation of art objects and ancient artifacts as well as other cultural heritage materials containing small copper particles that has not been previously considered.

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.

Sea spray aerosol (SSA) represents one of the largest aerosol components in our atmosphere. SSA plays a major role in influencing climate; however the overall impacts remain poorly understood due to the overall chemical complexity. SSA is comprised of a mixture of inorganic and organic components in varying proportions that change as a function of particle size and seawater composition. In this study, nascent SSA particles were produced using breaking waves, resulting in compositions and sizes representative of the open ocean. The composition of individual SSA particles ranging in size from ca. 0.15 to 10 μm is measured using Raman microspectroscopy, while the interfacial composition of collections of size-resolved particles is probed by sum frequency generation (SFG). Raman spectra of single particles have bands in the 980 to 1030 cm−1 region associated with the symmetric stretch of the sulfate anion, the 2800 to 3000 cm−1 region associated with carbon–hydrogen stretches, and from 3200–3700 cm−1 associated with the oxygen–hydrogen stretches of water. The relative intensities of these features showed a strong dependence on particle size. In particular, submicrometer particles exhibited a larger amount of organic matter compared to supermicrometer particles. However, for external surfaces of homogeneous SSA particles (i.e. particles without a solid inclusion), and also the interfaces of mixed-phase particles, there was a strong SFG response in the aliphatic C–H stretching region for both sub- and supermicrometer particles. This finding suggests that organic material present in supermicrometer particles primarily resides at the interface. The presence of methylene contributions in the SFG spectra indicated disordered alkyl chains, in contrast to what one might expect for a surfactant layer on a sea salt particle. Changes in peak frequencies and relative intensities in the C–H stretching region are seen for some particles after the addition of bacteria, phytoplankton, and growth medium to the seawater. This study provides new insights into the bulk and surface composition of SSA particles and represents a step forward in our understanding of this globally abundant aerosol. It also provides insights into the development of model systems for SSA that may more accurately represent the organic layer at the surface.

It is clear that mineral dust particles can impact a number of global processes including the Earth's climate through direct and indirect climate forcing, the chemical composition of the atmosphere through heterogeneous reactions, and the biogeochemistry of the oceans through dust deposition. Thus, mineral dust aerosol links land, air, and oceans in unique ways unlike any other type of atmospheric aerosol. Quantitative knowledge of how mineral dust aerosol impacts the Earth's climate, the chemical balance of the atmosphere, and the biogeochemistry of the oceans will provide a better understanding of these links and connections and the overall impact on the Earth system. Advances in the applications of analytical laboratory techniques have been critical for providing valuable information regarding these global processes. In this mini review article, we discuss examples of current and emerging techniques used in laboratory studies of mineral dust chemistry and climate and potential future directions.

Metal and metal oxide nanomaterials are found in many consumer products for use in a wide range of applications including catalysis, sensors and contaminant remediation. Because of the extensive use of metal-based nanomaterials, there are concerns that these materials have the potential to get into the environment sometime during production, distribution, use and/or disposal. In particular, there exists the potential that they will make their way into water systems, e.g. drinking water systems, ground water systems, estuaries and lakes. In this review, some of the uncertainties in understanding nanoparticle behavior, which is often due to a lack of fundamental knowledge of the surface structure and surface energetics for very small particles, are discussed. Although classical models may provide guidance for understanding dissolution and aggregation of nanoparticles in water, it is the detailed surface structure and surface chemistry that are needed to accurately describe the surface free energy, a large component of the total free energy, in order to fully understand these processes. Without this information, it is difficult to develop a conceptual framework for understanding the fate, transport and potential toxicity of nanomaterials. Needed research areas to fill this void are discussed.

Alkaline earth-based oxides are important materials in the storage of carbon dioxide. Here we present a template-free synthesis method for mesoporous magnesium oxide (MgO) via the thermal decomposition of anhydrous magnesium acetate. Characterization of the crystalline phase, particle size, pore size and surface area for mesoporous MgO samples was accomplished using a variety of techniques and methods including scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), powder X-ray diffraction (XRD), and nitrogen adsorption analysis. The results showed that mesoporous MgO prepared from anhydrous magnesium acetate had a high surface area in the range of 120–136 m2 g−1 and a narrow pore size distribution in the range of 3–4 nm. The pore structure was composed of small primary MgO nanoparticle aggregates with interparticle connections. In situ transmission FTIR spectroscopy was used to investigate CO2 adsorption on mesoporous MgO. This spectroscopic investigation showed that mesoporous MgO exhibited enhanced CO2 adsorption capacity relative to commercially available MgO nanoparticles. This difference was attributed mainly to an increase in surface area. Differences in surface carbonate/bicarbonate speciation were observed between the mesoporous and commercial MgO samples and were related to structural differences for the smaller nanoparticles.

Surface tension, an important property of liquids, is easily measured for bulk samples. However, for droplets smaller than one micron in size, there are currently no reported measurements. In this study, atomic force microscopy (AFM) and force spectroscopy have been utilized to measure surface tension of individual submicron sized droplets at ambient pressure and controlled relative humidity (RH). Since the surface tension of atmospheric aerosols is a key factor in understanding aerosol climate effects, three atmospherically relevant systems (NaCl, malonic and glutaric acids) were studied. Single particle AFM measurements were successfully implemented in measuring the surface tension of deliquesced particles on the order of 200 to 500 nm in diameter. Deliquesced particles continuously uptake water at high RH, which changes the concentration and surface tension of the droplets. Therefore, surface tension as a function of RH was measured. AFM based surface tension measurements are close to predicted values based on bulk measurements and activities of these three chemical systems. Non-ideal behaviour in concentrated organic acid droplets is thought to be important and the reason for differences observed between bulk solution predictions and AFM data. Consequently, these measurements are crucial in order to improve atmospheric climate models as direct measurements hitherto have been previously inaccessible due to instrument limitations.

Current practices of initial nanoparticle characterization with respect to particle size, shape, surface and bulk composition prior to experiments to test, for example, cellular interaction or toxicity, will not accurately describe nanomaterials in a given medium. The use of initial characterization data in subsequent analyses inherently assumes that nanoparticles are static entities. However, nanoparticle characterization, which is crucial in all studies related to their applications and implications, should also include information about the dynamics of the interfacial region between the nanomaterial surface and the surrounding medium. The objective of this tutorial review is to highlight the importance of in situ characterization of metal oxide nanoparticle surfaces in complex media. In particular, several examples of TiO2 (5 nm) and α-Fe2O3 (2 nm) nanoparticles, in different environmental and biological media, are presented so as to show the importance of the milieu to oxide surface composition. The surface composition is shown to be controlled by the adsorption of biological components (proteins and amino acids), inorganic oxyanions (phosphates and carbonates) and environmental ligands (humic acid). The extent of surface adsorption depends on the solution phase composition and the affinity of different components to adsorb to the nanoparticle surface. The examples presented here show that there is a range of possible surface interactions, adsorption energetics and adsorption modes including reversible adsorption, irreversible adsorption and co-adsorption.

Nanotechnology is no longer in its infancy and has made significant advances since the implementation of the National Nanotechnology Initiative (NNI) in 2000. Incorporation of nanotechnology in many fields including information technology, medicine, materials, energy, catalysis and cosmetics has led to an increase in engineered nanomaterial (ENM) production, and consequently, increased nanomaterial use. In comparison, the generation of concrete and consistent evidence related to the environmental health and safety of nanomaterials (NanoEHS) is lacking. The main factors contributing to the slower progress in NanoEHS versus conventional EHS are related to the complexity, property transformations, life cycles and behavior of nanomaterials even in carefully controlled environments. Therefore, new systematic, integrated research approaches in NanoEHS are needed for overcoming this complexity and bridging current knowledge gaps. A workshop on “NanoEHS: Fundamental Science Needs” brought together scientists and engineers to identify current fundamental science challenges and opportunities within NanoEHS. Detailed discussions were conducted on identifying the fundamental properties that are critical in NanoEHS, differentiating between conventional and NanoEHS studies as well as understanding, the effect of dynamic transformations on nanometrology, role of dosimetry and mechanistic data gaps in nanotoxicology. An important realization that even simple nanoscale materials can be complex when considering NanoEHS implications was noted several times during the workshop. Despite this fact, a number of fundamental research areas to further the scientific foundation to address NanoEHS needs are suggested.

Correction for ‘Humidity-dependent surface tension measurements of individual inorganic and organic submicrometre liquid particles’ by Holly S. Morris et al., Chem. Sci., 2015, 6, 3242–3247.