Nitrate Ion Photolysis in Thin Water Films in the Presence of Bromide Ions

Revealing the photochemical pathways of nitrate in water through first-principles simulations

The nitrate anion (NO3-) is abundant in environmental aqueous phases, including aerosols, surface waters, and snow, where its photolysis releases nitrogen oxides back into the atmosphere. Nitrate photolysis occurs via two channels: (1) the formation of NO2 and O- and (2) the formation of NO2- and O(3P). The occurrence of two reaction channels with very low quantum yield (∼1%) highlights the critical role of the solvation environment and spin-forbidden electronic transitions, which remain unexplained at the molecular level. We investigate the two photolysis channels in water using quantum chemical calculations and first-principles molecular dynamics simulations with hybrid density functional theory and enhanced sampling. We find that spin-forbidden absorption to the triplet state (T1) is possible but occurs at a rate ∼15 times weaker than the spin-allowed transition to the singlet state (S1). A metastable solvation cage complex requires additional thermal energy to dissociate the N-O bond, allowing for recombination or non-radiative deactivation. Our results explain the temperature dependence of photolysis, linked to hydrogen bond rearrangement in the solvation shell. This work provides new molecular insights into nitrate photolysis and its low quantum yield under environmental conditions.

Effects of cations on the structure, dynamics and vibrational sum frequency generation spectroscopy of liquid/vapor interfaces of aqueous solutions of monovalent and divalent metal nitrates

We have employed molecular dynamics (MD) simulations and theoretical vibrational sum frequency generation spectroscopy (VSFG) to investigate the structure and interactions of water and ions at liquid/vapor interfaces of aqueous solutions of monovalent and divalent metal nitrates of NaNO3, Mg(NO3)2 and Ca(NO3)2. Structural properties, such as the number density profiles, average number of hydrogen bonds, slab radial distribution functions (SRDFs), and tetrahedral order parameter, are calculated to investigate the structure of water in bulk and at the interfaces. The ion pairing tendency is determined in terms of cation-anion SRDF and is found to be following the order Mg(NO3)2 < Ca(NO3)2 < NaNO3 both at the interfacial and bulk regions. An ionic double layer is found to be formed at the interface. A weak propensity of anions is found to be at the interface, while the cations are found to be present below the interfacial region. Three prominent features are observed in the VSFG spectrum of the liquid/vapor interfaces of metal nitrate solutions: a free (dangling) O-H peak at 3750 cm-1, a peak at 3589 cm-1 due to O-H groups hydrogen bonded to nitrates, and a broad peak at 3200-3500 cm-1 due to O-H modes hydrogen bonded to water. The charge density of cations affects the intensity of the 3200-3500 cm-1 peak. The presence of ions is found to have very little effect on the position and intensity of the dangling peak as compared to that for the neat water/vapor interface. The ionic double layers generate electric fields that reorganize and reorient the water molecules towards the vapor. This upward reorientation of water leads to a positive region in the VSFG spectrum of O-H modes which are hydrogen bonded to water in contrast to that of the neat water/vapor interface. The O-H groups hydrogen bonded to the nitrates are mostly oriented downwards and the strength of such hydrogen bonds is found to be weaker than those hydrogen bonded to water.

Surface Nitrate Enrichment and Enhanced HONO Production from Ionic Surfactant Aggregation at the Aqueous-Air Interface

Significant discrepancies persist between field observations and model simulations regarding the strength of marine-derived HONO sources, underscoring the urgency to resolve unidentified HONO sources. In this study, sodium dodecyl sulfate (SDS) was chosen as a proxy for marine surfactants to investigate its impact on aqueous nitrate photolysis for the first time. Remarkable increases in HONO and NO2 production rates by factors of 3.3 and 5.6, respectively, along with a 1.9-fold rise in NO2- concentration, were observed at a very low SDS concentration of 0.01 mM, strongly illustrating the promoting effect on nitrate photolysis. Furthermore, at an SDS concentration of 2 mM, intriguingly aligned with the critical micelle concentration, there was an additional 41.7% increase in HONO production rates. Vertically resolved Raman measurements indicated that SDS anions at the aqueous-air interface attracted NO3- closer to the aqueous surfaces, increasing the amount of incompletely solvated surface nitrate. Importantly, the anionic surfactant exhibited a greater promoting effect on HONO production compared to other typical nitrate photochemistry systems with the addition of a marine dissolved organic matter proxy, halogen, photosensitizer, or OH scavenger. These findings offer new insights into marine-derived HONO sources and should be considered in model simulations concerning the budgets of NOx, OH, and O3.

Coarse particles compensate for missing daytime sources of nitrous acid and enhance atmospheric oxidation capacity in a coastal atmosphere

Large missing sources of daytime atmospheric nitrous acid (HONO), a vital source of hydroxyl radicals (OH) through its photolysis, frequently exist in global coastal regions. In this study, ambient HONO and relevant species were measured at a coastal site in the Pearl River Delta (PRD), China, during October 2019. Relatively high concentrations (0.32 ± 0.19 ppbv) and daytime peaks at approximately 13:00 of HONO were observed, and HONO photolysis was found to be the dominant (55.5 %) source of the primary OH production. A budget analysis of HONO based on traditional sources suggested large unknown sources during the daytime (66.4 %), which had a significant correlation with the mass of coarse particles (PM2.5-10) and photolysis frequency (J(NO2)). When incorporating photolysis of the abundant nitrate measured in coarse particles with a reasonable enhancement factor relative to fine particles due to favorable aerosol conditions, the missing daytime sources of HONO could be fully compensated by coarse particles serving as the largest source at this coastal site. Our study revealed great potential of coarse particles as a strong daytime HONO source, which has been ignored before but can efficiently promote NOx recycling and thus significantly enhance atmospheric oxidation capacity.

Significantly Accelerated Photochemical Perfluorooctanoic Acid Decomposition at the Air-Water Interface of Microdroplets

The efficient elimination of per- and polyfluoroalkyl substances (PFASs) from the environment remains a huge challenge and requires advanced technologies. Herein, we demonstrate that perfluorooctanoic acid (PFOA) photochemical decomposition could be significantly accelerated by simply carrying out this process in microdroplets. The almost complete removal of 100 and 500 μg/L PFOA was observed after 20 min of irradiation in microdroplets, while this was achieved after about 2 h in the corresponding bulk phase counterpart. To better compare the defluorination ratio, 10 mg/L PFOA was used typically, and the defluorination rates in microdroplets were tens of times faster than that in the bulk phase reaction system. The high performances in actual water matrices, universality, and scale-up applicability were demonstrated as well. We revealed in-depth that the great acceleration is due to the abundance of the air-water interface in microdroplets, where the reactants concentration enrichment, ultrahigh interfacial electric field, and partial solvation effects synergistically promoted photoreactions responsible for PFOA decomposition, as evidenced by simulated Raman scattering microscopy imaging, vibrational Stark effect measurement, and DFT calculation. This study provides an effective approach and highlights the important roles of air-water interface of microdroplets in PFASs treatment.

Daytime Atmospheric Halogen Cycling through Aqueous-Phase Oxygen Atom Chemistry

Halogen atoms are important atmospheric oxidants that have unidentified daytime sources from photochemical halide oxidation in sea salt aerosols. Here, we show that the photolysis of nitrate in aqueous chloride solutions generates nitryl chloride (ClNO₂) in addition to Cl₂ and HOCl. Experimental and modeling evidence suggests that O(³P) formed in the minor photolysis channel from nitrate oxidizes chloride to Cl₂ and HOCl, which reacts with nitrite to form ClNO₂. This chemistry is different than currently accepted mechanisms involving chloride oxidation by OH and could shift our understanding of daytime halogen cycling in the lower atmosphere.

Extensive field evidence for the release of HONO from the photolysis of nitrate aerosols

Particulate nitrate ([Formula: see text]) has long been considered a permanent sink for NO_x (NO and NO₂), removing a gaseous pollutant that is central to air quality and that influences the global self-cleansing capacity of the atmosphere. Evidence is emerging that photolysis of [Formula: see text] can recycle HONO and NO_x back to the gas phase with potentially important implications for tropospheric ozone and OH budgets; however, there are substantial discrepancies in "renoxification" photolysis rate constants. Using aircraft and ground-based HONO observations in the remote Atlantic troposphere, we show evidence for renoxification occurring on mixed marine aerosols with an efficiency that increases with relative humidity and decreases with the concentration of [Formula: see text], thus largely reconciling the very large discrepancies in renoxification photolysis rate constants found across multiple laboratory and field studies. Active release of HONO from aerosol has important implications for atmospheric oxidants such as OH and O₃ in both polluted and clean environments.

Combined effects of high relative humidity and ultraviolet irradiation: Enhancing the production of gaseous NO2 from the photolysis of NH4NO3

Free radicals and nitrogen-containing species produced by nitrate photolysis can affect various atmospheric chemical processes, and thereby the photochemical behavior of atmospheric nitrate aerosols has been attracting much attention. However, the photolysis mechanism of NH₄NO₃ and its products under different atmospheric conditions remain unclear. In this study, the effects of relative humidity (RH), pH, NH₃, ultraviolet (UV) light intensity and halogen ions (Cl^-, Br^- and I^-) on the photolysis of particulate NH₄NO₃ have been investigated through a flow tube reactor. The results show that RH can significantly enhance the production of gaseous NO₂ from the photolysis of NH₄NO₃ when RH is higher than its deliquescence RH, but almost no NO₂ is generated under dry conditions. Under high RH and UV light, the main product of NH₄NO₃ photolysis is NO₂, rather than NO and HONO, and another main species HNO₃ which mainly comes from the hydrolysis of product NO₂ in the gas path was detected. Almost no NO₂ and HNO₃ are produced under high RH without UV light or low RH with UV light, showing the combined effect of high RH and UV irradiation on the photolysis of NH₄NO₃. In addition, under high RH, the lower the pH and the stronger the light intensity, the higher the NO₂ production. Furthermore, surprising yields of NO and HONO are detected in the presence of halogen ions, especially in the presence of I^-, indicating the important role of halogen ion in the nitrate photolysis. These results provide new insights into the photolysis of atmospheric nitrate aerosols, and may contribute to elucidating the formation and migration of atmospheric nitrate aerosols and the potential mechanisms of the occurrence and evolution of atmospheric pollution and ozone pollution.

An unexpected large continental source of reactive bromine and chlorine with significant impact on wintertime air quality

Halogen atoms affect the budget of ozone and the fate of pollutants such as hydrocarbons and mercury. Yet their sources and significances in polluted continental regions are poorly understood. Here we report the observation of unprecedented levels (averaging at 60 parts per trillion) of bromine chloride (BrCl) at a mid-latitude site in North China during winter. Widespread coal burning in rural households and a photo-assisted process were the primary source of BrCl and other bromine gases. BrCl contributed about 55% of both bromine and chlorine atoms. The halogen atoms increased the abundance of 'conventional' tropospheric oxidants (OH, HO₂ and RO₂) by 26%-73%, and enhanced oxidation of hydrocarbon by nearly a factor of two and the net ozone production by 55%. Our study reveals the significant role of reactive halogen in winter atmospheric chemistry and the deterioration of air quality in continental regions where uncontrolled coal combustion is prevalent.

DFT Study of Microsolvated [NO3·(H2O)n]- (n = 1-12) Clusters and Molecular Dynamics Simulation of Nitrate Solution

Investigation of the process of the NO₃^- anion solvation is central to understanding the chemical and physical properties of its aqueous solutions. The importance of this topic can be seen in atmospheric chemistry, as well as in nuclear waste processing research. In this work, we used a particle swarm optimization technique driven by density functional theory to sample the potential energy surface of various microsolvated [NO₃·(H₂O)_n]^- (n = 1-12) clusters. We found that the charge transfer plays a crucial role in the stabilization of the investigated species. Moreover, by conducting ab initio molecular dynamics simulations, we showed that at low concentrations (∼0.2 M) the NO₃^- species tend to be located on the surface of water solution. We also observed that the contact ion pair K⁺-NO₃^- undergoes a fast dissociation and each of the ions is solvated separately. As a result, from our calculations, we expect that at low concentration there could be oppositely signed concentration gradients for NO₃^- and K⁺ ions in a thin water film.

Aqueous phase oxidation of bisulfite influenced by nitrate and its photolysis

Nitrate aerosol is ubiquitous in the atmosphere. Nitrate in the particulate and aqueous phase can affect various atmospheric chemical processes through its hygroscopicity and photolysis. The impacts of nitrate photolysis on the heterogeneous oxidation of SO₂ have been attracting attention. However, the influence of nitrate on heterogeneous aqueous phase formation of atmospheric sulfate aerosol is still not very clear. In this study, the effects of nitrate on aqueous phase oxidation of bisulfite under different conditions were investigated. Results show that nitrate photolysis can promote the oxidation of bisulfite to sulfate, especially in the presence of O₂. It is found that pH plays a significant role in the reaction, and ammonium sulfate has significant impacts on the enhancement of aqueous phase sulfate production through regulating the pH of solution. An apparent synergism is found among halogen chemistry, nitrate and its photochemistry and S (IV) aqueous oxidation, especially the oxidation of halide ions by nitrate and its photolysis and by the intermediate products produced by the free radical chain oxidation of S (IV) in acidic solution, leading to the coupling of the redox cycle of halogen with the oxidation of bisulfite, which promotes the continuous aqueous oxidation of bisulfite and the formation of sulfate. In addition, the role of nitrate itself in the aqueous phase oxidation of bisulfite is revealed. These results provide a new insight into the heterogeneous aqueous phase oxidation pathways and mechanisms of SO₂ in cloud and fog droplets and haze particles.

Photochemical reactions on aerosols at West Antarctica: A molecular case-study of nitrate formation among sea salt aerosols

Environmental implications of climate change are complex and exhibit regional variations both within and between the polar regions. The increase of solar UV radiation flux over Antarctica due to stratospheric ozone depletion creates the optimal conditions for photochemical reactions on the snow. Modeling, laboratory, and indirect field studies suggest that snowpack process release gases to the atmosphere that can react on sea salt particles in remote regions such as Antarctica, modifying aerosol composition and physical properties of aerosols. Here, we present evidence of photochemical processing in West Antarctica aerosols using microscopic and chemical speciation of individual atmospheric particles. Individual aerosol particles collected at the Brazilian module Criosfera 1 were analyzed by scanning transmission X-ray microscopy with near edge X-ray absorption fine structure spectroscopy (STXM/NEXAFS) combined with computer-controlled scanning electron microscopy (CCSEM) with energy-dispersive X-ray (EDX) microanalysis. The displacement of chlorine relative to sodium was observed over most of the sea salt particles. Particles with a chemical composition consistent with NaCl-NO₃ contributed up to 30% of atmospheric particles investigated. Overall, this study provides evidence that the snowpack and particulate nitrate photolysis should be considered in dynamic partition equilibrium in the troposphere. These findings may assist in reducing modeling uncertainties and present new insights into the aerosol chemical composition in the polar environment.

Photoinduced Oxidation Reactions at the Air-Water Interface

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

Molecular reactions at aqueous interfaces

This Review aims to critically analyse the emerging field of chemical reactivity at aqueous interfaces. The subject has evolved rapidly since the discovery of the so-called 'on-water catalysis', alluding to the dramatic acceleration of reactions at the surface of water or at its interface with hydrophobic media. We review critical experimental studies in the fields of atmospheric and synthetic organic chemistry, as well as related research exploring the origins of life, to showcase the importance of this phenomenon. The physico-chemical aspects of these processes, such as the structure, dynamics and thermodynamics of adsorption and solvation processes at aqueous interfaces, are also discussed. We also present the basic theories intended to explain interface catalysis, followed by the results of advanced ab initio molecular-dynamics simulations. Although some topics addressed here have already been the focus of previous reviews, we aim at highlighting their interconnection across diverse disciplines, providing a common perspective that would help us to identify the most fundamental issues still incompletely understood in this fast-moving field.

The unexpected effect of aqueous ion pairs on the forbidden n →π* transition in nitrate

Aqueous nitrate is ubiquitous in the environment, found for example in stratospheric clouds, tropospheric particulate matter, rain and snow, fertilized fields, rivers and the ocean. Its photolysis is initiated by absorption into the strongly forbidden n →π* transition. Photolysis reactivates deposited nitrate, releasing nitrogen oxides, and UV light is commonly used to break down nitrate pollution. The transition is doubly forbidden unless its symmetry is broken, giving a powerful means of probing the interactions of nitrate with its environment and of using experiment to validate the results of theory. In this study we demonstrate the remarkably different effects of the addition of a series of mono- and di-valent metal chlorides on the nitrate UV transition. While they all shift the transition to shorter wavelengths, the shift changes significantly from one to another. For the monovalent series Li⁺, Na⁺, K⁺, the blue shift decreases down the column being strongest for Li⁺ and weakest for K⁺. For the divalent series Mg²⁺, Ca²⁺, Ba²⁺, the opposite effect is observed with the energy shift of Ba²⁺ being an order of magnitude larger than for Mg²⁺. The absorption intensity also changes; the addition of Na⁺ and K⁺ decrease intensity whereas Li⁺ increases intensity. For the divalent cations an increase is seen for all three members of the series Mg²⁺, Ca²⁺ and Ba²⁺. Paradoxically, the effect of addition of CaCl₂ to the solution is to decrease the environmental photolysis rate of nitrate; despite the increase in intensity, Ca²⁺ blue shifts the peak position above the tropospheric photolysis threshold around 300 nm. Using computational chemistry we conclude that the effects are due to the microscopic interactions of the nitrate anion and not continuum effects. Two microscopic mechanisms are investigated in detail, the formation of a nitrate monohydrate cluster and a contact ion pair. The contact ion pair shows the potential for significant impact on the energy and intensity of the transition.

Enhanced Sulfate Production by Nitrate Photolysis in the Presence of Halide Ions in Atmospheric Particles

Heterogeneous oxidation of SO₂ is an effective production pathway of sulfate in the atmosphere. We recently reported a novel pathway for the heterogeneous oxidation of SO₂ by in-particle oxidants (OH, NO₂, and NO₂^-/HNO₂) produced from particulate nitrate photolysis (Environ. Sci. Technol. 2019, 53, 8757-8766). Particulate nitrate is often found to coexist with chloride and other halide ions, especially in aged sea-salt aerosols and combustion aerosols. Reactive uptake experiments of SO₂ with UV-irradiated nitrate particles showed that sulfate production rates were enhanced by a factor of 1.4, 1.3, and 2.0 in the presence of Cl^-, Br^-, and I^-, respectively, compared to those in the absence of halide ions. The larger sulfate production was attributed to enhanced nitrate photolysis promoted by the increased incomplete solvation of nitrate at the air-particle interface due to the presence of surface-active halide ions. Modeling results based on the experimental data showed that the nitrate photolysis rate constants increased by a factor of 2.0, 1.7, and 3.7 in the presence of Cl^-, Br^-, and I^-, respectively. A linear relation was found between the nitrate photolysis rate constant, j_NO3-, and the initial molar ratio of Cl^- to NO₃^-, [Cl^-]₀/[NO₃^-]₀, as j_NO3- = 9.7 × 10^-5[Cl^-]₀/[NO₃^-]₀ + 1.9 × 10^-5 at [Cl^-]₀/[NO₃^-]₀ below 0.2. The present study demonstrates that the presence of halide ions enhances sulfate production produced during particulate nitrate photolysis and provides insights into the enhanced formation of in-particle oxidants that may increase atmospheric oxidative capacity.

Amorphous phase state diagrams and viscosity of ternary aqueous organic/organic and inorganic/organic mixtures

A Dimer Coagulation, Isolation and Coalescence (DCIC) technique is used to probe the phase behaviour and glass transition temperatures of ternary aerosol mixtures.

Introductory lecture: atmospheric chemistry in the Anthropocene

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

Rapid cycling of reactive nitrogen in the marine boundary layer

Nitrogen oxides are essential for the formation of secondary atmospheric aerosols and of atmospheric oxidants such as ozone and the hydroxyl radical, which controls the self-cleansing capacity of the atmosphere. Nitric acid, a major oxidation product of nitrogen oxides, has traditionally been considered to be a permanent sink of nitrogen oxides. However, model studies predict higher ratios of nitric acid to nitrogen oxides in the troposphere than are observed. A 'renoxification' process that recycles nitric acid into nitrogen oxides has been proposed to reconcile observations with model studies, but the mechanisms responsible for this process remain uncertain. Here we present data from an aircraft measurement campaign over the North Atlantic Ocean and find evidence for rapid recycling of nitric acid to nitrous acid and nitrogen oxides in the clean marine boundary layer via particulate nitrate photolysis. Laboratory experiments further demonstrate the photolysis of particulate nitrate collected on filters at a rate more than two orders of magnitude greater than that of gaseous nitric acid, with nitrous acid as the main product. Box model calculations based on the Master Chemical Mechanism suggest that particulate nitrate photolysis mainly sustains the observed levels of nitrous acid and nitrogen oxides at midday under typical marine boundary layer conditions. Given that oceans account for more than 70 per cent of Earth's surface, we propose that particulate nitrate photolysis could be a substantial tropospheric nitrogen oxide source. Recycling of nitrogen oxides in remote oceanic regions with minimal direct nitrogen oxide emissions could increase the formation of tropospheric oxidants and secondary atmospheric aerosols on a global scale.

Photolysis of Nitric Acid and Nitrate on Natural and Artificial Surfaces

Photolysis of nitric acid and nitrate (HNO3/nitrate) was investigated on the surfaces of natural and artificial materials, including plant leaves, metal sheets, and construction materials. The surfaces were conditioned in the outdoor air prior to experiments to receive natural depositions of ambient HNO3/nitrate and other atmospheric constituents. The photolysis rate constant (JHNO3(s)) of the surface HNO3/nitrate was measured based on the production rates of nitrous acid (HONO) and nitrogen oxides (NOx). The JHNO3(s) values, from 6.0 × 10(-6) s(-1) to 3.7 × 10(-4) s(-1), are 1 to 3 orders of magnitude higher than that of gaseous HNO3. The HONO was the major product from photolysis of HNO3/nitrate on most plant leaves, whereas NOx was the major product on most artificial surfaces. The JHNO3(s) values decreased with HNO3/nitrate surface density and could be described by a simple analytical equation. Within a typical range of HNO3/nitrate surface density in the low-NOx forested areas, photolysis of HNO3/nitrate on the forest canopy can be a significant source for HONO and NOx for the overlying atmosphere.

Heterogeneous oxidation of nitrite anion by gas-phase ozone in an aqueous droplet levitated by laser tweezers (optical trap): is there any evidence for enhanced surface reaction?

Optical trapping of micron-sized droplet morphology and heterogeneous kinetics with gas-phase ozone with nitrite in a wall-less apparatus.

The effect of cations on NO2 production from the photolysis of aqueous thin water films of nitrate salts

Cations are shown to enhance nitrate photochemistry by changing the concentrations of nitrate ions in the interface region.

A pinch of salt is all it takes: chemistry at the frozen water surface

Chemical interactions at the air-ice interface are of great importance to local atmospheric chemistry but also to the concentrations of pollutants deposited onto natural snow and ice. However, the study of such processes has been hampered by the lack of general, surface-specific probes. Even seemingly basic chemical properties, such as the local concentration of chemical compounds, or the pH at the interface, have required the application of assumptions about solute distributions in frozen media. The measurements that have been reported have tended for the most part to focus on entire ice or snow samples, rather than strictly the frozen interface with the atmosphere. We have used glancing-angle laser spectroscopy to interrogate the air-ice interface; this has yielded several insights into the chemical interactions there. The linear fluorescence and Raman spectra thus measured have the advantage of easy interpretability; careful experimentation can limit their probe depth to that which is relevant to atmospheric heterogeneous processes. We have used these techniques to show that the environment at the interface between air and freshwater ice surfaces is distinct from that at the interface between air and liquid water. Acids such as HCl that adsorb to ice surfaces from the gas phase result in significantly different pH responses than those at liquid water surfaces. Further, the solvation of aromatic species is suppressed at freshwater ice surfaces compared with that at liquid water surfaces, leading to extensive self-association of aromatics at ice surfaces. Photolysis kinetics of these species are much faster than at liquid water surfaces; this can sometimes (but not always) be explained by red shifts in the absorption spectra of self-associated aromatics increasing the extent to which solar radiation is absorbed. The environment presented by frozen saltwater surfaces, in contrast, appears to be reasonably well-described by liquid water. The extent of hydrogen bonding and the solvation of adsorbed species are similar at liquid water surfaces and at frozen saltwater surfaces. Adsorbed acids and bases evoke similar pH responses at frozen saltwater ice surfaces and liquid water surfaces, and photochemical kinetics of at least some aromatic compounds at frozen saltwater ice surfaces are well-described by kinetics in liquid water. These differences are not observed in experiments that interrogate the entire ice sample (i.e., that do not distinguish between processes occurring in liquid regions within bulk ice and those at the air-ice interface). Our work has shown that in general, the chemistry occurring at salty frozen interfaces is well described as being cold aqueous chemistry, whereas that seen at the pure ice interface is not. These findings have significant implications for heterogeneous atmospheric processes in ice-covered environments.

Production of gas phase NO2 and halogens from the photolysis of thin water films containing nitrate, chloride and bromide ions at room temperature

Nitrate and halide ions coexist in particles generated in marine regions, around alkaline dry lakes, and in the Arctic snowpack. Although the photochemistry of nitrate ions in bulk aqueous solution is well known, there is recent evidence that it may be more efficient at liquid-gas interfaces, and that the presence of other ions in solution may enhance interfacial reactivity. This study examines the 311 nm photolysis of thin aqueous films of ternary halide-nitrate salt mixtures (NaCl-NaBr-NaNO3) deposited on the walls of a Teflon chamber at 298 K. The films were generated by nebulizing aqueous 0.25 M NaNO3 solutions which had NaCl and NaBr added to vary the mole fraction of halide ions. Molar ratios of chloride to bromide ions were chosen to be 0.25, 1.0, or 4.0. The subsequent generation of gas phase NO2 and reactive halogen gases (Br2, BrCl and Cl2) were monitored with time. The rate of gas phase NO2 formation was shown to be enhanced by the addition of the halide ions to thin films containing only aqueous NaNO3. At [Cl(-)]/[Br(-)] ≤ 1.0, the NO2 enhancement was similar to that observed for binary NaBr-NaNO3 mixtures, while with excess chloride NO2 enhancement was similar to that observed for binary NaCl-NaNO3 mixtures. Molecular dynamics simulations predict that the halide ions draw nitrate ions closer to the interface where a less complete solvent shell allows more efficient escape of NO2 to the gas phase, and that bromide ions are more effective in bringing nitrate ions closer to the surface. The combination of theory and experiments suggests that under atmospheric conditions where nitrate ion photochemistry plays a role, the impact of other species such as halide ions should be taken into account in predicting the impacts of nitrate ion photochemistry.

Role of nitrite in the photochemical formation of radicals in the snow

Photochemical reactions in snow can have an important impact on the composition of the atmosphere over snow-covered areas as well as on the composition of the snow itself. One of the major photochemical processes is the photolysis of nitrate leading to the formation of volatile nitrogen compounds. We report nitrite concentrations determined together with nitrate and hydrogen peroxide in surface snow collected at the coastal site of Barrow, Alaska. The results demonstrate that nitrite likely plays a significant role as a precursor for reactive hydroxyl radicals as well as volatile nitrogen oxides in the snow. Pollution events leading to high concentrations of nitrous acid in the atmosphere contributed to an observed increase in nitrite in the surface snow layer during nighttime. Observed daytime nitrite concentrations are much higher than values predicted from steady-state concentrations based on photolysis of nitrate and nitrite indicating that we do not fully understand the production of nitrite and nitrous acid in snow. The discrepancy between observed and expected nitrite concentrations is probably due to a combination of factors, including an incomplete understanding of the reactive environment and chemical processes in snow, and a lack of consideration of the vertical structure of snow.

Locating protonated amines in clathrates

The structures and inherent stabilities of hydrated, protonated ammonia, select protonated primary, secondary, and tertiary amines as well as tetramethylammonium with 19-21 water molecules were investigated using infrared photodissociation (IRPD) spectroscopy and blackbody infrared radiative dissociation (BIRD) at 133 K. Magic number clusters (MNCs) with 20 water molecules were observed for all ions except tetramethylammonium, and the BIRD results indicate that these clusters have stable structures, which are relatively unaffected by addition of one water molecule but are disrupted in clusters with one less water molecule. IRPD spectra in the water free O-H stretch region are consistent with clathrate structures for the MNCs with 20 water molecules, whereas nonclathrate structures are indicated for tetramethylammonium as well as ions at the other cluster sizes. The locations of protonated ammonia and the protonated primary amines either in the interior or at the surface of a clathrate were determined by comparing IRPD spectra of these ions to those of reference ions; Rb(+) and protonated tert-butylammonia with 20 water molecules were used as references for an ion in the interior and at the surface of a clathrate, respectively. These results indicate that protonated ammonia is in the interior of the clathrate, whereas protonated methyl- and n-heptylamine are at the surface. Calculations suggest that the number of hydrogen bonds in these clusters does not directly correlate with structural stability, indicating that both the number and orientation of the hydrogen bonds are important. These experimental results should serve as benchmarks for computational studies aimed at elucidating ion effects on the hydrogen-bonding network of water molecules and the surface activity of ions.

Thermodynamics of iodide adsorption at the instantaneous air-water interface

We performed molecular dynamics simulations using both polarizable and non-polarizable force fields to study the adsorption of iodide to the air-water interface. A novel aspect of our analysis is that the progress of ion adsorption is measured as the distance from the instantaneous interface, which is defined by a coarse-graining scheme proposed recently by Willard and Chandler ["Instantaneous liquid interfaces," J. Phys. Chem. B 114, 1954-1958 (2010)]. Referring structural and thermodynamic quantities to the instantaneous interface unmasks molecular-scale details that are obscured by thermal fluctuations when the same quantities are referred to an average measure of the position of the interface, such as the Gibbs dividing surface. Our results suggest that an ion adsorbed at the interface resides primarily in the topmost water layer, and the interfacial location of the ion is favored by enthalpy and opposed by entropy.

Enhanced Surface Partitioning of Nitrate Anion in Aqueous Bromide Solutions

The proximity of nitrate anions to the air-water interface is thought to strongly influence their photodissociation quantum yield, due to a reduced solvent cage effect at the water surface. Although nitrate in aqueous solution exhibits little or no surface affinity, the release of gas phase NO2 (nitrate's primary photodissociation product) has been reported to be enhanced when halides, in particular bromide, are also present in solution. Here, we use glancing-angle Raman spectroscopy to investigate whether solutions containing both nitrate and halides show different propensities for nitrate at the air-water interface. We find that bromide enhances, and chloride has little effect on (or perhaps suppresses) the surface partitioning of nitrate anions.

Production of gas phase NO₂ and halogens from the photochemical oxidation of aqueous mixtures of sea salt and nitrate ions at room temperature

Nitrate and halide ions coexist in a number of environmental systems, including sea salt particles, the Arctic snowpack, and alkaline dry lakes. However, little is known about potential synergisms between halide and nitrate ions. The effect of sea salt on NO(3)(-) photochemistry at 311 nm was investigated at 298 K using thin films of deliquesced NaNO(3)-synthetic sea salt mixtures. Gas phase NO(2), NO, and halogen products were measured as a function of photolysis time using NO(y) chemiluminescence and atmospheric pressure ionization mass spectrometry (API-MS). The production of NO(2) increases with the halide-to-nitrate ratio, and is similar to that for mixtures of NaCl with NaNO(3). Gas phase halogen production also increased with the halide-to-nitrate ratio, consistent with NO(3)(-) photolysis yielding OH which oxidizes halide ions in the film. Yields of gas phase halogens and NO were strongly dependent on the acidity of the solution, while that of NO(2) was not. An additional halogen formation mechanism in the dark involving molecular HNO(3) is proposed that may be important in other systems such as reactions on surfaces. These studies show that the yield of Br(2) relative to NO(2) during photolysis of halide-nitrate mixtures could be as high as 35% under some atmospheric conditions.