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

Measurement of Henry's law constants of ethyl nitrate in deionized water, synthetic sea salt solutions, and n-octanol

Ethyl nitrate (EN; C2H5ONO2) is an important component of atmospheric "odd nitrogen" (NOy) whose main source is marine emissions. To correctly describe its air-water transfer and model its global distribution, accurate values for its temperature- and salinity-dependent Henry's law solubility constants are needed. Here, we report Henry's law (HScp) constants for EN in deionized (DI) water, synthetic sea salt solutions (SSS), and n-octanol at temperatures between 278.2 K and 303.2 K. For DI water, HScp constants of (2.03 ± 0.06) M atm-1 at 293.2 K and (4.88 ± 0.13) M atm-1 at 278.2 K were observed (all stated uncertainties are at the 1σ level). The data are best described by ln(HScp(aq)/[Matm-1]) = -(16.2 ± 0.4)+(4.94 ± 0.11) × 103/T and ln(HScp(octanol)/[Matm-1]) = -(11.1 ± 1.9)+(4.15 ± 0.33) × 103/T, from which the octanol-water partition coefficient (KOW) was calculated. A temperature-independent salting-out factor of 1.25 ± 0.03 and Setschenow constant of KS = (0.33 ± 0.04) mol kg-1 were determined for SSS. Liquid-phase losses of EN were negligible in all solvents (kl < 1 × 10-4 s-1). The HScp(aq) values agree with results by Kames (1993) but are between 2% (at 303.2 K) and 19% (at 278.2 K) lower than the widely used parameterization by Kames and Schurath (1992), indicating a systemic bias in the EN literature and modelling of the Earth's nitrogen cycle.

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.

Review of air-water interface adsorption and reactions between trace gaseous organic and oxidant compounds

The surface chemistry of the atmospheric aerosol through homogeneous and heterogeneous catalytic reactions in the bulk water and the air-water surface is reviewed. Water plays a critical role as a substrate or an actual reactant in atmospheric reactions. The atmospheric aerosol differs in shape and surface area. Many gaseous reactive species and oxidants react at the air-water surface. Different thermodynamic methods to estimate partitioning coefficients are explored. The Gibbs free energy is reduced when reactant gaseous species react with oxidant at the air-water surface; this phenomenon is explained using examples. Langmuir-Hinshelwood reaction mechanism to quantify the heterogeneous reaction rate at the air-water interface is discussed. Critical comparisons of various sampling techniques used to analyze adsorption and reaction at the water surface are presented. The heterogeneous reaction rate at the air-water surface is significantly higher than in the bulk water phase due to a cage effect, higher rate of reactions, and lower Gibbs free energy of adsorption.

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.

Pollution-Derived Br2 Boosts Oxidation Power of the Coastal Atmosphere

The bromine atom (Br^•) has been known to destroy ozone (O₃) and accelerate the deposition of toxic mercury (Hg). However, its abundance and sources outside the polar regions are not well-known. Here, we report significant levels of molecular bromine (Br₂)─a producer of Br^•─observed at a coastal site in Hong Kong, with an average noontime mixing ratio of 5 ppt. Given the short lifetime of Br₂ (∼1 min at noon), this finding reveals a large Br₂ daytime source. On the basis of laboratory and field evidence, we show that the observed daytime Br₂ is generated by the photodissociation of particulate nitrate (NO₃^-) and that the reactive uptake of dinitrogen pentoxide (N₂O₅) on aerosols is an important nighttime source. Model-calculated Br^• concentrations are comparable with that of the OH radical─the primary oxidant in the troposphere, accounting for 24% of the oxidation of isoprene, a 13% increase in net O₃ production, and a nearly 10-fold increase in the production rate of toxic Hg^II. Our findings reveal that reactive bromines play a larger role in the atmospheric chemistry and air quality of polluted coastal and maritime areas than previously thought. Our results also suggest that tightening the control of emissions of two conventional pollutants (NO_x and SO₂)─thereby decreasing the levels of nitrate and aerosol acidity─would alleviate halogen radical production and its adverse impact on air quality.

Photodissociation of particulate nitrate as a source of daytime tropospheric Cl2

Chlorine atoms (Cl) are highly reactive and can strongly influence the abundances of climate and air quality-relevant trace gases. Despite extensive research on molecular chlorine (Cl₂), a Cl precursor, in the polar atmosphere, its sources in other regions are still poorly understood. Here we report the daytime Cl₂ concentrations of up to 1 ppbv observed in a coastal area of Hong Kong, revealing a large daytime source of Cl₂ (2.7 pptv s⁻¹ at noon). Field and laboratory experiments indicate that photodissociation of particulate nitrate by sunlight under acidic conditions (pH < 3.0) can activate chloride and account for the observed daytime Cl₂ production. The high Cl₂ concentrations significantly increased atmospheric oxidation. Given the ubiquitous existence of chloride, nitrate, and acidic aerosols, we propose that nitrate photolysis is a significant daytime chlorine source globally. This so far unaccounted for source of chlorine can have substantial impacts on atmospheric chemistry.

This study unravels an important daytime Cl₂ source in the extra-polar atmosphere and shows that photolysis of particle nitrate at high acidity produced unprecedented levels of Cl₂, boosting the oxidative power and air pollutants like O₃.

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

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

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.

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.

Photochemistry of glyoxylate embedded in sodium chloride clusters, a laboratory model for tropospheric sea-salt aerosols

Although marine aerosols undergo extensive photochemical processing in the troposphere, a molecular level understanding of the elementary steps involved in these complex reaction sequences is still missing.

Although marine aerosols undergo extensive photochemical processing in the troposphere, a molecular level understanding of the elementary steps involved in these complex reaction sequences is still missing. As a defined laboratory model system, the photodissociation of sea salt clusters doped with glyoxylate, [Na_nCl_n–2(C₂HO₃)]⁺, n = 5–11, is studied by a combination of mass spectrometry, laser spectroscopy and ab initio calculations. Glyoxylate acts as a chromophore, absorbing light below 400 nm via two absorption bands centered at about 346 and 231 nm. Cluster fragmentation dominates, which corresponds to internal conversion of the excited state energy into vibrational modes of the electronic ground state and subsequent unimolecular dissociation. Photochemical dissociation pathways in electronically excited states include CO and HCO elimination, leading to [Na_n–xCl_n–x–2HCOO]⁺ and [Na_nCl_n–2COO˙]⁺ with typical quantum yields in the range of 1–3% and 5–10%, respectively, for n = 5. The latter species contains CO₂˙^– stabilized by the salt environment. The comparison of different cluster sizes shows that the fragments containing a carbon dioxide radical anion appear in a broad spectral region of 310–380 nm. This suggests that the elusive CO₂˙^– species may be formed by natural processes in the troposphere. Based on the photochemical cross sections obtained here, the photolysis lifetime of glyoxylate in a dry marine aerosol is estimated as 10 h. Quantum chemical calculations show that dissociation along the C–C bond in glyoxylic acid as well as glyoxylate embedded in the salt cluster occurs after reaching the S₁/S₀ conical intersection, while this conical intersection is absent in free glyoxylate ions.

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.

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.

Adsorption and heterogeneous reactions of ClONO2 and N2O5 on/with NaCl aerosol

The adsorption and heterogeneous reactions of ClONO₂ and N₂O₅ on the NaCl (100) surface have been investigated by performing density functional theory (DFT) calculations.

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.

Effects of stratospheric ozone depletion, solar UV radiation, and climate change on biogeochemical cycling: interactions and feedbacks

Solar UV radiation and climate change interact to influence and determine the environmental conditions for humans on planet Earth.

Release of nitrous acid and nitrogen dioxide from nitrate photolysis in acidic aqueous solutions

Nitrate (NO3(-)) is an abundant component of aerosols, boundary layer surface films, and surface water. Photolysis of NO3(-) leads to NO2 and HONO, both of which play important roles in tropospheric ozone and OH production. Field and laboratory studies suggest that NO3¯ photochemistry is a more important source of HONO than once thought, although a mechanistic understanding of the variables controlling this process is lacking. We present results of cavity-enhanced absorption spectroscopy measurements of NO2 and HONO emitted during photodegradation of aqueous NO3(-) under acidic conditions. Nitrous acid is formed in higher quantities at pH 2-4 than expected based on consideration of primary photochemical channels alone. Both experimental and modeled results indicate that the additional HONO is not due to enhanced NO3(-) absorption cross sections or effective quantum yields, but rather to secondary reactions of NO2 in solution. We find that NO2 is more efficiently hydrolyzed in solution when it is generated in situ during NO3(-) photolysis than for the heterogeneous system where mass transfer of gaseous NO2 into bulk solution is prohibitively slow. The presence of nonchromophoric OH scavengers that are naturally present in the environment increases HONO production 4-fold, and therefore play an important role in enhancing daytime HONO formation from NO3(-) photochemistry.

Is Nitrate Anion Photodissociation Mediated by Singlet-Triplet Absorption?

Photolysis of the nitrate anion is involved in the oxidation processes in the hydrosphere, cryosphere, and stratosphere. While it is known that the nitrate photolysis in the long-wavelength region proceeds with a very low quantum yield, the mechanism of the photodissociation remains elusive. Here, we present the quantitative modeling of singlet-singlet and singlet-triplet absorption spectra in the atmospherically relevant region around 300 nm, and we argue that a spin-forbidden transition between the singlet ground state and the first triplet state contributes non-negligibly to the nitrate anion photolysis. We further propose that the nitrate anion excited into the first singlet excited state relaxes nonradiatively into its ground state. The full understanding of the nitrate anion photolysis can improve modeling of the asymmetric solvation in the atmospheric processes, e.g., photolysis on the surfaces of ice or snow.

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.

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.