Can We Model Snow Photochemistry? Problems with the Current Approaches

Freezing-induced acidification of sea ice brine

The acidity of sea ice and snow plays a key role in the chemistry of the cryosphere; an important example lies in the photochemical catalytic release of reactive bromine in polar regions, facilitated at pHs below 6.5. We apply in-situ acid-base indicators to probe the microscopic acidity of the brine within the ice matrix in artificial sea water at a range of concentrations (0.35-70 PPT) and initial pHs (6-9). The results are supported by analogous measurements of the most abundant salts in seawater: NaCl, Na2SO4, and CaCO3. In the research herein, the acidity is expressed in terms of the Hammett acidity function, H2-. The obtained results show a pronounced acidity increase in sea water after freezing at -15 °C and during the subsequent cooling down to -50 °C. Importantly, we did not observe any significant hysteresis; the values of acidity upon warming markedly resembled those at the corresponding temperatures at cooling. The acidity increase is attributed to the minerals' crystallization, which is accompanied by a loss of the buffering capacity. Our observations show that lower salinity sea water samples (≤ 3.5 PPT) reach pH values below 6.5 at the temperature of -15 °C, whereas higher salinity ices attain such values only at -30 °C. The ensuing implications for polar chemistry and the relevance to the field measurements are discussed.

Direct Observation of Anthracene Clusters at Ice Surfaces

Heterogeneous processes can control atmospheric composition. Snow and ice present important, but poorly understood, reaction media that can greatly alter the composition of air in the cryosphere in polar and temperate regions. Atmospheric scientists struggle to reconcile model predictions with field observations in snow-covered regions due in part to experimental challenges associated with monitoring reactions at air-ice interfaces, and debate regarding reaction kinetics and mechanisms has persisted for over a decade. In this work, we use wavelength-resolved fluorescence microscopy to determine the distribution and chemical speciation of the pollutant anthracene at environmentally relevant frozen surfaces. Our results indicate that anthracene adsorbs to frozen surfaces in monomeric form, but that following lateral diffusion, molecules ultimately reside within brine channels at saltwater ice surfaces, and in micron-sized clusters at freshwater ice surfaces; emission profiles indicate extensive self-association. We also measure anthracene photodegradation kinetics in aqueous solution and artificial snow prepared from frozen freshwater and saltwater solutions. Our results suggest that anthracene─and likely other aromatic pollutants─undergo bimolecular photodegradation at the surface of freshwater ice and sea ice, but not at the surface of frozen organic matter. These results will improve predictions of pollutant fate and exposure risk in the cryosphere. The techniques used can be applied to numerous surfaces within and beyond the atmospheric sciences.

Chemical Morphology Controls Reactivity of OH Radicals at the Air-Ice Interface

At the air-ice interface, some aromatic compounds such as benzene and anthracene are surprisingly unreactive toward OH. This may be a consequence of the poor solvation of these compounds at the interface, resulting in clustering there. We test this hypothesis by comparing the reaction of OH with pyrene, a 4-ring polyaromatic hydrocarbon (PAH), to reactions of OH with the more water-soluble compounds coumarin and 7-hydroxycoumarin (7OHC). We observe that OH reacts readily with coumarin and 7OHC at both liquid and frozen air-water interfaces. Pyrene, a much less soluble compound, reacts with OH at the liquid surface but not at the air-ice interface. We report evidence of pyrene aggregation at the ice surface based on its broadened and red-shifted emission spectrum alongside fluorescence mapping of anthracene, a closely related 3-ring PAH, which shows bunching at the ice surface. By contrast, fluorescence mapping shows that coumarin is fairly homogeneously distributed at the air-ice interface. Together, these results suggest that the limited reactivity of some compounds toward OH at the ice surface may be a consequence of their propensity to self-aggregate, demonstrating that chemical morphology can play an important role in reactions at the ice surface.

Appearance and Disappearance of Quasi-Liquid Layers on Ice Crystals in the Presence of Nitric Acid Gas

The surfaces of ice crystals near the melting point are covered with thin liquid water layers, called quasi-liquid layers (QLLs), which play crucial roles in various chemical reactions in nature. So far, there have been many spectroscopic studies of such chemical reactions on ice surfaces, however, revealing the effects of atmospheric gases on ice surfaces remains an experimental challenge. In this study, we chose HNO3 as a model atmospheric gas, and directly observed the ice basal faces by advanced optical microscopy under partial pressure of HNO3 (~10−4 Pa), relevant to those found in the atmosphere. We found that droplets (HNO3-QLLs) appeared on ice surfaces at temperatures ranging from −0.9 to −0.2 °C with an increase in temperature, and that they disappeared at temperatures ranging from −0.6 to −1.3 °C with decreasing temperature. We also found that the size of the HNO3-QLLs decreased immediately after we started reducing the temperature. From the changes in size and the liquid–solid phase diagram of the HNO3-H2O binary system, we concluded that the HNO3-QLLs did not consist of pure water, but rather aqueous HNO3 solutions, and that the temperature and HNO3 concentration of the HNO3-QLLs also coincided with those along a liquidus line.

Should We Not Further Study the Impact of Microbial Activity on Snow and Polar Atmospheric Chemistry?

Since 1999, atmospheric and snow chemists have shown that snow is a very active photochemical reactor that releases reactive gaseous species to the atmosphere including nitrogen oxides, hydrocarbons, aldehydes, halocarbons, carboxylic acids and mercury. Snow photochemistry therefore affects the formation of ozone, a potent greenhouse gas, and of aerosols, which affect the radiative budget of the planet and, therefore, its climate. In parallel, microbiologists have investigated microbes in snow, identified and quantified species, and sometimes discussed their nutrient supplies and metabolism, implicitly acknowledging that microbes could modify snow chemical composition. However, it is only in the past 10 years that a small number of studies have revealed that microbial activity in cold snow (< 0 °C, in the absence of significant amounts of liquid water) could lead to the release of nitrogen oxides, halocarbons, and mercury into the atmosphere. I argue here that microbes may have a significant effect on snow and atmospheric composition, especially during the polar night when photochemistry is shut off. Collaborative studies between microbiologists and snow and atmospheric chemists are needed to investigate this little-explored field.

Coexistence of Physisorbed and Solvated HCl at Warm Ice Surfaces

The interfacial ionization of strong acids is an essential factor of multiphase and heterogeneous chemistry in environmental science, cryospheric science, catalysis research and material science. Using near ambient pressure core level X-ray photoelectron spectroscopy, we directly detected a low surface coverage of adsorbed HCl at 253 K in both molecular and dissociated states. Depth profiles derived from XPS data indicate the results as physisorbed molecular HCl at the outermost ice surface and dissociation occurring upon solvation deeper in the interfacial region. Complementary X-ray absorption measurements confirm that the presence of Cl^- ions induces significant changes to the hydrogen bonding network in the interfacial region. This study gives clear evidence for nonuniformity across the air-ice interface and questions the use of acid-base concepts in interfacial processes.

Active molecular iodine photochemistry in the Arctic

During springtime, the Arctic atmospheric boundary layer undergoes frequent rapid depletions in ozone and gaseous elemental mercury due to reactions with halogen atoms, influencing atmospheric composition and pollutant fate. Although bromine chemistry has been shown to initiate ozone depletion events, and it has long been hypothesized that iodine chemistry may contribute, no previous measurements of molecular iodine (I₂) have been reported in the Arctic. Iodine chemistry also contributes to atmospheric new particle formation and therefore cloud properties and radiative forcing. Here we present Arctic atmospheric I₂ and snowpack iodide (I^-) measurements, which were conducted near Utqiaġvik, AK, in February 2014. Using chemical ionization mass spectrometry, I₂ was observed in the atmosphere at mole ratios of 0.3-1.0 ppt, and in the snowpack interstitial air at mole ratios up to 22 ppt under natural sunlit conditions and up to 35 ppt when the snowpack surface was artificially irradiated, suggesting a photochemical production mechanism. Further, snow meltwater I^- measurements showed enrichments of up to ∼1,900 times above the seawater ratio of I^-/Na⁺, consistent with iodine activation and recycling. Modeling shows that observed I₂ levels are able to significantly increase ozone depletion rates, while also producing iodine monoxide (IO) at levels recently observed in the Arctic. These results emphasize the significance of iodine chemistry and the role of snowpack photochemistry in Arctic atmospheric composition, and imply that I₂ is likely a dominant source of iodine atoms in the Arctic.

Concentrations of a triplet excited state are enhanced in illuminated ice

Photochemical reactions influence the fates and lifetimes of organic compounds in snow and ice, both through direct photoreactions and via photoproduced transient species such as hydroxyl radical (˙OH) and, perhaps, triplet excited states of organic compounds (i.e., triplets). While triplets can be important oxidants in atmospheric drops and surface waters, little is known of this class of oxidants in frozen samples. To investigate this, we examined the photoreaction of phenol with the triplet state of 3,4-dimethoxybenzaldehyde (³DMB*), a product from biomass combustion, in illuminated laboratory ices. Our results show that the rate of phenol loss due to ³DMB* is, on average, increased by a factor of 95 ± 50 in ice compared to the equivalent liquid sample. We find that this experimentally measured freeze concentration factor, F_EXP, is independent of total solute concentration and temperature, in contrast to what is expected from a liquid-like region whose composition follows freezing point depression. We also find that F_EXP for triplets is independent of pH, although the rates of phenol loss increase with decreasing pH in both solution and ice. The enhancement in the rate of phenol loss in/on ice indicates that concentrations of triplet excited states are enhanced in ice relative to solution and suggests that this class of oxidants might be a significant sink for organics in snow and ice.

Spectroscopic Properties of Anisole at the Air-Ice Interface: A Combined Experimental-Computational Approach

A combined experimental and computational approach was used to investigate the spectroscopic properties of anisole in aqueous solutions and at the ice-air interface in the temperature range of 77-298 K. The absorption, diffuse reflectance, and emission spectra of ice samples containing anisole prepared by different techniques, such as slow freezing (frozen aqueous solutions), shock freezing (ice grains), or anisole vapor deposition on ice grains, were measured to evaluate changes in the contaminated ice matrix that occur at different temperatures. It was found that the position of the lowest absorption band of anisole and its tail shift bathochromically by ∼4 nm in frozen samples compared to liquid aqueous solutions. On the other hand, the emission spectra of aqueous anisole solutions were found to fundamentally change upon freezing. While one emission band (∼290 nm) was observed under all circumstances, the second band at ∼350 nm, assigned to an anisole excimer, appeared only at certain temperatures (150-250 K). Its disappearance at lower temperatures is attributed to the formation of crystalline anisole on the ice surface. DFT and ADC(2) calculations were used to interpret the absorption and emission spectra of anisole monomer and dimer associates. Various stable arrangements of the anisole associates were found at the disordered water-air interface in the ground and excited states, but only those with a substantial overlap of the aromatic rings are manifested by the emission band at ∼350 nm.

Nitrate Concentration near the Surface of Frozen Aqueous Solutions

Photolysis of nitrate plays an important role in the emission of nitrogen oxides from snow and ice, which affects the composition of the overlying atmosphere. In order to quantify these reactions, it is necessary to know how much nitrate is available for photolysis near the surfaces of snow and ice. The concentration of nitrate excluded from frozen solutions of nitric acid, sodium nitrate, and magnesium nitrate was measured with attenuated total reflection infrared spectroscopy. Liquid water and nitrate were observed at and near the bottom surface of frozen aqueous solutions during annealing from -18 to -2 °C. At -2 °C, the nitrate concentration was determined to be ∼1.0 mol/L for frozen NaNO(3) and Mg(NO(3))(2) solutions and ∼0.8 mol/L for frozen HNO(3) solutions. At lower temperatures, nitrate concentration ranged from 1.6 to 3.7 mol/L. Ideal thermodynamics overestimates nitrate concentration at colder temperatures where the brine is highly concentrated for all solutions. The nitrate concentration at ice surfaces is well described by bulk freezing point depression data close to the melting point of ice and for nitric acid at colder temperatures. Effects of temperature and counterions and implications for modeling snow chemistry are discussed.

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.

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.

Observation of a brine layer on an ice surface with an environmental scanning electron microscope at higher pressures and temperatures

Observation of a uranyl-salt brine layer on an ice surface using backscattered electron detection and ice surface morphology using secondary-electron detection under equilibrium conditions was facilitated using an environmental scanning electron microscope (ESEM) at temperatures above 250 K and pressures of hundreds of Pa. The micrographs of a brine layer over ice grains prepared by either slow or shock freezing provided a complementary picture of the contaminated ice grain boundaries. Fluorescence spectroscopy of the uranyl ions in the brine layer confirmed that the species exists predominately in the solvated state under experimental conditions of ESEM.

Kinetics of heterogeneous reactions of ozone with representative PAHs and an alkene at the air-ice interface at 258 and 188 K

The kinetics of the reaction of an alkene (E-stilbene) and three polycyclic aromatic hydrocarbons (perylene, anthracene and fluoranthene), as examples of environmental pollutants, with ozone on the surface of ice grains (also called "artificial snow"), produced by shock-freezing of aqueous solutions, was studied at submonolayer pollutant coverages (c = 1.5 × 10(-8) to 3 × 10(-10) mol kg(-1)) and two different temperatures (258 and 188 K). This work supports and extends our previous discovery of a remarkable increase in the apparent ozonation rates with decreasing temperature. The ozonation kinetic results were evaluated using the Langmuir-Hinshelwood model and, in one case, the Eley-Rideal kinetic model. It is shown that the apparent rate enhancement is related to the specific nature of the ice surface at different temperatures, which influences the availability of contaminants to gaseous ozone, and to inherent reactivities of the contaminants. The maximum pseudofirst-order rate constants and the lifetimes of the studied compounds are provided. At a typical atmospheric ozone concentration in polar areas (50 ppbv), the lifetimes were estimated to be on the order of hours (258 K) or tens of minutes (188 K) for alkenes, and hundreds (258 K) or tens (188 K) of days for PAHs, thus approximately of the same magnitude or longer than those found for the gas-phase reactions. We imply that this rate enhancement at lower temperatures is a general phenomenon, and we provide data to implement heterogeneous reactions in snow in models that predict the extent of chemical reactions occurring in cold environments.