Air-snow interactions and atmospheric chemistry

Determination of the specific surface area of snow using ozonation of 1,1-diphenylethylene

We measured the kinetics of ozonation reaction of 1,1-diphenylethylene (DPE) in artificial snow, produced by shock freezing of DPE aqueous solutions sprayed into liquid nitrogen. It was demonstrated that most of the reactant molecules are in direct (productive) contact with gaseous ozone, thus the technique produces snow with organic molecules largely ejected to the surface of snow grains. The kinetic data were used to evaluate the snow specific surface area (∼70 cm(2) g(-1)). This number is a measure of the availability of the molecules on the surface for chemical reaction with gaseous species. The experimental results were consistent with the Langmuir-Hinshelwood type reaction mechanism. DPE represents environmentally relevant compounds such as alkenes which can react with atmospheric ozone, and are relatively abundant in natural snow. For typical atmospheric ozone concentrations in polar areas (20 ppbv), we estimated that half-life of DPE on the surface of snow grains is ∼5 days at submonolayer coverages and -15 °C.

Organic carbon in Antarctic snow: spatial trends and possible sources

Organic carbon records in Antarctic snow are sparse despite the fact that it is of great significance to global carbon dynamics, snow photochemistry, and air-snow exchange processes. Here, surface snow total organic carbon (TOC) along with sea-salt Na(+), dust, and microbial load of two geographically distinct traverses in East Antarctica are presented, viz. Princess Elizabeth Land (PEL, coast to 180 km inland, Indian Ocean sector) and Dronning Maud Land (DML, ∼110-300 km inland, Atlantic Ocean sector). TOC ranged from 88 ± 4 to 928 ± 21 μg L(-1) in PEL and 13 ± 1 to 345 ± 6 μg L(-1) in DML. TOC exhibited considerable spatial variation with significantly higher values in the coastal samples (p < 0.001), but regional variation was insignificant within the two transects beyond 100 km (p > 0.1). Both distance from the sea and elevation influenced TOC concentrations. TOC also showed a strong positive correlation with sea-salt Na(+) (p < 0.001). In addition to marine contribution, in situ microorganisms accounted for 365 and 320 ng carbon L(-1) in PEL and DML, respectively. Correlation with dust suggests that crustal contribution of organic carbon was marginal. Though TOC was predominantly influenced by marine sources associated with sea-spray aerosols, local microbial contributions were significant in distant locations having minimal sea-spray input.

A novel synthesis of the N-13 labeled atmospheric trace gas peroxynitric acid

Radioactively labeled trace gases have been successfully used to study heterogeneous chemistry of atmospheric relevance. Here we present a new synthesis of gas-phase peroxynitric acid labeled with 13N (H13NO4) to study the interaction of HNO4 with ice and snow surfaces. A yield of about 30% for HNO4 was determined. The main by-products were HNO3 and HNO2. Exposure of an ice packed bed flow tube to these species revealed that the interaction with the surface scale in the order HNO3ߙ>ߙHNO4ߙ=ߙHNO2ߙ>ߙNO2.

Photoinduced reduction of divalent mercury in ice by organic matter

Reduction of divalent mercury and subsequent emission to the atmosphere has been identified as loss process from surface snow, but its mechanism and importance are still unclear. The amount of mercury that stays in the snow pack until spring is of significance, because during snow melt it may be released to the aquatic environment and enter the food web. Better knowledge of its fate in snow might further assist the interpretation of ice core data as paleo-archive. Experiments were performed under well-controlled laboratory conditions in a coated wall flow tube at atmospheric pressure and irradiated with light between 300 nm and 420 nm. Our results show that the presence of benzophenone and of oxalic acid significantly enhances the release of mercury from the ice film during irradiation, whereas humic acid is less potent to promote the reduction. Further it was found that oxygen or chloride, and acidic conditions lowered the photolytically induced mercury release in the presence of benzophenone, while the release got larger with increasing temperatures.

Microorganisms in dry polar snow are involved in the exchanges of reactive nitrogen species with the atmosphere

The snowpack is a complex photochemical reactor that emits a wide variety of reactive molecules to the atmosphere. In particular, the photolysis of nitrate ions, NO(3)(-), produces NO, NO(2), and HONO, which affects the oxidative capacity of the atmosphere. We report measurements in the European High Arctic where we observed for the first time emissions of NO, NO(2), and HONO by the seasonal snowpack in winter, in the complete or near-complete absence of sunlight and in the absence of melting. We also detected unusually high concentrations of nitrite ions, NO(2)(-), in the snow. These results suggest that microbial activity in the snowpack is responsible for the observed emissions. Isotopic analysis of NO(2)(-) and NO(3)(-) in the snow confirm that these ions, at least in part, do not have an atmospheric origin and are most likely produced by the microbial oxidation of NH(4)(+) coming from clay minerals into NO(2)(-) and NO(3)(-). These metabolic pathways also produce NO. Subsequent dark abiotic reactions lead to NO(2) and HONO production. The snow cover is therefore not only an active photochemical reactor but also a biogeochemical reactor active in the cycling of nitrogen and it can affect atmospheric composition all year round.

Solution structure of NaNO3 in water: diffraction and molecular dynamics simulation study

The structure of a series of aqueous sodium nitrate solutions (1.9-7.6 M) was studied using a combination of experimental and theoretical methods. The results obtained from diffraction (X-ray, neutron) and molecular dynamics simulation have been compared and the capabilities and limitations of the methods in describing solution structure are discussed. For the solutions studied, diffraction methods were found to perform very well in description of hydration spheres of the sodium ion but do not yield detailed structural information on the anion's hydration structure. Molecular dynamics simulations proved to be a suitable tool in the detailed interpretation of the hydration sphere of ions, ion pair formation, and bulk structure of solutions.

The role of the snowpack on the fate of alpha-HCH in an atmospheric chemistry-transport model

A dynamic snowpack module was implemented in the Danish Eulerian Hemispheric Model Persistant Organic Pollutants (DEHM-POP), an atmospheric chemistry-transport model designed to study the environmental fate of persistent organic pollutants in the Northern Hemisphere. The role of the snowpack on the fate of alpha-hexachlorocyclohexane (alpha-HCH) was investigated by making simulations both with and without the formation of a snowpack and comparing model results with data from 21 air monitoring sites. The inclusion of a dynamic snowpack module in the DEHM-POP model generally improves the fit between modeled and observed alpha-HCH air concentrations for the winter and spring seasons and the overall correlation coefficient between predicted and observed concentrations are improved at 8 of the sites. The predicted snowpack concentrations are in good agreement with the few available snow measurements from the Arctic. The presence of a snowpack increases surface boundary layer air concentrations of alpha-HCH at midlatitudes, while the effect is more pronounced in the Arctic due to the longer periods of snow cover. The results indicate that the snowpack module in DEHM-POP acts as a fast-exchanging temporary storage medium for alpha-HCH, as significant fractions were rapidly revolatilized back into the atmosphere following deposition with snowfall, although the current parametrization for vapor-exchange probably over emphasizes this process. Nonetheless, increased air concentrations observed between March and May ("spring maximum events"; SME) at several high latitude monitoring stations are also predicted by the model. The model results indicate that the SMEs are associated with the revolatilization of previously deposited chemical from the snowpack, following a reduction in the capacity of the snowpack to retain alpha-HCH with increasing temperatures toward the end of the winter period, rather than the actual melting of the snowpack. The SMEs are not predicted at all the Arctic monitoring sites by the model, and the significance of the snowpack in controlling these in the model is, therefore, open to question given the uncertainties in the snow-air partition coefficient (K(sa)) and the reliance of the model on a one-layer snowpack rather than a multilayered snowpack.

Organic contaminant amplification during snowmelt

The release of organic contaminants from melting snow poses risks to aquatic and terrestrial organisms and to humans who rely on drinking water and food production from regions that are seasonally snow-covered. Measured and model-predicted spring peak concentrations in waters receiving snowmelt motivate a thorough investigation of organic contaminant behaviour during melting. On the basis of the current understanding of snow metamorphosis, snowmelt hydrology and chemical partitioning in snow, this critical review aims to provide a qualitative picture of the processes involved in the release of organic contaminants from a melting snowpack. The elution sequence of organic substances during snowmelt is strongly dependent on their environmental partitioning properties and the physical properties of the snowpack. Water-soluble organic contaminants can be discharged in greatly elevated concentrations at an early stage of melting, while the bulk of the hydrophobic chemicals attached to particles is often released at the end of the melt period. Melting of a highly metamorphosed and deep snowpack promotes such shock load releases, whereas a shallow snow cover over a relatively warm ground experiencing irregular melting over the winter season is unlikely to generate notable peak releases of organic substances. Meltwater runoff over frozen ground directly transfers contaminant shock loads into receiving water bodies, while permeable soils buffer and dilute the contaminants. A more quantitative understanding of the behaviour of organic contaminants in varying snowmelt scenarios will depend on controlled laboratory studies combined with field investigations. Reliable numerical process descriptions will need to be developed to integrate water quality and contaminant fate models.

Adsorption of phenanthrene on natural snow

The snowpack is a reservoir for semivolatile organic compounds (SVOCs) and, in particular, for persistent organic pollutants (POPs), which are sequestered in winter and released to the atmosphere or hydrosphere in the spring. Modeling these processes usually assumes that SVOCs are incorporated into the snowpack by adsorption to snow surfaces, but this has never been proven because the specific surface area (SSA) of snow has never been measured together with snow composition. Here we expose natural snow to phenanthrene vapors (one of the more volatile POPs) and measure for the first time both the SSA and the chemical composition of the snow. The results are consistent with an adsorption equilibrium. The measured Henry's law constant is H(Phen)(T) = 2.88 x 10(22) exp(-10660/7) Pa m2 mol(-1), with Tin Kelvin. The adsorption enthalpy is delta H(ads) = -89 +/- 18 kJ mol(-1). We also perform molecular dynamics calculations of phenanthrene adsorption to ice and obtain AHads = -85 +/- 8 kJ mol(-1), close to the experimental value. Results are applied to the adsorption of phenanthrene to the Arctic and subarctic snowpacks. The subarctic snowpack, with a low snow area index (SAI = 1000), is a negligible reservoir of phenanthrene, butthe colder Arctic snowpack, with SAI = 2500, sequesters most of the phenanthrene present in the (snow + boundary layer) system.

Release of Oxygen Atoms and Nitric Oxide Molecules from the Ultraviolet Photodissociation of Nitrate Adsorbed on Water Ice Films at 100 K

Production of O((3)P(J), J = 2, 1, 0) atoms from the 295-320 nm photodissociation of NO(3)- adsorbed on water polycrystalline ice films at 100 K was directly confirmed using the resonance-enhanced multiphoton ionization technique. Detection of the O atom signals required an induction period after deposition of HNO3 onto the ice film held at 130 K due to the slow ionization rate of HNO(3) to H+ and NO(3)- with a rate constant of k = (5.3 +/- 0.2) x 10(-3)s(-1). Translational energy distributions of the O atoms were represented by a combination of two Maxwell-Boltzmann energy distributions with translational temperatures of 2000 and 100 K. Direct detection of NO from the secondary photodissociation process was also successful. On the atmospheric implications, the influence of the direct release of the oxygen atoms into the air from NO(3)- adsorbed on the natural snowpack was included in an atmospheric model calculation on the mixing ratios of ozone and nitric oxide at the South Pole, and the results compared favorably with the field data.

Computational chemistry development of a unified free energy Markov model for the distribution of 1300 chemicals to 38 different environmental or biological systems

Predicting tissue and environmental distribution of chemicals is of major importance for environmental and life sciences. Most of the molecular descriptors used in computational prediction of chemicals partition behavior consider molecular structure but ignore the nature of the partition system. Consequently, computational models derived up-to-date are restricted to the specific system under study. Here, a free energy-based descriptor (DeltaG(k)) is introduced, which circumvent this problem. Based on DeltaG(k), we developed for the first time a single linear classification model to predict the partition behavior of a broad number of structurally diverse drugs and other chemicals (1300) for 38 different partition systems of biological and environmental significance. The model presented training/predicting set accuracies of 91.79/88.92%. Parametrical assumptions were checked. Desirability analysis was used to explore the levels of the predictors that produce the most desirable partition properties. Finally, inversion of the partition direction for each one of the 38 partition systems evidences that our models correctly classified 89.08% of compounds with an uncertainty of only +/-0.17% independently of the direction of the partition process used to seek the model. Other 10 different classification models (linear, neural networks, and genetic algorithms) were also tested for the same purposes. None of these computational models favorably compare with respect to the linear model indicating that our approach capture the main aspects that govern chemicals partition in different systems.

Photodecarbonylation of dibenzyl ketones and trapping of radical intermediates by copper(II) chloride in frozen aqueous solutions

This paper presents a quantitative and qualitative study of the Norrish type I reaction of dibenzyl ketone (DBK) and 4-methyldibenzyl ketone (MeDBK), producing the benzyl radicals and consequently recombination products, in frozen aqueous solutions over a broad temperature range (-80 to 20 degrees C). This work extends previous research on the cage effects in various constrained media to provide information about the dynamics and reactivity of the photochemically generated intermediates at the grain boundaries of ice matrix. As the temperature of aqueous solutions decreases, the solute concentrations become high at layers covering ice crystals, causing efficient molecular segregation. The cage effect experiments have shown that diffusion of the benzyl radicals within such reaction aggregates is still remarkably efficient at temperatures below -50 degrees C, independently of the initial ketone concentration in the range of 10(-6)-10(-4) mol L(-1). In addition, the study of trapping the benzyl radicals formed in situ by CuCl2 was used as a qualitative probe of heterogeneous bimolecular reactions in the frozen aqueous matrix and on its surface. Molecules of both solutes were found to be segregated from the ice phase to the same location and underwent chemical reactions within diffusion and intermediates lifetimes limits. Understanding the fundamental physicochemical processes in ice is unquestionably important in related environmental or cosmochemical investigations.

Molecular Dynamics Simulations of the Solution−Air Interface of Aqueous Sodium Nitrate

Molecular dynamics simulations have been used to investigate the behavior of aqueous sodium nitrate in interfacial environments. Polarizable potentials for the water molecules and the nitrate ion in solution were employed. Calculated surface tension data at several concentrations are in good agreement with measured surface tension data. The surface potential of NaNO3 solutions at two concentrations also compare favorably with experimental measurements. Density profiles suggest that NO3- resides primarily below the surface of the solutions over a wide range of concentrations. When the nitrate anions approach the surface of the solution, they are significantly undercoordinated compared to in the bulk, and this may be important for reactions where solvent cage effects play a role such as photochemical processes. Surface water orientation is perturbed by the presence of nitrate ions, and this has implications for experimental studies that probe interfacial water orientation. Nitrate ions near the surface also have a preferred orientation that places the oxygen atoms in the plane of the interface.

Arctic air pollution: origins and impacts

Notable warming trends have been observed in the Arctic. Although increased human-induced emissions of long-lived greenhouse gases are certainly the main driving factor, air pollutants, such as aerosols and ozone, are also important. Air pollutants are transported to the Arctic, primarily from Eurasia, leading to high concentrations in winter and spring (Arctic haze). Local ship emissions and summertime boreal forest fires may also be important pollution sources. Aerosols and ozone could be perturbing the radiative budget of the Arctic through processes specific to the region: Absorption of solar radiation by aerosols is enhanced by highly reflective snow and ice surfaces; deposition of light-absorbing aerosols on snow or ice can decrease surface albedo; and tropospheric ozone forcing may also be contributing to warming in this region. Future increases in pollutant emissions locally or in mid-latitudes could further accelerate global warming in the Arctic.