Can We Model Snow Photochemistry? Problems ... - ACS Publications

Feature Article pubs.acs.org/JPCA

Can We Model Snow Photochemistry? Problems with the Current Approaches Florent Domine,*,†,‡ Josué Bock,§ Didier Voisin,§ and D. J. Donaldson∥ †

Takuvik Joint International Laboratory, Université Laval (Canada) and CNRS (France), Pavillon Alexandre Vachon, 1045 Avenue de La Médecine, Québec, QC G1V 0A6, Canada ‡ Department of Chemistry, Université Laval, Pavillon Alexandre Vachon, 1045 Avenue de La Médecine, Québec, QC G1V 0A6, Canada § Université Joseph Fourier−Grenoble 1/CNRS, Laboratoire de Glaciologie et Géophysique de l’Environnement, UMR 5183, Grenoble, F-38041, France ∥ Department of Chemistry, University of Toronto, and Department of Physical and Environmental Sciences, University of Toronto Scarborough, Scarborough, Toronto, ON, Canada ABSTRACT: Snow is a very active photochemical reactor that considerably affects the composition and chemistry of the lower troposphere in polar regions. Snow photochemistry models have therefore been recently developed to describe these processes. In all those models, the chemically active medium is a brine formed at the surface of snow crystals by impurities whose presence cause surface melting. Reaction and photolysis rate coefficients are those measured in dilute liquid solutions. Here, we critically examine the basis for these models by considering the structure of ice crystal surfaces, the processes involved in the interactions between impurities and ice crystals, the location of impurities in snow, and the reactivity of impurities in the various media present in snow. We conclude that the brine formed by impurities can only be present in grooves at grain boundaries and cannot cover ice crystal surfaces because of insufficient ice wettability. It is then very likely that most reactions in snow do not take place in liquids, but rather either on an actual ice surface highly different from a liquid or in particulate matter contained in snow, such as organic particles that are thought to contain most snow chromophores. We discuss why some snow models appear to adequately reproduce some observations, concluding that they are insufficiently constrained and that the use of adjustable parameters allows acceptable fits. We discuss the complexity of developing a snow model without adjustable parameters and with a predictive value. We conclude that reaching this goal in the near future is a tremendous challenge. Modeling attempts focused on snow where the impact of organic particles is minimal, such as on the east Antarctic plateau, represents the best chance of midterm success.

I. INTRODUCTION Snow is a porous medium made of air, ice crystals, and trace amounts of impurities. At or very near 0 °C, liquid water is also present. Some of the trace impurities present in real environmental snow are photochemically active, making snow a photochemical reactor that releases products to the atmosphere, considerably affecting its composition and chemistry, most notably in polar regions.1,2 A noteworthy and large scale effect of snow photochemistry is the release of halogen atoms that leads to ozone destruction and the oxidation and deposition to the snow of elemental mercury, during so-called ozone depletion events (ODEs)3 and atmospheric mercury depletion events (AMDEs).4 These regularly affect the pan-Arctic area and coastal Antarctica.5 Through its impact on atmospheric composition, snow photochemistry also impacts climate. Processes involved include changes in greenhouse gas concentrations such as ozone and also the release of oxygenated organics that can favor the formation and growth of aerosol that act as cloud condensation nuclei, which contribute to cloud cover, © 2013 American Chemical Society

modifying the budget of shortwave radiation in the atmosphere.6,7 Given the widespread impacts of snow photochemistry and its interactions with climate change, accurate modeling of the relevant processes is becoming a necessity. The challenge is huge as many unknowns prevent a good understanding of the actual processes taking place; indeed, we do not even know where most reactants are located in the snow,2,8,9 what their reactivities are there, and what the relevant reaction rate coefficients are there. Attempts at modeling have therefore required simplifications and the use of adjustable parameters, with the undisputable benefit of producing models that often acceptably reproduce data against which they are tested. Current snow chemical models are based on the widely accepted existence of the so-called quasi-liquid layer (hereafter, QLL) present at the surface of ice crystals.10 The universal Received: December 14, 2012 Revised: March 26, 2013 Published: April 18, 2013 4733

dx.doi.org/10.1021/jp3123314 | J. Phys. Chem. A 2013, 117, 4733−4749

The Journal of Physical Chemistry A

Feature Article

approximation is that all or a fixed fraction of reactants are located in the QLL and that reaction and photolysis rate coefficients are close to those measured in dilute liquid solutions.11−14 The purpose of this article is not to review the literature on snow photochemistry and on the numerous laboratory experiments intended to provide relevant basic data. Instead, our goal is to present a critical examination of the main hypotheses made in recent snow modeling studies in the light of selected relevant aspects of snow and ice physical chemistry and snow microphysics, using existing data, some of which are presented with extra details. Our main objective is to attempt to reach a conclusion on the validity of current snow photochemistry models. Finally, we discuss the possibility to build predictive snow models without adjustable parameters. But first, to expose the context of this discussion for a wide readership, a prerequisite is the description of the processes of formation of snow in the atmosphere and of its evolution on the Earth’s surface, where many impurities are present and can be incorporated in the snow.

that snow chemical composition can show dramatic changes after precipitation. Finally, warm episodes can lead to the appearance of water at the ice−air interface, and this happens first at the intersection of three grains.33 This water can dissolve gases present in snow interstitial air. Snow on the surface (ground or sea ice) is therefore a mixture of mostly ice and air, but it also contains a large variety of impurities in the form of dissolved and adsorbed gases and particles imbedded within the crystal, or located on their surface or at grain boundaries and triple junctions. It is important to note that some species trapped within crystals will not be as easily accessible for reaction with atmospheric species as others located on ice crystal surfaces. Furthermore, a large fraction of snow impurities are located in particles, with limited contact to the ice surface.34 An illustration of the location of impurities in snow is shown in Figure 1.

II. PROCESSES OF FORMATION AND EVOLUTION OF SNOW Most snow found on the surface comes from precipitation of crystals formed in clouds. Direct accretion on the surface is due to surface hoar and rime caused, respectively, by water vapor condensation and supercooled droplet freezing directly onto the snow surface.15,16 The first stage in hydrometeor (i.e., a condensed water particle, either solid or liquid) formation is usually the heterogeneous nucleation of a liquid water droplet on a condensation nucleus, which can be formed by a sulphuric acid aerosol, mineral dust, organic particles, or biological fragments such as a whole or part of a bacteria.17 During subsequent droplet growth, soluble atmospheric trace gases such as mineral or organic acids, aldehydes, and other polar molecules are incorporated.18 Droplets can also scavenge other particles.19 The formation of an ice crystal requires the presence of an ice nucleus (IN), and these form only a very small fraction of atmospheric particles, so that liquid droplets often freeze at temperatures below −30 °C. The most efficient IN are fragments of bacterial membranes, which can initiate freezing at −2 °C.20 At the center of an atmospheric particle, there is therefore usually an IN, and other particles can also be present. After nucleation, ice particles grow at the expense of liquid ones because the vapor pressure of ice is lower than that of water. During growth, ice-soluble species such as HCl, HNO3, and HCHO are incorporated into the ice crystalline lattice to form solid solutions.21,22 Liquid droplets can also freeze onto the growing crystal,23 while retaining a fraction of their solutes.18,24 However, although observed in polar regions,25 riming is often not an important process there, so we neglect it in future discussions. During precipitation, snow crystals efficiently scavenge aerosol particles below the cloud26 and can also adsorb trace gases. Once on the Earth surface, snow crystals undergo sublimation and condensation cycles caused mostly by temperature gradients in the snow cover. These result in massive structural modifications regrouped under the term “snow metamorphism”,27 which is accompanied by chemical changes,28 as release and uptake of dissolved and adsorbed species take place. Particles and dissolved gases can also end up at grain boundaries or triple junctions.29,30 Wind propagating in the snow cover also deposits large amounts of particles,31,32 so

Figure 1. Selected possible locations of chemical impurities in snow. Chemical species can be located in several phases such as in the bulk or surface of ice crystals, in several particle types, and in liquid brines produced, e.g., by the melting of sea salt particles at temperatures above the eutectic. The embedded condensation and ice nuclei can be mineral particles, sulphuric acid, bacterial fragments, and many other species.

III. CURRENT STATE OF SNOW MODELS Treating chemical reactions in all the phases present in snow is a difficult and complex enterprise and all the required basic data is not available. Therefore, at present, most snow chemistry models make the approximation that all the reactants found in snow are present in the QLL and that reaction and photolysis rate coefficients are those measured in liquid water.11−13 The QLL is assumed to form the interface between the snow or ice and the overlying atmosphere or snow interstitial air. A possible variation assumes that a fixed fraction of reactants are in the QLL,14,35 and this fraction is adjusted to optimize agreement with data.14 In those models, the thickness of the QLL is either set arbitrarily 11,14,36 or is determined from the ionic concentrations of the snow.12,13 In this last case, the more or less implicit assumption is that all ions present in snow form a brine at the surface by virtue of the freezing point depression,37,38 and this brine is then a true liquid. All current models have been tested against one or a small number of data sets and usually have adjustable parameters that are optimized 4734



Feature Article

thermodynamic sense. Because of this property gradient within this QLL, any measurement of a property will be an average over some thickness, which may depend on the measurement method and may partly explain why different methods indicate different thicknesses. In passing, the term “quasi-liquid layer” implicitly suggests the existence of a homogeneous layer and is therefore misleading. A term such as “disordered surface transition region” may have been more appropriate. In any case, sticking with current terminology, we conclude for our purpose that there is a QLL at the surface of pure ice. Most of its properties are intermediate between those of ice and of water, while others (e.g., molecules orientation at the surface) are simply different from those of water.48 The QLL thickness is a few nanometers at −10 °C, increasing with temperature. Within the QLL, there is a gradient in physical properties between the ice−QLL and the QLL−air interfaces, and only very near 0 °C, when the QLL is very thick, may some of these properties resemble those of liquid water.53 B. Surface Layer of Ice with Impurities. In the presence of solutes such as acids and salts, phase diagrams predict that a liquid solution will exist below 0 °C in coexistence with ice, down to a eutectic temperature. Strictly speaking, equilibrium multicomponent phase diagrams describe the equilibria present among the bulk phases, which are present in a defined system. From our perspective, they are deficient in several ways: first, they are silent as to the location of the various phases present at equilibrium. Thus, such diagrams do not, in fact, predict that a brine layer will exist at the interface with air, only that a brine of a particular composition should exist within the total system at equilibrium. Second, quantitative formulations, which are used to predict brine compositions, typically assume ideal solution conditions, clearly not the case in a supercooled, concentrated brine solution. Unfortunately, the relevant activity coefficients (and their temperature and composition dependence) are not generally known except for the simplest systems. Finally, the surface energy terms in the system free energy formulation are ignored in the development of the phase diagram. Because our interest is strongly in the properties of the interfaces between solid water and air (the atmosphere at the snowpack surface or any vapor and air present in liquid inclusions) or solid and liquid water (at grain boundaries and liquid inclusions or at the air interface with a brine), this represents a significant deficiency in the phase diagram approach to understanding snow chemistry. For the moment, we ignore the above and limit ourselves to a perfect ice single crystal. We argue here that in real environmental snows, the small amount of liquid formed cannot cover the whole surface of the snow crystals because of imperfect ice wettability. This issue has been ignored in snow models, but it is critical. Its effect can be illustrated by the easily reproducible experiment involving a system where the effect is more pronounced: water on glass. If in principle 1 mL of water can cover a glass surface of 1 m2 by forming a 1 μm thick layer, this is in fact not possible because interface energies are such that water cannot be spread over the glass and will cover only a very tiny fraction of the 1 m2 area. The same applies to water or a brine on ice, although to a significantly different extent, as explained in this section. The consequence is that a brine cannot represent the interface between snow crystals and air in real snow. To demonstrate this, we first use natural examples where both snow composition and specific surface area (SSA) were measured to evaluate the reasonable range of film

for the specific case studied so that their predictive value is limited. Photochemistry also depends on actinic fluxes in snow. Radiative transfer models have recently been used for calculating UV−vis actinic fluxes in snowpacks.39 These show an important influence of light-absorbing impurities such as black carbon on photochemical production. Such calculations need effective optical properties for the snow−impurities mixture, which depend on the impurity concentrations, but also on their location on or in the snow grains. Most radiative transfer models for snow with impurities are based on an assumption that impurities and snow form an external mixture, in which they contribute separately and independently to the total optical response.40 To evaluate these assumptions, we need to consider the physical chemistry of ice surfaces in the case of pure ice and when impurities are present, to discuss how chemical species interact with snow crystals and what the reaction rate coefficients are there.

IV. RELEVANT ASPECTS OF SNOW PHYSICAL CHEMISTRY AND MICROPHYSICS A. Surface Layer of Pure Ice. The suggestion of the presence of a liquid or liquid-like layer on the ice surface dates back to Faraday.41 Various techniques have evidenced the existence of a disordered layer, as reviewed by Petrenko and Witworth,42 and include atomic force microscopy,43,44 X-ray diffraction,45 ellipsometry,46 nuclear magnetic resonance,47 and sum frequency generation vibrational spectroscopy.48,49 The thickness of this layer measured on pure ice varies with the technique employed and increases with increasing temperature. Its detection starts between −35 and −3 °C for most techniques, and its thickness is between 1 and 50 nm at −10 °C.42 Theories aiming to explain the existence of the QLL abound,50−52 and it is not our purpose to review these, but many of them have stirred heated debates,53−55 none of them reaching a consensus. The aspect that most interests us here is whether there is a difference in structure and properties between the QLL and bulk water. Baker and Dash53 mention that “it is convenient in theoretical descriptions as a first approximation to assume the surface film properties are those of bulk liquid” and indeed many theories make that approximation.56 However, it is well established that the QLL and bulk liquid water have physical properties that very significantly differ. For example, the electrical conductivity of the QLL is 6 orders of magnitude larger than that of liquid water,52 the self-diffusion coefficient of H2O molecules in the QLL is 4 orders of magnitude smaller than in water and 2 orders of magnitude greater than in bulk ice,47 and the viscosity of the QLL is much greater than that of supercooled water.44 From a chemistry perspective, glancing-angle Raman spectra of water molecules in the air−ice interface region suggest a different hydrogen bonding environment present there from that found in either liquid or solid water.57 This is also indicated by molecular dynamics simulations of the solvation of organics in this region.58 Considering that the QLL is a homogeneous medium with well-defined properties under given conditions is even debatable. Indeed, as well illustrated by molecular dynamics calculations,59,60 the QLL is a transition layer whose structure varies continuously between the ice− QLL and QLL−air interfaces, so that its dynamic properties feature a gradient with depth.61 The QLL is not homogeneous and therefore cannot even be defined as a phase in the 4735



Feature Article

sites such as grain boundaries. Similarly to Cullen and Baker,69 we used sublimation in a scanning electron microscopy vacuum chamber to reveal possible nonvolatile impurities. The snow samples studied and details of the methods are given in Domine et al.27 Most samples did not show visible impurities at grain boundaries. However, Figure 2 shows two samples where

thickness in snow. We then address wettability by considering contact angles. Solute concentrations in snow show large variations. Domine et al.32 analyzed a range of coastal Arctic snows, from dilute ones little affected by sea salt to concentrated ones on sea ice. Total ion concentrations in bulk melted snow ranged from 4.5 to 978 μM (we omit here snow samples directly in contact with seawater), and the SSA of some of these samples have also been reported.62,63 As a first approximation, we treat the snow samples as an H2O−NaCl mixture of the same total ion concentration as the analyzed snow. Using the H2O−NaCl phase diagram,64 we calculate that a snow sample with a total solute concentration of 4.5 μM has a liquid mass fraction of 10−6 at −10 °C. That snow (the middle 29 April layer in Domine et al.32) had a SSA of 50 m2 kg−1, so that a continuous liquid film would have a thickness of 0.02 nm, i.e., much less than a monolayer of solution. In this case, it is therefore not even necessary to consider wetting to conclude that liquids cannot cover the whole surface of snow crystals, a conclusion also reached by Weller et al.65 from analyses of snow from central Antarctica. For the most concentrated samples (top of 27 April layer on 8 May), the ionic concentration reaches 978 μM with a SSA of 18 m2 kg−1, leading to a film thickness of about 12 nm, which may cover the whole surface if wettability allows. This can be tested by considering relationships between layer thickness and contact angle. Hobbs66 gives the thickness d of an equilibrium liquid film spread onto a solid surface as σ d 2 = (1 − cos(θ )) SL ρL g (1)

Figure 2. Observation of impurities at grain boundaries in recent snow. Top: after a short sublimation time, nonvolatile impurities are revealed near a triple junction. Bottom: after a long sublimation time, a filament of impurities becomes visible around a neck between two crystals. Snow crystals are from samples A9 (top) and A7 (bottom) described in Domine et al.27

where σSL is the interfacial energy between the solid and liquid, ρL the density of the liquid, and g gravity. Very few measurements of the contact angle of a solution or water on an ice surface are available. Ketcham and Hobbs33 measured a water−ice contact angle of 1°, and Hobbs66 subsequently recommended that value in his monograph. On the basis of freezing experiments, Knight67 mentions that the “contact angle of water on ice must be greater than zero.” However, he cautions that he uses an “effective” contact angle since his system is not in equilibrium. Blackford et al.68 studied the equilibrium between a 0.043 M NaCl solution and ice and measured a contact angle of θ = 5°, a fairly high value. Applying eq 1 to the case of Arctic snow above and using σSL = 33 mJ m−2,33 we conclude that the contact angle must be less than 0.001° if the film covers the whole surface, a most unlikely low number. Therefore, for ionic concentrations found in almost any snow, the liquid film due to the presence of ionic solutes cannot form a uniform layer over the surface. The conditions may even be less favorable to the formation of liquids on the surface if grain boundaries are considered. First, grain boundaries form grooves on the ice surface and most observations of small amounts of liquid in snow, including of water in snow, showed that any visible amount of liquid first forms in grooves at grain boundaries and triple junctions before wetting grain surfaces.33 Second, a significant fraction of impurities may be sequestered at grain boundaries and are thus not available to form liquids on the surface. Cullen and Baker69 showed that in solid polycrystalline ice (i.e., with essentially no porosity, unlike snow) a large fraction of the impurities were located at grain boundaries. This, however, needs to be confirmed for snow because in ice there is no ice−air interface to host impurities as in snow, so that they have to find other

sublimation did reveal some. In one of them, a long sublimation time lead to the formation of a filament of impurities. Note that in no case did we observe clear signs of the presence of a liquid pool at a triple junction, which suggests that actual liquids in dry snow, if they do form, cover a negligible fraction of the surface. The presence of grain boundaries may also explain the interesting results of Cho et al.37 These authors studied the composition of frozen NaCl solutions using nuclear magnetic resonance and concluded that a liquid brine exists in the frozen sample down to temperatures lower than the eutectic, with preferential migration of this brine to the top and bottom of the NMR tube. This observation is often invoked to justify the presence of liquids on snow crystal surfaces at all environmental temperatures. Moreover, Cho et al.37 showed that the composition of the liquid brine below the eutectic temperature was predicted correctly by extrapolating the phase diagram to lower temperatures, which added credence to the observations and their thermodynamic description. Subsequently, Kuo et al.38 improved this thermodynamic approach by introducing nonideal solutions, but this did not alter the concept. The applicability of these observations to natural snow is not certain. First of all, Cho et al.37 used solute concentrations orders of magnitude greater than natural snow. Second, their ice samples were made by the fast freezing of very small amounts of solution, so that very small crystals formed, and their system was significantly different from single crystals. The 4736



Feature Article

energy terms for grain boundaries were therefore important, and these favored the presence of a liquid in equilibrium with the small crystals. Indeed, Wettlaufer and Worster70 showed that in confined geometries, surface energy terms decrease the system total energy and thus lower the solution melting temperature. We deduce from these considerations that the results of Cho et al.37 may not apply to natural snow. The conclusion of this section is that true liquids may coexist with snow, but for the overwhelming majority of natural snows, their amount will be so small that they cannot form a continuous liquid layer on the snow surface. Rather, such brines would be located in grooves at grain boundaries and triple junction, where surface energy terms lower the eutectic temperature compared to that obtained for single crystals. The majority of snow crystal surfaces will therefore be ice with a QLL, and possibly adsorbed species, but not a liquid brine. Exceptionally high salt concentrations are required for a homogeneous liquid to cover snow crystals. C. Location of Impurities in Snow. Snow contains an incompletely explored number of molecular and ionic impurities such as short chain aldehydes and ketones,71,72 organic acids,73,74 mineral ions,75−77 hydrogen peroxide,78,79 mercuric compounds,80,81 and heavier organic molecules that range from C6 hydrocarbons to large multifunctional molecules and include persistent organic pollutants such as pesticides.82−84 Snow also contains particles that acted as condensation nuclei, were scavenged during precipitation, or were gravitationally deposited or filtered out of air windpumped through snow. These particles include sea salt,32,85 mineral dust,86,87 and organic particles whose complex composition is only starting to be elucidated. Recent work has shown that reactions may take place within aerosols to form oligomeric and macromolecular structures with a wide variety of functional groups such as carbonyls, imines, and acids with humic-like and fulvic-like structures,88−94 which are often chromophores. Finally, snow also contains microbes82,95,96 and macroscopic debris coming from plants and soils.34 To fully understand the reactivity of all these species, their location in snow must be known. Impurities may form a solid solution with ice and are therefore embedded within the ice crystalline lattice. They may also be adsorbed on the ice surface, located in grain boundaries or at triple junctions, be dissolved in liquids that may form there, and finally may be present in particles, therefore in a phase unrelated to ice. Figure 1 shows selected locations for reactants in snow, some of them being discussed in more detail in subsequent sections. To illustrate that the dominant assumption in snow models (that reactants are in a brine layer that behaves as a liquid) is often not valid, we will detail three examples: formaldehyde (HCHO), the smallest organic molecule found in snow, the nitrate ion (NO3−), and C2+ organic molecules. Formaldehyde. Laboratory work showing that HCHO forms a solid solution with water ice22 suggests that this molecule in natural snow could be located within the ice crystal lattice. It could also be contained within organic particles. Detailed simultaneous measurements of HCHO in snow and the atmosphere in two distinct arctic locations22,97 have shown that the temporal variations of [HCHO] in snow could be reproduced well by modeling the solid-state diffusion of HCHO in and out of snow crystals22,97 without using any adjustable parameter, as shown in Figure 3. Organic matter was also measured and found to be fairly abundant,34 negating the argument that formaldehyde was present in ice merely for lack

Figure 3. Time series of HCHO concentrations in surface snow at Barrow, Alaska, in April 2009. As a test of the hypothesis that HCHO forms a solid solution with ice in natural snow, [HCHO] could be modeled well using measured gas phase values, measured snow specific surface area, and the diffusion coefficient and solubility of HCHO in ice, measured in the laboratory (modified from Barret et al.97).

of a better solvent. Furthermore, in both these studies, the implicit assumption that the fraction of HCHO present on snow crystal surfaces is totally negligible, as indicated by previous laboratory and molecular dynamics studies,98,99 has been verified. This indicates that HCHO in snow is most likely located within ice crystals, so that models of HCHO in snow cannot assume that it is dissolved in a liquid layer at the surface. Nitrate. The nitrate ion NO3− is often considered central to snow photochemistry, as it is an important source of OH radicals. Again, most models assume its presence on the surface, dissolved in a liquid layer. However, just like HCl and probably also HF,100,101 nitrate can form a solid solution with ice, at least when it is present as nitric acid,102,103 so that at least a fraction of the nitrate is located within ice crystals. Further confirming this point, Wren and Donaldson104 showed in laboratory experiments that the exclusion of the nitrate anion to ice surfaces during freezing of nitrate solutions is not complete or even well described by the corresponding phase diagram. Moreover, snow analyses indicate that the interactions between the nitrate ion and snow are complex and probably depend on the chemical form in which it was incorporated, i.e., acid or salt. To illustrate our point, we first use the snow analyses obtained on the Ross sea coast in Antarctica, described in Beine et al.105 Those authors show correlations in snow composition between NO3− and Na+ for aged snows. The correlation is excellent (R2 = 0.97) from which it was deduced that NO3− is associated with sea salt particles. It is indeed logical to expect gas phase HNO3 to be scavenged by the ubiquitous sea salt aerosol at this coastal site, according to the reaction NaCl(solid) + HNO3(gas) → NaNO3(solid) + HCl(gas)

(2)

NO3−/Na+

The ratio is then determined by the mixing ratio of HNO3, the size distribution of the sea salt aerosol (conditioning its surface/volume ratio and therefore the rate of reaction 2), and the contact time between HNO3 and the aerosol in the atmosphere and in the snow. The conditions around this Ross sea site were presumably fairly stable to lead to such a good correlation. However, snow analyses performed in Svalbard by Amoroso et al.,96 presented in more detail here, lead to a similar conclusion. Figure 4 shows the correlation, based on 324 snow analyses between 20 February and 11 April 4737



Feature Article

NO3− can be expected to be located at the snow crystal surfaces or within crystals. If these tentative interpretations are correct, we conclude that the environment of NO3− in snow will be highly variable. Depending on the exposure to sea salt, on the partial pressure of HNO3, on temperature, and on the snow’s specific surface area, nitrate can be dissolved within snow crystals, adsorbed onto their surface, present in highly concentrated liquid brines in grooves at triple junctions, in grain boundaries, or present on sea salt or mineral particles within the snowpack. Its reactivity can then be expected to be highly variable and, in all cases, potentially very different from that in dilute solutions. C2+ Organic Molecules. Even though initial studies of snow photochemistry identified NO3− as a crucial chromophore,108 subsequent work showed that even in fairly pristine areas like central Greenland, unidentified chromophores, presumably organics, were responsible for most of the UV light absorption in snow.109 Confirmation of this was provided by the extensive studies during the 2009 OASIS campaign in Barrow, Alaska, where absorption spectra and chemical and physical analyses of snow confirmed that organics were likely to be the main absorbers in snow.34,78,110,111 Understanding the location of organics in snow is therefore critical to predict their reactivity. Here, we discuss acetaldehyde, acetone, and large macromolecules such as HULIS (HUmic LIke Substances112). We also discuss large identified molecules such as polycyclic aromatic hydrocarbons (PAHs) and some chlorinated organics, classified as persistent organic pollutants (POPs), even though they may not all absorb at UV wavelengths. Acetaldehyde and acetone have been measured in polar snow, simultaneously with snow physical properties such as specific surface area (SSA), so that tests of their mechanism of incorporation in snow and, in particular, adsorption could be made.72,113 On the basis of the concentration dependences on temperature, atmospheric mixing ratios, and snow SSA, it was concluded that these molecules most likely did not form a solid solution with ice and were not adsorbed on its surface. This last conclusion was corroborated by laboratory measurements of adsorption of acetone and acetaldehyde on ice,98,114−116 which showed that the equilibrium concentrations expected assuming that adsorbed species are in equilibrium with the gas phase were almost 2 orders of magnitude lower than measured values. Houdier et al.72 and Domine et al.113 therefore concluded that the only reasonable remaining location for these molecules were in organic aerosols deposited to snow or were an analytical artifact, where the melting of snow for analysis resulted in the hydrolysis of macromolecules that produced carbonyls. Despite the remaining uncertainties, the message from these studies is that small carbonyls most likely are not in direct contact with ice crystals and are more likely entities in organic particles. Even though no relevant data are available, large molecules such as PAHs and POPs are intuitively not expected to form solid solutions with ice because their large size would cause huge distortions to the crystalline lattice and because they are often hydrophobic. Many laboratory and modeling studies have made the assumption or demonstrated that these compounds were at least in part adsorbed onto snow crystals.58,84,117−122 It is also well-known that these molecules, which have a low vapor pressure, also tend to sorb to organic aerosols123 and this process has also been taken into account to describe or model physical interactions of these molecules with snow.124−126 Detailed field studies indicated that adsorption alone could not

Figure 4. Correlation between sodium and nitrate ions in Svalbard snow in winter and spring 2006. The points are regrouped along three linear correlations, indicative of different values of atmospheric variables such as sea salt aerosol size distribution and nitric acid mixing ratios.

2006, for snow layers taken from the surface to a depth of 50 cm. Overall, there seems to be a poor correlation, but we find that in fact the points are regrouped along 3 lines, each with a good correlation. We interpret this as the result of sea salt fractionation under three main reaction conditions, whose differences can be variations in HNO3 mixing ratios and in the size distribution of the sea salt (due presumably to differences in air mass origins). The conclusion from these paragraphs is that in these coastal Antarctic and Svalbard snows, NO3− is associated with sea salt particles. If the temperature is above the eutectic, we expect these particles to form liquid droplets that will preferentially accumulate in grooves at grain boundaries. Below the eutectic, sea salt will remain as individual particles, where NO3− will possibly have a very different reactivity from that in highly concentrated brines. In any case, there is no reason to expect the reactivity to be similar to that in dilute solutions, as assumed in current snow photochemistry models. This of course applies to snow in coastal areas, where HNO3 is expected to be scavenged by sea salt particles. Similar processes can be expected to take place in the presence of alkaline mineral dust that would also scavenge HNO3. For example, on limestone particles, we would have106 CaCO3(solid) + 2HNO3(gas) → Ca(NO3)2(solid) + H 2O(gas) + CO2(gas)

(3)

The situation may be very different in inland areas such as the east Antarctic plateau or the Greenland ice cap. We do not present new data from those locations here, but the data presented by Beine et al.105 for fresh snows may be indicative of what might happen in inland areas. Beine et al.105 showed that, for fresh snows, there is almost no correlation between NO3− and Na+ (R2 = 0.19). Their interpretation is that NO3− in snow comes from gaseous HNO3 that adsorbed on or dissolved in snow in the air mass that generated the precipitation. Backtrajectories indeed showed that air masses originated from the Antarctic plateau, with reduced sea salt concentrations. Since solubilization and adsorption have different temperature and partial pressure dependences,102,107 the ratio of dissolved/ adsorbed NO3− will vary with these conditions. In any case, in the absence of sea salt or other particles that scavenge HNO3, 4738



Feature Article

explain all the observations127 and that other incorporation processes were involved, confirming the possibility that a fraction of these organics were contained in organic particles. Given the wide range of properties of the molecules considered and the limited data available, no clear-cut conclusion can be reached. It nevertheless appears likely that both adsorption to snow crystal surfaces and adsorption onto or incorporation into organic particles do take place, to variable extents depending on the molecule, temperature, snow SSA, and particles SSA, concentrations, and chemical properties. The main point here is that it does not appear possible to assume that these molecules are solely adsorbed onto ice surfaces. Given their general hydrophobicity, their solution into liquid pools at triple junctions is probably a negligible process, so that again it does not seem reasonable to treat their reactivity as if they were in dilute liquid solutions. Particulate matter, including organic particles, can be present in snow as condensation nuclei. It can also be scavenged from the atmosphere during precipitation, be dry-deposited, or be wind-pumped into snow. Organic particulate matter is chemically extremely complex. Even though numerous individual molecules have been identified,128 a significant fraction of the particle mass is thought to be formed by macromolecules whose actual chemical formula cannot today be determined. However, thanks to laboratory work and detailed analytical studies on natural aerosols, many reaction pathways leading to the formation of organic aerosols and functional groups have been identified.129 Reactions include oxidation both by gas phase species and within the particle and nonoxidative accretion processes. Functional groups produced include carbonyls, heterocycles, esters, and various (hemi)acetals. The ability to follow the compositional evolution of aerosols has made progress due to the widespread use of highresolution mass spectrometry93 but simplifying variables such as the predominance of given mass peaks have to be used to classify composition. The current understanding is that by aging, aerosol composition becomes fulvic-like and humiclike.130 How organic particles interact with ice has been little explored, except from the viewpoint of ice nucleation.131−133 It is not known whether some molecules will spread onto the ice surface and become available for reactions there or whether the particle will retain its integrity, but the idea that organic molecules will not necessarily be located on ice surfaces is supported by laboratory experiments: for several aromatic compounds (benzene and some PAHs) self-association among the organics was found to be preferred to uniform spreading on the ice−air interface.58,122,134 Some molecules such as small organic acids, which are known to adsorb onto ice surfaces,135−137 may migrate from organic particles and partition to the ice surface. However, given the reduced mobility of water molecules on the ice surface relative to liquid water,44 interactions will be limited, and we speculate that most of the constituents of an organic particle will remain within the particle, but it is clear that experimental investigations are required on this point. In summary to this section, current knowledge on the interactions of C2+ organic molecules with snow is very limited, especially given the complexity of atmospheric and snow organic matter. We do know that HCHO dissolves in ice and that many compounds adsorb onto ice surfaces. Compounds in particles may end up on the ice surface, but intuitively, we expect that the macromolecules that form most of the particle mass will remain within an individualized particle. With regards

to the possible formation of a true liquid, it can be envisaged that some species such as organic acids could contribute to the formation of a brine according to their phase diagram. But again, as mentioned above for mineral ions, such a liquid would cover a very small fraction of the snow crystals surface, limiting its impact on snow photochemistry. Since most chromophores are suspected to originate from organic particles, we do not know what the environments of these chromophores will be. Actual absorption spectra, cross-sections, and quantum yields may even be very different from those measured in melted snow, as melting may significantly change electronic or molecular structure, as exemplified by the solubilization of aldehydes, which forms diols. D. Reactivity of Chemical Species in Snow. The previous section indicates that reactants in snow can be located in several environments: ice crystalline lattice, ice crystal surface, grain boundaries, liquid brines, and various types of particles (organic, sea salt, mineral, etc.). One expects chemical reactivity to be different in all these media so that a preliminary question before assigning a rate or photolysis coefficient to a reactive process is: where are the reactants? Our discussion of NO3− indicates that it may even depend on snow bulk composition and on other variables such as temperature and atmospheric composition, so that the answer may not be simple and unique. An inadequate determination of the species environment in snow can have dramatic consequences on the understanding of its reactivity, and this problem can also manifest itself in the design of laboratory experiments, where the method of sample preparation can result in different reactant locations. This is well illustrated by the case of NO3−. Early studies of NO3− photolysis138−140 have prepared samples by rapidly freezing liquid solutions of NO3− salts, but because of the lower solubility of species in ice than in water, rapid freezing leads to solute exclusion and to its accumulation at the water−ice interface. Furthermore, ice growth instability of the Mullins−Sekerka type141 takes place, with the formation of dendrites and the trapping of pockets of highly concentrated solution between dendrites, as illustrated in Figure 5. In those experiments, it is then very likely that NO3− ended up in liquid pockets, as implied by the results of Wren and Donaldson.104 It is therefore not surprising that NO3− displayed properties similar to those expected from supercooled water (Figure 6). For example, the measured OH quantum yield from UV photolysis of these frozen solutions was 3.9 × 10−3 at 253 K.138−140 When by contrast NO3− was introduced by vapor deposition of HNO3 onto ice, meaning that NO3− was then mostly or totally located on the ice surface, the measured quantum yield was 0.6, i.e., ∼150 times higher.142 Other factors may affect NO3− reactivity. Recent laboratory experiments143,144 have shown that if NO3− is located in brines, similar to those that could be formed by the melting of sea salt particles on ice surfaces, the quantum yield of NO2 is greatly dependent on the composition of the solution. It is increased in NaBr/NaNO3 solutions because NO3− presence near the surface is favored. At room temperature, Richards et al.144 suggest that the NO2 yield could be as high as 1, largely due to NO3− presence near the surface, compared to a value of 0.011 in the absence of NaBr. These effects were found to be very dependent on the solution ionic composition. For example, if K+ replaced Na+ in the solution, the enhancement in NO2 production was not observed. The great variety of ionic compositions observed for snows in coastal and inland areas75 4739



Feature Article

where NO3− has a high photolysis yield, and high yields of the photolysis products NO and NO2 were then observed.145−148 On the contrary, with alkaline snow,105 NO3− was most likely attached to sea salt particles, which melted upon impact on the snow crystals at the mild temperatures of that study, so that NO3− was then in liquid pockets where, depending on their composition, the photolysis quantum yield could be either low or high. The production of the secondary product HONO, the one gaseous nitrogen species measured in coastal Antarctica,105 was much lower than expected. This may be explained by snow composition, which was dominated by sea salt with a significant input from mineral dust. It is reasonable to suggest that the specific composition at the time of that study was such that NO3− photolysis was not very efficient. A photolysis coefficient measured on laboratory ice samples whose conditions of formation do not mimic those of the snow considered may therefore not be used blindly in snow chemistry models. Other studies indicate that the reactivity of molecules on the ice surface is very different from that in or on liquid water. Kahan and Donaldson122 compared the photolysis rates of anthracene and naphthalene on ice and water surfaces. The direct photolysis was much faster (by up to an order of magnitude) at the ice surface, independent of temperature between ∼−5 and −20 °C. In a further study, those authors149 prepared two different types of ice samples. First, ice cubes were made by freezing liquid solutions, so that most of the anthracene was located in liquid pockets within the ice. Then such ice cubes were crushed into granules, so that anthracene from the broken inclusion could diffuse onto the ice surface. Figure 7 shows that the reactivity in the liquid pockets is similar to that in liquid solutions, while photolysis is much faster on the ice surfaces of the granules. Kahan et al.134 went one step further and showed that the reactivity at the air−ice interface could be tuned from the rapid photolysis on the pure ice surface, to the slower rate on water surfaces, by creating a liquid

Figure 5. Processes taking place in the rapid freezing of a 10 ppm NO3− solution, as it may happen in laboratory experiments intended to study NO3− photolysis. NO3− is rejected from the ice crystalline network and accumulates at the freezing front. Rapid growth leads to instabilities and dendrite formation, with the trapping of concentrated NO3− liquid pockets between dendrites. Some NO3− may be incorporated within the ice crystals, under nonequilibrium conditions. Predicting the actual NO3− environments is difficult.

could then lead to highly variable NO2 and NO production efficiencies, which could not be explained by using a single value of NO3− photolysis quantum yield. This is consistent with the impact of snow composition on the yield of photolysis products observed in field studies. In acidic snow, NO3− was probably supplied to snow as HNO3, and part of it was therefore located on ice crystal surfaces,

Figure 6. Temperature dependence of the NO3− photolysis quantum yields measured in laboratory ice samples prepared by freezing NO3− solutions or by vapor deposition of HNO3 onto vapor-deposited ice. The minus, square, plus, and triangle are liquid solution data from Jankowski et al.,182 Warneck and Wurzinger,183 Zellner et al.,184 and Zepp et al.,185 respectively. The circles and cross are solution and frozen solution data from Chu and Anastasio138 and Dubowski et al.,186 respectively. The diamonds are frozen solution data from Dubowski et al.139 The asterisk is adsorbed HNO3 data from Zhu et al.142 4740



Feature Article

Figure 7. Photochemical decay of anthracene present at different aqueous environments. The loss rate on the pure ice surface is identical to that on the granules. Reprinted with permission from ref 149.

performed using concentrations orders of magnitude higher than those found in natural snow, so that the actual behavior of these molecules in snow may be different because selfassociation may not take place at the much lower concentrations of natural snow. Nevertheless, these results show that, for these hydrophobic organic compounds at least, solvation at the air−ice surface is not favored with respect to solvation in a more “organic” environment. Reactant concentrations and the interactions among the various organic and inorganic compounds in their vicinity are therefore another factor to take into account when designing laboratory experiments and when using laboratory results to model snow photochemistry. The conclusion from this section is that, as expected, the reactivity of a compound in snow will be dramatically dependent on its location and also on its concentration. Examples developed here show that the reactivity on the ice surface is very different from that in dilute liquid solutions. It is highly likely that reactivity in particles and in the ice crystalline lattice will also be very different from liquid reactivity, and some reaction rates in the brine liquid droplets expected to form at triple junctions may also be different from those in dilute solutions and those of lower ionic strength. It therefore does not seem possible to model snow photochemistry using the most widely used approach so far, assuming that reactions take place in a liquid layer. Before we examine critically the performance of existing snow photochemistry models and evaluate their claim that they reproduce observations, we wish here to also discuss a crucial aspect of snow photochemistry: the determination of UV and visible light fluxes in snow. This aspect is not always detailed in relevant models, and recent work has detected difficulties that will need to be resolved before a sound treatment of actinic fluxes is achieved. E. Light Fluxes in Snow. Theoretical and experimental studies of interactions between electromagnetic radiation and snow have been performed for decades, with emphasis on energy balance and remote sensing applications.152−155 With regard to photochemistry, the aspect of interest is the transmission of UV and visible radiation through snow. The validity of relevant radiative transfer model, e.g., using the δEddington or the discrete ordinate methods, has been verified

brine layer on the surface. This is shown in Figure 8 in the case of harmine, an aromatic compound chosen because it still

Figure 8. Harmine photolysis at the surface of aqueous salt solutions. The blue symbols show the result at the liquid surface, and the red symbols show the frozen case. With no added salt, the loss rate is slower at the liquid surface than at the frozen surface; this difference vanishes as the increased salt content forces a liquid brine layer to form on the surface (modified from Kahan et al.134).

strongly fluoresces in concentrated ionic solutions. Here, its observed loss rate is shown as a function of the prefreezing salt concentration in solution. At a prefreezing NaCl concentration of ∼200 mM, there is sufficient brine present on the ice surface that the surface effectively acts as a liquid. Together these results indicate (1) that there is a clear difference between the reactivity of the ice and water surfaces and (2) that the mode of preparation of ice samples certainly affects observed reactivity, so that great care must be taken when designing and interpreting laboratory experiments and when applying them to snow modeling. Another factor worth mentioning here is the effect of concentration. Several studies have reported that hydrophobic molecules such as dibenzylketone and PAHs self-associated on the surface to form islands so that the environment of a molecule was essentially concentration-independent in the range studied.122,150,151 However, these experiments were 4741



Feature Article

experimentally,156 so that current theory can in principle be applied to snow photochemistry. Actinic fluxes in snow can be calculated if some physical properties (particularly the specific surface area and density, but grain shape may also have an effect157) and the concentration and optical properties of absorbers in snow are known.8,155 In the UV−vis, snow albedo and actinic fluxes are highly dependent on the concentration and optical properties of absorbing impurities.158 However, difficulties have been recently encountered in calculating reliable actinic fluxes, for example, during the OASIS2009 campaign in Barrow, Alaska. During that campaign, several measurements were made in relation to snow physical, chemical, and optical properties. First, identifiable chromophores were quantified in the snow, including Black Carbon (BC), water-soluble HULIS, and their absorption spectrum,34 as well as the total absorbance of melted filtered snow.111 Second, snow SSA and density were measured.159 Third, optical measurements were made for snow albedo and e-folding depth.160 A snowpack radiative transfer model161 was then used to retrieve from these optical measurements a scattering and an absorption coefficient for the snow. However, BC and HULIS concentrations necessary to reproduce the retrieved absorption coefficient were, respectively, in general 5 and 20 times higher (up to 100 times higher in some cases) than what was measured in the same or similar snowpacks. Several reasons can be found for such high discrepancies. First, the measurements of BC in snow were based on a thermo-optical method,162 which measures elemental carbon independently of its optical properties. In the case of aerosols, this is known to be possibly different from optical BC determination.163 As for the HULIS measurements, they were limited to the water-soluble fraction of HULIS, which could account for some discrepancy. Moreover, absorption per unit mass of absorber in snow has been shown to depend on location,40 with internally mixed BC up to 2.5 times more absorbing per unit mass than externally mixed BC. Finally, even the proper optical properties of absorbing materials such as BC deposited in the snow are not so accurately known and depend on physical characteristics such as size, optical properties of the bulk material, and porosity, as detailed, for example, in Bohren.164 Optical properties of absorbers also depend on their immediate surrounding, as recent studies have shown for BC,165,166 a reference material,167 or individual species such as benzene.151 We must conclude from this section that actinic fluxes in snow in the UV−vis wavelength range cannot yet be calculated reliably because of inadequate characterization of the chemical, optical, and physical properties of absorbers, and of their location in snowpacks, i.e., within or outside snow grains.

and nitrite concentrations measured in photolyzed artificial snow. In a subsequent erratum, Jacobi35 adjusted in a novel manner these five rate constants since the bulk to QLL volume ratio used to convert measured bulk concentrations into modeled QLL concentrations was wrong in the original publication. In both cases, experimental data were reasonably well reproduced, despite a factor of ∼30 between modeled QLL concentrations in the two versions of this work. This illustrates that the experimental data sets used to test snow chemistry models are insufficient to provide enough constraints on the adjusted parameter values. The correct reproduction of the data is therefore entirely due to these adjustments. Thomas et al.14 proceeded in a different way, with an imposed QLL volume and adjustable fractions of reactants between the QLL and the (unreactive) bulk crystal. The nitrate fraction in the QLL is crucial to allow a good reproduction of the data, but the optimum fraction chosen by the authors (∼6% of total NO3− in the QLL) and whose value is required to get a good fit, is inconsistent with other data. For example, it is not consistent with the equilibrium of HNO3 between the gas phase and the liquid layer, and respecting this equilibrium would yield much too low NO values, as shown in their supplement. We also note that the equilibrium of HNO3 between the gas and ice phases102 is not respected either and that HNO3 gas phase values are not well reproduced. The study of Thomas et al.14 therefore allows a good agreement only with selected data, and the adjusted parameter value produces an internal inconsistency in their model. To conclude on this aspect, adjustable parameters allow an acceptable fit to selected data. However, when these adjustable parameters are too numerous,12 the models become underconstrained and without predictive ability. With only a single adjustable parameter,14 only part of the data is reproduced, and this produces internal inconsistencies. More knowledge of each elementary process is needed to build new models and to be able to interpret the disagreements between model and observations in terms of missing or inaccurate processes. More knowledge on the location and physical state of reactants is also essential. Regarding radiative transfer in snowpacks, the main uncertainties arise from poor knowledge of the precise optical properties of absorbers in the snow, such as BC, dust, and brown carbon, by themselves and in the snow environment. A second source of uncertainty relates to the actual location of absorbers in the snowpack and the nature of their immediate surroundings and therefore to the process by which they were incorporated into snow. Aerosol and gases scavenged in a cloud would be expected to be internally mixed, whereas aerosol and gases scavenged below-cloud or dry-deposited to the snowpack would be externally mixed, as would be chromophores originating from mixing with soil. Either way, they may largely be relocated during snow metamorphism, which can be fast enough to mobilize a large fraction of the ice mass.168 B. Are New Models Possible? The task to build a physically based snow model is tremendous. There are about the same number of reactions as in gas phase or liquid phase atmospheric chemistry, except that, here, several phases are present, and some of them, such as organic particles, are not even well characterized. Regarding reactions on ice surfaces, only a small number have been measured.107 Furthermore, measuring a reaction rate coefficient or an absorption spectrum on an ice surface is arguably more difficult than in the gas or liquid phase, for example, because reacting mixtures are more

V. MODELING SNOW PHOTOCHEMISTRY A. Evaluation of Existing Models. Again, current models are based on the assumption that snow chemistry takes place solely in the QLL, which is assumed to cover the surface of snow crystals.12,14 Although this concept appears unrealistic as shown in section IV, these models provide acceptable reproduction of some data, which may be invoked to claim their sound foundation. However, we argue here that several adjustable parameters make such reproduction possible, without enough constraints to conclude as to whether they are correct or not. Bock and Jacobi12 proposed a reaction mechanism for nitrate photochemistry in snow and adjusted five reaction or photolysis rate coefficients to reproduce nitrate 4742



Feature Article

riming has never been reported there, the intervention of the liquid phase can probably be neglected; (3) since this is the place furthest removed from most sources of trace gases, the number of reactants is reduced; and (4) there are a number of bases where extensive atmospheric and snow chemistry field studies have been or are being conducted, such as Dome C178 and South Pole.179 It is clear that, for progress, the simultaneous monitoring of snow and atmospheric composition are in order. Ideally, what is of greatest interest is the composition of snow interstitial air rather than that of the atmosphere above the snow. However, analyzing snow interstitial air remains a challenge, as this requires pumping air from the snow, which inevitably draws air from the atmosphere or other snow layers.180 Novel analytical methods requiring low air flow would definitely assist in a precise identification of processes. In parallel, laboratory studies of reactions taking place on ice surfaces must be undertaken to obtain adequate rate coefficients. It seems essential that these experiments be performed on ice samples that are prepared from vapor deposition and not the freezing of solution, given our previous discussion on the importance of the mode of formation of ice and of reactant location on reactivity. Additional work to determine the partitioning of species between the ice surface and its volume, not only at equilibrium but as a function of the history of ice crystals,21 appears essential. On the Antarctic plateau, NO3− and H2O2 can reasonably be expected to be the main chromophores, which considerably simplifies chemistry, and detailed studies of the reactivity of these species in adequately prepared samples are mandatory. Consistency tests between models, field, and laboratory measurements should be possible for NO3− since unique data on the exchange processes of this ion between the snow and the atmosphere are starting to be available. Data are furthermore constrained by the use of isotopic methods, which identify the processes.181 In summary, an enhanced awareness that snow is a multiphase system (each type of particle present in the snow representing a distinct phase) where the location of reactants is difficult to predict and where the reactivity of reactants dramatically depends on their location is a prerequisite to propose a sound strategy to achieve snow models with a predictive value. The snow cannot be considered as a simple two-phase system: interstitial air and a QLL that is treated as a liquid. We believe the simplest natural system to develop a novel snow modeling approach is the East Antarctic plateau. Building a photochemical model of central Antarctic snow with no adjustable parameter will represent a laudable accomplishment.

difficult to prepare and because nonvolatile reactants cannot be easily sampled for analysis. Other difficulties include (i) understanding the partitioning of a species between the different phases: ice, air, particles, and liquid water when the temperature is near 0 °C; (ii) measuring solid phase processes such as diffusion, solubility, and possibly photolysis and reaction rates; (iii) evaluating the effect of kinetics and the presence of nonequilibrium conditions between phases; and (iv) elucidating reactions in particles, notably organic ones. With regards to the first three aspects, the formation of nonliquid condensed matter such as ice and organic particles with several constituents often leads to nonequilibrium concentrations, for example, because ice crystal growth can be too rapid for adsorbed species that become incorporated in bulk to fully equilibrate.21,169 In ice, equilibration takes place by solid state diffusion, a slow process. Barret et al.97 have shown that, in the case of HCHO, out-of-equilibrium situations persisted for several days, possibly much more. A detailed understanding of the kinetics and thermodynamics of incorporation of species in ice and the possible role of impurities on ice crystal growth from the gas phase are required to correctly implement these aspects into useful models. It does not appear required to list all the other obvious difficulties, but we feel that the likely impact of chemistry within organic particles should be briefly discussed. Again, organic particles are currently thought to host most of the chromophores in snow.34,111 Reactions therefore probably take place in these complex organic mixtures. Despite ongoing progress regarding their composition,93,129,130 the stage where their reactivity can be understood and predicted has not been reached yet. Most models dealing with aerosol chemistry concentrate on gas phase chemistry and transfer of oxidized species to the condensed phase170 but lack a description of organic reactivity in the condensed phase. Some descriptions that exist for organic reactivity in the aqueous phase171 would not be properly adapted for describing the organic reactivity in aerosol inclusions in an ice matrix, for the reasons developed above. Lengthy chemical models containing thousands of lumped reactions have been developed for complex organic mixtures,172 but they assumed a homogeneous phase, and advancing these to incorporate structure and local molecular environments in a nonliquid medium, as is done for some complex chemical and biological systems,173−175 remains a distant goal. Given all this complexity, it is plainly clear that today we are not able to produce a mechanistic snow chemistry model with a predictive capacity. This goal even seems elusive in the near future. Initial developments should logically be devoted to sites where the chemical composition is the least complicated, where extensive measurements can be made, and these field and modeling studies would ideally be complemented by welldesigned laboratory experiments. The simplest natural systems regarding snow chemistry are those with the smallest number of reactant and phases. The greatest difficulty to describe chemical reactivity is arguably caused by organic particles. Study sites where their concentration is minimal should be selected first. The East Antarctic plateau, where concentrations of organics is the lowest on Earth,176 therefore appears as the most promising location for successful modeling of snow chemistry. Other reasons why we recommend this location include (1) the low concentration not only of organic particles but also of most other particle types such as sea salt and dust;177 (2) since

■

AUTHOR INFORMATION

Corresponding Author

*(F.D.) E-mail: fl[email protected]. Notes

The authors declare no competing financial interest. 4743



Feature Article

Biographies

Didier Voisin is an associate Professor at the University of Grenoble and does his research at the Glaciology Laboratory. His main interests are in atmospheric organic compounds, in particular in the aerosol phase. He has developed analytical methods to measure complex organic macromolecules such as HUmic-LIke Substances (HULIS) and has studied them in aerosols and in snow. He has recently become interested in the impact of organic absorbers on the optical budget of snow and takes part in polar field studies.

Florent Domine is currently a senior scientist at Takuvik Laboratory at Université Laval in Quebec City and an adjunct professor in the Department of Chemistry. For the past 20 years, his main interests have been focused on ice and snow chemistry and physics. An important aspect of his work has been to combine measurements of snow chemical composition and of physical properties such as specific surface area in order to understand changes in snow composition and to explain snow reactivity. He has taken part in many polar field studies, including the large ALERT 2000 and Barrow 2009 international campaigns. He has also been actively involved in laboratory studies of the adsorption and diffusion of reactive trace gases on/in ice.

James Donaldson is a Professor in the Department of Chemistry and Professor of Chemistry in the Department of Physical and Environmental Sciences at the University of Toronto. His research interests lie in the physical chemistry of environmental processes, in particular, the heterogeneous chemistry and photochemistry of surfaces exposed to the atmosphere. Using a novel glancing-angle laser spectroscopic probe, his research group has investigated the physicochemical properties of the air−ice interface and the kinetics of chemical reactions taking place there.

■

ACKNOWLEDGMENTS

■

REFERENCES

Many of the ideas and some of the data presented here were possible thanks to polar campaigns for which funds were provided by the French Polar Institute (IPEV) to F.D. The scanning electromicrographs of Figure 1 were obtained at the Crystallography Department of the University of Göttingen, Germany, thanks to Prof. W. F. Kuhs and colleagues. The chemical analyses of Figure 3 were performed in Svalbard by Antonietta Ianniello, from CNR Rome. D.J.D. acknowledges financial support from NSERC and CFCAS for studies on ice surface chemistry.

Josué Bock did his Ph.D. at the Glaciology Laboratory in Grenoble. One of his focuses was the modeling of snow photochemistry. He also did field work in Barrow, Alaska, where he focused on snow physics measurements. He is currently working on models of Antarctic snow chemistry and of ice core interpretation.

(1) Domine, F.; Shepson, P. B. Air−Snow Interactions and Atmospheric Chemistry. Science 2002, 297, 1506−1510. (2) Grannas, A. M.; Jones, A. E.; Dibb, J.; Ammann, M.; Anastasio, C.; Beine, H. J.; Bergin, M.; Bottenheim, J.; Boxe, C. S.; Carver, G.; et al. An Overview of Snow Photochemistry: Evidence, Mechanisms and Impacts. Atmos. Chem. Phys. 2007, 7, 4329−4373. (3) Bottenheim, J. W.; Netcheva, S.; Morin, S.; Nghiem, S. V. Ozone in the Boundary Layer Air over the Arctic Ocean: Measurements during the TARA Transpolar Drift 2006−2008. Atmos. Chem. Phys. 2009, 9, 4545−4557. (4) Ariya, P. A.; Dastoor, A. P.; Amyot, M.; Schroeder, W. H.; Barrie, L.; Anlauf, K.; Raofie, F.; Ryzhkov, A.; Davignon, D.; Lalonde, J.; Steffen, A. The Arctic: a Sink for Mercury. Tellus, Ser. B 2004, 56, 397−403. (5) Jones, A. E.; Anderson, P. S.; Wolff, E. W.; Roscoe, H. K.; Marshall, G. J.; Richter, A.; Brough, N.; Colwell, S. R. Vertical Structure of Antarctic Tropospheric Ozone Depletion Events: 4744



Feature Article

during the ALERT 2000 Campaign. Atmos. Environ. 2002, 36, 2767− 2777. (26) Magono, C.; Endoh, T.; Ueno, F.; Kubota, S.; Itasaka, M. Direct Observations of Aerosols Attached to Falling Snow Crystals. Tellus 1979, 31, 102−114. (27) Domine, F.; Lauzier, T.; Cabanes, A.; Legagneux, L.; Kuhs, W. F.; Techmer, K.; Heinrichs, T. Snow Metamorphism As Revealed by Scanning Electron Microscopy. Microsc. Res. Tech. 2003, 62, 33−48. (28) Domine, F.; Taillandier, A. S.; Houdier, S.; Parrenin, F.; Simpson, W. R.; Douglas, T. A. In Physics and Chemistry of Ice; Kuhs, W. F., Ed.; Royal Society of Chemistry: Cambridge, U.K., 2007; pp 27−46. (29) Mulvaney, R.; Wolff, E. W.; Oates, K. Sulphuric Acid at Grain Boundaries in Antarctic Ice. Nature 1988, 331, 247−249. (30) Rempel, A. W.; Waddington, E. D.; Wettlaufer, J. S.; Worster, M. G. Possible Displacement of the Climate Signal in Ancient Ice by Premelting and Anomalous Diffusion. Nature 2001, 411, 568−571. (31) Harder, S. L.; Warren, S. G.; Charlson, R. J.; Covert, D. S. Filtering of Air through Snow As a Mechanism for Aerosol Deposition to the Antarctic Ice Sheet. J. Geophys. Res. 1996, 101, 18729−18743. (32) Domine, F.; Sparapani, R.; Ianniello, A.; Beine, H. J. The Origin of Sea Salt in Snow on Arctic Sea Ice and in Coastal Regions. Atmos. Chem. Phys. 2004, 4, 2259−2271. (33) Ketcham, W. M.; Hobbs, P. V. An Experimental Determination of Surface Energies of Ice. Philos. Mag. 1969, 19, 1161−1173. (34) Voisin, D.; Jaffrezo, J.-L.; Houdier, S.; Barret, M.; Cozic, J.; King, M. D.; France, J. L.; Reay, H. J.; Grannas, A.; Kos, G.; Ariya, P. A.; Beine, H. J.; Domine, F. Carbonaceous Species and Humic Like Substances (HULIS) in Arctic Snowpack during OASIS Field Campaign in Barrow. J. Geophys. Res. 2012, 117, D00R19. (35) Jacobi, H. W. Correction to ″Development of a Mechanism for Nitrate Photochemistry in Snow″. J. Phys. Chem. A 2011, 115, 14717− 14719. (36) Liao, W.; Tan, D. 1-D Air-Snowpack Modeling of Atmospheric Nitrous Acid at South Pole during ANTCI 2003. Atmos. Chem. Phys. 2008, 8, 7087−7099. (37) Cho, H.; Shepson, P. B.; Barrie, L. A.; Cowin, J. P.; Zaveri, R. NMR Investigation of the Quasi-Brine Layer in Ice/Brine Mixtures. J. Phys. Chem. B 2002, 106, 11226−11232. (38) Kuo, M. H.; Moussa, S. G.; McNeill, V. F. Modeling Interfacial Liquid Layers on Environmental Ices. Atmos. Chem. Phys. 2011, 11, 9971−9982. (39) Reay, H. J.; France, J. L.; King, M. D. Decreased Albedo, eFolding Depth and Photolytic OH Radical and NO2 Production with Increasing Black Carbon Content in Arctic Snow. J. Geophys. Res. 2012, 117, D00R20. (40) Flanner, M. G.; Liu, X.; Zhou, C.; Penner, J. E.; Jiao, C. Enhanced Solar Energy Absorption by Internally-Mixed Black Carbon in Snow Grains. Atmos. Chem. Phys. 2012, 12, 4699−4721. (41) Faraday, M. On Regelation, and on the Conservation of Force. Philos. Mag. 1859, 17, 162−169. (42) Petrenko, V. F.; Whitworth, R. W. Physics of Ice; Oxford University Press: Oxford, U.K., 1999. (43) Döppenschmidt, A.; Butt, H. J. Measuring the Thickness of the Liquid-Like Layer on Ice Surfaces with Atomic Force Microscopy. Langmuir 2000, 16, 6709−6714. (44) Pittenger, B.; Fain, S. C.; Cochran, M. J.; Donev, J. M. K.; Robertson, B. E.; Szuchmacher, A.; Overney, R. M. Premelting at Ice− Solid Interfaces Studied via Velocity-Dependent Indentation with Force Microscope Tips. Phys. Rev. B 2001, 63, 134102. (45) Dosch, H.; Lied, A.; Bilgram, J. H. Glancing-Angle X-RayScattering Studies Of The Premelting Of Ice Surfaces. Surf. Sci. 1995, 327, 145−164. (46) Furukawa, Y.; Yamamoto, M.; Kuroda, T. Ellipsometric Study of the Ice Surface-Structure Just Below the Melting-Point. J. Phys. 1987, 48, 495−501. (47) Mizuno, Y.; Hanafusa, N. Studies of Surface-Properties of Ice Using Nuclear-Magnetic-Resonance. J. Phys. 1987, 48, 511−517.

Characteristics and Broader Implications. Atmos. Chem. Phys. 2010, 10, 7775−7794. (6) Antony, R.; Thamban, M.; Krishnan, K. P.; Mahalinganathan, K. Is Cloud Seeding in Coastal Antarctica Linked to Bromine and Nitrate Variability in Snow? Environ. Res. Lett. 2010, 5, 014009. (7) Ziemba, L. D.; Dibb, J. E.; Griffin, R. J.; Huey, L. G.; Beckman, P. Observations of Particle Growth at a Remote, Arctic Site. Atmos. Environ. 2010, 44, 1649−1657. (8) Domine, F.; Albert, M.; Huthwelker, T.; Jacobi, H. W.; Kokhanovsky, A. A.; Lehning, M.; Picard, G.; Simpson, W. R. Snow Physics As Relevant to Snow Photochemistry. Atmos. Chem. Phys. 2008, 8, 171−208. (9) McNeill, V. F.; Grannas, A. M.; Abbatt, J. P. D.; Ammann, M.; Ariya, P.; Bartels-Rausch, T.; Domine, F.; Donaldson, D. J.; Guzman, M. I.; Heger, D.; et al. Organics in Environmental Ices: Sources, Chemistry, and Impacts. Atmos. Chem. Phys. 2012, 12, 9653−9678. (10) Dash, J. G.; Rempel, A. W.; Wettlaufer, J. S. The Physics of Premelted Ice and Its Geophysical Consequences. Rev. Mod. Phys. 2006, 78, 695−741. (11) Michalowski, B. A.; Francisco, J. S.; Li, S. M.; Barrie, L. A.; Bottenheim, J. W.; Shepson, P. B. A Computer Model Study of Multiphase Chemistry in the Arctic Boundary Layer during Polar Sunrise. J. Geophys. Res. 2000, 105, 15131−15145. (12) Bock, J.; Jacobi, H. W. Development of a Mechanism for Nitrate Photochemistry in Snow. J. Phys. Chem. A 2010, 114, 1790−1796. (13) Boxe, C. S.; Saiz-Lopez, A. Multiphase Modeling of Nitrate Photochemistry in the Quasi-Liquid Layer (QLL): Implications for NO(x) Release from the Arctic and Coastal Antarctic Snowpack. Atmos. Chem. Phys. 2008, 8, 4855−4864. (14) Thomas, J. L.; Stutz, J.; Lefer, B.; Huey, L. G.; Toyota, K.; Dibb, J. E.; von Glasow, R. Modeling Chemistry in and above Snow at Summit, Greenland - Part 1: Model Description and Results. Atmos. Chem. Phys. 2011, 11, 4899−4914. (15) Colbeck, S. C. On the Micrometeorology of Surface Hoar Growth on Snow in Mountainous Area. Boundary-Layer Meteorol. 1988, 44, 1−12. (16) Podolskiy, E. A.; Nygaard, B. E. K.; Nishimura, K.; Makkonen, L.; Lozowski, E. P. Study of Unusual Atmospheric Icing at Mount Zao, Japan, Using the Weather Research and Forecasting Model. J. Geophys. Res. 2012, 117, D12106. (17) Andreae, M. O.; Rosenfeld, D. Aerosol−Cloud−Precipitation Interactions. Part 1. The Nature and Sources of Cloud-Active Aerosols. Earth-Sci. Rev. 2008, 89, 13−41. (18) Voisin, D.; Legrand, M.; Chaumerliac, N. Scavenging of Acidic Gases (HCOOH, CH3COOH, HNO3, HCl, and SO2) and Ammonia in Mixed Liquid−Solid Water Clouds at the Puy de Dome Mountain (France). J. Geophys. Res. 2000, 105, 6817−6835. (19) Pruppacher, H. R.; Klett, J. D. Microphysics of Clouds and Precipitation, 2nd ed.; Springer: Dordrecht, The Netherlands, 1997. (20) Christner, B. C.; Cai, R.; Morris, C. E.; McCarter, K. S.; Foreman, C. M.; Skidmore, M. L.; Montross, S. N.; Sands, D. C. Geographic, Seasonal, and Precipitation Chemistry Influence on the Abundance and Activity of Biological Ice Nucleators in Rain and Snow. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 18854−18854. (21) Domine, F.; Thibert, E. Mechanism of Incorporation of Trace Gases in Ice Grown from the Gas Phase. Geophys. Res. Lett. 1996, 23, 3627−3630. (22) Barret, M.; Houdier, S.; Domine, F. Thermodynamics of the Formaldehyde−Water and Formaldehyde−Ice Systems for Atmospheric Applications. J. Phys. Chem. A 2011, 115, 307−317. (23) Rango, A.; Foster, J.; Josberger, E. G.; Erbe, E. F.; Pooley, C.; Wergin, W. P. Rime and Graupel: Description and Characterization As Revealed by Low-Temperature Scanning Electron Microscopy. Scanning 2003, 25, 121−131. (24) Iribarne, J. V.; Pyshnov, T. The Effect of Freezing on the Composition of Supercooled Droplets 0.1. Retention of HCl, HNO3, NH3 and H2O2. Atmos. Environ., Part A 1990, 24, 383−387. (25) Cabanes, A.; Legagneux, L.; Domine, F. Evolution of the Specific Surface Area and of Crystal Morphology of Arctic Fresh Snow 4745



Feature Article

(48) Wei, X.; Miranda, P. B.; Shen, Y. R. Surface Vibrational Spectroscopic Study of Surface Melting of Ice. Phys. Rev. Lett. 2001, 86, 1554−1557. (49) Wei, X.; Miranda, P. B.; Zhang, C.; Shen, Y. R. Sum-Frequency Spectroscopic Studies of Ice Interfaces. Phys. Rev. B 2002, 66, 085401. (50) Fletcher, N. H. Surface Structure of Water and Ice 0.2. a Revised Model. Philos. Mag. 1968, 18, 1287−1300. (51) Wettlaufer, J. S.; Worster, M. G.; Wilen, L. A. Premelting Dynamics: Geometry and Interactions. J. Phys. Chem. B 1997, 101, 6137−6141. (52) Ryzhkin, I. A.; Petrenko, V. F. Quasi-Liquid Layer Theory Based on the Bulk First-Order Phase Transition. J. Exp. Theor. Phys. 2009, 108, 68−71. (53) Baker, M. B.; Dash, J. G. Surface Layers on Ice: Comment. J. Geophys. Res. 1996, 101, 12929−12931. (54) Knight, C. A. Surface Layers on Ice. J. Geophys. Res. 1996, 101, 12921−12928. (55) Knight, C. A. Surface Layers on Ice: Reply. J. Geophys. Res. 1996, 101, 12933−12936. (56) Ryzhkin, I. A.; Petrenko, V. F. Violation of Ice Rules near the Surface: A Theory for the Quasiliquid Layer. Phys. Rev. B 2002, 65, 012205. (57) Kahan, T. F.; Reid, J. P.; Donaldson, D. J. Spectroscopic Probes of the Quasi-Liquid Layer on Ice. J. Phys. Chem. A 2007, 111, 11006− 11012. (58) Ardura, D.; Kahan, T. F.; Donaldson, D. J. Self-Association of Naphthalene at the Air−Ice Interface. J. Phys. Chem. A 2009, 113, 7353−7359. (59) Girardet, C.; Toubin, C. Molecular Atmospheric Pollutant Adsorption on Ice: A Theoretical Survey. Surf. Sci. Rep. 2001, 44, 163− 238. (60) Pfalzgraff, W.; Neshyba, S.; Roeselova, M. Comparative Molecular Dynamics Study of Vapor-Exposed Basal, Prismatic, and Pyramidal Surfaces of Ice. J. Phys. Chem. A 2011, 115, 6184−6193. (61) Furukawa, Y.; Nada, H. Anisotropic Surface Melting of an Ice Crystal and Its Relationship to Growth Forms. J. Phys. Chem. B 1997, 101, 6167−6170. (62) Domine, F.; Cabanes, A.; Legagneux, L. Structure, Microphysics, and Surface Area of the Arctic Snowpack near Alert during the ALERT 2000 Campaign. Atmos. Environ. 2002, 36, 2753−2765. (63) Cabanes, A.; Legagneux, L.; Domine, F. Rate of Evolution of the Specific Surface Area of Surface Snow Layers. Environ. Sci. Technol. 2003, 37, 661−666. (64) Akinfiev, N. N.; Mironenko, M. V.; Grant, S. A. Thermodynamic Properties of NaCl Solutions at Subzero Temperatures. J. Solution Chem. 2001, 30, 1065−1080. (65) Weller, R.; Traufetter, F.; Fischer, H.; Oerter, H.; Piel, C.; Miller, H. Postdepositional Losses of Methane Sulfonate, Nitrate, and Chloride at the European Project for Ice Coring in Antarctica DeepDrilling Site in Dronning Maud Land, Antarctica. J. Geophys. Res. 2004, 109, D07301. (66) Hobbs, P. V. Ice Physics; Oxford University Press: New York, 1974. (67) Knight, C. A. An Exploratory Study of Ice-Cube Spikes. J. Glaciol. 2005, 51, 191−200. (68) Blackford, J. R.; Jeffree, C. E.; Noake, D. F. J.; Marmo, B. A. Microstructural Evolution in Sintered Ice Particles Containing NaCl Observed by Low-Temperature Scanning Electron Microscope. Proc. Inst. Mech. Eng., Part L 2007, 221, 151−156. (69) Cullen, D.; Baker, I. Observation of Impurities in Ice. Microsc. Res. Tech. 2001, 55, 198−207. (70) Wettlaufer, J. S.; Worster, M. G. Premelting Dynamics. Ann. Rev. Fluid. Mech. 2006, 38, 427−452. (71) Hutterli, M. A.; Rothlisberger, R.; Bales, R. C. Atmosphere-toSnow-to-Firn Transfer Studies of HCHO at Summit, Greenland. Geophys. Res. Lett. 1999, 26, 1691−1694. (72) Houdier, S.; Perrier, S.; Domine, F.; Cabanes, A.; Legagneux, L.; Grannas, A. M.; Guimbaud, C.; Shepson, P. B.; Boudries, H.; Bottenheim, J. W. Acetaldehyde and Acetone in the Arctic Snowpack

during the ALERT2000 Campaign. Snowpack Composition, Incorporation Processes and Atmospheric Impact. Atmos. Environ. 2002, 36, 2609−2618. (73) Legrand, M.; Deangelis, M. Origins and Variations of Light Carboxylic-Acids in Polar Precipitation. J. Geophys. Res. 1995, 100, 1445−1462. (74) Roberts, J. L.; van Ommen, T. D.; Curran, M. A. J.; Vance, T. R. Methanesulphonic Acid Loss during Ice-Core Storage: Recommendations Based on a New Diffusion Coefficient. J. Glaciol. 2009, 55, 784− 788. (75) Krnavek, L.; Simpson, W. R.; Carlson, D.; Domine, F.; Douglas, T. A.; Sturm, M. The Chemical Composition of Surface Snow in the Arctic: Examining Marine, Terrestrial, and Atmospheric Influences. Atmos. Environ. 2012, 50, 349−359. (76) Maupetit, F.; Delmas, R. J. Snow Chemistry of High-Altitude Glaciers in the French Alps. Tellus, Part B 1994, 46, 304−324. (77) Toom-Sauntry, D.; Barrie, L. A. Chemical Composition of Snowfall in the High Arctic: 1990−1994. Atmos. Environ. 2002, 36, 2683−2693. (78) Beine, H. J.; Anastasio, C.; Domine, F.; Douglas, T.; Barret, M.; France, J.; King, M.; Hall, S.; Ullmann, K. Soluble Chromophores in Marine Snow, Seawater, Sea Ice and Frost Flowers near Barrow, Alaska. J. Geophys. Res. 2012, 117, D00R15. (79) McConnell, J. R.; Bales, R. C.; Stewart, R. W.; Thompson, A. M.; Albert, M. R.; Ramos, R. Physically Based Modelling of Atmosphere-to-Snow-to-Firn Transfer of H2O2 at South Pole. J. Geophys. Res. 1998, 103, 10561−10570. (80) Douglas, T. A.; Sturm, M.; Simpson, W. R.; Blum, J. D.; AlvarezAviles, L.; Keeler, G. J.; Perovich, D. K.; Biswas, A.; Johnson, K. Influence of Snow and Ice Crystal Formation and Accumulation on Mercury Deposition to the Arctic. Environ. Sci. Technol. 2008, 42, 1542−1551. (81) Poulain, A. J.; Lalonde, J. D.; Amyot, M.; Shead, J. A.; Raofie, F.; Ariya, P. A. Redox Transformations of Mercury in an Arctic Snowpack at Springtime. Atmos. Environ. 2004, 38, 6763−6774. (82) Ariya, P. A.; Domine, F.; Kos, G.; Amyot, M.; Cote, V.; Vali, H.; Lauzier, T.; Kuhs, W. F.; Techmer, K.; Heinrichs, T.; Mortazavi, R. Snow: A Photobiochemical Exchange Platform for Volatile and SemiVolatile Organic Compounds with the Atmosphere. Environ. Chem. 2011, 8, 62−73. (83) Meyer, T.; Lei, Y. D.; Wania, F. Measuring the Release of Organic Contaminants from Melting Snow under Controlled Conditions. Environ. Sci. Technol. 2006, 40, 3320−3326. (84) Herbert, B. M. J.; Halsall, C. J.; Jones, K. C.; Kallenborn, R. Field Investigation into the Diffusion of Semi-Volatile Organic Compounds into Fresh and Aged Snow. Atmos. Environ. 2006, 40, 1385−1393. (85) Wolff, E. W.; Legrand, M. R.; Wagenbach, D. Coastal Antarctic Aerosol and Snowfall Chemistry. J. Geophys. Res. 1998, 103, 10927− 10934. (86) Painter, T. H.; Barrett, A. P.; Landry, C. C.; Neff, J. C.; Cassidy, M. P.; Lawrence, C. R.; McBride, K. E.; Farmer, G. L. Impact of Disturbed Desert Soils on Duration of Mountain Snow Cover. Geophys. Res. Lett. 2007, 34, L12502. (87) Deangelis, M.; Legrand, M. Origins and Variations of Fluoride in Greenland Precipitation. J. Geophys. Res. 1994, 99, 1157−1172. (88) Noziere, B.; Dziedzic, P.; Cordova, A. Formation of Secondary Light-Absorbing ″Fulvic-Like’’ Oligomers: A Common Process in Aqueous and Ionic Atmospheric Particles? Geophys. Res. Lett. 2007, 34, L21812. (89) Noziere, B.; Esteve, W. Light-Absorbing Aldol Condensation Products in Acidic Aerosols: Spectra, Kinetics, and Contribution to the Absorption Index. Atmos. Environ. 2007, 41, 1150−1163. (90) Tolocka, M. P.; Jang, M.; Ginter, J. M.; Cox, F. J.; Kamens, R. M.; Johnston, M. V. Formation of Oligomers in Secondary Organic Aerosol. Environ. Sci. Technol. 2004, 38, 1428−1434. (91) Bhatia, M. P.; Das, S. B.; Longnecker, K.; Charette, M. A.; Kujawinski, E. B. Molecular Characterization of Dissolved Organic 4746



Feature Article

Matter Associated with the Greenland Ice Sheet. Geochim. Cosmochim. Acta 2010, 74, 3768−3784. (92) Laskin, J.; Laskin, A.; Roach, P. J.; Slysz, G. W.; Anderson, G. A.; Nizkorodov, S. A.; Bones, D. L.; Nguyen, L. Q. High-Resolution Desorption Electrospray Ionization Mass Spectrometry for Chemical Characterization of Organic Aerosols. Anal. Chem. 2010, 82, 2048− 2058. (93) Nizkorodov, S. A.; Laskin, J.; Laskin, A. Molecular Chemistry of Organic Aerosols through the Application of High Resolution Mass Spectrometry. Phys. Chem. Chem. Phys. 2011, 13, 3612−3629. (94) Noziere, B.; Dziedzic, P.; Cordova, A. Products and Kinetics of the Liquid-Phase Reaction of Glyoxal Catalyzed by Ammonium Ions (NH4+). J. Phys. Chem. A 2009, 113, 231−237. (95) Hodson, A.; Anesio, A. M.; Tranter, M.; Fountain, A.; Osborn, M.; Priscu, J.; Laybourn-Parry, J.; Sattler, B. Glacial Ecosystems. Ecol. Monogr. 2008, 78, 41−67. (96) Amoroso, A.; Domine, F.; Esposito, G.; Morin, S.; Savarino, J.; Nardino, M.; Montagnoli, M.; Bonneville, J. M.; Clement, J. C.; Ianniello, A.; Beine, H. J. Microorganisms in Dry Polar Snow Are Involved in the Exchanges of Reactive Nitrogen Species with the Atmosphere. Environ. Sci. Technol. 2010, 44, 714−719. (97) Barret, M.; Domine, F.; Houdier, S.; Gallet, J. C.; Weibring, P.; Walega, J.; Fried, A.; Richter, A. Formaldehyde in the Alaskan Arctic Snowpack: Partitioning and Physical Processes Involved in Air−Snow Exchanges. J. Geophys. Res. 2011, 116, D00R03. (98) Winkler, A. K.; Holmes, N. S.; Crowley, J. N. Interaction of Methanol, Acetone and Formaldehyde with Ice Surfaces between 198 and 223 K. Phys. Chem. Chem. Phys. 2002, 4, 5270−5275. (99) Hantal, G.; Jedlovszky, P.; Hoang, P. N. M.; Picaud, S. Calculation of the Adsorption Isotherm of Formaldehyde on Ice by Grand Canonical Monte Carlo Simulation. J. Phys. Chem. C 2007, 111, 14170−14178. (100) Haltenorth, H.; Klinger, J. Solubility of Hydrofluoric-Acid in Ice Ih Single-Crystals. Solid State Commun. 1977, 21, 533−535. (101) Thibert, E.; Domine, F. Thermodynamics and Kinetics of the Solid Solution of HCl in Ice. J. Phys. Chem. B 1997, 101, 3554−3565. (102) Thibert, E.; Domine, F. Thermodynamics and Kinetics of the Solid Solution of HNO3 in Ice. J. Phys. Chem. B 1998, 102, 4432− 4439. (103) Domine, F.; Thibert, E.; Silvente, E.; Legrand, M.; Jaffrezo, J. L. Determining Past Atmospheric HCl Mixing Ratios from Ice Core Analyses. J. Atmos. Chem. 1995, 21, 165−186. (104) Wren, S. N.; Donaldson, D. J. Exclusion of Nitrate to the Air− Ice Interface During Freezing. J. Phys. Chem. Lett. 2011, 2, 1967−1971. (105) Beine, H. J.; Amoroso, A.; Domine, F.; King, M. D.; Nardino, M.; Ianniello, A.; France, J. L. Surprisingly Small HONO Emissions from Snow Surfaces at Browning Pass, Antarctica. Atmos. Chem. Phys. 2006, 6, 2569−2580. (106) Hanisch, F.; Crowley, J. N. Heterogeneous Reactivity of Gaseous Nitric Acid on Al2O3, CaCO3, and Atmospheric Dust Samples: A Knudsen Cell Study. J. Phys. Chem. A 2001, 105, 3096− 3106. (107) Crowley, J. N.; Ammann, M.; Cox, R. A.; Hynes, R. G.; Jenkin, M. E.; Mellouki, A.; Rossi, M. J.; Troe, J.; Wallington, T. J. Evaluated Kinetic and Photochemical Data for Atmospheric Chemistry: Volume VHeterogeneous Reactions on Solid Substrates. Atmos. Chem. Phys. 2010, 10, 9059−9223. (108) Honrath, R. E.; Peterson, M. C.; Guo, S.; Dibb, J. E.; Shepson, P. B.; Campbell, B. Evidence of NOx Production within or upon Ice Particles in the Greenland Snowpack. Geophys. Res. Lett. 1999, 26, 695−698. (109) Anastasio, C.; Robles, T. Light Absorption by Soluble Chemical Species in Arctic and Antarctic Snow. J. Geophys. Res. 2007, 112, D24304. (110) Domine, F.; Gallet, J. C.; Barret, M.; Houdier, S.; Voisin, D.; Douglas, T.; Blum, J. D.; Beine, H.; Anastasio, C. The Specific Surface Area and Chemical Composition of Diamond Dust near Barrow, Alaska. J. Geophys. Res. 2011, 116, D00R06.

(111) Beine, H. J.; Anastasio, C.; Esposito, G.; Patten, K.; Wilkening, E.; Domine, F.; Voisin, D.; Barret, M.; Houdier, S.; Hall, S. Soluble, Light-Absorbing Species in Snow at Barrow, Alaska. J. Geophys. Res. 2011, 116, D00R05. (112) Graber, E. R.; Rudich, Y. Atmospheric HULIS: How HumicLike Are They? A Comprehensive and Critical Review. Atmos. Chem. Phys. 2006, 6, 729−753. (113) Domine, F.; Houdier, S.; Taillandier, A. S.; Simpson, W. R. Acetaldehyde in the Alaskan Subarctic Snowpack. Atmos. Chem. Phys. 2010, 10, 919−929. (114) Domine, F.; Rey-Hanot, L. Adsorption Isotherms of Acetone on Ice between 193 and 213 K. Geophys. Res. Lett. 2002, 29, 1873. (115) Petitjean, M.; Mirabel, P.; Le Calve, S. Uptake Measurements of Acetaldehyde on Solid Ice Surfaces and on Solid/Liquid Supercooled Mixtures Doped with HNO3 in the Temperature Range 203−253 K. J. Phys. Chem. A 2009, 113, 5091−5098. (116) Abbatt, J. P. D.; Bartels-Rausch, T.; Ullerstam, M.; Ye, T. J. Uptake of Acetone, Ethanol and Benzene to Snow and Ice: Effects of Surface Area and Temperature. Environ. Res. Lett. 2008, 3, 045008. (117) Roth, C. M.; Goss, K. U.; Schwarzenbach, R. P. Sorption of Diverse Organic Vapors to Snow. Environ. Sci. Technol. 2004, 38, 4078−4084. (118) Daly, G. L.; Wania, F. Simulating the Influence of Snow on the Fate of Organic Compounds. Environ. Sci. Technol. 2004, 38, 4176− 4186. (119) Domine, F.; Cincinelli, A.; Bonnaud, E.; Martellini, T.; Picaud, S. Adsorption of Phenanthrene on Natural Snow. Environ. Sci. Technol. 2007, 41, 6033−6038. (120) Herbert, B. M. J.; Villa, S.; Halsall, C. Chemical Interactions with Snow: Understanding the Behavior and Fate of Semi-Volatile Organic Compounds in Snow. Ecotoxicol. Environ. Saf. 2006, 63, 3−16. (121) Hoff, J. T.; Wania, F.; Mackay, D.; Gillham, R. Sorption of Nonpolar Organic Vapors by Ice and Snow. Environ. Sci. Technol. 1995, 29, 1982−1989. (122) Kahan, T. F.; Donaldson, D. J. Photolysis of Polycyclic Aromatic Hydrocarbons on Water and Ice Surfaces. J. Phys. Chem. A 2007, 111, 1277−1285. (123) Simcik, M. F.; Franz, T. P.; Zhang, H. X.; Eisenreich, S. J. GasParticle Partitioning of PCBs and PAHs in the Chicago Urban and Adjacent Coastal Atmosphere: States of Equilibrium. Environ. Sci. Technol. 1998, 32, 251−257. (124) Franz, T. P.; Eisenreich, S. J. Snow Scavenging of Polychlorinated Biphenyls and Polycyclic Aromatic Hydrocarbons in Minnesota. Environ. Sci. Technol. 1998, 32, 1771−1778. (125) Lei, Y. D.; Wania, F. Is Rain or Snow a More Efficient Scavenger of Organic Chemicals? Atmos. Environ. 2004, 38, 3557− 3571. (126) Wania, F.; Hoff, J. T.; Jia, C. Q.; Mackay, D. The Effects of Snow and Ice on the Environmental Behaviour of Hydrophobic Organic Chemicals. Environ. Pollut. 1998, 102, 25−41. (127) Burniston, D. A.; Strachan, W. J. M.; Hoff, J. T.; Wania, F. Changes in Surface Area and Concentrations of Semivolatile Organic Contaminants in Aging Snow. Environ. Sci. Technol. 2007, 41, 4932− 4937. (128) Fu, P.; Kawamura, K.; Barrie, L. A. Photochemical and Other Sources of Organic Compounds in the Canadian High Arctic Aerosol Pollution during Winter-Spring. Environ. Sci. Technol. 2009, 43, 286− 292. (129) Kroll, J. H.; Seinfeld, J. H. Chemistry of Secondary Organic Aerosol: Formation and Evolution of Low-Volatility Organics in the Atmosphere. Atmos. Environ. 2008, 42, 3593−3624. (130) Ng, N. L.; Canagaratna, M. R.; Zhang, Q.; Jimenez, J. L.; Tian, J.; Ulbrich, I. M.; Kroll, J. H.; Docherty, K. S.; Chhabra, P. S.; Bahreini, R.; Murphy, S. M.; Seinfeld, J. H.; Hildebrandt, L.; Donahue, N. M.; DeCarlo, P. F.; Lanz, V. A.; Prevot, A. S. H.; Dinar, E.; Rudich, Y.; Worsnop, D. R. Organic Aerosol Components Observed in Northern Hemispheric Datasets from Aerosol Mass Spectrometry. Atmos. Chem. Phys. 2010, 10, 4625−4641. 4747



Feature Article

(131) Hoose, C.; Kristjansson, J. E.; Burrows, S. M. How Important Is Biological Ice Nucleation in Clouds on a Global Scale? Environ. Res. Lett. 2010, 5, 024009. (132) Poeschl, U.; Martin, S. T.; Sinha, B.; Chen, Q.; Gunthe, S. S.; Huffman, J. A.; Borrmann, S.; Farmer, D. K.; Garland, R. M.; Helas, G.; Jimenez, J. L.; King, S. M.; Manzi, A.; Mikhailov, E.; Pauliquevis, T.; Petters, M. D.; Prenni, A. J.; Roldin, P.; Rose, D.; Schneider, J.; Su, H.; Zorn, S. R.; Artaxo, P.; Andreae, M. O. Rainforest Aerosols As Biogenic Nuclei of Clouds and Precipitation in the Amazon. Science 2010, 329, 1513−1516. (133) Koehler, K. A.; DeMott, P. J.; Kreidenweis, S. M.; Popovicheva, O. B.; Petters, M. D.; Carrico, C. M.; Kireeva, E. D.; Khokhlova, T. D.; Shonija, N. K. Cloud Condensation Nuclei and Ice Nucleation Activity of Hydrophobic and Hydrophilic Soot Particles. Phys. Chem. Chem. Phys. 2009, 11, 7906−7920. (134) Kahan, T. F.; Kwamena, N. O. A.; Donaldson, D. J. Different Photolysis Kinetics at the Surface of Frozen Freshwater vs. Frozen Salt Solutions. Atmos. Chem. Phys. 2010, 10, 10917−10922. (135) Sokolov, O.; Abbatt, J. P. D. Adsorption to Ice of n-Alcohols (Ethanol to 1-Hexanol), Acetic Acid, and Hexanal. J. Phys. Chem. A 2002, 106, 775−782. (136) Jedlovszky, P.; Hantal, G.; Neurohr, K.; Picaud, S.; Hoang, P. N. M.; von Hessberg, P.; Crowley, J. N. Adsorption Isotherm of Formic Acid on the Surface of Ice, As Seen from Experiments and Grand Canonical Monte Carlo Simulation. J. Phys. Chem. C 2008, 112, 8976−8987. (137) von Hessberg, P.; Pouvesle, N.; Winkler, A. K.; Schuster, G.; Crowley, J. N. Interaction of Formic and Acetic Acid with Ice Surfaces between 187 and 227 K. Investigation of Single Species- and Competitive Adsorption. Phys. Chem. Chem. Phys. 2008, 10, 2345− 2355. (138) Chu, L.; Anastasio, C. Quantum Yields of Hydroxyl Radical and Nitrogen Dioxide from the Photolysis of Nitrate on Ice. J. Phys. Chem. A 2003, 107, 9594−9602. (139) Dubowski, Y.; Colussi, A. J.; Hoffmann, M. R. Nitrogen Dioxide Release in the 302 nm Band Photolysis of Spray-Frozen Aqueous Nitrate Solutions. Atmospheric Implications. J. Phys. Chem. A 2001, 105, 4928−4932. (140) Honrath, R. E.; Guo, S.; Peterson, M. C.; Dziobak, M. P.; Dibb, J. E.; Arsenault, M. A. Photochemical Production of Gas Phase NOx from Ice Crystal NO3. J. Geophys. Res. 2000, 105, 24183−24190. (141) Mullins, W. W.; Sekerka, R. F. Morphological Stability of a Particle Growing by Diffusion or Heat Flow. J. Appl. Phys. 1963, 34, 323−329. (142) Zhu, C. Z.; Xiang, B.; Chu, L. T.; Zhu, L. 308 nm Photolysis of Nitric Acid in the Gas Phase, on Aluminum Surfaces, and on Ice Films. J. Phys. Chem. A 2010, 114, 2561−2568. (143) Richards, N. K.; Finlayson-Pitts, B. J. Production of Gas Phase NO2 and Halogens from the Photochemical Oxidation of Aqueous Mixtures of Sea Salt and Nitrate Ions at Room Temperature. Environ. Sci. Technol. 2012, 46, 10447−10454. (144) Richards, N. K.; Wingen, L. M.; Callahan, K. M.; Nishino, N.; Kleinman, M. T.; Tobias, D. J.; Finlayson-Pitts, B. J. Nitrate Ion Photolysis in Thin Water Films in the Presence of Bromide Ions. J. Phys. Chem. A 2011, 115, 5810−5821. (145) Beine, H. J.; Domine, F.; Simpson, W.; Honrath, R. E.; Sparapani, R.; Zhou, X. L.; King, M. Snow-Pile and Chamber Experiments during the Polar Sunrise Experiment ’Alert 2000’: Exploration of Nitrogen Chemistry. Atmos. Environ. 2002, 36, 2707− 2719. (146) Beine, H. J.; Honrath, R. E.; Domine, F.; Simpson, W. R.; Fuentes, J. D. NOx during Background and Ozone Depletion Periods at Alert: Fluxes above the Snow Surface. J. Geophys. Res. 2002, 107, 4584. (147) Davis, D.; Nowak, J. B.; Chen, G.; Buhr, M.; Arimoto, R.; Hogan, A.; Eisele, F.; Mauldin, L.; Tanner, D.; Shetter, R.; Lefer, B.; McMurry, P. Unexpected High Levels of NO Observed at South Pole. Geophys. Res. Lett. 2001, 28, 3625−3628.

(148) Jones, A. E.; Weller, R.; Anderson, P. S.; Jacobi, H. W.; Wolff, E. W.; Schrems, O.; Miller, H. Measurements of NOx Emissions from the Antarctic Snowpack. Geophys. Res. Lett. 2001, 28, 1499−1502. (149) Kahan, T. F.; Zhao, R.; Jumaa, K. B.; Donaldson, D. J. Anthracene Photolysis in Aqueous Solution and Ice: Photon Flux Dependence and Comparison of Kinetics in Bulk Ice and at the Air− Ice Interface. Environ. Sci. Technol. 2010, 44, 1302−1306. (150) Kurkova, R.; Ray, D.; Nachtigallova, D.; Klan, P. Chemistry of Small Organic Molecules on Snow Grains: The Applicability of Artificial Snow for Environmental Studies. Environ. Sci. Technol. 2011, 45, 3430−3436. (151) Kahan, T. F.; Donaldson, D. J. Benzene Photolysis on Ice: Implications for the Fate of Organic Contaminants in the Winter. Environ. Sci. Technol. 2010, 44, 3819−3824. (152) Hall, D. K.; Sturm, M.; Benson, C. S.; Chang, A. T. C.; Foster, J. L.; Garbeil, H.; Chacho, E. Passive microwave remote and insitu measurements of Arctic and sub-arctic snow covers in Alaska. Remote Sens. Environ. 1991, 38, 161−172. (153) Fily, M.; Bourdelles, B.; Dedieu, J. P.; Sergent, C. Comparison of in Situ and Landsat Thematic Mapper Derived Snow Grain Characteristics in the Alps. Remote Sens. Environ. 1997, 59, 452−460. (154) Warren, S. G. Optical-Properties of Snow. Rev. Geophys. 1982, 20, 67−89. (155) Kokhanovsky, A. A.; Zege, E. P. Scattering Optics of Snow. Appl. Opt. 2004, 43, 1589−1602. (156) Phillips, G. J.; Simpson, W. R. Verification of Snowpack Radiation Transfer Models Using Actinometry. J. Geophys. Res. 2005, 110, D08306. (157) Picard, G.; Arnaud, L.; Domine, F.; Fily, M. Determining Snow Specific Surface Area from near-Infrared Reflectance Measurements: Numerical Study of the Influence of Grain Shape. Cold Reg. Sci. Technol. 2009, 56, 10−17. (158) Warren, S. G.; Wiscombe, W. J. A Model for the Spectral Albedo of Snow 0.2. Snow Containing Atmospheric Aerosols. J. Atmos. Sci. 1980, 37, 2734−2745. (159) Domine, F.; Gallet, J.-C.; Bock, J.; Morin, S. Structure, Specific Surface Area and Thermal Conductivity of the Snowpack around Barrow, Alaska. J. Geophys. Res. 2012, 117, D00R14. (160) France, J. L.; Reay, H. J.; King, M. D.; Voisin, D.; Jacobi, H. W.; Domine, F.; Beine, H.; Anastasio, C.; MacArthur, A.; Lee-Taylor, J. Hydroxyl Radical and NOx Production Rates, Black Carbon Concentrations and Light-Absorbing Impurities in Snow from Field Measurements of Light Penetration and Nadir Reflectivity of Onshore and Offshore Coastal Alaskan Snow. J. Geophys. Res. 2012, 117, D00R12. (161) Lee-Taylor, J.; Madronich, S. Calculation of Actinic Fluxes with a Coupled Atmosphere-Snow Radiative Transfer Model. J. Geophys. Res. 2002, 107, 4796. (162) Cavalli, F.; Viana, M.; Yttri, K. E.; Genberg, J.; Putaud, J. P. Toward a Standardised Thermal-Optical Protocol for Measuring Atmospheric Organic and Elemental Carbon: the EUSAAR Protocol. Atmos. Meas. Tech. 2010, 3, 79−89. (163) Jeong, C. H.; Hopke, P. K.; Kim, E.; Lee, D. W. The Comparison between Thermal-Optical Transmittance Elemental Carbon and Aethalometer Black Carbon Measured at Multiple Monitoring Sites. Atmos. Environ. 2004, 38, 5193−5204. (164) Bohren, C. F. Applicability of Effective-Medium Theories to Problems of Scattering and Absorption by Nonhomogeneous Atmospheric Particles. J. Atmos. Sci. 1986, 43, 468−475. (165) Gyawali, M.; Arnott, W. P.; Lewis, K.; Moosmueller, H. In Situ Aerosol Optics in Reno, NV, USA during and after the Summer 2008 California Wildfires and the Influence of Absorbing and NonAbsorbing Organic Coatings on Spectral Light Absorption. Atmos. Chem. Phys. 2009, 9, 8007−8015. (166) Adler, G.; Riziq, A. A.; Erlick, C.; Rudich, Y. Effect of Intrinsic Organic Carbon on the Optical Properties of Fresh Diesel Soot. Proc. Natl. Acad. Sci. U.S.A. 2010, 107, 6699−6704. (167) Baumgardner, D.; Popovicheva, O.; Allan, J.; Bernardoni, V.; Cao, J.; Cavalli, F.; Cozic, J.; Diapouli, E.; Eleftheriadis, K.; Genberg, P. 4748



Feature Article

308 and 351 nm in the temperature-range 278−353 K. J. Atmos. Chem. 1990, 10, 411−425. (185) Zepp, R. G.; Hoigne, J.; Bader, H. Nitrate-induced photooxidation of trace organic-chemicals in water. Environ. Sci. Technol. 1987, 21, 443−450. (186) Dubowski, Y.; Colussi, A. J.; Boxe, C.; Hoffmann, M. R. Monotonic increase of nitrite yields in the photolysis of nitrate in ice and water between 238 and 294 K. J. Phys. Chem. A 2002, 106, 6967− 6971.

J.; et al. Soot Reference Materials for Instrument Calibration and Intercomparisons: A Workshop Summary with Recommendations. Atmos. Meas. Tech. 2012, 5, 1869−1887. (168) Pinzer, B. R.; Schneebeli, M.; Kaempfer, T. U. Vapor Flux and Recrystallization during Dry Snow Metamorphism under a Steady Temperature Gradient As Observed by Time-Lapse Micro-Tomography. Cryosphere 2012, 6, 1141−1155. (169) Domine, F.; Rauzy, C. Influence of the Ice Growth Rate on the Incorporation of Gaseous HCl. Atmos. Chem. Phys. 2004, 4, 2513− 2519. (170) Aumont, B.; Valorso, R.; Mouchel-Vallon, C.; Camredon, M.; Lee-Taylor, J.; Madronich, S. Modeling SOA Formation from the Oxidation of Intermediate Volatility n-Alkanes. Atmos. Chem. Phys. 2012, 12, 7577−7589. (171) Tilgner, A.; Herrmann, H. Radical-Driven Carbonyl-to-Acid Conversion and Acid Degradation in Tropospheric Aqueous Systems Studied by CAPRAM. Atmos. Environ. 2010, 44, 5415−5422. (172) Domine, F.; Bounaceur, R.; Scacchi, G.; Marquaire, P. M.; Dessort, D.; Pradier, B.; Brevart, O. Up to What Temperature Is Petroleum Stable? New Insights from a 5200 Free Radical Reactions Model. Org. Geochem. 2002, 33, 1487−1499. (173) Singh, R. N.; Kumar, A.; Tiwari, R. K.; Rawat, P.; Baboo, V.; Verma, D. Molecular Structure, Heteronuclear Resonance Assisted Hydrogen Bond Analysis, Chemical Reactivity and First Hyperpolarizability of a Novel Ethyl-4-{(2,4-dinitrophenyl)-hydrazon -ethyl}-3,5-dimethyl-1H-pyrrole-2-carboxylate: A Combined DFT and AIM Approach. Spectrochim. Acta, Part A 2012, 92, 295−304. (174) McGinnis, J. L.; Dunkle, J. A.; Cate, J. H. D.; Weeks, K. M. The Mechanisms of RNA SHAPE Chemistry. J. Am. Chem. Soc. 2012, 134, 6617−6624. (175) Fertinger, C.; Hessenauer-Ilicheva, N.; Franke, A.; van Eldik, R. Direct Comparison of the Reactivity of Model Complexes for Compounds 0, I, and II in Oxygenation, Hydrogen-Abstraction, and Hydride-Transfer Processes. Chem.Eur. J. 2009, 15, 13435−13440. (176) Preunkert, S.; Legrand, M.; Stricker, P.; Bulat, S.; Alekhina, I.; Petit, J. R.; Hoffmann, H.; May, B.; Jourdain, B. Quantification of Dissolved Organic Carbon at Very Low Levels in Natural Ice Samples by a UV-Induced Oxidation Method. Environ. Sci. Technol. 2011, 45, 673−678. (177) Udisti, R.; Dayan, U.; Becagli, S.; Busetto, M.; Frosini, D.; Legrand, M.; Lucarelli, F.; Preunkert, S.; Severi, M.; Traversi, R.; Vitale, V. Sea Spray Aerosol in Central Antarctica. Present Atmospheric Behaviour and Implications for Paleoclimatic Reconstructions. Atmos. Environ. 2012, 52, 109−120. (178) Preunkert, S.; Ancellet, G.; Legrand, M.; Kukui, A.; Kerbrat, M.; Sarda-Esteve, R.; Gros, V.; Jourdain, B. Oxidant Production over Antarctic Land and Its Export (OPALE) project: An Overview of the 2010−2011 Summer Campaign. J. Geophys. Res. 2012, 117, D15307. (179) Eisele, F.; Davis, D. D.; Helmig, D.; Oltmans, S. J.; Neff, W.; Huey, G.; Tanner, D.; Chen, G.; Crawford, J.; Arimoto, R.; Buhr, M.; Mauldin, L.; Hutterli, M.; Dibb, J.; Blake, D.; Brooks, S. B.; Johnson, B.; Roberts, J. M.; Wang, Y. H.; Tan, D.; Flocke, F. Antarctic Tropospheric Chemistry Investigation (ANTCI) 2003 Overview. Atmos. Environ. 2008, 42, 2749−2761. (180) Albert, M. R.; Shultz, E. F. Snow and Firn Properties and Air− Snow Transport Processes at Summit, Greenland. Atmos. Environ. 2002, 36, 2789−2797. (181) Frey, M. M.; Savarino, J.; Morin, S.; Erbland, J.; Martins, J. M. F. Photolysis Imprint in the Nitrate Stable Isotope Signal in Snow and Atmosphere of East Antarctica and Implications for Reactive Nitrogen Cycling. Atmos. Chem. Phys. 2009, 9, 8681−8696. (182) Jankowski, J. J.; Kieber, D. J.; Mopper, K. Nitrate and nitrite ultraviolet actinometers. Photochem. Photobiol. 1999, 70, 319−328. (183) Warneck, P.; Wurzinger, C. Product quantum yields for the 305-nm photodecomposition of NO3− in aqueous-solution. J. Phys. Chem. 1988, 92, 6278−6283. (184) Zellner, R.; Exner, M.; Herrmann, H. Absolute OH quantum yields in the laser photolysis of nitrate, nitrite and dissolved H2O2 at 4749


Can We Model Snow Photochemistry? Problems ... - ACS Publications

Recommend Documents