Nitrate Photolysis in Salty Snow - The Journal of Physical Chemistry A

Sep 21, 2016 - Here, we report a systematic study of the amounts of NO, NO2, and total nitrogen oxides (NOy) emitted from illuminated snow samples as ...
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Nitrate Photolysis in Salty Snow Karen J. Morenz,† Qianwen Shi,‡ Jennifer G. Murphy,† and D. James Donaldson*,†,‡ †

Department of Chemistry and ‡Department of Physical and Environmental Sciences, University of Toronto, 80 Saint George Street, Toronto, Ontario M5S 3H6, Canada S Supporting Information *

ABSTRACT: Nitrate photolysis from snow can have a significant impact on the oxidative capacity of the local atmosphere, but the factors affecting the release of gas-phase products are not well understood. Here, we report a systematic study of the amounts of NO, NO2, and total nitrogen oxides (NOy) emitted from illuminated snow samples as a function of both nitrate and total salt (NaCl and Instant Ocean) concentration. The results provide experimental evidence that the release of nitrogen oxides to the gas phase is directly related to the expected nitrate concentration in the brine at the surface of the snow crystals. With no added salts, steady-state release of gas-phase products increases to a plateau value with increasing prefreezing nitrate concentration; with the addition of salts, the steady-state gasphase nitrogen oxides generally decrease with increasing prefreezing NaCl or Instant Ocean concentration. In addition, for these frozen mixed nitrate (25 mM)−salt (0−500 mM) solutions, there is an increase in gas-phase NO2 seen at low added salt amounts, with NO2 production enhanced by up to 42% at low prefreezing [NaCl] (≤25 mM) and by up to 89% at prefreezing Instant Ocean concentrations lower than 200 mM [Cl−]. This enhancement may be important to the atmospheric oxidative capacity in polar regions.

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INTRODUCTION With a rapidly changing and increasingly human-impacted Arctic, understanding the impact of snow and ice chemistry on local atmospheric processes is becoming of great importance. Several recent reviews1−4 outline our level of understanding of air−ice−snow interactions, as well as the unknowns still to be understood. Snow can act as a photochemical reactor; it is a porous, high-surface area medium that can contain photoactive contaminants.1,2,5,6 Nitrate photolysis from snow has been shown to be an important factor in the oxidative capacity of the atmosphere in pristine arctic regions, according to reactions depicted in Scheme 1, especially since gas-phase products of

In aqueous solutions, ions and other impurities are thought to be excluded from the growing ice crystal into a liquid (or liquid-like) brine layer of small volume and thus very high salt concentration. Although most models and many experiments assume such a liquid-like region exists solely at the air−ice boundary, exclusion also takes place into liquid “pockets”, which are generally thought to be formed at boundaries between single crystals.1,3,9−12 The salt-water phase diagrams that are used to predict the liquid content and concentration of frozen salt solutions are based solely on bulk thermodynamics and are silent regarding the location of the liquid portion of the total frozen solution.1 Nevertheless, when exclusion takes place to the air−ice boundary, there is ready access of the excluded species to the atmosphere and so gas-phase products of any reactions may readily escape.7,11 In the case of nitrate anion photochemistry, the hypothesis that chemistry takes place in a liquid region with access to the atmosphere is supported by the observation that there is no discontinuity in NO2 and NO2− production rates in water/ice at the freezing point of water, indicating that the photolysis of nitrate continues to be in a liquid-phase medium.15,16 Atmospheric models which include photochemical nitrogen oxide release from snowpacks have assumed the relevant nitrate concentration to be that which is predicted to exist in the “brine layer” excluded from the freezing aqueous matrix.17 However, this assumption has not been convincingly demonstrated experimentally, and several studies have shown that the exclusion of salts from the ice crystal is not as simple as a

Scheme 1. Reactions Relating to Nitrate Photolysis and Its Impact on Atmospheric Oxidative Capacity

nitrate photolysis are observed in much higher quantities from snow packs than they are from liquid aqueous solutions of comparable nitrate concentrations.2,5,7 In fact, gaseous nitrogen oxides and ozone levels similar to smog events in polluted midlatitudes have been measured in the troposphere at the South Pole.8,9 However, the mechanism of nitrate photolysis from snow packs is not well understood,7 nor even is the exact morphology of the surface of frozen fresh or salt waters.1,7,10 © XXXX American Chemical Society

Received: July 4, 2016 Revised: September 21, 2016

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DOI: 10.1021/acs.jpca.6b06685 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A bulk phase diagram predicts, and at least in the case of magnesium nitrate, the brine is less concentrated than expected according to its phase diagram.1,12 Most studies of frozen nitrate photochemistry have focused on gas-phase products arising from otherwise pristine frozen samples.3,13 Studies in frozen mixed salt−nitrate solutions have concentrated on halide activation by the OH product of nitrate photolysis;4,15 little has been reported about how the presence of salts might influence nitrate photochemistry and gas-phase nitrogen oxide production. Some studies have observed that the presence of halide ions, specifically bromide, may enhance the amount of nitrate available at the surface of a liquid solution.18,19 Richards et al.18 noted that increasing NaBr in illuminated aqueous nitrate solution at 298 ± 2 K caused an increase in NO2 and Br2 production. On the basis of molecular dynamics simulations, they suggest that this is because bromide will pull sodium ions to the surface, in turn pulling nitrate ions to the surface. Subsequently, Hong et al.19 corroborated this using glancing angle Raman spectroscopy to demonstrate an increase in nitrate concentration in the water surface region when bromide was also present in solution. However, it is not known if the presence of halides in prefreezing solutions will enhance the amount of nitrate available at the surface for photochemical reaction in frozen samples. In the following we present a systematic study of the production of gas-phase nitrogen oxides from photolysis of laboratory-made nitrate snow samples, first varying the nitrate concentration (in unbuffered acidic and nominally neutral snows) and subsequently changing the added salt (halide) concentration at fixed nitrate concentration. We find in general that the amounts of gas-phase products follow the nitrate concentration calculated to be in the excluded liquid-like region of the snow. A remarkable increase in the NOx production at lower (environmentally relevant) concentrations of sea salt departs from this general trend.

Snow samples were placed at the bottom of a 200 mL double-jacketed glass reaction chamber that could be illuminated through a quartz window at the top. The temperature of the experiment (261 or 253 K) was maintained by flowing coolant through the jacket using a recirculating chiller; the snow temperature was directly measured using a thermometer to be within 2 °C of the coolant temperature. To test for any effect of annealing, some samples were allowed to equilibrate to the chamber temperature for 1 h; no difference in results was seen between such samples and those equilibrated for only 10 min. Illumination was provided by a 1 kW xenon arc lamp, which roughly mimics actinic solar radiation, which was passed through a 20 cm water bath to filter out IR, then reflected downward through the snow sample. “Zero” air flowed from the bottom of the snow chamber at 200 sccm, after passing through a humidifier containing frozen deionized water at the same temperature as the reaction vessel. After passing through the snow sample, the air flow containing gas-phase products was taken from the top of the chamber and sent to a dual-channel chemiluminescence instrument (Air Quality Design Inc.) for detection of NO and NO2 (Channel 1) and NOy (total nitrogen oxides, Channel 2). Channel 1 operated as an NO detector, with NO2 detected via the increase in signal obtained following selective photolysis of the input air sample. Channel 2 used a heated Mo catalyst to transform NOy to NO for detection. The conversion efficiencies of both channels were measured before each experiment. The output from the chemiluminescence detector was sent to a laboratory computer for analysis. See the Supporting Information (SI) for a diagram of the set up as well as a full description of the detection method. An experiment consisted of loading the snow chamber and measuring the signal in each of the NO, NO2, and NOy channels in the dark until a stable reading was obtained. The signal was always measured in cycles, where gas coming from the snow chamber was measured for 2 min; then a background was established for the following 30 s of each measurement cycle by measuring the signal in the absence of chemiluminescence. After a stable reading was achieved from the snow chamber in darkness, the lamp was allowed to illuminate the snow sample from above, and the gas-phase concentrations were monitored for 30 min, at which time the lamp was blocked and the dark signal recovered. Figure S1 in the SI illustrates a typical experimental run. The NO, NO2, and NOy production from each sample was calculated by averaging the readings from at least the final four channel readout cycles using that day’s calibration data and sample mass. The possible production of NO from NO2 photolysis in the snow chamber and its headspace was examined by mixing 20 sccm of NO2 gas (6 ppm) with 180 sccm of zero air from the humidifier and flowing this mixture through the snow chamber packed with 25 g of snow made with DI water at 261 K. The difference between the illuminated and the dark NO2 amounts reaching the detector was 20%; this was roughly consistent with the concurrent increase in NO. Details are provided in Table S3 of the SI.

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EXPERIMENTAL SECTION All snow precursor solutions were prepared by serial dilution from stock solutions prepared by dissolving a measured mass of the relevant salt(s) or acid in ultrapure deionized water. Sodium nitrate (Sigma-Aldrich), sodium chloride (A.C.P.), and nitric acid (A.S.P.) were of reagent grade and used without purification. Instant Ocean (Spectrum Brands) has composition: chloride (47.53 wt %), sodium (26.45 wt %), sulfate (6.41 wt %), magnesium (3.19 wt %), calcium (1.00 wt %), potassium (0.952 wt %), bicarbonate (0.356 wt %), and bromide (0.16 wt %).20 Solutions were not buffered, and the pH was not measured for any. Following Wren et al.,21 we expect the pH of the sodium nitrate solutions to lie between 5 and 6. Snow was formed in a similar manner to that reported in Wren et al.21 by spraying the solutions through a spritzer bottle cap into a Dewar flask filled with liquid nitrogen and then collected by scooping out the resulting frozen imperfect planar crystals, which were approximately 1 × 1 mm as judged by visual inspection. Samples and blanks were stored in plastic bottles at 263 K for at least 20 h before measurement, during which time the snow crystals fused to neighboring crystals to create porous clumps of snow, varying in size. Each sample (of approximately 25 g) was weighed before being transferred to the reaction chamber, and larger clumps of snow were physically broken up in order to be able to transfer the sample from the storage bottle to the reaction chamber.

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RESULTS AND DISCUSSION a. Dependence of Gas-Phase Products on Prefreezing Nitrate Concentration in Neutral and Acidic NitrateOnly Snows. We find that the overall production of gas-phase nitrogen oxides has a broadly similar dependence on nitrate concentration for both sodium nitrate and nitric acid precursors B

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Figure 1. Experimentally measured production of (a) NO, (b) NO2, and (c) NOy as a function of prefreezing nitrate concentration from illuminated NaNO3- and HNO3-containing snow at 261 K. Error bars indicate standard error, n ≥ 4 for all nitrate concentrations except 15 mM, where n ≥ 2. Note the variation in y-axis scales between plots.

exclusively from gas-phase photolysis of NO2 in the headspace of the snow reaction chamber, as this process will also photodissociate the NO2 released by photolysis of nitrate in the acidic snow. We note that the illumination from the Xe lamp used here most probably extended below 300 nm in wavelength, possibly exciting the tail of a strong nitrate absorption feature centered around 200 nm. If this is the case, the large amount of NO seen in near-neutral conditions, which was not observed by Osthoff and co-workers,22,23 could arise from a higher production of NO2− from the shorter wavelength excitation used here. Since nitrite photolysisforming NOis an order of magnitude more favorable than that of nitrate in the wavelength range studied here, this could explain the NaNO3 results. Under acidic conditions, the nitrite is expected to combine very rapidly with protons to form HONO, which will contribute to the high observed NOy, but whose gas-phase photolysis to NO + OH is not favored in the present experimental arrangement. Data presented in Abida and Osthoff22 show significantly less NOy release from basic (pH = 9.5) than acidic (pH = 4.5) snow, with NO2 comprising the major component in both cases. A somewhat earlier report from the same group23 showed that, under strongly acidic conditions (pH = 2.5), the NO2 fraction decreased to about one-third of the NOy, with a small amount (12%) of NO, and significant release of other nitrogen oxides (NOz = NOy − NOx), including HONO, HO2NO2, and HOONO.23 In our experiments, the presence of NOz (perhaps including HONO formed by reaction 5 in Scheme 1 or via gasphase recombination of NO + OH) is suggested by the observed concentration of NOy being larger than that of NOx. HONO is not expected to suffer the same extent of gas-phase

at both temperatures investigated. Figure 1 displays the gasphase concentrations of (a) NO, (b) NO2, and (c) NOy as a function of prefreezing [NaNO3] and [HNO3] obtained for snow illumination at 261 K. The corresponding measurements at 253 K are presented in Figure S2 of the SI. In all cases, the amounts of NO2 and NOy evolved increase with increasing prefreezing nitrate up to a plateau reached at about 25 mM prefreezing [NO3−]. Also, in all cases, the total amount of nitrogen oxides (NOy) is greater than NOx (NO + NO2) at all prefreezing nitrate concentrations, and this difference is larger at 261 K than 253 K. In a broad sense, these results are in good agreement with those of Abida and Osthoff,22 who reported nitrogen oxide photoproduction from (mostly) acidic snow samples at −20 °C. Although not discussed by those authors, they present supplementary data that shows that at bulk pH fixed near ∼4.5 the observed gas-phase NO2 and NOy amounts increase to a plateau in a very similar manner as seen here for the acidic samples, with NO2 comprising most, but not all, of the observed NOy. A plot of these data is displayed as Figure S3. In spite of the similarities in total NOy results seen from the nitric acid- and sodium nitrate-doped snows, inspection of Figure 1 shows a striking difference in the observed NO/NOx ratios obtained using the different precursors. The NOx observed from the sodium nitrate snow is predominantly (70−80%) composed of NO; by contrast, the gas-phase nitrogen oxides measured from the acid snow are almost entirely made up of NO2, with a very small and decreasing amount of NO seen as the prefreezing nitrate concentration increases. This observation suggests that the high levels of NO measured in the NaNO3 experiments cannot originate C

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Figure 2. Experimentally measured production of (a) NO, (b) NO2, and (c) NOy as a function of prefreezing nitrate concentration from illuminated NaNO3-containing snow at 261 K. Solid lines show the predicted nitrate concentration in the brine, calculated using eq 1. Error bars indicate standard error, n ≥ 4 for all nitrate concentrations except 15 mM, where n = 2. Note the variation in y-axis scales between plots.

liquid at 261 K from eq 1 allows estimation of the nitrate concentration in the liquid-like region. Then approximating the snow crystals as 1 mm × 1 mm × 0.25 mm rectangular prisms and dividing the liquid amount by the surface area gives an estimate of the maximum brine thickness at the surface of each snow crystal. These results are reported in Table S1 of the SI. Note that these estimates give a maximum bound, as eq 1 assumes ideal solution behavior and that all liquid portions are at the surface of the ice,11 rather than also appearing in interior pockets or grain boundaries.1,2,12 The solid lines displayed in Figure 2 (and in Figure S4 in the SI) show the estimated nitrate concentration in the excluded liquid fraction, as calculated in this manner. In spite of the assumptions behind eq 1, the results shown in Figure 2 suggest that gas-phase nitrogen oxide production is well correlated with the calculated nitrate concentration in the liquid-like region, as evidenced by the very similar dependences on prefreezing nitrate concentration observed, at least up to 50 mM prefreezing concentration. Figure S4 shows that this good agreement holds as well at the lower temperature. b. Nitrogen Oxide Emission from Salty Snows. In order to explore the relationship between nitrogen oxide release and halide concentration as well as to further test the hypothesis that brine concentration is a determining factor in the rate of nitrogen oxide production, snow samples were prepared with a constant 25 mM NaNO3 concentration and varying amounts of sodium chloride or Instant Ocean. Concentrations of added salts between 1 mM and 0.5 M in chloride were used, which roughly corresponds to seawater salt concentration. The influence of varying the prefreezing chloride concentration on

photodissociation as NO2 under illumination from an actiniclike lamp. Since the observed NO cannot arise exclusively from NO2 photolysis in the gas phase (reaction 6 in Scheme 1), we suggest that some fraction of the NO seen in the sodium nitrate experiments must be from reactions 3 and 7 listed above. In the case of the acidic snow, in addition to HONO,13 the NOy may well contain some combination of pernitric, pernitrous, and nitric acids, consistent with Osthoff and co-workers.22,23 Note that we cannot eliminate the possibility that any initially released NO may suffer different degrees of oxidation in the gas phase depending on the composition of the snow sample, also contributing to NOy and altering the observed NO. The concentration of nitrate excluded from the frozen matrix to the interface has been proposed to be a key factor in the rate of production of gas-phase products from nitrate photolysis.1,5,7,10−12,14,15 We test this by comparing the observed gasphase product concentrations to the excluded nitrate concentration, estimated using eq 111,12 as an upper limit to the liquid fraction at the interface, assuming ideal solutions and no liquid inclusions at grain boundaries, etc. φLR =

M H2ORTf TC bulk 1000Hf°(T − Tf )

(1)

Here φLR is the liquid fraction of the total amount of H2O, MH2O is the molar mass of H2O, R is the ideal gas constant, Tf is the freezing temperature of pure water, T is the temperature of the experiment, Cbulk is the concentration of the prefreezing solution, and ΔHf0 is the standard enthalpy of fusion of water. Multiplying the total volume by the fraction expected to be D

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Figure 3. Measured NO (top), NO2 (middle), and NOy (bottom) as a function of prefreezing chloride concentration at constant prefreezing [NO3−] = 25 mM. Solid lines show the predicted brine nitrate concentrations calculated using eq 1: (lleft) data from NaCl-doped samples; (right) Instant Ocean.

added Instant Ocean than is seen with added NaCl, rising to about 90% higher than the no-salt case at 25−50 mM added [Cl−], before slowly diminishing with additional increasing Instant Ocean. In this case, the NO2 levels do not strongly follow the predicted brine nitrate concentration. This greatly increased NO2 dominates the NOx amounts over most of the prefreezing nitrate concentrations studied here and pushes the observed dependence of NOy on [Cl−] to be more slowly varying with added salt than predicted by the nitrate concentration alone. Although the general decrease in NOy with increasing added salt concentrations is consistent with a decreasing nitrate concentration in the liquid-like region of the snow samples, the increase in the NO2 component at low sodium chloride concentrations is striking. The production of gas-phase NO2 from nitrate photolysis in aqueous solution is limited by its hydrolysis16 and also the recombination reaction of NO2 with OH (formed in reactions 1−3 of Scheme 1), reforming nitrate and H+. This limitation is well illustrated by turning “on” and “off” the recombination reaction in a simple box model calculation (see SI for details); a decrease of over an order of magnitude in NO2 emission is predicted due to this recombination reaction and reported in Table S5. However, the recombination may be reduced via scavenging of OH by

gas-phase nitrate photolysis products at 261 K is illustrated in Figure 3 for both salt additives. Unlike the sodium nitrate-only samples, for these salty snows the observed gas-phase NOx amount is roughly equal to that of NOy, although NO is still the dominant contributor to the observed NOx level. A listing of the observed nitrogen oxides as a function of the salt concentrations is provided in Table S2. Figure 3 shows that above about 50 mM chloride concentration, all product amounts decrease with added salt concentration for both the NaCl- and the Instant Ocean-doped snows. For the NaCl-doped snow, good overall agreement is obtained between the amounts of gas-phase nitrogen oxides emitted and the concentration of nitrate in the brine calculated using eq 1, as displayed by the solid lines in the figure. Notably, however, Figure 4 illustrates that at low concentrations of added NaCl (≤10 mM), the observed NO2 increases by up to 42% (at 1 mM NaNO3 in the precursor) over the no-salt case; it then decreases with increasing prefreezing NaCl, closely tracking the calculated nitrate concentration in the brine. The observed NO emitted from Instant Ocean-doped samples shows very similar behavior to that seen from the NaCl-doped samples, closely tracking the predicted excluded nitrate concentration at prefreezing [Cl−] ≥ 10 mM. However, the NO2 concentration increases much more as a function of E

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Figure 4. Percent increase over snow with no added NaCl or Instant Ocean of NO2 as a function of prefreezing chloride concentration at constant prefreezing [NO3−] = 25 mM: (left) data from NaCl-doped samples; (right) Instant Ocean.

presented in Figure 3 suggest that environmentally relevant levels of bromide are effective OH scavengers in excluded brine solutions, even at the higher pH values expected at the frozen seawater surface. This is tested in the box model by the addition of bromide to the chloride−nitrate solution, so that the Br−:Cl− ratio is about 10−3. With the addition of bromide, the calculated NO2 production increases by 20% over that with chloride alone, effectively eliminating the scavenging effect of OH recombination. The final line of Table S5 reports this result. This finding suggests that our understanding of polar boundary layer oxidation chemistry may be deficient. To

other reactive species present in solution. In particular, halide anions may react with OH radicals, as shown in Scheme 2, reducing the concentration of OH available to deplete NO2,18,25 as has been previously suggested.26 This scavenging effect is nicely demonstrated in the box model through the addition of reaction 9, where a chloride concentration of only 4% of the nitrate concentration is sufficient to bring the calculated NO2 up to about 80% of its value in the absence of the recombination reaction as displayed in the third line of Table S5. Consistent with this expectation, in the present experiments, the observed NO2 increases with a small addition of chloride. At the same time, the measured total nitrogen oxidesNOydecreases, demonstrating the overall importance of the diluting effect of increasing brine volume. These results seem fully consistent with the nitrate concentration in the excluded brine playing the dominant role in the amounts of photochemical products emitted to the gas phase, with this being modulated at low chloride concentrations by a chemical effect of increased NO2 formation (or rather decreased NO2 scavenging) in the brine. Thus, we do not find evidence for an enhanced exclusion of nitrate to the snow surface due to the presence of codissolved NaCl in the prefreezing solution. Although the trend in nitrogen oxide release from Instant Ocean-containing snow is generally similar to that seen from NaCl snow, the enhanced NO2 observed from snow with lower Instant Ocean concentrations is much greater than that seen with NaCl alone, causing NO2 to rise to levels close to those observed for NO. While NaCl is the major component of Instant Ocean, Instant Ocean also contains other seawater species, including bicarbonate ions that buffer the pH to about 8 as well as other salts.19 Even though a low pH is known to be important for halide activation from ice,24,22 Wren and Donaldson12 showed that the pH of the brine excluded from frozen Instant Ocean solutions remains close to its prefreezing value of 8. Results from Richards and Finlayson-Pitts27 indicate that solution pH has very little effect on nitrate photolysis between pH 4 and 9. In frozen solutions, Abida and Osthoff22 noted a decrease in NOy production from snow buffered to pH 9.5. Therefore, a different pH environment in the Instant Ocean snow is likely not responsible for the enhanced NO2 production seen at lower salt concentrations. However, Instant Ocean contains a small fraction of bromide ions, approximately 0.15% of the number of chloride ions.19 Although bromide is a trace component, its much more rapid reaction with OH in comparison to chloride (see Scheme 2) causes reaction with bromide to be very favorable.24 This facile reaction is known to selectively promote bromide activation from frozen salty snow, even at very low Br−:Cl− ratios, as long as the pH lies below about 5.21,24 The first step in this activation involves reaction of OH with bromide; the results

Scheme 2. Halide Quenching of OH Radicals

date, most attention has been given to nitrate photolysis in pristine snows and halide activation in salty ice samples, representing sea ice. The latter studies have shown that efficient production of gas-phase halogen species requires pH levels to be lower than 6, eliminating fresh sea ice as an important source of local atmospheric oxidants. The present results show that reactive nitrogen oxides are emitted more readily from sunlit sea ice (or salty snow) at its native pH than they are from saltfree, nonacidic snow samples, thus providing a previously unconsidered extra oxidant source. The impact of this on polar boundary layer chemistry could be significant, because in a warming Arctic environment there is expected to be an increase in the amount of new (first-year) sea ice, higher in salt than aged sea ice, which contains a greater proportion of low-salt snow, at the same time as an expected increased anthropogenic loading of nitrate precursors to that ice.

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CONCLUSIONS We provided experimental evidence that the photolytic production of nitrogen oxides from frozen nitrate solutions is directly dependent on the calculated salt concentration in brine at the surface of the snow. In salt-containing snow samples at low chloride concentrations, nitrogen oxide production is enhanced relative to its level with no salts added, with the enhancement observed using Instant Ocean seawater surrogate approximately twice that seen using pure NaCl. We suggest that chemistry involving the halides present in salt is responsible for the enhancement in the gas-phase products and that the small amount of bromide present in seawater is responsible for the additional enhancement seen for those solutions. These observations provide a strong argument that NaCl solutions F

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(14) Kahan, T. F.; Reid, J. P.; Donaldson, D. J. Spectroscopic Probes of the Quasi-Liquid Layer on Ice. J. Phys. Chem. A 2007, 111 (43), 11006−11012. (15) 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 (20), 4928−4932. (16) Boxe, C. S.; Colussi, A. J.; Hoffmann, M. R.; Murphy, J.G.l; Wooldridge, P. J.; Bertram, T. H.; Cohen, R. C. Photochemical Production and Release of Gaseous NO2 from Nitrate-Doped Water Ice. J. Phys. Chem. A 2005, 109 (38), 8520−8525. (17) Boxe, C. S.; Saiz-Lopez, A. Multiphase Modeling of Nitrate Photochemistry in the Quasi-liquid Layer (QLL): Implications for NOx Release from the Arctic and Coastal Antarctic Snowpack. Atmos. Chem. Phys. 2008, 8, 4855−4864. (18) 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 (23), 5810−5821. (19) Hong, A. C.; Wren, S. N.; Donaldson, D. J. Enhanced Surface Partitioning of Nitrate Anion in Aqueous Bromide Solutions. J. Phys. Chem. Lett. 2013, 4 (17), 2994−2998. (20) Langer, S.; Pemberton, R. S.; Finlayson-Pitts, B. J. Diffuse Reflectance Infrared Studies of the Reaction of Synthetic Sea Salt Mixtures with NO2: A Key Role for Hydrates in the Kinetics and Mechanism. J. Phys. Chem. A 1997, 101 (7), 1277−1286. (21) Wren, S. N.; Donaldson, D. J.; Abbatt, J. P. D. Photochemical Chlorine and Bromine Activation from Artificial Saline Snow. Atmos. Chem. Phys. 2013, 13, 9789−9800. (22) Abida, O.; Osthoff, H. D. On the pH dependence of photoinduced volatilization of nitrogen oxides from frozen solutions containing nitrate. Geophys. Res. Lett. 2011, 38(16).10.1029/ 2011GL048517 (23) Abida, O.; Mielke, L. H.; Osthoff, H. D. Observation of Gasphase Peroxynitrous and Peroxynitric Acid during the Photolysis of Nitrate in Acidified Frozen Solutions. Chem. Phys. Lett. 2011, 511, 187−192. (24) Abbatt, J.; Oldridge, N.; Symington, A.; Chukalovskiy, V.; McWhinney, R. D.; Sjostedt, S.; Cox, R. A. Release of Gas-Phase Halogens by Photolytic Generation of OH in Frozen Halide-Nitrate Solutions: An Active Halogen Formation Mechanism? J. Phys. Chem. A 2010, 114 (23), 6527−6533. (25) Wittmer, J.; Bleicher, S.; Zetzsch, C. Iron(III)-Induced Activation of Chloride and Bromide from Modeled Salt Pans. J. Phys. Chem. A 2015, 119 (19), 4373−4385. (26) Das, R.; Dutta, B. K.; Maurino, V.; Vione, D.; Minero, C. Environ. Chem. Lett. 2009, 7, 337−442. (27) 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 (19), 10447−10454.

should not be assumed to be analogous to seawater in laboratory measurements or models.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.6b06685. Experimental details, supplementary figures and tables; box model results (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +1 416 978 3603. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by grants from NSERC, to whom we are grateful. We thank Dr. Alex Moravek and Angela Hong for technical advice.

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REFERENCES

(1) Dominé, F.; Bock, J.; Voisin, D.; Donaldson, D. J. Can We Model Snow Photochemistry? Problems with the Current Approaches. J. Phys. Chem. A 2013, 117, 4733−4749. (2) George, C.; Ammann, M.; D’Anna, B.; Donaldson, D. J.; Nizkorodov, S. A. Heterogeneous Photochemistry in the Atmosphere. Chem. Rev. 2015, 115 (10), 4218−4258. (3) Bartels-Rausch, T.; Jacobi, H.-W.; Kahan, T. F.; Thomas, J. L.; Thomson, E. S.; Abbatt, J. P. D.; Ammann, M.; Blackford, J. R.; Bluhm, H.; Boxe, C.; et al. A Review of Air-ice Chemical and Physical Interactions (AICI): Liquids, Quasi-liquids, and Solids in Snow. Atmos. Chem. Phys. 2014, 14, 1587−1633. (4) Abbatt, J. P. D.; Thomas, J. L.; Abrahamsson, K.; Boxe, C.; Granfors, A.; Jones, A. E.; King, M. D.; Saiz-Lopez, A.; Shepson, P. B.; Sodeau, J.; et al. Halogen Activation via Interactions with Environmental Ice and Snow in the Polar Lower Troposphere and Other Regions. Atmos. Chem. Phys. 2012, 12, 6237−6271. (5) Dominé, F.; Shepson, P. B. Air-Snow Interactions and Atmospheric Chemistry. Science 2002, 297 (5586), 1506−1510. (6) 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. (7) Marcotte, G.; Marchand, P.; Pronovost, S.; Ayotte, P.; Laffon, C.; Parent, P. Surface-Enhanced Nitrate Photolysis on Ice. J. Phys. Chem. A 2015, 119 (10), 1996−2005. (8) Crawford, J. H.; Davis, D. D.; Chen, G.; Buhr, M.; Oltmans, S.; Weller, R.; Mauldin, L.; Eisele, F.; Shetter, R.; Lefer, B.; Arimoto, R.; Hogan, A. Evidence for Photochemical Production of Ozone at the South Pole Surface. Geophys. Res. Lett. 2001, 28, 3641−3644. (9) Helmig, D.; Johnson, B.; Oltmans, S. J.; Neff, W.; Eisele, F.; Davis, D. D. Elevated Ozone in the Boundary-Layer at South Pole. Atmos. Environ. 2008, 42 (12), 2788−2803. (10) Kahan, T. F.; Wren, S. N.; Donaldson, D. J. A Pinch of Salt Is All It Takes: Chemistry at the Frozen Water Surface. Acc. Chem. Res. 2014, 47, 1587−1594. (11) 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 (43), 11226−11232. (12) Wren, S. N.; Donaldson, D. J. Exclusion of Nitrate to the Air-Ice Interface During Freezing. J. Phys. Chem. Lett. 2011, 2, 1967−1971. (13) Bartels-Rausch, T.; Donaldson, D. J. HONO and NO2 Evolution from Irradiated Nitrate-Doped Ice and Frozen Nitrate Solutions. Atmos. Chem. Phys. Discuss. 2006, 6, 10713−10731. G

DOI: 10.1021/acs.jpca.6b06685 J. Phys. Chem. A XXXX, XXX, XXX−XXX