Exclusion of Nitrate to the Air–Ice Interface During Freezing - The

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Exclusion of Nitrate to the Air
Ice Interface During Freezing Sumi N. Wren and D. J. Donaldson* Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario M5S 3H6, Canada

bS Supporting Information ABSTRACT: During freezing, the majority of solutes are rejected from the growing ice lattice and are concentrated at grain boundaries or nodes within the ice crystal or at the ice crystal surface itself. The degree of solute enrichment as well as the location of the rejected solutes has important consequences for reactions occurring in or on frozen media. We have used glancing-angle Raman spectroscopy to probe the exclusion of nitrate to the air
ice interface during freezing. This work represents the first use of this technique to measure solutes at the ice surface. Our results show that nitrate is excluded to the ice surface but not to the extent predicted by equilibrium thermodynamics. These findings have important implications for understanding the mechanism of snowpack nitrate photolysis. SECTION: Atmospheric, Environmental and Green Chemistry

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hemistry occurring in the snowpack can significantly influence the composition of the overlying boundary layer.1 Fluxes of NOx (NOx = NO + NO2) and HONO have been measured over arctic, antarctic, and midlatitude snowpacks, and it is now well established that chemical processes initiated by the photolysis of snowpack nitrate are responsible. (See Grannas et al.2 and references therein.) Although nitrate snowpack photochemistry has been the subject of several recent laboratory studies,3
10 the mechanism of NOx and HONO production from frozen media is not well understood. The uptake and release of nitrogen oxides from sunlit snow is important because these compounds play an important role in determining the oxidative capacity of the overlying polar boundary layer. Furthermore, because nitrate (NO3
) is ubiquitous in snow, the ice-core nitrate record is used to reconstruct past climate and atmospheric composition.11 A good understanding of the chemical cycling of nitrogen species between the air and ice is therefore required for the accurate interpretation of the ice-core nitrate data. During freezing, the majority of solutes are rejected from the growing ice lattice and become concentrated either at grain boundaries or nodes within the ice crystal or at the air
ice interface. Solute exclusion into these liquid regions (LRs) may significantly increase their concentrations over those in the bulk sample, with important consequences for reactions occurring there. Several aqueous phase bimolecular and heterogeneous reactions have been shown to be accelerated upon freezing12
15 with enhanced reaction rates attributed to freeze-concentration effects. The specific location of rejected solutes (at the ice surface or in liquid pockets/grain boundaries) is important for understanding chemistry in or on frozen media because this will determine the availability of these solutes for reaction, exchange with the atmosphere, or both. Furthermore, there is some evidence to suggest that reactivity at the air
ice interface is r 2011 American Chemical Society

distinctly different from reactivity in the liquid-like pockets that may exist within an ice crystal.16,17 Nitrate at the ice surface has been the subject of only a few spectroscopic studies to date.18,19 In the present experiments, we have used glancing-angle Raman spectroscopy to probe the exclusion of nitrate anions to the ice surface. This technique (which is sensitive to the upper ∼50
100 nm of the sample surface20) is less sensitive than interface-specific techniques such as sum frequency generation (SFG) spectroscopy or X-ray photoelectron spectroscopy (XPS), which have subnanometer resolution, but is nevertheless appropriate for this study given the large liquid fractions that are expected (vide infra). This study represents the first use of glancing-angle Raman spectroscopy to probe solutes at the air
ice interfacial region. Figure 1 shows glancing-angle Raman spectra acquired at the surface of a 100 mM Mg(NO3)2(aq) sample prior to (dashed line) and after (solid line) freezing it from room temperature to 268 K. The nitrate symmetric stretching band (ν-sym NO3
), located at ∼1000 cm
1 21 and indicated by an arrow in the Figure, is barely distinguishable in the spectrum acquired prior to freezing, as expected based on the dependence of the nitrate peak intensity on [Mg(NO3)2] at the liquid surface (Figure S2 of the Supporting Information). As illustrated in Figure 1, the frozen sample shows a much more clearly defined ν-sym NO3
peak. The larger peak area at the frozen surface is indicative of an enriched nitrate concentration there, suggesting that nitrate is excluded to the ice surface during freezing. Figure 2 shows that cooling the same sample to 258 K (solid black trace) did not affect the spectrum nor did warming the Received: June 3, 2011 Accepted: July 19, 2011 Published: July 19, 2011 1967

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The Journal of Physical Chemistry Letters

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prefreezing or postmelting concentrations and an equilibrium thermodynamic analysis. Cho et al.22 present a formulation derived from ideal solution thermodynamics for calculating the fraction of water in the LR, jH2O(T) vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi !ffi u u MH O RTf T 2 ð1Þ Cbulk jH2 O ðTÞ ¼ t 1000Hfo T
Tf The concentration of solutes in the unfrozen LR, CLR, is then taken as representing total exclusion of solutes to the LR CLR ¼ Figure 1. Glancing-angle Raman spectra acquired at the surface of a 100 mM Mg(NO3)2(aq) sample prior to freezing (red dashed line) and of the same sample after freezing to 268 K (black solid line). Each plotted spectrum is an average of four individual spectra.

Figure 2. Glancing-angle Raman spectra acquired at the surface of frozen samples: 100 mM Mg(NO3)2 cooled to 258 K (solid black line), 100 mM Mg(NO3)2 subsequently warmed to 268 K (dashed red line), and 75 mM Mg(NO3)2 cooled to 258 K (dashed blue line). Each plotted spectrum is an average of four individual spectra. The area under the νsym NO3 peak (in arbitrary area units) is the same within experimental ̅ three spectra. error for all

sample back to 268 K, as displayed by the dashed red trace. Spectra acquired at the frozen surface also did not depend on the rate of freezing between 5 and 60 min. Within our experimental uncertainty, the area under the ν-sym NO3
peak was found to be independent of both temperature and the rate of cooling or warming for all frozen 100 mM Mg(NO3)2 samples. As illustrated by the dashed blue trace in Figure 2, it is also difficult to distinguish the spectrum acquired at the surface of a frozen 75 mM Mg(NO3)2 sample from the ones acquired at the surface of frozen 100 mM samples. Spectra acquired at the surface of 50 mM samples were also very similar. We made a few attempts to measure nitrate at the surface of frozen KNO3(aq) (50
500 mM) and frozen HNO3(aq) (100 mM) samples. Preliminary results showed little to no increase in the ν-sym NO3
feature upon freezing, and so further experiments were not undertaken. It is experimentally difficult to quantify solute concentrations in the LRs of ice; typically, they are calculated using measured

Cbulk jH2 O ðTÞ

ð2Þ

where the meaning of the symbols is given in the Supporting Information. This formulation has been adopted by several groups to model the kinetics of nitrate photolysis in frozen media.10,23 Alternatively, the concentration of Mg(NO3)2 in the unfrozen solution (CLR) may be estimated from the Mg(NO3)2
H2O phase diagram.24 At 268 K, this is ∼0.8 mol L
1 (∼12 wt %). According to the calibration plot for ν-sym NO3
peak area versus [Mg(NO3)2] (Figure S2, Supporting Information), a 0.8 mol L
1 Mg(NO3)2 solution should give rise to a peak area over an order of magnitude larger than the one we observe. The fact that we observe only a small increase in peak area upon freezing suggests that nitrate is not being excluded to the ice surface to the extent predicted by the phase diagram. Under the same conditions, eqs 1 and 2 predict a CLR of 16 mol L
1, which should give rise to an even larger nitrate peak area. In both approaches outlined above, CLR values are predicted to be a strong function of temperature: at 258 K, the phase diagram predicts a CLR of 1.6 mol L
1, and eqs 1 and 2 predict a CLR of 20 mol L
1. However, Figure 2 shows the ν-sym NO3
peak area to be independent of temperature between 258 and 268 K. The lack of temperature dependence further emphasizes the failure of the thermodynamic approaches to predict nitrate exclusion. Our conclusion that the nitrate concentration at the ice surface is less than expected relies on the relationship between the nitrate peak intensity and concentration being the same at the ice and water surfaces. We believe that this should hold in the present case because we expect a true liquid to exist at the air
ice interface under our conditions. If the entire LR exists at the surface, then from the phase diagram we estimate a liquid thickness at the surface of up to 100 μm; using eq 1, we estimate a thickness of up to ∼10 μm. (See the Supporting Information for details.) Given our glancing-angle Raman probe depth of 50
100 nm, even if only 1% of the total liquid fraction lies at the surface, then we expect to be probing a liquid environment at the frozen salt surfaces under our conditions. Our results suggest two possibilities: either nitrate is preferentially excluded to liquid pockets or grain boundaries within the ice sample rather than to the ice surface or some of the nitrate is effectively incorporated into the ice matrix. Given the low solubility of nitrate in ice,25 we favor the first possibility. These findings suggest that caution should be taken in (a) using an equilibrium thermodynamic approach to calculate nitrate enrichment factors for the LRs or (b) assuming that the LR is located exclusively at the ice surface. A recent study by Cheng et al.26 proposes that factors other than thermodynamics (such as the 1968

dx.doi.org/10.1021/jz2007484 |J. Phys. Chem. Lett. 2011, 2, 1967–1971

The Journal of Physical Chemistry Letters instability created during freezing by the rejected solute) may govern the location and nature of the unfrozen regions in ice. The behavior we observe for nitrate differs from that observed for halides. An NMR spectroscopic investigation by Cho et al.22 showed that during the freezing of NaCl(aq) solutions there is a preferential migration of ions to LRs located at the top and bottom of the NMR tube. A molecular dynamics simulation by Carignano et al.27 also shows that Na+ and Cl
ions are rejected from freezing ice into LRs located at both air
ice interfaces. Furthermore, in the case of halides, there is evidence to suggest that enriched surface concentrations can be sufficiently predicted using an equilibrium thermodynamic analysis. An XPS study by Krepelova et al.28 shows that at temperatures above the eutectic, an unfrozen NaCl brine phase is present at the outermost surface of the sample (probe depth ∼1.1 nm). According to their analysis, the composition of the unfrozen solution reflects the expected bulk composition based on the NaCl
water phase diagram. We recently studied the heterogeneous ozonation of bromide at the surface of frozen NaBr samples prepared in the same fashion as in the present experiments (frozen from the bottom up).14 The reaction kinetics we observed are consistent with the formation of a brine layer at the sample surface with a bromide concentration predicted by the NaBr
H2O phase diagram.29 Fewer studies have directly investigated the exclusion of nitrogen oxides during freezing. Takenaka et al.15 froze solutions of nitrous acid in the presence of oxygen from the bottom up. The unfrozen solution was separated from the ice during freezing, and the authors monitored the concentration
time profiles of nitrite and nitrate (the nitrite oxidation product) in each phase. They found that the concentration of nitrite in the unfrozen solution increased by only ∼20%. This result is consistent with the behavior we observe for nitrate in this experiment: there is exclusion to the surface but not to a large extent. Krepelova et al.18 used NEXAFS to interrogate nitrate formed by NO2 hydrolysis at a 230 K ice surface. Although this is a very different situation than nitrate exclusion from solution, the authors concluded that at low nitrate amounts the nitrate was not uniformly distributed on the surface and was not in equilibrium with gas-phase nitric acid. It is unclear why halides may be strongly excluded to the ice surface whereas nitrate and nitrite are not. The different behavior may be related to the aqueous surface affinity of the ions. There is a wealth of experimental30
32 and theoretical33 evidence to suggest that the heavy halide anions (Br
and I
) exhibit a strong surface affinity. By contrast, several studies suggest that the NO3
anion exhibits a near-neutral surface affinity34,35 or is even depleted at the air
aqueous interface.36,37 However, a correlation of exclusion to the air
ice interface with ion aqueous surface affinity is only speculative, and further work is required to understand the mechanism of solute exclusion during freezing. Atmospheric Implications. Laboratory investigations have attempted to elucidate where nitrate photolysis occurs in ice. Several of these studies suggest that nitrate photolysis occurs in a liquid-like region.3,10 However, the location of this region, whether it be at the air
ice interface or within the bulk ice matrix has not yet been determined, although yields of gas-phase products (NO2) may suggest that nitrate photolysis is occurring at the surface.3 Field and experimental observations also suggest that ice/snow morphology and the location of NO3
is crucial to reactivity.4,38 In the summer months, polar temperatures can exceed 0 °C, resulting in surface melting.39 In addition, temperature gradients

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within the snowpack can lead to snowpack metamorphism.40 When wet metamorphism occurs, liquid water is remobilized, the morphology of the ice grains changes, and the distribution of solutes within the ice grains may be affected. Therefore, although nitrate may initially be present at the ice grain surface (e.g., from the adsorption and dissolution of gas-phase HNO3), its location is likely to change with time. The average snowpack nitrate concentration measured at Summit, Greenland (2000) was 4.4 μM, and a typical summertime temperature was
20 °C.41 Jacobi and Hilker10 used [1] and [2] to calculate an LR concentration of 230 mM under these conditions, which represents an enrichment of over four orders of magnitude. However, our results suggest that surface concentrations of nitrate may be significantly lower than expected from application of [1], [2], and equilibrium phase diagrams. Given the magnitude of the calculated enrichment, assuming that this LR resides at the surface of ice crystals may result in gross misinterpretation of the mechanism of snowpack nitrate photochemistry. In conclusion, we have directly measured nitrate anion at the ice surface using glancing-angle Raman spectroscopy. Our results suggest that nitrate is excluded to the ice surface during freezing but that its concentration there is much lower than that predicted using an equilibrium thermodynamic analysis; the concentration of solutes in the unfrozen solution existing at the ice surface may be significantly different from the concentration of solutes found within liquid pockets or grain boundaries. Care should be taken in assuming that all solutes are excluded into a liquid layer existing at the ice surface. Predicting surface concentrations may be nontrivial, especially given that nitrate and halides appear to show different affinities for exclusion to the ice surface. Further work is needed to connect the mechanism of snowpack nitrate photolysis with nitrate exclusion and where the reaction is taking place.

’ EXPERIMENTAL METHODS The reaction chamber and glancing-angle Raman technique have been described in detail elsewhere.14,20 In brief, Raman scattering was induced at the sample surface using the 355 nm output of a frequency-tripled Nd/YAG laser (pulse repetition 10 Hz, pulse energy ∼0.6 mJ). The laser beam impinged the sample surface at a glancing angle (>85° from the surface normal). Raman scattering was collected perpendicular to the surface using a 7 mm diameter liquid light guide suspended ∼5 mm above the sample. The collected light was imaged onto the entrance slit (0.5 mm diameter) of a 1/4 m monochromator, and the transmitted intensity was detected and amplified by a photomultiplier tube. A 355 nm laser-line long-pass filter was placed between the exit slit of the monochromator and the photomultiplier tube to reduce interference from Rayleigh scattering. The electronic signal was read by a digital oscilloscope that averaged the intensity versus time signal over 64 laser shots. A LabView program sampled a 50 ns window centered on the intensity versus time peak, and this value was stored for subsequent analysis. Raman spectra were collected by scanning the monochromator in steps (∼25 cm
1). Aqueous Mg(NO3)2 (0.05 to 1.50 M) solutions were prepared by dissolving Mg(NO3)2 3 6H2O(s) in 18 MΩ deionized water. A 4 mL aliquot of solution was then spread onto a thin piece of stainless steel shimstock resting on the chamber floor, and the sample was frozen either rapidly (i.e., within 5 min) or slowly (i.e., over ∼1 h, at a rate of ∼0.2 K min
1). Frozen 1969

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The Journal of Physical Chemistry Letters samples were fairly smooth and flat on top and had rounded edges (surface area ∼9 to 10 cm2 and thickness ∼0.3 to 0.5 cm).

’ ASSOCIATED CONTENT

bS

Supporting Information. Experimental details, glancingangle Raman spectra acquired at the surface of aqueous Mg(NO3 )2 solutions with bulk concentrations, area under the ν-sym NOh3 peak as a function of Mg(NO3)2 concentration, supplemental references. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Tel/Fax: 416-978-3603.

’ ACKNOWLEDGMENT This research was funded by NSERC and CFCAS. S.N.W. thanks NSERC for a PGS-D scholarship. We thank Dr. T. F. Kahan for useful discussions. ’ REFERENCES (1) Domine, F.; Shepson, P. B. Air
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