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Physical Characterization of Frozen Saltwater Solutions using Raman Microscopy Philip Patrick Anthony Malley, Subha Chakraborty, and Tara F Kahan ACS Earth Space Chem., Just Accepted Manuscript • DOI: 10.1021/ acsearthspacechem.8b00045 • Publication Date (Web): 23 May 2018 Downloaded from http://pubs.acs.org on June 3, 2018

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ACS Earth and Space Chemistry

Physical Characterization of Frozen Saltwater Solutions using Raman Microscopy Philip P. A. Malley, Subha Chakraborty, and Tara F. Kahan*

1-014 Center for Science and Technology, Syracuse University 111 College Place Syracuse, NY 13244 *Author to whom correspondence should be addressed: [email protected], (315) 443-3285. Fax: (315) 443-4070. 1-014 Center for Science and Technology, Syracuse University, Syracuse, NY 13244.

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Abstract: Ice is an important but poorly understood atmospheric reaction medium. Reactions in ice and at air-ice interfaces are often modelled using rate constants measured in liquid aqueous solution, despite evidence that reactivity in these two media can be very different. This approach may be valid at high ionic strengths (e.g. in sea ice) due to the formation of liquid brine. However, recent experiments indicate uneven solute distribution at ice surfaces, suggesting that liquid water does not completely wet ice surfaces at environmentally-relevant solute concentrations. We have investigated the distribution of liquid solution, solid ice, and solid salt (NaCl.2H2O, “hydrohalite”) at the surface of frozen aqueous sodium chloride (NaCl) solutions and frozen sea water using Raman microscopy. At temperatures above the eutectic (–21.1 °C), the ice surfaces were incompletely wetted except at the highest temperatures (approximately –5 °C). Liquid water at the surface took the form of either isolated patches or channels, depending on salt concentration and sample temperature; liquid fractions ranged from approximately 11% to 85%. 3-Dimensional (“volume”) maps showed similar liquid fractions and channel widths at all depths investigated (up to 100 µm) as at the surface for each sample composition. Below –21.1 °C, no liquid was observed in any sample. Instead, hydrohalite was observed with surface coverages ranging from 13% to 100% depending on salt concentration; surface coverage was independent of temperature between –30 °C and –22 °C. Accounting for the presence of two distinct reaction environments at the surface of salty ice might improve predictions of physical and chemical processes in snow-covered regions.

Key words: heterogeneous chemistry, polar boundary layer, cryosphere, snow pack, Sargasso Sea, photochemistry, quasi-liquid layer, model


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Introduction Ice has long been recognized as an important reaction medium in diverse fields including food preservation, astrochemistry, and stratospheric ozone depletion. In the mid-1990s, researchers discovered that reactions within terrestrial snowpacks could greatly alter the composition of the atmosphere.1-3 Atmospheric models have consequently incorporated reactions in snowpacks and at air-ice interfaces to improve predictions of atmospheric composition in snow-covered regions. In these models, ice is often treated as an inert substance coated by a layer of liquid water; all snowpack chemistry occurs within this liquid phase (e.g. References 4-5). This simplification allows researchers to use rate constants measured in aqueous solutions to model reactions in snow and ice. While this approach significantly reduces the complexity of modelling snowpack chemistry, its validity has been questioned. Specifically, the assumption that reactivity in snow (or at air-ice interfaces) is always well-described by kinetics measured in aqueous solution is not strongly supported.6-9 Many physical and chemical processes occur very differently at liquid and frozen water surfaces. For example, a number of aromatic pollutants have been reported to photolyze more rapidly at ice surfaces than at liquid water surfaces, and some aromatic pollutants that do not photolyze in aqueous solution can photolyze at ice surfaces.10-13 Heterogeneous reaction kinetics have also been reported to differ at frozen and liquid water surfaces: Ozonation of several aromatic species has been reported to be much faster at ice surfaces than at liquid water surfaces, while hydroxyl radicals (OH), which react rapidly with aromatic species at liquid water surfaces, have been reported to be unreactive toward a range of aromatic species at ice surfaces.10, 14-17 If solutes cause the surface to be coated in a liquid “brine,” then these physical and chemical processes will occur as though the surface is a liquid rather than a solid, and can be modelled as



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such. There is some evidence that this might be the case: The photolysis rate constant of the aromatic dye harmine was larger at frozen freshwater surfaces than in aqueous solution, but no enhancement compared to in aqueous solution was observed at the surface of frozen NaCl solutions when initial NaCl concentrations exceeded 0.2 M). These results were interpreted as indicating that the reaction environment experienced by harmine transitioned from that of an ice surface to a liquid brine as the NaCl concentration increased.18 While the fractional volume of liquid water in ice can be predicted based on solute concentration and temperature, this does not provide enough information to determine the composition of salty ice surfaces.19-20 It has been suggested that ice surfaces may not be completely wetted even at high solute concentrations.7 Incomplete wetting would result in an ice surface consisting of discrete regions of bare ice and liquid brine. In support of this, several experimental studies have reported inhomogeneous solute distribution at salty ice surfaces.21-24 Scanning electron microscopy (SEM) studies of sintered frozen NaCl solutions showed isolated liquid patches at ice surfaces and at grain boundaries above the eutectic temperature, and “webs” of solid salt below the eutectic temperature, while environmental SEM was recently used to image evaporating laboratory-prepared frost flowers.22, 25 In the latter work, liquid brine was inferred to coat thin (~2 µm) fingers of ice at high temperatures, and salt (likely NaCl or NaCl•2H2O) crystals were observed even at temperatures as high as –6 °C, which is above the eutectic temperature of the NaCl-water system (–21.1 °C).26 Another environmental SEM study showed that a uranyl salt was distributed in channels at ice surfaces, likely in liquid regions at grain boundaries.21 Atomic force microscopy (AFM) and optical images indicate the presence of isolated “clumps” or islands of solid salt surrounded by bare ice below the eutectic temperature.24, 27 X-ray fluorescence (XRF) has recently been used to investigate the spatial


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distribution of solutes in frozen samples. Significant spatial heterogeneity of several elements including Br and Cl was reported in sea ice cores at depths ranging from the top centimeter to almost 2 m.28 Another study reported that transition metals were distributed in channels when they were frozen from aqueous solutions containing NaCl.23 If surface wetting of frozen NaCl solutions is incomplete, as these studies suggest, at least two distinct reaction environments would be present at ice surfaces – pure ice and a liquid brine. As discussed above, reactivity can differ greatly at ice and liquid water surfaces, and atmospheric models might be improved by accounting for multiple distinct reaction environments at salty ice surfaces. Raman microscopy has become a popular technique to study the spatially-resolved composition of atmospheric condensed phases. Unlike most traditional surface science techniques, it can operate at atmospheric pressure, and can be used with liquid and solid samples, including high-vapour pressure compounds. It is less likely to cause beam damage than other techniques such as electron microscopy and x-ray microscopy. Raman microscopy has been used to study the composition of laboratory-generated and ambient particulate matter, and recently it has been used to investigate the composition of micro-inclusions in sea ice and to investigate the speciation of nitrate in and at the surface of frozen aqueous solutions.29-31 In addition to detecting and identifying solutes in ice and at ice surfaces, Raman spectroscopy can be used to distinguish between liquid water and ice, as discussed in Reference 32. In this work we use Raman microscopy to detect liquid water (or brine) in frozen aqueous NaCl solutions at a range of concentrations and temperatures relevant to Earth’s surface. We determine the fraction of liquid at the ice surface and in the bulk sample, as well as the spatial distribution of the liquid. These results will help inform future atmospheric models that include ice as a reaction medium.



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Methods Materials: Sodium chloride (Sigma Aldrich ≥99.5%) solutions were prepared by dissolving a measured quantity of NaCl in 18.2 MΩ cm deionized water to a concentration of either 0.6 M or 0.02 M. Frozen sea ice was prepared from Sargasso Sea water. The seawater was collected in 2012, was filtered to remove particulate matter, and was refrigerated until use.33 Sample Preparation and Raman Imaging: A custom-built cold chamber was used for all experiments. The chamber consisted of a stainless steel plate and a tightly-fitting stainless steel cover with a quartz window in the ceiling. A Peltier device attached to the bottom of the chamber controlled the temperature. A drop of solution (approximately 100 µL) was placed on the chamber floor, and a thermocouple was inserted into the sample through a port on the side of the chamber. All samples were initially frozen at a temperature between –25 and –30 °C, then slowly (0.5 °C min-1) warmed to the desired temperature. This was done to ensure consistent results, as different sample preparation methods (rapid sample warming or freezing the sample directly to the desired temperature) resulted in inconsistent liquid fractional surface coverage. Frozen samples were allowed to equilibrate at the set temperature for 10 minutes prior to acquiring spectra. Raman spectra were acquired with a Renishaw InVia Raman microscope using a 532 nm continuous wave laser with a 50× long working distance objective. The microscope was optically focused at the sample surface prior to each experiment, and an optical image was acquired. Scattered light was collected through the microscope objective, then passed through a 1800 lines/mm visible range grating. The spectrum was imaged onto a 1015 pixel CCD camera. Spectra were acquired between 2502 and 3824 cm-1. The depth of focus in our experiments is ~4 µm and the measurement diameter is ~2 µm. Several experiments were performed to ensure that


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exposure to the laser beam did not alter the physical properties of the ice surface. These are discussed in the Supplementary Information (SI). 150 µm × 150 µm maps were acquired with 3 µm steps along the x and y axes with an acquisition time of 0.25 s per spectrum. Collection of a single map took ~41 minutes. Threedimensional (“volume”) maps were obtained using the same parameters as the surface area maps but were repeated in 10 µm steps along the z-axis from the ice surface (z = 0 µm) to a depth of 100 µm. Since these step sizes are larger than the depth of focus and measurement diameter of our system, we do not expect adjacent pixels to contribute to spectra acquired at a given location. Figure 1a shows spectra of deionized water at room temperature, at –17 °C, and at –26 °C. Distinct differences are observed in the spectra of liquid and solid samples. Stronger hydrogen bonding between water molecules in ice shifts the Raman peak to lower energies (~3135 cm-1, compared to ~3426 cm-1 in liquid water). There is little difference in the spectra acquired at the two temperatures in the frozen samples. Figure 1b shows spectra of a 0.6 M aqueous NaCl solution acquired at similar temperatures. While the room temperature spectrum looks very similar to that of deionized water, the spectra acquired in ice have very different appearances. At –14.7 °C, the spectrum of the NaCl solution resembles that of liquid water much more so than ice, with peak intensity at 3425 cm-1, likely due to the formation of a liquid brine. We note that spectra of frozen aqueous NaCl solutions never perfectly matched those of liquid water; a small “ice-like” peak at 3135 cm-1 was always observed. This feature is likely due to contributions from nearby ice within the observational volume described above. The spectrum in Figure 1b acquired at –25 °C resembles that of ice, but two additional peaks are observed at 3421 cm-1 and 3539 cm-1. These peaks are due to the formation of NaCl•2H2O (“hydrohalite”), which



is thermodynamically stable at temperatures below the eutectic point of the NaCl-H2O system (–

Normalized intensity

21.1 °C).26

Normalized intensity

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(a)

(b)

Figure 1. Raman spectra of (a) deionized water acquired at 25 °C, −16.7 °C, and –25.6 °C; and (b) an aqueous 0.6 M NaCl solution acquired at 24.6 °C, −14.7 °C, and −24.6 °C. The intensity of each spectrum is normalized to that at 3136 cm−1.

Spectra were analyzed in two ways. In the first (“Method 1”), we examined covariance of each spectrum with a reference spectrum of pure ice and normalized the covariance by the product of the total counts in the entire sample spectrum and the reference spectrum. This normalized covariance, ρ, represents the spectral deviation from pure ice and hence provides qualitative information about the presence of any species (including liquid water) other than pure ice at a particular coordinate. We plot surface maps of (1 – ρ) over the entire scan-area after scaling between 0 and 1 such that (1 – ρ) = 0 and (1 – ρ) = 1 on the scan-area would correspond to minimum and maximum spectral deviation from pure ice. A description of the calculations is provided in the SI. In the second method (“Method 2”), each spectrum was assigned as liquid 8 ACS Paragon Plus Environment

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water (or a liquid brine), ice, or hydrohalite based on the relative intensities at energies corresponding to peaks in the spectra of liquid water, ice, and hydrohalite. The criteria used for this analysis are shown in the SI. This method does not provide information about the relative contribution of each potential component to the spectra; it classifies each spectrum based on the dominant component. Fractional liquid surface coverage was determined as the ratio of “water-like” pixels to the total number of pixels in a map. For maps generated using Method 2, this entailed dividing the number of spectra classified as water (i.e. teal pixels) by the total number of pixels in a map. For maps generated using Method 1, a threshold value is required to distinguish between “waterlike” and “ice-like” spectra. The threshold value chosen can greatly affect the calculated liquid fraction. To choose the threshold value, we calculated the expected liquid volume fraction of frozen aqueous 0.6 M NaCl solutions at -15 ºC and -8 ºC using Equation 1.19 Φ௟ (ܶ) =

[ே௔஼௟]എ

[1]

[ே௔஼௟]೅

where Φl(T) is the fractional liquid coverage at a given temperature, [NaCl]o is the concentration of NaCl in liquid solution, and [NaCl]T is the NaCl concentration in the liquid fraction of the ice at a given temperature. [NaCl]T, which is greater than [NaCl]o due to freeze exclusion, is determined from Equation 2.19 [ܰܽ‫= ் ]݈ܥ‬

ଵ଴଴଴୼ு೑എ

ெಹమ ೀ ோ்೑

ቀ

்೑ ି் ்

ቁ

[2]

Here, ΔHfo is the enthalpy of fusion, MH2O is the molar mass of water, R is the gas constant, and Tf is the temperature of fusion. We calculated Φl in bulk ice at -15 ºC and -8 ºC at a range of threshold values, and chose the threshold value that yielded liquid fractions closest to those predicted by Equation 1 at those temperatures. The threshold value used in this work was 0.85,



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meaning that spectra that deviated by at least 15% from a spectrum of pure ice were considered to be liquid water. Liquid fractions determined from maps created using Method 2 consistently yielded greater values of Φl than those determined from maps created using Method 1, both at the air-ice interface and in bulk ice. Method 1 was therefore used for most analysis. Method 2 was used for experiments in which maps were acquired below the eutectic temperature, as Method 2 can distinguish between liquid water, ice, and hydrohalite. A scaling factor of 0.85 was applied to the values of Φl determined from these experiments to bring them into agreement with the values determined using Method 1. The widths of liquid channels at air-ice interfaces and within bulk ice were estimated by measuring the width approximately perpendicular to the channel direction at several locations along the length of the channel.

Results and Discussion Surface Maps of Salt Solutions Above the Eutectic Temperature Figure 2 shows Raman maps acquired at –15 °C at the surface of frozen 0.02 M and 0.6 M NaCl solutions, which correspond to NaCl concentrations expected in snow contaminated with road salt and in sea ice, respectively.34 Figure 2 also shows a map of a frozen seawater sample at the same temperature. Distinct regions of solid ice and liquid solution are apparent at the surface of each sample. No liquid was observed at the surface of frozen deionized water in the absence of salt, as shown in the SI. The deviations from ice observed in Figure 2 are due to the formation of a liquid brine, and not to the so-called “disordered surface region” or “quasiliquid layer” that exists at the top few layers of ice.6 Unlike techniques such as sum-frequency


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vibrational spectroscopy or glancing-angle Raman spectroscopy, our method is not sensitive or selective to this region.32, 35

(a)

(b)

(a) (b) (c)

(c)



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Figure 2. Correlation map of spectra acquired by raster-scan over the surface of a (a) 0.02 M NaCl solution at −13.7 °C; (b) 0.6 M NaCl solution at −13. 5°C; and (c) seawater at −13.5 °C. The scan-size is 150 µm × 150 µm, with a 3 µm step size. The normalized covariance ρ of the spectra at each coordinate of the samples and that of DI water at the coordinate (0, 0) is evaluated and (1 – ρ) is plotted as a surface-plot as shown in the color-bar. The value of (1 – ρ) indicates the dissimilarity between the acquired spectra and the reference spectrum corresponding to DI water at – 18 °C; increasing brightness corresponds to increasing liquid-like nature of the surface.

The deviation from ice is generally greatest in the middle of the liquid regions, whether they are channels or “islands.” This may indicate that liquid regions are deepest in the centre of the feature, as reported by a recent XRF study.23 The deviation from ice is much greater in the 0.6 M solution and the seawater than in the 0.02 M solution, perhaps suggesting deeper liquid channels at the higher salt concentrations. The maps shown in Figure 2 are also shown in the SI with binary analysis (i.e. classifying each spectrum as either liquid water or ice). This analysis was used to calculate liquid surface fraction for each experimental condition. Less liquid is observed at the surface of 0.02 M NaCl solutions than at the surface of 0.6 M solutions; average fractional liquid surface coverages at 15 °C were 13 ± 3% and 19 ± 6% respectively. The fractional liquid surface coverage of frozen seawater was 22 ± 4%, which is similar to that measured at the surface of frozen 0.6 M NaCl solutions at the same temperature. This suggests that NaCl is responsible for the majority of surface melting observed in sea ice, despite the presence of other solutes. These results are in agreement with previous studies that suggest incomplete wetting of ice surfaces by NaCl, but this is the first study to show this at seawater NaCl concentrations.22-24, 27-28 Figure 3 shows the fraction of the ice surface covered by liquid as a function of temperature for 0.02 and 0.6 M NaCl solutions. Maps acquired at each temperature for an individual sample are also shown. As the temperature increases, isolated patches of liquid extend and merge to form channel-like structures in the 0.6 M samples. Liquid at the surface of frozen 0.02 M NaCl solutions generally took the form of patches or islands rather than channels at all


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temperatures. Our results are consistent with suggestions that solute-induced melting occurs at grain boundaries between ice crystals.21, 23 A positive deviation from the liquid volume fractions predicted by Equations 1 and 2 is observed for the 0.02 M solutions and for the warmest 0.6 M solutions. We hesitate to ascribe this to a greater liquid fraction at the air-ice interface than predicted by thermodynamics, however. First, liquid fractions measured within the bulk agreed with those at the surface for a given sample (see below). Second, significant uncertainty is associated with choosing a threshold value to distinguish between liquid water and ice. We therefore focus on the relative amounts of liquid at the air-ice interface under different conditions rather than on the exact liquid fractions calculated. -20.3 oC 0.6 M

-15.5 oC 0.6 M

-10.2 oC 0.6 M

-15.1 oC 0.02 M

-5.1 oC 0.6 M

-4.9 oC 0.02 M



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Figure 3. Temperature dependence of the percent coverage of liquid at air-ice interfaces. Raman maps acquired at temperatures corresponding to the data points in the Figure are also shown. The 4 upper maps are from a 0.6 M NaCl solution and the 2 lower maps are from a 0.02 M NaCl solution. The maps are aligned along the x-axis with the corresponding data point on the graph. Maps were analyzed using Method 1. Black indicates ice and teal indicates liquid water.

Figure 4 shows the average width of liquid channels observed at the surface of frozen aqueous 0.6 M NaCl solutions. Over the span of 10 ºC (from -20.3 to -10.2 ºC), the average width increased by approximately a factor of 3. This is similar to the 2.5× increase in the fractional liquid surface coverage observed over this temperature range.

30 20 10 0 -22

-20

-18

-16

-14

-12

-10

-8

-6

-4

-2

Temperature (oC) Figure 4. Average channel width at the surface of frozen aqueous 0.6 M NaCl solutions. Error bars indicate the standard deviation about the mean of at least 3 trials.

Volume Maps We acquired volume (3-dimensional) maps of frozen 0.02 M and 0.6 M aqueous NaCl solutions and of seawater. Figure 5 shows maps of each sample type at temperatures of approximately -15.5 °C. Liquid regions were observed at all depths. We calculated the liquid fractions of frozen 0.6 M NaCl solutions at depths of –40 and –80 µm; they were 16 ± 6% and 23 ± 8%, which are in good agreement with the fractional coverage of 19 ± 6% measured at the surface. Channel widths at the two depths were 10 ± 9 µm and 11 ± 6 µm, which are also in excellent agreement with the width of 8 ± 4 µm measured at the surface at the same temperature. 14 ACS Paragon Plus Environment

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This demonstrates that not all solutes are excluded to the ice surface, in agreement with recent reports that the concentrations of solutes such as Cl- and NO3- measured at ice surfaces are lower than expected if all of the solute was excluded to the ice surface.30-31, 36-37 Recent x-ray tomography studies have reported liquid volume fractions below 5% in sea ice samples at –20 °C.38 We did not acquire volume maps at –20 °C, but liquid fractional surface coverage (which appears to be similar to bulk liquid volume fraction) was 12 ± 7% for 0.6 M NaCl solutions at – 20.3 °C. This discrepancy could be due to different histories and formation processes of the ice samples, which have been reported (and which we have observed) to affect liquid volume fraction.39 It could also be due to the choice of threshold used to distinguish between water and ice in the two studies. The thermodynamically-predicted liquid volume fraction at this temperature (based on Equation 1) is 12%, in good agreement with our experimental value.



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Figure 5. Volume maps of (a) a 0.02 M aqueous NaCl solution at –15.5 °C, (b) a 0.6 M aqueous NaCl solution at – 15.4 °C, and (c) seawater at –15.6 °C. Maps were analyzed using Method 1. Teal indicates liquid water.

X-ray tomography studies of frozen sea ice show vertical liquid channels extending to depths of several mm.38, 40 Conversely, a different x-ray tomography study showed that cesium chloride in frozen aqueous solutions was concentrated in distinct unconnected pockets rather than in channels.39 Our results are consistent with both studies. We observed liquid channels in frozen 0.6 M aqueous NaCl solutions and in frozen seawater (some of which extended at least 100 µm in depth), while liquid water in frozen 0.02 M NaCl solutions was primarily in isolated 16 ACS Paragon Plus Environment

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patches. The study investigating cesium chloride distributions used 1.0 mM concentrations.39 At this low concentration, there is likely not enough liquid present to form the channels that are observed at seawater NaCl concentrations. Finally, we note that under our experimental conditions less than 1% of the volume of frozen deionized water was in liquid form (as shown in the SI), which indicates that the liquid observed in the volume maps presented in Figure 5 is due to solute-induced melting.

Surface Maps of Frozen Salt Solutions Below the Eutectic Temperature At temperatures below the eutectic (–21.1 °C), NaCl–H2O mixtures at equilibrium will consist of a combination of solid ice and solid salt (including the hydrohalite, NaCl•2H2O, which we are able to detect using Raman spectroscopy as shown in Figure 1b).26 Liquid water (or brine) is not expected to be present, although an NMR study reported the presence of liquid water at temperatures as low as –45 °C in frozen 0.5 M NaCl solutions.19 Figure 6 shows surface maps of frozen NaCl solutions and seawater at approximately – 31°C. At this temperature, we do not see any evidence of liquid (which would be represented as teal pixels on the maps if it was detected). Instead, we see distinct regions of pristine ice and hydrohalite. The surface coverage of hydrohalite varied significantly between samples and ranged from 40 to 100% for the 0.6 M NaCl solutions and from 45 to 80% for the seawater. Coverage was much lower for the 0.02 M NaCl solutions, ranging from 13 to 20%. At temperatures lower than the eutectic, the ice is opaque due to the presence of hydrohalite, and volume maps could not be obtained.



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Figure 6. Surface maps of (a) 0.6 M NaCl (–32.0 °C), (b) Sargasso Sea Water (–29.5 °C), and (c) 0.02 M NaCl (– 30.9 °C). Maps were analyzed using Method 2. Black indicates ice, teal indicates liquid water (not observed), and magenta indicates hydrohalite.

Surface Maps of Frozen Salt Solutions Near the Eutectic Temperature Figure 7 shows a surface map of a 0.6 M NaCl solution acquired just below the eutectic temperature, as well as a map of the same sample region acquired after warming the sample to just above the eutectic temperature. The transition from solid hydrohalite to liquid brine when the temperature rises above the eutectic temperature is evident. At all temperatures investigated, hydrohalite and liquid were never observed simultaneously in the same sample.


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Figure 7. Raman maps of a 0.6 M NaCl solution at –21.8 °C (top) and –18.7 °C (bottom). Maps were analyzed using Method 2. Black indicates ice, teal indicates liquid water, and magenta indicates hydrohalite.

We performed a series of experiments to investigate the transition between liquid brine and hydrohalite near the eutectic temperature using 0.6 M NaCl solutions. For these experiments, a Raman map was acquired at a temperature below the eutectic, and then the temperature was slowly raised by a few degrees. After equilibrating the sample at the new temperature, another map was acquired, and this process was repeated until several maps were acquired at temperatures ranging from approximately –32 °C to –15 °C. Then maps were acquired as the temperature was slowly decreased. Figure 8a shows the fraction of the ice surface containing hydrohalite at each temperature, and Figure 8b shows the fractional liquid surface coverage. The hydrohalite coverage remained constant at temperatures between –32 and –21.7 °C. As the temperature increased above –21.1 °C, the extent of hydrohalite coverage decreased rapidly. Negligible hydrohalite was detected at temperatures above the eutectic. As we cooled the sample, we observed a reproducible hysteresis: hydrohalite was not observed until the temperature decreased below –22 °C – a full degree lower than the eutectic temperature. The 19 ACS Paragon Plus Environment


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same hysteresis reported for hydrohalite formation is observed for fractional liquid surface coverage (Fig. 8b). Whereas the surface coverage of hydrohalite was constant at temperatures below the eutectic, at temperatures greater than the eutectic the liquid fraction increased with increasing temperature.

Figure 8. Average fraction of the surface covered with (a) hydrohalite and (b) liquid water as a function of sample temperature. Red triangles indicate increasing temperature, and blue circles indicate decreasing temperature. Error bars represent the standard deviation about the mean of at least 3 trials. The red and blue arrows indicate the direction of the temperature ramp (warming or cooling). Fractional surface coverages were determined using Method 2.

The observed hysteresis is likely due to the fact that hydrohalite formation requires the proper conformation of salt and water molecules. Similar hystereses are reported for several atmospheric phase changes such as the efflorescence of aqueous aerosols and the homogeneous


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freezing of water.41-43 This hysteresis was observed whether the solution was cooled slowly between map acquisition temperatures, or at a rapid rate (0.5 °C min-1 or 0.5 °C s-1).

Atmospheric Implications The results of this work have two major implications for the way air-ice interactions are treated in atmospheric models. The first is that we observe an even distribution of liquid water throughout the entire ice sample, whereas models generally position all of the water (and solutes) at the air-ice interface. Only solutes in liquid regions at the air-ice interface (or connected to it) can participate in heterogeneous reactions. The volume of the liquid brine at the air-ice interface, and the number of solute molecules in this brine, could be adjusted in atmospheric models. The second implication of our results is that ice surfaces will not be completely coated in liquid water, even at high solute concentrations and high temperatures. Some fraction of the surface will always consist of bare ice. This spatial heterogeneity may affect gas-surface interactions, as two distinct environments – pure ice and liquid brine (or pure ice and solid hydrohalite at low temperatures) – will be present. Physical and reactive uptake coefficients of gases to these two environments, as well as reaction kinetics of adsorbed / absorbed species, may be very different. Altering the volume of liquid accessible to the air in atmospheric models is relatively straightforward. Accounting for the effects this will have on reactivity is not. For one thing, the distribution of solutes between different liquid regions may not be uniform. For example, nitrate was recently shown to reside in liquid compartments within frozen aqueous solutions containing seawater levels of NaCl.30 Nitrate’s surface concentration was ~20% greater than that in the bulk in frozen solutions prepared from 0.5 M NaCl, but was 80% greater in solutions prepared from an aquarium sea salt mixture. This suggests that the concentration of reactive species at air-ice



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interfaces may depend on the identity of ionic species present, and not solely on the extent of brine formation. It is also possible that some solutes will not reside within liquid regions in the ice. X-ray fluorescence mapping of ice core samples from Antarctica suggested that Cl, K, and Ca were located primarily within grain boundaries, but Br and Sr had different lateral distributions at all ice core depths investigated.28 Finally, understanding that two distinct reaction environments (bare ice and liquid brine) exist at air-ice interfaces under most environmental conditions may be helpful for interpreting and predicting heterogeneous reaction kinetics in snow-covered regions, but more information is necessary to incorporate this into models in a meaningful way. Specifically, the distribution of reactive solutes between these two media should be known, as should the physical and reactive uptake coefficients of gas-phase species. Further work is required to better understand reactivity on salty ice surfaces and to improve predictions of chemistry in snow-covered regions.

Acknowledgments: This work was funded by NSF award 1454959. The authors thank Dr. David J. Kieber for providing Sargasso Sea Water for the experiments.

Supporting Information: Quality control experiments, description of spectral analysis methods, surface and volume maps of deionized water.


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