Interfacial supercooling and the precipitation of hydrohalite in frozen

Laboratory experiments are presented on the phase change at the surface of sodium chloride – water mixtures at 14 temperatures between 259 K and 240 K. Chloride is a ubiquitous component of polar coastal surface snow. The chloride 15 embedded in snow is involved in reactions that modify the chemical composition of snow as well as ultimately impact the 16 budget of trace gases and the oxidative capacity of the overlying atmosphere. Multiphase reactions at the snow – air interface 17 have found particular interest in atmospheric science. Undoubtedly, chemical reactions proceed faster in liquids than in solids; 18 but it is currently unclear when such phase changes occur at the interface of snow with air. In the experiments reported here, 19 a high selectivity to the upper few nanometres of the frozen solution – air interface is achieved by using electron yield near20 edge X-ray absorption fine structure (NEXAFS) spectroscopy. We find that sodium chloride at the interface of frozen solutions, 21 which mimic sea-salt deposits in snow, remain as supercooled liquid down to 240 K. Below this temperature, hydrohalite 22 exclusively precipitates, anhydrous sodium chloride is not detected. In this work, we present the first NEXAFS spectrum of 23 hydrohalite. The hydrohalite is found to be stable while increasing the temperature towards the eutectic temperature of 253 K. 24 Taken together, this study reveals no differences in the phase changes of sodium chloride at the interface as compared to the 25 bulk. That sodium chloride remains liquid at the interface upon cooling down to 240 K, which spans the most common 26 temperature range in Polar marine environments, has consequences for interfacial chemistry involving chlorine as well as for 27 any other reactant for which the sodium chloride provides a liquid reservoir at the interface of environmental snow. 28 Implications for the role of surface snow on atmospheric chemistry are discussed. 29 30 https://doi.org/10.5194/tc-2020-253 Preprint. Discussion started: 27 November 2020 c © Author(s) 2020. CC BY 4.0 License.

The relevance of halogen multiphase chemistry for the atmosphere is not limited to chlorine. A more recent example is the 57 oxidation of bromide. Bromide is present in sea-salt, is a key reactant in ozone depletions in polar atmospheres (Simpson et 58 al., 2015), and participates in atmospheric chlorine chemistry by forming interhalogen compounds (Finlayson-Pitts, 2003). 59 Oldridge and Abbatt (2011) have shown that a Langmuir-Hinshelwood type surface reaction of ozone with bromide occurs at 60 the liquid -air interface simultaneously with a corresponding bulk reaction in the temperature range of 263 K to 248 K. A 61 surface-active reaction intermediate was found to explain the high interfacial reactivity for the case of the reaction with ozone 62 , while other bromine species may directly exhibit surface propensity on their own (Gladich et al., 2020). 63 Clearly, this line of research shows how reaction kinetics and mechanisms differ at the interface from those in the bulk and 64 that heterogeneous chemistry is a key driver in atmospheric chemistry. 65

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In the cryosphere, where the snowpack is strongly impacting the chemistry in the overlaying atmosphere (Dominé and Shepson,67 2002; Thomas et al., 2019), halogen compounds are also found within the snow. Sea-salt components, a source of halogens in 68 snow in costal snowpack, might originate from migration from underlying sea-ice or from deposition of wind-transported sea-69 spray aerosol (Dominé et al., 2004). One characteristic of the cryosphere are its subfreezing temperatures and the consequent 70 https://doi.org/10.5194/tc-2020-253 Preprint. Discussion started: 27 November 2020 c Author(s) 2020. CC BY 4.0 License.
we base the discussion of phase changes in this work on the relative humidity, a measure for the partial pressure of water, that 105 the sample was exposed to. 106 107 Figure 2: Phase diagram of the NaCl-water binary system. The data show the freezing point depression of sodium-chloride 108 solutions (yellow filled circles) and give the concentration of an aqueous sodium chloride solution in equilibrium with water 109 in the temperature range of 273 K to 254 K (Rumble). The dark blue lines indicate the phase boundaries (Koop et al., 2000b;110 Rumble), that is it denotes the so-called liquidus and solidus line, respectively, and thus shows the temperature and 111 concentration range where ice and aqueous sodium chloride solution co-exist. The eutectic temperature of sodium chloride -112 water binaries is 251.9 K (Koop et al., 2000a). 113 114 Koop et al. (2000a) were the first to show that because precipitation of sodium chloride can be kinetically hindered, i.e. 115 precipitation may not occur even though temperature has dropped below the eutectic temperature where the solid is the 116 thermodynamically favoured phase, supercooled sodium chloride solutions in the presence of ice can prevail down to 240 K. 117 Cho et al. (2002) have observed a liquid fraction in sodium chloride -water mixtures at even lower temperatures of between 118 228 K and 273 K based on the evaluation of 1 H-NMR signals. Cho et al. (2002) proposed the presence of liquid below the 119 eutectic temperature to be an interfacial phenomenon, stabilized by surface forces in analogy to the disordered interface 120 observed for neat ice surfaces when approaching the melting point ( (Krepelova et al., 2010a), it was shown by probing the X-ray absorption of oxygen atoms in sodium chloride -water 130 binary mixtures that the hydrogen bonding network did not reveal the presence of any liquid features at the interface below 131 the eutectic temperature. Interpretation of these oxygen NEXAFS spectra was complicated by the appearance of crystal water 132 in the hydrohalite and by the presence of adsorbed H2O. Also, the oxygen spectra might be dominated by ice that is present in 133 equilibrium with the sodium chloride solution and thus small fractions of liquid might have been difficult to detect. In this 134 work, we therefore discuss Cl K-edge NEXAFS that have previously been used to inspect the chemical speciation of chlorine 135 in glasses and in coal (Huggins and Huffman, 1995;Evans et al., 2008). Interest in the local environment of chloride at the 136 interface in sodium chloride -water binary mixtures comes also from earlier work on nitric and hydrochloric acid adsorbed at 137 the ice-air interface. We have shown that nitrate and chloride forms solvation shells with a hydrogen-bonding structure similar 138 to that in aqueous solution in the interfacial region of ice at concentrations low enough to prevent melting (Krepelova et

Sample Preparation and Water Dosing 152
To prepare a sample, 1 µl of a 2.12 g sodium chloride (Fluka Trace Select 38979-25G-F) solution in 80 ml of water (Fluka 153 Trace Select 142100-12-F) was dropped at the centre of the sample holder and dried at 60 °C. The sample holder was then 154 moved into the flow-through cell and kept at UHV and at 60 °C to 80 °C for 45 minutes to remove volatile impurities. Water 155 vapour was dosed to the flow-through cell via a 0.8 mm i.d. steel capillary from the vapour above liquid water (Fluka Trace 156 Select 142100-12-F) in a vacuum-sealed, temperature-controlled glass reservoir. Before dosing, the water was degassed by 4 157 freeze-pump-thaw cycles. Pressure in the flow-through cell was monitored by a capacitance manometer (Baratron 626A) with 158 a measurement range from 5 × 10 −4 to 10 mbar and an accuracy of 0.25 % of the reading. Temperature was monitored with a 159 Pt-1000 sensor located at the edge of the sample holder. The sensor was calibrated prior to the experiments by growing ice on 160 the sample holder and noting its vapour pressure which is a direct measure of the temperature at the sample spot (Marti and 161 Mauersberger, 1993). During the experiments, the calibration was confirmed when ice was present. At 253 K, the offset 162 between temperature reading and calibration was found to be 4.3 ± 0.2 K. 163

X-ray exited Electron Spectroscopy 164
Partial Auger-Meitner electron-yield NEXAFS spectra at the Cl K-edge were acquired with a fixed kinetic energy window of 165 2370 eV to 2390 eV, which includes the KL2,3L2,3 Auger-Meitner peak of chlorine. The pass energy and dwell time were set 166 to 200 eV and 300 ms, respectively. The distance of the sample to the electron analyser inlet (working distance) was 1 mm 167 and the electron analyser was operated with an electron sampling aperture with a diameter of 500 µm. NEXAFS spectra were 168 measured by sweeping the incident X-ray photon energy across the chlorine K-edge from 2815 eV to 2845 eV with steps 169 ranging from 0.2 eV to 1 eV. The NEXAFS spectra were processed by dividing by the photon flux (I0) as derived in-situ using 170 a Ni coated membrane, by subtracting the mean pre-edge intensity as background, and by normalising to the mean intensity at 171 2830 eV to 2833 eV X-ray photon energy. Photoemission spectra (XPS) of O1s, Cl2p, Na1s, C1s, and Au4f were recorded at 172 an incident X-ray photon energy of 2200 eV and a with a pass energy of 100 eV and a dwell time of 120 ms. To quantify, a 173 linear background was applied and the photoemission signal was integrated in Matlab without any peak fitting. 174 1 Results and Discussion 175 NEXAFS of brine, halite, and hydrohalite 176 Figure 3 shows chlorine K-edge X-ray absorption spectra of NaCl salt and of frozen NaCl-water binary mixtures in the 177 presence of ice. NEXAFS spectroscopy probes the X-ray absorption of chlorine atoms, that is the resonant excitation of core 178 electrons into unoccupied molecular orbitals. As exactly those outer orbitals are forming chemical bonds, NEXAFS spectra 179 directly reflect changes to the local chemical environment and structural arrangement. NEXAFS spectroscopy of halogen salts 180 has thus been used to discuss their phase and chemical speciation in geological examples (Huggins and Huffman, 1995;Evans 181 et al., 2008). In this work, the X-ray absorption spectra were derived by recording the intensity of Auger-Meitner electrons at 182 2370-2390 eV, which corresponds to the KL2,3L2,3 transition in chlorine (Cleff and Mehlhorn, 1969). Detecting electrons, as 183 done in this work, makes X-ray absorption inherently surface sensitive, because electrons have a limited escape depth in matter 184 . The escape depth can be quantified by relating it to the inelastic mean free path (IMFP) of electrons in 185 matter and to the take-off angle of detected relative to the surface normal. The IMFP of electrons with a kinetic energy of 186 2380 eV is about 7 nm in NaCl and in ice (Tanuma et al., 1991). The take-off angle of electron detection is 30° in our set-up 187 (Orlando et al., 2016), this gives an escape depth of 6 nm, meaning that cumulatively, 95% of the electrons detected originate 188 from 18 nm, with an exponentially decreasing contribution from the surface towards the bulk. In the following, we report the 189 NEXAFS spectra derived from this interfacial region of NaCl-water binary mixtures to discuss changes in the solvation of 190 chloride by water as we explore the regions of the phase diagram where precipitation of sodium chloride has been described 191 for bulk samples. NaCl samples. The atomic ratio of the total carbon to oxygen from water present at the interface was below 0.25, except for 210 samples in Fig. 3A and F where total carbon to oxygen atomic ratios were 0.5-0.75. Adsorbed water molecules were present 211 at the interface of all samples, and ice or liquid water formed in some samples, as gaseous water was present in all experiments 212 with pressures between 0.3 mbar to 1.8 mbar. The atomic ratio was derived based on the measured C1s/O1s photoemission 213 intensities and a calibration to account for the analyser efficiency and total X-ray photoionization cross section using C1s/O1s 214 photoemission intensities of 0.8 mbar CO2 gas following a procedure used before (Krepelova et al., 2013). Direct comparison 215 between the individual samples and estimation of surface coverages of the carbon impurities is hampered by the varying water 216 content at the sample's interface as adsorption and water uptake varies with the individual relative humidity settings. The spectrum of sodium chloride in aqueous solution, Fig. 3B, shows a broader absorption peak in region I compared to the 232 spectra of the halite and a second feature at 2840 eV (region III) (Huggins and Huffman, 1995). In this work, the NEXAFS 233 spectrum of NaCl in aqueous solution was recorded in a solution-ice binary mixture based on the phase diagram (see 234 Efflorescence at the Interface). Based on freezing point depression data, the concentration of sodium chloride in such an 235 aqueous solution in equilibrium with ice is 3.5 mol l -1 (Rumble, 2019). The spectrum in Fig. 3B generally agrees with the X-236 ray absorption spectra reported for 0.1 mol l -1 and 1 mol l -1 aqueous solutions (Fig. 3 Huggins and Huffman (1995)) as it 237 captures the general decrease in intensity in region I with excitation energy and the increase in absorption in region III (Fig.  238 3). Discussing differences in the hydration structure of chloride at the water-air interface as compared to the bulk solution is Upon cooling further to 241 K, the spectrum changed significantly. The NEXAFS spectrum in Fig. 3D shows a wide peak in 252 region I with two well resolved features about 3 eV apart. That both features have a similar intensity, makes this spectrum 253 clearly distinct from those of an aqueous NaCl solution with its decreasing trend of absorption in region I. Compared to the 254 spectrum of aqueous NaCl solution (Fig. 3B, C), the absorption edge is shifted to higher photon energies in the spectrum in 255 Fig. 3D. The absence of a feature in region II makes the spectrum in Fig. 3D distinct from the spectrum of anhydrous NaCl 256 salt. Notably, the spectrum quality is greatly improved and is similar to that of the solid, anhydrous halite sample (Fig. 3A). 257 One factor impacting the spectral quality is the stability of the sample during the NEXAFS acquisition. The analysis of the Cl 258 2p photoemission spectra acquired before and after each NEXAFS run showed that the amount of chlorine detected in the 259 sample volume fluctuated by less than 10%, between 0 % and 9 %, in samples shown in Fig. 3

C, D, E, and F (Appendix A). 260
For comparison, samples in Fig. 3 A and B showed a decrease of 39 % and 43 % in the integrated Cl 2p signal intensity, 261 respectively, from prior-to after the NEXAFS was recorded. Possible reasons for these trends are an increase in adsorbed 262 water with time masking the intensity of the underlying chlorine (Fig. 3A) and changes in the distance of the sample to the 263 electron analyser with time leading to a reduction in the intensities of all compounds (Fig. 3 B). Both of these processes would 264 also affect the NEXAFS signal by inducing changes in intensity with time. Direct quantitative comparison is beyond the scope 265 of this work, and is hampered by the different probing depth of XPS and NEXAFS as given by the kinetic energy. 266

267
We assign the spectrum in Fig. 3D to that of sodium chloride dihydrate (hydrohalite, NaCl•2H 2 O). The NEXAFS spectrum of 268 the hydrohalite has to the best of our knowledge not been described before. The double peak feature in region I we observe

Efflorescence at the interface 337
Now that we have identified halite, the hydrohalites, and the aqueous solution by means of the NEXAFS spectra at the 338 interfacial region, we discuss their observation in the phase diagram. Figure 4A shows the sodium chloride -water phase 339 diagram in the temperature -relative humidity space as initially constructed by Koop et al. (2000a). In this work, the relative 340 humidity (RH) is referenced to the vapour pressure of (supercooled) water as parameterised by Marti and Mauersberger (1993).

Liquid below eutectic and nucleation 347
In a typical experiment, anhydrous salt was exposed to increased relative humidity at a fixed temperature of 259 K. The relative 348 humidity was increased by increasing the flux of water vapour into the experimental cell. Once the relative humidity reached 349 72 %, the sample started to dissolve by water up-take from the gas-phase and an aqueous solution was formed (brine). This 350 phase change was evident by the sample becoming shiny and then forming transparent spheres as observed by an endoscope 351 digital camera (Fig. 5B'). Then, the relative humidity was further increased and/or temperature was lowered to cross the ice 352 stability line until ice nucleation occurred at a modest oversaturation. Ice nucleation was evident by a sharp pressure drop from 353 the pressure dosed to the cell to the water vapor pressure of ice at that temperature. Generally, in the presence of ice, the partial 354 pressure of water in the flow-through cell is given by the vapor pressure of ice and thus a sole function of temperature. If the 355 water vapour pressure upstream of the flow through cell exceeds this value, the ice on the sample holder is growing, if it is set 356 below, the ice sublimates. Based on the calibration of the dosing reservoir temperature and partial pressure of water in the 357 flow-through cell (in absence of ice), the incoming H2O vapour flux was adjusted such that the equilibrium pressure in the cell 358 matched the vapour pressure of ice. 359 360 A NEXAFS spectrum was acquired at 259 K and 1.82 mbar on the ice stability line and taken as reference for aqueous sodium 361 chloride solution (Fig. 3B). When the temperature was lowered while adapting the flux of water into the set-up to match the 362 vapour pressure of ice at 248 K -249 K, thus 4 K to 5 K below the eutectic temperature (Fig. 4B), the NEXAFS spectra 363 revealed that the chloride at the air-ice interface is in an environment identical to aqueous chloride (Fig. 3C). While cooling 364 further and adjusting the vapor pressure to match that of ice at each temperature, a sudden change in the sample appearance, 365 becoming less transparent, was observed by the digital endoscope, indicating efflorescence of the sample. A NEXAFS 366 spectrum recorded reveals that hydrohalite has precipitated from the brine at 240.5 K and 74 % RH (Fig. 3D). Krepelova et al. 367 (2010a) has studied phase changes of sodium chloride at the interfacial region of sodium chloride -water binary mixtures 368 previously. They have probed the oxygen with XPS and partial electron yield NEXAFS spectroscopy and concluded that, in 369 the presence of ice, hydrohalite forms about 11 K below the eutectic (filled star in Fig. 4). Consistent with that, the chloride 370 has a local environment indistinguishable from that of the hydrohalite 11.4 K below the eutectic temperature and in the 371 presence of ice in the current study. Our results of precipitation in the presence of ice surfaces agree with the crystallization 372 temperature observed by Koop et al. (2000a). We lack a direct comparison to the bulk, because the electron yield NEXAFS 373 spectroscopy used in our work is inherently surface sensitive. Cooling sodium chloride solutions of varying concentration, 374 To investigate whether or not precipitated sodium chloride is the stable form at the interface at temperatures close to the 385 eutectic temperature, the sample at 240 K (green filled triangle, Fig. 4B) was warmed towards the eutectic temperature while 386 staying in the ice stability domain. Acquiring a NEXAFS spectrum (Fig. 3E) that resembles that of hydrohalite, shows that the 387 solid salt is the thermodynamically stable form also at temperatures close to the eutectic. We therefore interpret the existence 388 of liquid during the previous cooling of the sample (Fig. 4B, orange circle, Fig. 3C) at the interface as supercooled solution. 389 The sample was kept at this condition for 3 h and showing that liquid can exist for extended times at the air-ice interface below 390 the eutectic temperature and that the temperature alone is not sufficient to predict its presence. Rather the thermal history of 391 the snow needs to be considered. 392 393 For samples that were cooled to temperatures that triggered efflorescence, the chlorine NEXAFS spectra show that the 394 hydrohalite is the dominating phase at the interface of frozen sodium chloride -water binary mixtures. Cho et al. (2002) have 395 shown that when frozen aqueous solutions were warmed, a liquid fraction was observed below the eutectic temperatures. In 396 their experiments, ice was frozen in NMR tubes lowering the temperature to 228 K in 15 min. which is significantly colder 397 than the efflorescence temperatures observed here and by Koop et al. (2000a). After 10 minutes, the samples were warmed 398 and NMR signals were recorded. Interestingly, Cho et al. (2002)  started with an aqueous solution that was formed in-situ and was kept in equilibrium with a vapour pressure of roughly 404 1.9 mbar. The chloride concentration in such solutions is close to the concentration in a solution at 1.8 mbar and at 259 K, 405 where ice nucleation occurred and where the freezing point depression data give a concentration of 3.5 mol l -1 . This 406 concentration can be directly compared to the concentration in the initial solutions of Cho et al. (2002), which ranged from 407 below 0.01 mol l -1 to 0.5 mol l -1 . This back-of-the-envelope calculation thus suggests that the concentration of the solutions 408 from which ice nucleated in the experiments reported here exceeded those described by Cho et al. (2002) for which no liquid 409 fraction was observed. Now, the concentration of the initial solution from which ice precipitated, determines the ice to brine 410 ratio after ice formation. This is, because as the concentration of the brine is a sole function of temperature, the volume of the 411 brine relative to that of ice is given by the water to sodium chloride ratio in the initial solution. One might speculate that with 412 large amounts of brine relative to ice, that is concentrations of initial solutions from which ice nucleates > 0.5 mol l -1 , patches 413 and inclusions are larger in size than for more dilute solutions. The size of these patches or inclusions is of relevance, as surface 414 forces reduce the melting point only for inclusions in the nanometre range (Nye, 1991 i.e., the concentration of HCl was too low to melt the ice. Oxygen K-edge NEXAFS spectra showed that a substantial fraction 430 of the water molecules at the air-ice interface is arranged in a hydrogen-bonding structure like that of liquid water. The 431 NEXAFS spectrum of sodium chloride -ice mixtures (Fig. 3E) at 249 K are indistinguishable from those of hydrohalite. Taken 432 the spectra quality and the small difference in the shape of the liquid and of the hydrohalite spectrum, it is beyond the scope 433 of this work to elaborate whether the NEXAFS spectrum in Fig. 3E might be understood by deconvoluting it in its hydrohalite 434 and brine components and by this reveal a fraction of the chloride being embedded in a brine-like hydrogen bonding network. 435 The appearance of hydrohalite at air-ice interfaces might also be of interest to sea ice research, because precipitation of 500 hydrohalite increases the albedo of sea ice. During the Snowball Earth period, 700 million years before present, climatic 501 conditions may have favoured the existence of hydrohalite with its climatic feedback (Light et al., 2009)