Interfacial supercooling and the precipitation of hydrohalite in frozen NaCl solutions as seen by X-ray absorption spectroscopy

Abstract

Abstract. Laboratory experiments are presented on the phase change at the surface of sodium chloride–water mixtures at temperatures between 259 and 241 K. Chloride is a ubiquitous component of polar coastal surface snow. The chloride embedded in snow is involved in reactions that modify the chemical composition of snow as well as ultimately impact the budget of trace gases and the oxidative capacity of the overlying atmosphere. Multiphase reactions at the snow–air interface have been of particular interest in atmospheric science. Undoubtedly, chemical reactions proceed faster in liquids than in solids; but it is currently unclear when such phase changes occur at the interface of snow with air. In the experiments reported here, a high selectivity to the upper few nanometres of the frozen solution–air interface is achieved by using electron yield near-edge X-ray absorption fine-structure (NEXAFS) spectroscopy. We find that sodium chloride at the interface of frozen solutions, which mimic sea-salt deposits in snow, remains as supercooled liquid down to 241 K. At this temperature, hydrohalite exclusively precipitates and anhydrous sodium chloride is not detected. In this work, we present the first NEXAFS spectrum of hydrohalite. The hydrohalite is found to be stable while increasing the temperature towards the eutectic temperature of 252 K. Taken together, this study reveals no differences in the phase changes of sodium chloride at the interface as compared to the bulk. That sodium chloride remains liquid at the interface upon cooling down to 241 K, which spans the most common temperature range in Arctic marine environments, has consequences for interfacial chemistry involving chlorine as well as for any other reactant for which the sodium chloride provides a liquid reservoir at the interface of environmental snow. Implications for the role of surface snow in atmospheric chemistry are discussed.

Salt Enrichment and Dynamics in the Interface of Supercooled Aqueous Droplets

“…We would like now to discuss the consequences of the NaCl location for the reactivity of atmospheric aerosols. Donaldson et al have detected that brine exclusion to the surface quasi-liquid layer (QLL) of ice , affects the kinetics of photolysis reactions of adsorbed polycyclic aromatic hydrocarbons (e.g., anthracene, harmine). Similarly, it is expected that such organic molecules can also be adsorbed on aerosol surfaces.…”

A temperature‐dependent hydrating structure around SO42− investigated from Raman spectroscopy

“…The water–Cl − interaction may also include the D 2 O/H 2 O···Cl − hydrogen bond 34,37–41 except for electrostatic (or Coulomb) interaction 42,43 . The D 2 O/H 2 O–Cl − interaction should have a different strength from the SO 4 2− ···H–O–H/D–O–D hydrogen bond, as indicated from the different hydrating structures near the surface 44 and the different hydrate forms for Na 2 SO 4 and NaCl: Na 2 SO 4 ·10H 2 O (a stable compound at ambient conditions) and NaCl·2H 2 O 45 (formed only below ice point). It can be further expected that SO 4 2− has a larger hydrating scale than Cl − .…”

Low-temperature ice nucleation of sea spray and secondary marine aerosols under cirrus cloud conditions

“…Several previous studies have analyzed the temperature-and humiditydependent phase states of NaCl particles, since they are often used as a proxy for SSA. Previous studies have found that the phase state of NaCl after efflorescence depends on the temperature at which efflorescence occurred, with anhydrous NaCl observed for efflorescence temperatures above 273 K ( Bartels-Rausch et al, 2021;Koop et al, 2000b;Peckhaus et al, 2016;Wagner et al, 2012). Since the majority of aerosol drying, including the efflorescence of the salt components of pSSA and aSSA+SMA, occurred at room temperature (Fig.…”