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.
Highlights
Chemical cycling of halogens affects the composition of the troposphere and via this effect influences climate and impacts human health (Simpson et al, 2007; Abbatt et al, 2012; SaizLopez and von Glasow, 2012; Simpson et al, 2015)
We report the near-edge X-ray absorption fine-structure (NEXAFS) spectra derived from this interfacial region of NaCl–water binary mixtures to discuss changes in the solvation of chloride by water as we explore the regions of the phase diagram where precipitation of sodium chloride has been described for bulk samples
The upper few nanometres of the interfacial region of a sodium chloride–ice binary system was investigated in this study at various positions in the phase diagram
Summary
Chemical cycling of halogens affects the composition of the troposphere and via this effect influences climate and impacts human health (Simpson et al, 2007; Abbatt et al, 2012; SaizLopez and von Glasow, 2012; Simpson et al, 2015). Reactive chlorine species act as a powerful oxidant on atmospheric cycles that destroy or produce ozone and are relevant for the atmospheric oxidation capacity (Finlayson-Pitts, 2003; Thornton et al, 2010). Atmospheric ozone is of concern because it directly impacts atmospheric composition, health, and climate (Simpson et al, 2007). Prominent examples of these reactions in the atmosphere with chloride in sea salt or salt dust are shown in Fig. 1 and discussed in the following.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
