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The Cryosphere An interactive open-access journal of the European Geosciences Union
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Chemical reactions in sea-salt embedded in costal polar snow impact the composition and air-quality of atmosphere. Here, we investigate the phase changes of sodium chloride. This is of importance, because chemical reactions proceed faster in liquid solutions as compared to solid salt and the precise precipitation temperature of sodium chloride is still under debate. We focus on the upper nanometer of sodium chloride – ice samples because of its role as reactive interface in the environment.
Preprints
https://doi.org/10.5194/tc-2020-253
https://doi.org/10.5194/tc-2020-253

  27 Nov 2020

27 Nov 2020

Review status: this preprint is currently under review for the journal TC.

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

Thorsten Bartels-Rausch1, Xiangrui Kong1,a, Fabrizio Orlando1,b, Luca Artiglia1, Astrid Waldner1, Thomas Huthwelker2, and Markus Ammann1 Thorsten Bartels-Rausch et al.
  • 1Laboratory of Environmental Chemistry, Paul Scherrer Institut, Villigen PSI, Switzerland
  • 2Swiss Light Source (SLS), Paul Scherrer Institut, Villigen PSI, Switzerland
  • anow at: Department of Chemistry and Molecular Biology, Atmospheric Science, University of Gothenburg, Gothenburg, Sweden
  • bnow at: Omya International AG, Oftringen, Switzerland

Abstract. Laboratory experiments are presented on the phase change at the surface of sodium chloride – water mixtures at temperatures between 259 K and 240 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 found 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, remain as supercooled liquid down to 240 K. Below this temperature, hydrohalite exclusively precipitates, 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 253 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 240 K, which spans the most common temperature range in Polar 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 on atmospheric chemistry are discussed.

Thorsten Bartels-Rausch et al.

 
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Thorsten Bartels-Rausch et al.

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Data set on interfacial supercooling and the precipitation of hydrohalite in frozen NaCl solutions by X-ray absorption spectroscopy Bartels-Rausch, Thorsten https://doi.org/10.16904/envidat.164

Thorsten Bartels-Rausch et al.

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Short summary
Chemical reactions in sea-salt embedded in costal polar snow impact the composition and air-quality of atmosphere. Here, we investigate the phase changes of sodium chloride. This is of importance, because chemical reactions proceed faster in liquid solutions as compared to solid salt and the precise precipitation temperature of sodium chloride is still under debate. We focus on the upper nanometer of sodium chloride – ice samples because of its role as reactive interface in the environment.
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