Interfacial supercooling and the precipitation of hydrohalite in frozen NaCl solutions as seen by X-ray absorption spectroscopy
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Published:2021-04-23
Issue:4
Volume:15
Page:2001-2020
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ISSN:1994-0424
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Container-title:The Cryosphere
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language:en
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Short-container-title:The Cryosphere
Author:
Bartels-Rausch ThorstenORCID, Kong Xiangrui, Orlando Fabrizio, Artiglia Luca, Waldner Astrid, Huthwelker Thomas, Ammann MarkusORCID
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.
Funder
Schweizerischer Nationalfonds zur Förderung der Wissenschaftlichen Forschung
Publisher
Copernicus GmbH
Subject
Earth-Surface Processes,Water Science and Technology
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