Journal cover Journal topic
Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
doi:10.5194/acp-2017-35
© Author(s) 2017. This work is distributed
under the Creative Commons Attribution 3.0 License.
Research article
25 Jan 2017
Review status
This discussion paper is under review for the journal Atmospheric Chemistry and Physics (ACP).
Evaporating brine from frost flowers with electron microscopy, and implications for atmospheric chemistry and sea-salt aerosol formation
Xin Yang1, Vilém Neděla2, Jiří Runštuk2, Gabriela Ondrušková3, Ján Krausko3, Ľubica Vetráková3, and Dominik Heger3 1British Antarctic Survey, Natural Environment Research Council, Cambridge, UK
2Environmental Electron Microscopy Group, Institute of Scientific Instruments of the CAS, Brno, Czech Republic
3Department of Chemistry, Faculty of Science, Masaryk University, Kamenice 5/A8, 625 00 Brno Research, and Centre for Toxic Compounds in the Environment (RECETOX), Masaryk University, Kamenice 5/A29, 625 00 Brno
Abstract. An environmental scanning electron microscope was used for the first time to obtain well-resolved images, in both temporal and spatial dimensions, of lab-prepared frost flowers (FFs) under evaporation within the chamber temperature range from −5 °C to −18 °C and pressures above 500 Pa. Our scanning shows temperature-dependent NaCl speciation: the brine covering the ice was observed at all conditions, whereas the NaCl crystals were formed at temperatures below −10 °C as the brine oversaturation was achieved. Finger-like ice structures covered by the brine, with a diameter of several micrometres and length of tens to one hundred micrometres, are exposed to the ambient air. The brine-covered fingers are highly flexible and cohesive. The exposure of the liquid brine on the micrometric fingers indicates a significant increase in the brine surface area compared to that of the flat ice surface at high temperatures, whereas the NaCl crystals can become sites of heterogeneous reactivity at lower temperatures. There is no evidence that, without external forces, salty FFs could automatically fall apart to create a number of sub-particles at the scale of micrometres as the exposed brine fingers seem cohesive and hard to break in the middle. The fingers tend to combine together to form large spheres and then join back to the mother body, eventually forming a large chunk of salt after complete dehydration. A present microscopic observation rationalizes several previously unexplained observations, namely, that FFs are not a direct source of sea salt aerosols and that saline ice crystals under evaporation could accelerate the heterogeneous reactions of bromine liberation.

Citation: Yang, X., Neděla, V., Runštuk, J., Ondrušková, G., Krausko, J., Vetráková, Ľ., and Heger, D.: Evaporating brine from frost flowers with electron microscopy, and implications for atmospheric chemistry and sea-salt aerosol formation, Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2017-35, in review, 2017.
Xin Yang et al.
Xin Yang et al.

Video supplement

S1 Video to complete Figure 3.
X. Yang, V. Neděla, J. Runštuk, G. Ondrušková, J. Krausko, Ľ. Vetráková, and D. Heger
doi:10.5446/21430
S2 Video to complete Figure 5.
X. Yang, V. Neděla, J. Runštuk, G. Ondrušková, J. Krausko, Ľ. Vetráková, and D. Heger
doi:10.5446/21431
S4 Video to complete Figure 7.
X. Yang, V. Neděla, J. Runštuk, G. Ondrušková, J. Krausko, Ľ. Vetráková, and D. Heger
doi:10.5446/21432
S5 A video of visualizing the formation of NaCl crystals on the top of the surface brine layer of FF related to Figure 8.
X. Yang, V. Neděla, J. Runštuk, G. Ondrušková, J. Krausko, Ľ. Vetráková, and D. Heger
doi:10.5446/21433
Xin Yang et al.

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Short summary
A unique environmental electron microscope was used for monitoring the evaporation of salty frost flowers. We observe a cohesive villous brine surface layer facilitating the formation of NaCl microcrystals at temperatures below −10 °C as the brine oversaturation is achieved. This finding confirms the increased surface area and thus also the enhanced heterogeneous reactivity; however, no support for the easiness of fragmentation to produce aerosols can be provided.
A unique environmental electron microscope was used for monitoring the evaporation of salty...
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