Sublimation of frozen CsCl solutions in ESEM: determining the number and size of salt particles relevant to sea-salt aerosols
- 1Environmental Electron Microscopy Group, Institute of Scientific Instruments of the Czech Academy of Sciences, Brno, Czech Republic
- 2British Antarctic Survey, Natural Environment Research Council, Cambridge, UK
- 3Department of Chemistry, Faculty of Science, Masaryk University, Brno, Czech Republic
- 1Environmental Electron Microscopy Group, Institute of Scientific Instruments of the Czech Academy of Sciences, Brno, Czech Republic
- 2British Antarctic Survey, Natural Environment Research Council, Cambridge, UK
- 3Department of Chemistry, Faculty of Science, Masaryk University, Brno, Czech Republic
Abstract. We identified the factors that, during the sublimation of a frozen CsCl solution, are important in generating fine salt particles as a possible source of salt aerosol. The number, size, and structure of the particles that remain after ice sublimation were investigated with respect to the concentration of the salt in the sample, the freezing method, and the sublimation temperature. The last-named aspect is evidently of primary importance for the preference of fine salt crystals over a large compact piece of salt. Independently of the concentration and freezing method tested, the sublimation of the frozen samples above the eutectic temperature (Teu) yielded a large compact piece of salt, namely, an improbable source of aerosol particles. Small salt particles that might be a source of atmospheric aerosols were formed predominantly at the temperatures below Teu, and their structures strongly depended on the concentration of the salt. For example, the sublimation of those samples that exhibited less than 8 psu (0.05 M) often produced small aerosolizable isolated particles readily able to be windblown. Conversely, the sublimation of 78 psu (0.5 M) samples led to the formation of relatively stable and largely interconnected salt structures. Presumably, our findings have important implications because the formed salt particles may assume the role of cloud condensation nuclei and ice-nucleating particles, affect polar atmospheric radiative forcing, and facilitate heterogeneous atmospheric chemistry.
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Ľubica Vetráková et al.
Status: open (until 28 May 2022)
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RC1: 'Comment on tc-2021-376', Gernot Nehrke, 17 May 2022
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The manuscript represents a study on the growth of CsCl crystals within the chamber of an ESEM and, based on these observations, draws conclusions for the formation sea-salt aerosols. The relatively large effective cross section of Cs results in a good contrast in the BSD signal, allowing a good separation between the CsCl crystals and ice. The authors argue that CaCl can be used to study the formation of sea-salt crystals (mainly NaCl) due to the similarity in eutectic temperatures.
At this point I do not agree with the authors. The difference in solubility product of both minerals is huge (more than a factor of four). The formation of minerals and their size in from sea ice is a complex process that involves the presence of different mineral phases. For example, the mineral precipitation during the formation of frost-flowers follows a complex temperature profile. Often the increase in temperature is moderate in the beginning (the water is already close to the freezing temperature of sea-water) followed by a sharp drop in temperature. The cooling rates used in the ESEM study in which a droplet of distilled water containing various amounts of CsCl does not represent a situation that mimics the natural one. I don’t see an explanation how the different scenarios used (cooling rates, concentrations, and seeded vs. un-seeded) relate to natural processes. In an isolated, relatively clean system like the water droplet in an ESEM processes like supercooling are very likely, which will have a strong impact on the precipitation kinetics. This is not comparable to the natural system.
To summarize, I do not criticize the experiments itself, but I have my doubts that they allow any conclusions for the formation of “salt particles relevant to sea-salt aerosols). In my opinion the study would be more suitable for a journal like the “Journal of Crystal Growth” or “Crystal Growth and Design”.
Best regards
Gernot Nehrke
Ľubica Vetráková et al.
Ľubica Vetráková et al.
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