Review of “Sublimation of frozen CsCl solutions in ESEM: determining the number and size of salt particles relevant to sea-salt aerosols” submitted to ACP by Vetráková et al.

This study presents sublimation of CsCl solution to elucidate formation of fine salt particles and potential as origins of sea-salt aerosols in polar areas. This unique approach can provide us very interesting and important suggestion about sea-salt aerosols in polar areas. There are several interesting conclusions obtained from this study based on laboratory experiments using ESEM:

1) Fine salt particles formed through sublimation of salt solution under colder conditions.

2) Size and number concentrations of salt particles related to temperature and the concentrations of salt solution.

Overall, this topic and subject is suitable for scope of ACP. I am confident that this investigation has potential to demonstrate these interesting results. However, I found some weakness in the manuscript. Hence, I recommend major revision before publication in ACP. My specified suggestions and comments are listed below.

Major comments

1. Application of laboratory results to ambient conditions

In this study, sublimation processes were examined under several artificial conditions such as non-seeding, seeding, and LN frozen. Although I understand different conditions between laboratory experiments and fields, more careful discussion are required for robust conclusion. Specifically, authors need to discuss carefully whether the examined artificial conditions are available in polar areas, and what the most usual conditions are in polar areas. For example, there are a large amount of brine on younger sea ice. Actually, the wet surface is present on sea ice even below -20 °C (e.g., Hara et al., ACP, 2017). Therefore, “the edge” might not be realistic in such conditions. On the other hand, salinity of snow on the aged sea ice and multi-year sea ice are lower than that on the younger sea ice, as discussed in this study. However, I am not sure whether “brine” is present in the snow the aged sea ice and multi-year sea ice, or not. Is that available? At least, the snow on the aged sea ice and multi-year sea ice were not wet, as far as I observed that visibly.

1. Seeding

When solid materials are present in brine (i.e., seeding in this study), salts can be crystalized or precipitated on the solid materials. In this study, small ice crystals were used as “seeding” materials. Considering brine is attached to ice under ambient conditions on sea ice and in snow, “seeding” conditions are real situation. Or not? Which are realistic conditions (non-seeding or seeding) which authors considered. Additionally, upside surface of brine on sea ice and snow is attached usually to colder air in polar areas in contrast to colder surface in the bottom (i.e., cool stage) during the laboratory experiments. More careful explanation and discussion are required.

1. Evaporation and sublimation

Laboratory experiments showed clearly that many salt particles are observed on the substrates after sublimation and evaporation. This result is very interesting. Crystallization of the salt particles are found on the dry surface on the substrates where brine was absent via brine shift and sublimation as shown in Figs. in the manuscript. The dry surface might be not available under the conditions with presence of brine on sea ice and snow in polar areas.

1. Size and size distribution
2. In this study, maximum diameter is used as particle size. As shown in ESEM images, the morphology of salt particles was irregular not spherical. I understand the difficulty to measure the size of salt particles with irregular shape. However, more careful procedure to measure the size is required, for example, to measure minimum diameter and mean diameter. Also, variations of size distribution of maximum diameter, minimum diameter, and mean diameter should be discussed.

Specific comments

1. Line 187-189: Add information of Fig. No which you show.

1. Line 226-227: Size distribution of salt particles by non-seeding and seeding

“Seeding” preferred to form smaller (aerosolable) salt particles in this study. What are reasons of this different? More discussion is useful for readers.

1. Section 3.2: LN-frozen and full sublimation

LN-frozen samples were simulated with the complete freezing condition. Difference between LN-frozen samples and the other samples is very interesting to understand formation of small salt particles depending on freezing processes under the colder conditions. However, I wonder whether the complete freezing of brine can proceed in polar areas, particularly where sea-salt aerosols are released. As well as, I am not sure that full sublimation occur on the surface under the conditions with presence of brine. Add more careful explanation and discussion.

1. Section 3.5: Surface conditions of sea ice with brine and snow

In this section, implication to polar atmosphere and conditions was mentioned and discussed. Surface conditions in sea ice areas changed drastically depending on presence of brine, sea ice growth (aging), accumulation of snowfall, and so on, as shown by Hara et al. (2017). Similar to sea ice conditions, snow salinity (i.e., sea-salt concentrations) on sea-ice is varied largely by blowing snow and snowfall. Therefore, I strongly recommend that formation of aerosolable salt particles is discussed on every (or typical) surface conditions such as wet surface on younger sea ice by brine, conditions with presence of frost flower, wet saline snow on sea-ice, and low – middle saline snow on sea ice (aged seasonal sea ice or multi – year sea ice). Although some explanation and discussion were mentioned already in the manuscript, correspondence between laboratory results and the ambient conditions in the polar areas was interpreted hardly because of partly confusion.

1. Line 367: Temperature of mirabilite precipitation

Add reference which show the temperature of -6.4 °C.

1. Line 369-373:

Changes of surface conditions of sea-ice were shown in Hara et al. (2017).

Hara, K., Matoba, S., Hirabayashi, M., and Yamasaki, T.: Frost flowers and sea-salt aerosols over seasonal sea-ice areas in northwestern Greenland during winter–spring, Atmos. Chem. Phys., 17, 8577-8598, https://doi.org/10.5194/acp-17-8577-2017, 2017

1. Sizes of small salt particles shown in this study ranged in micrometer or larger than 10 mm. It is true that particles with size of micrometers are “aerosolable”. However, these particles may be attached to the surface of ice and snow in polar regions. What are specific processes to release sea-salt aerosols into the atmosphere? Because of less volatility of sea-salts, sea-salt aerosols must be released through the physical processes such as erosion during the strong winds and then sublimation. Although release of salt particles with size of micrometers can be explained by the processes, sea-salt aerosols in the Antarctic were distributed in smaller than 1 mm (Yang et al., 2019; Frey et al., 2020) and smaller than 100 nm (Hara et al., 2011). If possible, these issues should be discussed.

General comments:

Vetrákova et al. present a thorough study of sublimation from CsCl/water in an environmental SEM (ESEM). The research is well motivated, carried out with expertise, and features a good discussion. It is of good value for the scientific community, including atmospheric science. It is also well written, although slightly too much targetted to specialists.

The paper is rather large, and features a plethora of images and topics, and the results are described in great detail. It could profit either from substantial cutting, putting a narrower focus, or - in the other extreme - a more complete view. This would consist the "revision", while scientifically almost everything is very sound (see below).

In case of a more complete view, valuable information would be the phase diagrams NaCl/ice/water and CsCl/ice/water, discussions on crystal morphology of all species (would that include NaCl.2H2O?), a discussion of the regular patterns (e.g. fig. 7c), comparison to simple optical microscopy studies, (semi-)quantitative evaluations, etc.

Specific comments:

It is not always clear how much the paper builds on results from Závacká et al. (2022), and what is new (except of course the cation, and the advantages of using a heavy ion).

The discussion and interpretation of figure 2 is, compared to all others, very short. Specifically, the droplet-like features on figures 2a and 2b require an explanation. Rather few readers are experts in ESEM, let alone ESEM of multiphase systems!

The brine (section 3.3) must be supercooled, and indeed the droplet-like features suggest the liquid state. But is there any independent proof of the liquid nature?

Technical corrections:

The surface pretreatmenta are not provided. They are very important for the Peltier surface and for the silicon wafer (for which the reader requires additionally how it was fixed and thermally coupled to the Peltier).

Line 137: 650 Pa should not be called "ambient" pressure, which is ca. 101000 Pa.

Line 186: The temperature sensor and its setup are not described.