Using an environmental scanning electron microscope, we quantified for the first time aerosol‐sized salt particles formed during the sublimation of sea ice as a function of temperature and concentration. The sublimation temperature of the ice is a dominating physical factor to determine the size of the residua: Below −20°C, micron‐sized pieces of salt emerge, whereas above the temperature large chunks of salt are detected. Another such aspect influencing the distribution of sizes in salt particles is the concentration: Micron‐sized particles are observed exclusively at salinities below 3.5 psu, while below 0.085 psu particles with a median smaller than 6 μm arise from sea ices at any subzero temperature. Moreover, when a chunk of salt sublimes at less than −30°C to be dried and warmed later, a large number of sub‐micron crystals will appear. We relate our findings to the production of the polar atmospheric sea salt aerosols.

Frozen aqueous solutions are an important subject of study in numerous scientific branches including the pharmaceutical and food industry, atmospheric chemistry, biology, and medicine. Here, we present an advanced environmental scanning electron microscope methodology for research of ice samples at environmentally relevant subzero temperatures, thus under conditions in which it is extremely challenging to maintain the thermodynamic equilibrium of the specimen. The methodology opens possibilities to observe intact ice samples at close to natural conditions. Based on the results of ANSYS software simulations of the surface temperature of a frozen sample, and knowledge of the partial pressure of water vapor in the gas mixture near the sample, we monitored static ice samples over several minutes. We also discuss possible artifacts that can arise from unwanted surface ice formation on, or ice sublimation from, the sample, as a consequence of shifting conditions away from thermodynamic equilibrium in the specimen chamber. To demonstrate the applicability of the methodology, we characterized how the true morphology of ice spheres containing salt changed upon aging and the morphology of ice spheres containing bovine serum albumin. After combining static observations with the dynamic process of ice sublimation from the sample, we can attain images with nanometer resolution.

Abstract. We present a novel technique that elucidates the mechanism of the formation of small aerosolizable salt particles from salty frozen samples. We demonstrated that CsCl may be a suitable probe for sea salts due to its similar subzero properties and sublimation outcomes: CsCl substantially increased the visibility of the salt both during and after ice sublimation. Hence, we identified the factors that, during the sublimation of a frozen salty 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; we showed that the formation of small salt particles is generally restricted if the brine is liquid during the ice sublimation, i.e. at temperatures higher than the eutectic temperature (Teu). Small salt particles that might be a source of atmospheric aerosols were formed predominantly at temperatures below Teu, and their structures strongly depended on the concentration of the salt. For example, the sublimation of those samples that exhibited a concentration of less than 0.05 M often produced small aerosolizable isolated particles that are readily able to be windblown. Conversely, the sublimation of 0.5 M samples led to the formation of relatively stable and largely interconnected salt structures. Our findings are in good agreement with other laboratory studies which have unsuccessfully sought salt aerosols from, for example, frost flowers at temperatures above Teu. This study offers an explanation of the previously unexplored behaviour.

Abstract. Here we present a novel technique that enlightens the mechanism of formation of small aerosolizable salt particles from salty frozen samples. We demonstrated that CsCl may be a suitable probe for the sea salt due to their similar subzero properties and sublimation outcomes: Using CsCl substantially increased the visibility of the salt both during and after ice sublimation. Hence, we identified the factors that, during the sublimation of a frozen salty 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: We showed that the formation of the small salt particles is generally restricted if the brine is liquid during the ice sublimation, i.e., at temperatures higher than the eutectic temperature (Teu). Small salt particles that might be a source of atmospheric aerosols were formed predominantly at the temperatures below the 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. Our findings are in a good agreement with other laboratory studies unsuccessfully seeking for salt aerosols, e.g., from the frost flowers, at temperatures above the Teu. This study offers an explanation of this previously unexplained behaviour.

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