Technical note: Sublimation of frozen CsCl solutions in ESEM: determining the number and size of salt particles relevant to sea-salt aerosols

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.