Ice nucleation on particles in atmosphere and soil

 
In the works of Fukuta and Schaller (1982) and Wex et al. (2014) the study of ice nucleation on aerosol particles is presented. According to Fukuta and Schaller (1982) there “are presently three mechanisms of heterogeneous ice nucleation known by aerosol particles – deposition, condensation-freezing including immersion freezing, and contact freezing”. There is an attempt in Fukuta and Schaller (1982) to distinguish between condensation and immersion freezing as follows: “In the process of condensation-freezing nucleation, liquid water forms on the ice nucleus surface before freezing nucleation takes place in it. If the liquid has existed for some time on the nucleus surface before the freezing nucleation starts, the process is considered as immersion-freezing”.

In the works of Marcolli et al. (2007), Pinti et al. (2012) and Kaufmann et al. (2016) the results of heterogeneous immersed ice nucleation are presented. The differential scanning calorimeter (DSC, TA Instruments Q10) was used in that study, which allowed the determination of the phase transition temperatures in the range between 130 and 600 K with a precision of 0.01 K. The cooling and heating rates were of 10 and 1 K min^-1, respectively and the heat flux on the sample was measured. So, the determination of temperature of heterogeneous immersed ice nucleation is done by freezing samples of water – oil emulsion and water suspension. The water contained natural mineral particles and prepared modelling mineral powder. The portions of emulsion 4-15 mg and the droplets of suspension 1.8-2 mg were coated with oil and placed in an aluminium crystallising pan. Several cycles of freezing and thawing with given cooling intensity were conducted and the temperatures of phase transition were recorded according to the method developed by Marcolli et al., (2007). The freezing temperature was determined as the onset point of the freezing peak on the heat flux curve on a DSC thermogram as function of temperature (heterogeneous and homogeneous nucleation in the emulsified sample). The temperatures of heterogeneic ice nucleation lay within the range of 252-270 K and the temperatures of pure water ice nucleation lay on the graph below 252 K.

So we considered the analogy of heterogeneic ice nucleation in bulk immersion freezing of the atmospheric droplets containing aerosol particles and freezing of moisture within the wet grounds. For the experiment we used modelled sandy and kaolinite grounds with the weight wetness of about 25% and 90% and with the total mass of about 100 g and 80 g respectively. The samples were placed in stainless steel or plastic dishes and were cooled down in the refrigerated chamber with ambient temperature of about -5°C. The ground samples’ temperatures were measured and recorded. The temperatures of the moment of ice nucleation in the grounds were determined. The variation of the experimental sandy ground samples’ temperature with weight wetness of 25% placed in the refrigerated chamber can be seen on the graph.

The performed experiments of the ground samples freezing indicated that ice nucleation in the considered experimental samples of sandy and kaolinite grounds happen at the temperatures of about -4°C (close to the ambient temperature of the refrigerated chamber (-5°C)). This may be compared to the results of the heterogeneic ice nucleation on the particles of sand and kaolinite immersed in the water droplet presented in the works Marcolli et al., (2007), Pinti  et al. (2012) and Kaufmann et al. (2016). Relatively high ice nucleation temperature for the experimental samples of the modelled grounds can be also explained by the possibility of the interaction of not only ground moisture with the ground particles but also of the ground moisture with the stainless steel or plastic material of the dish which have higher ice nucleation activation material factor and the temperature of ice nucleation on it respectively.