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Crystallization of Supercooled Droplets of Aqueous Solutions

Measurements show that the droplet cloud can be in a supercooled state, i.e. to remain liquid at temperatures well below the melting temperature, especially if the water is not in contact with any solid surface, which would contribute to the crystallization process. The crystallization of supercooled droplets in the clouds may occur by two mechanisms: homogeneous and heterogeneous formation of ice nuclei.

Homogeneous freezing of supercooled solution droplets plays an important role at temperatures below -35 °C, i.e. on the clouds formation in the upper troposphere and lower stratosphere.

Relatively recently a thermodynamic theory of homogeneous crystallization of an aqueous solution was developed by Koop and other. The authors show that a homogeneous formation of ice nuclei in aqueous solutions depends on water activity of the solution, which depends on dissolved substances in the water and not on its nature. It was found that the influence of solution and pressure has very similar effect on the rate of ice nucleation.
In contrast to this theory, which uses the parameterization of dependence of the nucleation rate on water activity, we propose to calculate the homogeneous ice nucleation rate in the supercooled droplets of the solution by a formula similar to the classical expression for the calculation of the rate of ice nuclei formation in the pure water, but taking into account the presence of solutes.

In the lower and middle troposphere ice crystals appear mainly due to the heterogeneous mechanism of the ice nuclei formation on the surface of substrates in a volume of supercooled droplets. To start the process of heterogeneous crystallization the presence of foreign particles is needed. One significantly lowers the energy barrier for the formation of the solid phase on the surface of substrates in the volume of supercooled droplets.

The presence of substrates in the supercooled droplets increases their freezing temperature. However, not all atmospheric aerosols can serve as ice nuclei. Value of the surface area of the substrate has a big impact on the heterogeneous freezing. To take into account all features of the substrates allows such parameter as the specific linear energy. The most favorable conditions for the formation of ice nuclei are on the surfaces of the substrates with a low specific surface energy, which has low energy cost of a new phase particle formation.

Crystallization of citric acid

Substrate area calculator

Major axis (2*b):m
Minor axis (2*a):m
  
Substrate area:
(prolate spheroid)
m2


Chukin-Platonova Model of Supercooled Aqueous Solution Droplet Crystallization (CPM)