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Chemical kinetic theory and methodology is applied to examine the ice nucleating properties of silver iodide (AgI) and silver iodide-silver chloride (AgI-AgCl) aerosols in a large cloud chamber held at water saturation. This approach uses temporal data on ice crystals formation with changes in key nucleation parameters such as temperature, water vapor concentration and droplet concentrations. The inter-relationships between ice nucleation effectiveness, nucleation mechanisms, nuclei chemical and physical properties and the rate of appearance of ice crystals can be deduced. The theory and methodology can be applied to atmospheric experimentation. Ice nucleation effectiveness increases of up to three orders of magnitude over that of AgI aerosols can be achieved with AgI-AgCl solid solution aerosols. Both aerosols are shown to form ice crystals by predominantly contact nucleation at temperatures of −16°C and warmer. Nucleation of the ice phase following collision is identified as a very rapid process, so that the rate of appearance of ice crystals is controlled by the much slower transport rate of nuclei to cloud droplets. The higher efficiency of AgI-AgCl nuclei with respect to the standard AgI nuclei is attributed to an improvement in the relative rates of nucleation versus deactivation or solution following collision of the nuclei with cloud droplets. This increase is most probably due to epitaxy and/or surface “active site” improvements. At a temperature of −20°C, all tested aerosols formed ice crystals by a combination of contact nucleation and deposition nucleation. The percentage of ice crystals formed by deposition correlated well with a minimum particle size of 500 Å for an appreciable deposition rate.
DeMott et al. (Fri,) studied this question.