AgI-NaI Aerosols as Ice Nuclei

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  • 1 Radiophysics Laboratory, CSIRO, Sydney, Australia
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Abstract

The ice-nucleating properties of cloud-seeding aerosols produced by burning a solution af AgI and NaI in acetone have been studied in a laboratory cloud chamber. The ice-forming ability of these mixed aerosols is not influenced as markedly by the supersaturation in the test cloud as is the case with “pure” AgI aerosols. This is ascribed to the hygroscopic nature of the particles which promotes drop formation at low supersaturations and allows the AgI to nucleate by a freezing mechanism. The relationship between nucleus size and nucleation temperature has been investigated and is compared with theory. Fletcher's theory, as recently modified to take account of the soluble component in mixed AgI aerosols,does not explain the rapid decrease in nucleation temperature with decreasing particle size and the wide size range of nuclei active at a particular temperature. A theory which assumes that nucleating sites of widely differing efficiencies are randonly distributed over the surface of the nucleating material is favored.

Abstract

The ice-nucleating properties of cloud-seeding aerosols produced by burning a solution af AgI and NaI in acetone have been studied in a laboratory cloud chamber. The ice-forming ability of these mixed aerosols is not influenced as markedly by the supersaturation in the test cloud as is the case with “pure” AgI aerosols. This is ascribed to the hygroscopic nature of the particles which promotes drop formation at low supersaturations and allows the AgI to nucleate by a freezing mechanism. The relationship between nucleus size and nucleation temperature has been investigated and is compared with theory. Fletcher's theory, as recently modified to take account of the soluble component in mixed AgI aerosols,does not explain the rapid decrease in nucleation temperature with decreasing particle size and the wide size range of nuclei active at a particular temperature. A theory which assumes that nucleating sites of widely differing efficiencies are randonly distributed over the surface of the nucleating material is favored.

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