WMOD00298

• Title: Quantifying Ice Nucleation by Silver Iodide Aerosols
• Date: 5/1/1990
• State: CO
• Weather Modification - Doc Type: Report

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combustion of AgloNH4I-acetone-water solutions containing ammonium

perchlorate (30 mole % with respect to AgI was found to be optimal).

Both the AgI and AgI-AgC1 nuclei produced by solution combustion have

been shown to function primarily and preferentially as contact-freezing

nuclei in the high concentration, water saturated, conditions in the

CSU isothermal cloud chamber. Other experiments conducted at saturation

ratios between ice saturation and water saturation in the CSU dynamic

cloud chamber have demonstrated that these nuclei can initiate ice

formation by a deposition mechanism, at least at temperatures colder

than about -160C (DeMott et al., 1984b). Size effects were not

investigated in these preliminary studies. Feng et a1. (1989) have also

observed nucleation by these aerosols in cloudless air, as warm as

-10oC. However, humidity was not well controlled in their study.

Finnegan and Pitter (1988) have also shown that the AgI-AgCl aerosols

may act very rapidly by a condensation-freezing nucleation mode when

introduced directly into cloud, due to high supersaturations generated

with the addition of water vapor from combustion. Comprehensive

quantitative descriptions of these results for AgI-AgCl aerosols have
not been developed.

Solution combustion type ice nuclei generators can be made to

produce aerosols that form ice preferentially by condensation-freezing

or immersion-freezing nucleation mechanisms. One means by which this

occurs is when a comp1exing and solubilizing reagent such as sodium

iodide (NaI) is used together with AgI (Mossop and Jayaweera, 1969).

Blumenstein et al. (l987) have demonstrated that the widely used

AgloNaI aerosols nucleate ice by two different condensation-freezing

modes; one acts quickly (within seconds) in the presence of a transient