WMOD00298

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

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For deposition, results for nucleation activity are expressed by

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an active site density

Dd (cm-2) parameter. This quantity is computed'
ep
fraction of the aerosols nucleating ice by

using the cumulative

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2
deposition (Fd ) and the aerosol sizes (r ), by way of the equation,
ep .

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2
Fdep - 1 - EXP (-4 1[' r. Ddep)

(4.1)

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This method provides a way for quantifying nucleation size effects. The

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method shows that deposition nucleation rate should directly depend on

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the surface area of the aerosol particle because it assumes that

nucleating sites are uniformly distributed with surface area. This

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method was used by Schaller and Fukuta (1979), although they did not

use monodisperse aerosols in their studies to test the primary

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assumption. The validity of this approach is tested for the AgI

aerosols used in this study. A polynomial (multiplicative) function of

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ice supersaturation is fit to the computed site density, just as

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Huffman (1973) did for ice nucleus concentrations measured versus ice

supersaturation in an ice-thermal gradient diffusion chamber.

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4.1.2 Contact-Freezing Nucleation Experiments

The technique used to quantify contact-freezing nucleation

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activity of the ice nucleus aerosols employed the isothermal cloud

chamber. These experiments utilized the steady state and long time

response characteristics of this cloud chamber that are ideal for

studying this clearly time dependent mechanism.

As previously

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described, droplet concentrations are maintained in this chamber by

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continuous supply at a fixed temperature. Tests with known injected

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