Comparative Characterizations of the Ice Nucleus Ability of AgI Aerosols by Three Methods

Paul J DeMott, Arlin B Super, Gerhard Langer, David C Rogers, Jack T McPartland


Three methods were used to assess the ice nucleating ability of different AgI-based aerosols produced using two solution combustion generators. The standard method employed was an isothermal cloud chamber which has been historically used for "calibrations" of ice nucleus aerosol generators. Comparisons with historical data showed consistency over a 20 year period for one generator, but not for another. Data on rates of ice crystal formation were used to infer operative ice formation mechanisms, and make inferences to the relative utility of the different aerosols in the atmosphere. Comparative measurements of ice nucleus effectiveness at -20°C were made using two NCAR counters that have been used operationally to trace the aerosols tested. Results showed that these devices can give self-consistent results and typically measure about 33% of the ice nuclei measured in the larger cloud chamber and 25% of the total AgI aerosols present. These results varied depending on the ice nucleus aerosol tested, presumably due to differences in particle size and chemistry. Measurements of the ice-forming ability of one aerosol (AgCl0.22I0.78-0.125NaCl) were also made with a continuous flow ice-thermal diffusion chamber. These measurements showed that the water supersaturation dependence of the ice formation rate by condensation-freezing for these aerosols varies by more than three orders of magnitude between 0 and 7% supersaturation. Ice formation was nearly instantaneous above 7% supersaturation for all aerosols capable of acting as ice nuclei. The various measurements taken will permit the quantitative transfer of laboratory results on ice nucleus ability to a range of expected atmospheric conditions.

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