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<br />used operationally to trace the aerosols tested. Results showed that these devices can give self-consistent <br />results and typically measure about 33% of the ice nuclei measured in the larger cloud chamber and 25% <br />of the total AgI aerosols present. These results varied depending on the ice nucleus aerosol tested, <br />presumably due to differences in particle size and chemistry. Measurements of the ice-forming ability of <br />one aerosol (AgClo.22Io.7s-0.125NaCI) were also made with a continuous flow ice-thermal diffusion <br />chamber. These measurements showed that the water supersaturation dependence of the ice formation <br />rate by condensation-freezing for these aerosols varies by more than three orders of magnitude between <br />o and 7% supersaturation. Ice formation was nearly instantaneous above 7% supersaturation for all <br />aerosols capable of acting as ice nuclei. The various measurements taken will permit the quantitative <br />transfer of laboratory results on ice nucleus ability to a range of expected atmospheric conditions. <br /> <br />SUMMARY AND CONCLUSIONS <br /> <br />Yield values in the CSU isothermal cloud chamber were obtained for aerosols produced from com busting <br />2 and 3 weight % AgI solutions using the MSU Skyfire generator. The yields for aerosols from the 2% <br />AgI solutions were not substantially different than for 3% AgI solutions, indicating that no added benefit <br />exists for using 3% AgI solutions in warmer supercooled cloud conditions. Comparison of3% AgI <br />solution aerosols in the current tests with those tested in the MSU generator in 1972 gave excellent <br />agreement. Ventilation of the generator at higher wind speeds increased the generator yield across the <br />temperature spectrum tested by a factor of 2 to 10 times. This is an unusual result for most generators at <br />the warmest temperatures since the typical effect of ventilation is more rapid quenching of the bum and <br />production of smaller particle sizes which usually are less efficient at warmer temperatures. This may <br />merit further investigation. <br /> <br />Yield values obtained using the NA WC generator indicated greatly enhanced ice formation activity <br />warmer than -10 oC for AgI-NH4I-acetone water solutions (2% AgI) compared to earlier tests in 1978 and <br />1981. Apparently, some changes to generator combustion design have been made. As a consequence, a <br />chemical change to a solution to produce very efficient AgIo.7sClo.22-0.125NaCI composite nuclei, <br />produced only a slight enhancement in yield at the warmest temperature tested (-6 oc). Nevertheless, <br />these aerosols do appear to offer the potential for enhanced yield at even warmer temperatures. This <br />would have to be verified by further testing. Ventilation of the NA WC generator a higher wind speeds <br />was found to increase yield by about 10 times at -20 oC, but decrease the yield by 10 times at -6 oc. .This <br />was consistent with results obtained in previous calibrations and is believed to reflect the dependence of <br />activation on particle size (smaller during ventilation). <br /> <br />Examination of the ice formation rates in the isothermal cloud chamber indicated that the 2% AgI aerosols <br />produced by both the MSU and NA WC generators functioned by contact-freezing at temperatures warmer <br />than -16 oC in the ICC. Previous tests of the 3% AgI MSU generator aerosols for higher LWC and higher <br />droplet concentrations in the ICC indicated faster ice formation rates in proportion to droplet <br />concentration, quantitatively supporting this conclusion. Ice formation rates were faster at temperatures <br />below -16 oC. The ice formation mechanisms in this temperature regime coUld not be discerned. <br /> <br />Use of the solution containing ammonium iodide and paradichlorobenzene in the NA WC generator did <br />not force a rapid condensation-freezing nucleation process at water saturation in the ICC. Nevertheless, <br />evidence does indicate that a slower condensation-freezing process did occur. This ice formation process <br />could provide sub~tantial advantages in natural clouds over aerosols which realize their greatest activity as <br />contact-freezing nuclei. This is because the ice formation rates by contact-freezing are dependent on the <br />droplet concentration at water saturation, while the ice formation rates by condensation-freezing are not. <br />Consider, for example, a natural orographic cloud with 100 droplets cm-3 of similar sizes to those in the <br /> <br />56 <br /> <br />j <br />