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ICC tests. In this case, the contact-freezing rate constants expected in the natural cloud for MSU and NA WC natural draft AgI aerosols are obtained by multiplying the kacl values in Table 3 (-0.08 minor at T~ -16 oc) by the ratio of in the ICC versus natural droplet concentrations (100 cm.3 /2100 cm-3). For a 20 minute transit time at a constant temperature through the natural cloud, this implies that only about 7% of the potential yield (Tables 1 and 2) would be realized. For NA WC AgICI-0.125NaCI aerosols, the rate would be 0.1 min-] regardless of droplet concentrations. Thus, in 20 minutes transit at one temperature, 86% of the potential yield (Table 2) would occur. An order of magnitude difference in ice crystal forn:tation is inferred to be possible by switching aerosols from AgI to AgICI-0.125NaCt Future studies of these and other aerosols tested for this paper would benefit from analysis of dependence of ice formation rate on varied cloud droplet concentrations and humidities, in order to validate mechanisms. The measurements made with two NCAR counters coordinated to sample the same aerosols used in ICC tests indicated that the NCAR counters sampled ice nucleus aerosols at about one-third of the efficiency of the ICC at -20 oC after dilution airflow corrections are applied to the ICC data. In comparison to raw ICC results, such as are commonly reported from the CSU laboratory, the NCAR counters detected closer to two-thirds of the ice nuclei. There appeared to be differences in this counting efficiency factor as a function of the aerosol tested, but not enough data were collected to confirm this. Nevertheless, the counting efficiency noted is a factor of about ten higher than measured for two other NCAR counters sampling different AgI aerosols by Sackiw et at (1984). This could be related to differences in the counter response to the different aerosols tested in the two studies. Although the ice formation mechanism operating within the NCAR counters is unknown for the aerosols tested, the amount of ice crystals formed compared to the ICC are reasonable if one assumes that a contact-freezing process dominated, but was limited by the shorter residence time within the counters compared to the cloud chamber. ' The results of the ICC/NCAR counter cross-calibration should be extremely useful for quantifying the NCAR data in field experiment studies of cloud seeding using the aerosols tested. The excellent consistency notes between the two units tested was another positive result in this regard. We recommend cross-calibrations of the type performed here for NCAR counters before use in field operations. The CFD experiments on AgICI-0.125NaCl aerosols from the NA WC generator yielded several interesting results which demonstrate the utility of this technique alone or in concert with other measurements of ice nuclei. The results, which by their nature did not permit contact-freezing nucleation to be observed to any degree, indicated that these aerosols required greater than 4% water supersaturation to achieve 10% instantaneous activation by condensation-freezing at -16 oC. These conditions are probably met on some occasions during field generation due to the moisture released during combustion (Finnegan and Pitter, 1988). Combined with the ICC results, the CFD results also give vital information permitting. calculations of ice formation rates in real clouds. This will be the subject of future analyses and experimentation. The CFD results also show that it will be readily possible to easily and quantitatively describe differences in chemically different (e.g., more hygroscopic or more hydrophobic) ice nuclei in future experiments. Concerning the practical matter of using such an instrument simply for tracking the transport of manmade ice nuclei"by aircraft, the results simply that the device should be operated at a high sup-ersaturation to facilitate efficient detection. 57