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Donald E. Hagen, Rodney J. Anderson, and James L. Kassner Jr.

Abstract

Experimental data on ice nucleation, presented in an earlier paper, are analyzed to yield information about the homogeneous nucleation rate of ice from supercooled liquid and the heights of energy barriers to that nucleation. The experiment consisted of using an expansion cloud chamber to nucleate from the vapor a cloud of supercooled pure water drops and the observation of the fraction of drops which subsequently froze. The analysis employed standard classical homogeneous nucleation theory. The data are used to extract the first experimental measurement (albeit indirect) of the activation energy for the transfer of a water molecule across the liquid-ice interface at temperatures near −40°C. The results provide further evidence that the local liquid structure becomes more icelike as the temperature is lowered.

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Rodney J. Anderson, Ronald C. Miller, James L. Kassner Jr., and Donald E. Hagen

Abstract

Observations of the homogeneous nucleation of water vapor in an expansion cloud chamber have been carried out for the temperature range −50 to +17°C in the carrier gases argon and helium. We have found that the onset of the ice phase in freshly nucleated drops always occurs in the form of a two-stage process, condensation followed by homogeneous freezing at temperatures near −40°C. Ice particles appear as brilliant spherical particles in the cloud of liquid drops which scatter much less light. The critical gas temperature associated with the observation of ice nucleation depends on the type of carrier gas, the duration of the minimum final temperature, and whether there are ions or re-evaporation nuclei present. These effects and the analysis of the total homogeneous nucleation rate (liquid drops plus ice particles) strongly support the conclusion that the ice particles result from the freezing of liquid water drops which have been nucleated homogeneously from the vapor phase. A somewhat higher critical freezing temperature is observed in the absence of an electric clearing field. This probably is an indication that ice particles preferentially form on ions or simply that droplets which nucleate slightly earlier on ions have a chance to grow to a larger size, thus increasing the droplets’ probability of freezing. An ice memory effect has also been observed in nucleation which occurs on re-evaporation nuclei remaining from previous expansions. lens and re-evaporation nuclei raise the threshold temperature of ice nucleation about 1 and 2°C, respectively, above the critical spontaneous freezing temperature (−41°C). Consequently, they would be expected to have little impact on atmospheric processes.

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