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- Author or Editor: J. L. Kassner Jr. x
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Abstract
In recent years the thermal diffusion chamber has found wide application in studying condensation nuclei (CCN) which are effective in natural cloud and fog formation. Explicit solution of the equations governing the transient behavior of the chamber suggests the necessity for precise control of the temperature or the relative humidity of the incoming sample if meaningful measurements are to be obtained. A recent conclusion of Fitzgerald, that transient supersaturations exceeding the steady-state peak value may arise if the incoming sample is saturated at a temperature less than that of the (hot) top plate, is also verified. Further, it is pointed out that as long as turbulence occurs while introducing the sample, the transient behavior of the chamber remains indeterminate. This defect may be eliminated by giving special consideration to the method of sample introduction. The measurements of CCN concentration as a function of time at Rolla taken with a thermal diffusion chamber are also presented. The CCN concentration, in general, is found to follow local meteorological conditions. The trend of our data shows a qualitative agreement with that available in the literature. It is suggested that an intercomparison of the data taken on a carefully fabricated thermal diffusion chamber with those taken on the other types of cloud chamber would help to decide the potential value of the former as a CCN counter.
Abstract
In recent years the thermal diffusion chamber has found wide application in studying condensation nuclei (CCN) which are effective in natural cloud and fog formation. Explicit solution of the equations governing the transient behavior of the chamber suggests the necessity for precise control of the temperature or the relative humidity of the incoming sample if meaningful measurements are to be obtained. A recent conclusion of Fitzgerald, that transient supersaturations exceeding the steady-state peak value may arise if the incoming sample is saturated at a temperature less than that of the (hot) top plate, is also verified. Further, it is pointed out that as long as turbulence occurs while introducing the sample, the transient behavior of the chamber remains indeterminate. This defect may be eliminated by giving special consideration to the method of sample introduction. The measurements of CCN concentration as a function of time at Rolla taken with a thermal diffusion chamber are also presented. The CCN concentration, in general, is found to follow local meteorological conditions. The trend of our data shows a qualitative agreement with that available in the literature. It is suggested that an intercomparison of the data taken on a carefully fabricated thermal diffusion chamber with those taken on the other types of cloud chamber would help to decide the potential value of the former as a CCN counter.
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.
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.
Abstract
The clustering of water vapor about ions is important because of its relevance to atmospheric electrical processes. For this reason we have placed our emphasis particularly on the description of the size distribution (concentrations) and mobilities of the small ion clusters at various humidities. From our present theoretical study, we find that most of the hydronium ions H3O+ tend to associate with a small number of water molecules to form a hydrated ion cluster even at extremely low humidities in the range of 5 × 10−3 to 1%. At atmospherically more realistic humidities and at the room temperature, our computed number of water molecules in the hydrated ion clusters is predicted to be relatively small. It is then conjectured that ion-induced nucleation process (if it occurs) starts rather from the small hydrated ion clusters which initially existed even at extremely low humidities in the atmosphere. In addition, we also find that, in general, the hydrated ion clusters of small sizes corresponding to the mass range of 2–5 water molecules are responsible for the ion mobility range of 2–2.5 cm−2 (V s)−1. For reduced mobility below 2.0 cm2 (V s)−1, the mass of the hydrated ion cluster is predicted to be greater than that of approximately five water molecules. The simultaneous estimation of size distribution and mobility aids us in better understanding observed mobility spectra and the nature of atmospherically important prenucleation clusters, including the information of their electric conductivities in the atmosphere.
Abstract
The clustering of water vapor about ions is important because of its relevance to atmospheric electrical processes. For this reason we have placed our emphasis particularly on the description of the size distribution (concentrations) and mobilities of the small ion clusters at various humidities. From our present theoretical study, we find that most of the hydronium ions H3O+ tend to associate with a small number of water molecules to form a hydrated ion cluster even at extremely low humidities in the range of 5 × 10−3 to 1%. At atmospherically more realistic humidities and at the room temperature, our computed number of water molecules in the hydrated ion clusters is predicted to be relatively small. It is then conjectured that ion-induced nucleation process (if it occurs) starts rather from the small hydrated ion clusters which initially existed even at extremely low humidities in the atmosphere. In addition, we also find that, in general, the hydrated ion clusters of small sizes corresponding to the mass range of 2–5 water molecules are responsible for the ion mobility range of 2–2.5 cm−2 (V s)−1. For reduced mobility below 2.0 cm2 (V s)−1, the mass of the hydrated ion cluster is predicted to be greater than that of approximately five water molecules. The simultaneous estimation of size distribution and mobility aids us in better understanding observed mobility spectra and the nature of atmospherically important prenucleation clusters, including the information of their electric conductivities in the atmosphere.
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.
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.
