James L. Kassner

Homogeneous nucleation rate measurements for water over a wide range of temperature and nucleation rate

An expansion cloud chamber was used to measure the homogeneous nucleation rate for water over a wide range of temperature from 230–290 K and nucleation rates of 1–106 drops cm−3 s−1. The comprehensive and extensive nature of this data allows a much more detailed comparison between theory and experiment than has previously been possible. The expansion chamber technique employs continuous pressure measurement and an adiabatic pulse of supersaturation to give the time history of supersaturation and temperature during the nucleation. The resulting drop concentration is determined using photographic techniques. The experimental observations are presented in tabular form and from them an empirical nucleation rate formula is determined: J=S2 exp[328.124−5.582 43T+0.030 365T2−5.0319E−5T3 −(999.814−4.100 87 T+3.010 84E−3 T2)ln−2S], where J is the nucleation rate in units of drops cm−3 s−1. S is the supersaturation ratio and T is the temperature in K.

The nucleation of water vapor in the absence of particulate matter and ions

Abstract The nucleation rate of droplets from water vapor in a helium atmosphere was measured with an expansion cloud chamber as a function of temperature, supersaturation, and sensitive time. In addition to homogeneous nucleation, a form of heterogeneous nucleation was observed to occur above the ion limit, presumably upon neutral centers whose concentration is low and seemingly dependent upon the initial partial pressure of water vapor present. The semiphenomenological classical theory was modified to take into account the change in the free energy of formation of the critical cluster brought about by the presence of a single foreign molecule. The temperature dependence of these data closely confirms that found by a number of earlier investigators. The form of heterogeneous nucleation observed here makes the disparity in the temperature dependence of the nucleation rate of previous work understandable. As higher and higher nucleation rates are sampled, the centers upon which heterogeneous nucleation occurs are depleted and homogeneous nucleation becomes dominant. It was definitely established that the nucleation rate of water vapor is higher in an argon atmosphere than in a helium atmosphere. The latter result seems to indicate that the noncondensable gas plays a role in the clustering process and may possibly hydrate under appropriate circumstances.

Theory of Nucleation of Water. I. Properties of Some Clathrate-Like Cluster Structures

Abstract The tranquility of classical homogeneous nucleation theory has been disturbed by the introduction of statistical mechanical correction factors to a basically thermodynamic theory. These factors, which appear to be essential, destroy much of the agreement with experiment in the case of water vapor. A molecular model for the prenucleation water clusters is proposed with a view toward resolving some of these difficulties. As a first step, the properties of a few specific cluster configurations have, been examined. Clathrate-like structures containing 16 to 57 water molecules are discussed. The hydrogen bonds were treated as simple harmonic oscillators for the purpose of calculating normal mode frequencies. The Helmholtz free energy of formation of the cluster is calculated from the appropriate partition functions. For these clathrate-like structures the free energy of formation was not found to be a smoothly increasing function of the number of molecules but showed minima corresponding to closed cages.

Role of various water clusters in IR absorption in the 8–14-μm window region

A controversy exists over the cause of the IR absorption in the 8-14-microm wavelength window region. One explanation for the continuum absorption is to consider the accumulation of the far wings of strong water monomer absorption lines. On the other hand, other researchers believe that the IR absorption originates from the water dimer complex. Some other researchers claim that either water clusters of larger size or hydrated aerosols are responsible for the continuum absorption. Here we explore the hitherto less-understood systems of water clusters, both homomolecular and heteromolecular, including the water dimer, to enhance our knowledge of the cause of the IR absorption. The present study indicates that homomolecular water clusters larger than the dimer are not responsible for the continuum absorption.

Homogeneous Nucleation Rate for Water

Homogeneous nucleation rate data for water extending over an exceptionally large domain of rate (J), supersaturation ratio (S), and temperature (T) was recently published. Because it spans a large J‐S‐T surface, this data constitutes a good test of nucleation theory. Here classical nucleation theory is used to analyze this data. By adjusting only the sticking coefficient, we are able to obtain a good fit between theory and experiment. It was necessary to include an increase in the water molecular density associated with the finite water compressibility.

Mobility of intermediate sized aqueous ions in a neutral gas

Abstract A mobility versus size relationship is calculated for aqueous ions of intermediate size in an argon gas. The method is based on the Chapman-Enskog transport theory. It involves the microscopic interaction potential between the aqueous ion and the neutral gas molecules. A model potential is used that accounts for the water molecules in the cluster-ion as well as its finite size. The results agree with the Langevin small ion theory at the small end of the size spectrum, give a lower mobility for intermediate sized ions, and then agree with the Stokes viscosity theory at the large end of the size spectrum. An explicit function containing adjustable parameters is fitted to the numerical results in the intermediate size region. The results are applied to data from a Wilson cloud chamber ion mobility experiment.

Study of prenucleation ion clusters: Correlation between ion mobility spectra and size distributions

Additional studies regarding our earlier electrothermodynamic theory are presented. Comparisons to recent expansion cloud chamber ion mobility measurements are made, indicating general agreement with observations. This theory predicts more stable and ordered structure for prenucleation ion–water cluster systems than accounted for by the classical Thomson’s theory. In the limiting case of the dielectric constant e=1, our monopole electrostatic energy term contributed by the foreign ion center precisely converges to that of Thomson. Predicted ion cluster size distributions are found to correlate well with ion cluster size spectra obtained from the ion mobility measurements of hydrated ion clusters and Champman–Enskog theory. In view of good correlation between the theory and observation, we believe that ion mobility study at sufficiently low electric field is a powerful tool for studying prenucleation dynamics.

University of Missouri–Rolla cloud simulation facility: Proto II chamber

The Graduate Center for Cloud Physics Research at UMR has developed a cloud simulation facility to study phenomena occurring in terrestrial clouds and fogs. The facility consists of a pair of precision cooled‐wall expansion chambers along with extensive supporting equipment. The smaller of these chambers, described in this article, is fully operational, and is capable of simulating a broad range of in‐cloud thermodynamic conditions. It is currently being used to study water drop growth and evaporation for drops nucleated (activated) on well‐characterized aerosol particles. Measurements have been made not only for continuous expansions (simulated updraft) but also for cyclic conditions, i.e., sequences of expansion‐compression cycles resulting in alternating drop growth and evaporation. The larger of the two cloud chambers is nearing completion and will provide a broader range of conditions than the smaller chamber. The facility is supported by a fully implemented aerosol laboratory which routinely produce...

FUNDAMENTAL STUDIES ON VAPOR PHASE WATER CLUSTERS

The vapor phase clustering of water molecules (both homo-molecular and heteromolecular) influences atmospheric electro-optics, certain electrical phenomena, and helps remove pollutants through gas-to-particle conversion. The University of Missouri-Rolla Center for Cloud Physics Research has devoted a concerted effort toward developing the molecular description of clusters. In the microcrystalline approximation a given cluster structure is described by a factorized statistical mechanical partition function which contains the binding energy, translation and rotation factors, internal vibrations and rotations, and a contribution to the entropy. New quantum mechanical procedures continue to improve the knowledge of structural details and the frequencies and oscillator strengths of internal motions. Molecular dynamics investigations sharpen the understanding of conformational changes, collision dynamics, energy transfer, and growth and decay reactions. The experimental program has evolved information which along with other work has guided the development of theory. Careful homogeneous nucleation rate measurements delineate shortcomings in the classical liquid drop theory. Ion mobility measurements yield additional clues about the properties of equilibrium ion clusters. So far no theory is completely successful in describing clustering phenomena. Recent findings indicate that the less stable clusters may be more important than previously realized. A realistic evaluation of the theory will be given and possible avenues for future work discussed.

AN INFRARED EXTINCTION CELL OF NOVEL DESIGN

The extinction of infrared radiation in the atmosphere may be attributed to molecules, molecular clusters, and aerosol particles. Reliable parameterization of the infrared extinction must take into account contributions of each of these species over the wide range of relative humidities and temperatures which occur in the atmosphere. Extinction by larger aerosol particles (r ± 0.01 μm) and the dependence of the extinction on relative humidity is reasonably well understood. The contributions being made by very small particles (particles too small to be modeled by using their bulk optical parameters) and molecular clusters are poorly known. An infrared extinction cell currently under construction in University of Missouri laboratory is described. The cell is a windowless, multiple-pass cell capable of obtaining path lengths approaching 1 kilometer. The design of the cell allows measurements to be made in both the subsaturated and supersaturated relative humidity regions. The contemplated initial studies include absorption by water vapor at high supersaturations (super-saturation ratios approaching 2), absorption by ion and H 2 SO 4 acid molecular clusters, and absorption by ultrafine aerosol particles. These studies are closely correlated with ongoing theoretical and simulation chamber studies being carried out in this laboratory.