Daniel R. White

Condensation Coefficient Measurement for Water in the UMR Cloud Simulation Chamber

Abstract A systematic series of condensation coefficient measurements of water have been made using the University of Missouri—Rolla cooled-wall expansion chamber which simulates the thermodynamics of cloud. This coefficient is seen to decrease from a value near unity, at the outset of simulation, to a value in the neighborhood of 0.01 toward the end of a simulation. Final values of this coefficient are sufficiently low as to contribute significantly to the broadening of the drop-size distribution in cloud.

Hydration properties of combustion aerosols

There has been considerable recent interest in the hydration properties of combustion aerosols. Both their warm cloud condensation behavior and ice-nucleating ability have important atmospheric implications. At the University of Missouri-Rolla we have a substantial cloud simulation facility designed for the laboratory study of atmospheric processes under realistic conditions (i.e., temperature, pressure, and supersaturation) and time scales. A combustion aerosol capability has been added to this facility. We can generate a variety of combustion aerosols, under controlled and observed conditions, characterize and shape (modify their size distribution) these aerosols, and then examine their hydration behavior under either warm or cold conditions. Here we describe this combustion system and present results (size distributions and critical supersaturation distributions) for aerosols resulting from the combustion of various liquid fuels. The hydration of these aerosols is found to obey Kohler theory, and solub...

Nucleation Experiments with Monodisperse NaCl Aerosols

Abstract Nucleation experiments with monodisperse NaCl aerosols showed good agreement with the Kohler theory relating the critical super-saturation Sc to the dry size. Aerosols produced by condensing NaCl showed the same Sc as those produced by evaporating aqueous NaCl solution droplets. This indicates that if there is an energy barrier in going from a dry NaCl particle to a solution droplet, this energy barrier is small. The fact that the evaporation aerosol particles are cubical crystals and the condensation aerosols are amorphous spheres is shown to make no difference in the nucleation threshold. The investigation also gives insights into the performance of the equipment used, especially the commercial electrostatic aerosol classifier and the vertical flow thermal diffusion chamber developed in this laboratory. When operating this chamber in the isothermal mode, a 36% upper limit was found on the uncertainty in Sc due to index of refraction sensitivity in sizing the water drops. Within this range of un...

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...

A system for collecting milligram quantities of cloud condensation nuclei

ABSTRACT An experimental system to collect cloud condensation nuclei (CCN) onto filters in amounts sufficient for chemical analysis is described. This experimental apparatus is designed to process ambient air at a rate of more than 1 m3/min. Two identical systems have been built. One is installed in a laboratory at Rolla, MO. The other is installed on an 11 m long trailer. The system isolates three size classes of CCN, having mass median diameters of 0.27, 0.12, and 0.075 μm, respectively, and mass collection rates of 11.5, 1.28, and 0.13 mg/day, respectively. The above sizes and collection rates are obtained from a computer simulation that does not account for particle evaporation, but tests of the system indicate that there is particle evaporation which produces some change in the sizes and collection rates. The system components were tested by sampling laboratory-generated CCN of different selected sizes, and the test results were used as inputs for the computer simulation of the system performance.

A correction to classical homogeneous nucleation theory for polar molecules exhibiting an electric double layer at the liquid surface

Abstract An oriented dipole surface layer is added to the classical liquid drop model of nucleation to account for the surface behavior of substances having polar nonsymmetrical molecules. The preliminary treatment of the change in free energy as given by Abraham is modified to include Fletchers exponential decay of the degree of orientation. An additional correction for the excess binding energy due to the presence of a foreign molecule in the prenucleation cluster is included to account for the inflections observed in the experimental results of Allen and Kassner. The resulting free energy of formation is combined with the kinetic treatment of Frenkel to obtain a nucleation rate law. The theoretical results are compared to the experimental results of Allen and Kassner as a function of both supersaturation and temperature. The agreement is good once the heterogeneous component is taken into account.