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D. J. Alofs and J. Podzimek

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D. J. Alofs and J. C. Carstens

Abstract

The performance of the conventional horizontal plate thermal diffusion chamber as a cloud nucleus counter was studied. Numerical calculations were performed in order to follow the simultaneous drop growth and sedimentation in the spatially and temporally nonuniform supersaturation field. The effect of nuclei distribution and smallest detectable drop size were investigated. The results indicate order of magnitude uncertainty in the count at 0.1% supersaturation, and a factor of 2 uncertainty at 1.0% supersaturation.

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D. J. Alofs and Yue-Tung Tue

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The performance of the Twomey-type cloud condensation nucleus (CCN) counter is evaluated by a numerical simulation and found to be better than that predicted in an earlier study. Due to a lack of data on CCN spectra at low supersaturations, the earlier study used extrapolations that subsequent measurements show are unrealistic for atmospheric CCN. The present study indicates that if the instrument has a 0.5 μm radius detector limit, the count is accurate to within 15% for operating supersaturations above 0.1% and for typical atmospheric aerosols.

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A. M. Sinnarwalla and D. J. Alofs

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A vertical plate, steady-flow, thermal diffusion chamber designed for use as a cloud condensation nucleus counter is described. In this instrument, phoretic forces are shown to limit the maximum available growth time to a value eight times larger than the growth time available in conventional horizontal plate chambers. This additional growth time is shown to be necessary when operating at supersaturations below 0.2%. Experiments and calculations concerning convection currents in the vertical chamber are also presented.

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P. C. Mahata and D. J. Alofs

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Condensation of water vapor an various surfaces was studied experimentally. For surfaces with an airwater contact angle θ less than 20° the experimentally determined values of critical supersaturation S c. agreed with those given by the Volmer theory. At higher θ the experimental values of S c. were below the Volmer theory values. When the applied supersaturation was less than S c, condensation was avoided for periods as long as 20 h. Thus, if adsorption of water tends to negate the Volmer theory, the process is a slow one. It was determined both by experiment and theoretical analysis that the effect of surface roughness is to decrease S c only Slightiy. These results suggest that most insoluble airborne particles are not likely to serve as cloud condensation nuclei.

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D. J. Alofs and R. L. Reisbig

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The wave-induced movement of oil lenses and other floats was studied by using mechanically generated gravity waves in a water tank. The measured surface drift velocities were in all cases higher than those predicted by the Stokes theory of deep water waves. For wave conditions at which the Stokes velocity is higher than 2 cm sec−1, the measured surface velocities were 35–150% higher than the Stokes velocity. The drift velocity was insensitive to float size when the float length was larger than one wavelength. Thin, flexible, plastic floats were found to have the same drift speed as similarly sized oil lenses. A discussion of the movement of oil spills on the open ocean is included.

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A. M. Sinnarwalla, D. J. Alofs, and J. C. Carstens

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Growth rate measurements were made for water drops grown on nuclei in atmospheric air samples taken in Rolla, Missouri. Rolla, having a population of 15,000 and very little industry, is relatively free of urban pollutants. The measurements were made in a vertical flow thermal diffusion chamber at supersaturations of 0.5 and 1%. The time to grow from near dry radius to the final radius (6 to 7.5 µm) was measured. If one assumes the thermal accommodation coefficient is unity, the measurements indicate an average value of 0.026 for the condensation coefficient. The temperature ranged from 22.5 to 25.7°C.

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P. C. Mahata, D. J. Alofs, and A. M. Sinnarwalla

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Analytical expressions are derived for temperature and vapor pressure profiles in a vertical-plate, steady-flow, thermal diffusion chamber designed to count cloud condensation nuclei. The influence of longitudinal diffusion of heat and water vapor on the supersaturation rise time is shown to be quite significant at low sample velocities. The effect of different velocity profile shapes is also investigated.

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D. E. Hagen, J. Schmitt, M. Trueblood, J. Carstens, D. R. White, and D. J. Alofs

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.

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D. J. Alofs, M. B. Trueblood, D. R. White, and V. L. Behr

Abstract

Nucleation experiments with monodisperse NaCl aerosols showed good agreement with the Köhler 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 uncertainty, the isothermal mode data agreed with the Köhler theory.

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