Experimental and Theoretical Studies of Cloud-Particle Growth Kinetics

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As a cloud droplet or ice crystal grows by condensation, water molecules flow from the vapor surrounding the particle to its surface and the heat released by condensation flows away from the particle. Classical theory treats the environment of the particle as a continuum, so that the heat and mass transfer are described by the diffusion equation. This is a valid approximation for drops that are much larger than the molecular mean free path, which is about 0.06 microns for normal sea level conditions. However, newly formed droplets have sizes typically between 0.1 and 1 microns. For these, the classical, continuum theory must be modified to include gas kinetic effects. The modification introduces two new parameters that characterize the transfer of heat and mass, called the condensation coefficient and the accommodation coefficient. The condensation coefficient may be described as the fraction of the molecules hitting the surface of the particle that stick to it. The accommodation coefficient may be thought of as the fraction of the molecules bouncing off the surface of a particle that have acquired the temperature of the particle. In general, these so-called kinetic coefficients must be determined experimentally, because there is no known way to derive their values theoretically. Previous attempts to evaluate them have given widely variable results. This project approaches the problem a new way, by suspending the particles by electrodynamic levitation under controlled conditions of temperature, pressure, and humidity, heating them with pulses of laser energy, and deducing the accommodation and condensation coefficients from the temperature change and the loss of mass by evaporation. Recent attention has focused on the possible influence of trace amounts of volatile solutes on cloud droplet growth and evaporation. Experiments using droplets doped with these materials will determine their effect on the kinetic coefficients. Sensitivity tests using a numerical cloud model will be carried out to evaluate the significance of the kinetic coefficients and their variability. This work is fundamental for understanding the formation, development, and dissipation of clouds in the atmosphere. Furthermore, an accurate accounting of the effects of clouds on solar radiation may require knowledge of the kinetic coefficients.