Simultaneous Measurement of Condensation and Thermal Accommodation Coefficients for Cloud Droplet Growth in Due Consideration of a New Moving Surface-Boundary Effect

Abstract

A droplet growth theory that describes a new effect of vapor and temperature field shift due to the growth-based movement of droplet surface boundary (moving boundary effect) was derived and found to enhance the growth rate as a function of supersaturation (S ? 1) and droplet radius (a) in second order. The theory plays the role of a missing link and resolves the gap that exists among the measured data; the high (S ? 1) used was found to overestimate thermal accommodation and condensation coefficients, α and ?. The theory provided the basis for bettering the measurement and correction for data analyzed without the moving boundary effect, and suggested the broadening effect in the droplet size spectrum by accelerating the growth on the larger end together with the growth slowdown effect of α, ? on the smaller. Measurements employing a horizontal-flow thermal diffusion chamber and the Mie scattering method using a He?Ne laser for droplet size estimation at temperature T = 277 K, (S ? 1) = 0.32%, a ≈ 2 ?m under two pressures of modulation, p = 100 and 860 hPa, with correction of the moving boundary effect for nuclei of NaCl and (NH4)2SO4, resulted in the simultaneous determination of α = 0.81 ± 0.07 and ? = 0.043 ± 0.016 on average. For paper smoke, α = 0.68 and ? = 0.022. Comparison with other data of the low moving boundary effect suggests a slight ? increase with temperature lowering. Based on the result of the present measurement and other theoretical and experimental works, the mechanism of ? being much smaller than α was suggested mostly as a kinetic process controlled by the removal rate of the released latent heat at the moment of molecular impact on the surface and vice versa for evaporation.