Measurements on the number concentrations of cloud condensation nuclei (CCN) and aerosol particles were carried out at Tsukuba during the dissipation stage of fog. Number concentrations of CCN in the supersaturation range of 0.24-1.24% and submicron aerosol particles with radii between 0.05 and 0.25 μm tended to increase during the dissipation of fog. Chemical elements in individual aerosol particles collected during the dissipation of fog were examined by using an electron microscope equipped with an energy-dispersive X-ray analyzer (EDX). The results of quantitative analysis indicated that sulfur was dominant in the submicrometer particles. Estimated radius growth rates of aerosol particles during the dissipation showed increase in proportion to aerosol radius, suggesting the importance of chemical reactions in fog droplets. The elemental composition of the residues of individual fog droplets was also examined with the EDX, together with that of large particles after the dissipation of fog. It was found from the quantitative analysis that the weight ratios of S/Fe and S/K in large particles were 3.5 and 5.3 times greater than those of fog droplet residues, respectively. It is suggested that SO2 oxidation took place in fog droplets. And evaporation of fog droplets will contribute to the increase in the concentrations of large particles and CCN in the lower atmosphere near the ground.
A linear instability theory in clouds is developed to explain both the formation mechanism of the uneven structure on the surface of the convective cloud and the mixing or entrainment process. This instability is a generalization of the conditional instability which is usually discussed in a stratification which is static and horizontally uniform. In clouds, on the other hand, the stratification of pressure, temperature, water-vapor mixing ratio and cloud water mixing ratio are tilted against the horizon and the wind blows. At the same time, a simple model of the parameterization of eddy effect inside a cloud is proposed by using this instability theory.