Observations of aerosols and droplets in California stratus II

Abstract

Size distributions of aerosols and droplets were observed in stratus in the hills south of San Francisco during the summers of 1968 and 1969. Instrumentation included a Royco light-scattering particle counter, a condensation nuclei counter, and a settling tunnel. The average size distributions showed primary modes at submicron sizes on clear days, during cloud formation and dissipation, and in the developed cloud. A broad secondary mode of about 3 droplets cm^-3μ^-1 was found at a radius of 5 μ in the cloud. A somewhat sharper mode of 0.9 cm^-3μ^-1 at 8-μ radius was found in the average size distribution for dissipating clouds. During periods of cloud onset, the secondary mode is not well developed. Timesection analyses of particle and droplet spectra are given, and these show that there is considerable temporal variability in the spectra. The time-section analyses fail to confirm the general trend of the droplet mode toward larger sizes with time, as predicted by some numerical calculations of condensation processes. Analyses of the chloride, sulfur compound, and nitrate content of the aerosols and droplets are presented. The amounts of sulfur compounds and nitrates relative to chlorides are too high for the principal source of these materials to have been sea salt, unless the chlorides had been lost through some chemical process.