Observations of aerosols and droplets in California stratus

Abstract

A program of aerosol and cloud droplet measurements in stratus clouds was carried out during the summer of 1968 in the hills south of San Francisco. Size distributions were determined with a Royco light-scattering particle counter, a condensation nuclei counter, and a settling tunnel. Two six-stage cascade impactors were used to collect samples for sulfur-compound and chloride analysis. The size distribution data showed a mode in the submicron size range. Secondary modes occurred at radii of about 6 to 8 ? in developed clouds and at somewhat smaller sizes during periods of cloud dissipation. The aerosol below the cloud did not show the secondary mode at the larger sizes. The location of the secondary mode is consistent with some of the distributions calculated from theory, but the observed concentrations of about 1 droplet per cc per ? of radius at the secondary mode are substantially less than calculated. One possible reason involves the mixed chemical composition of the nuclei; another is the high cooling rates used for the calculations. The observations show organic material to be present and a nonuniformity in the distribution of sulfur compounds and chlorides over the droplet spectrum. The average ratio of sulfur to chloride varies from about 4 for r = 0.2 ? to about 0.4 for r = 2 ? to about 1.3 for r = 4 ?. The results of some other studies of fog and stratus droplets are also reviewed briefly. DOI: 10.1111/j.2153-3490.1970.tb01940.x