Modification of laboratory fog with organic surface films

Abstract

Laboratory fogs were generated under controlled and reproducible conditions in a 600-meter3 chamber and seeded with aerosols of various compounds that form organic surface films at air-water interfaces. The fogs were programmed through several slow expansions and compressions to assure adequate time to permit the coating of the fog drops with a surface film and to avoid nucleation on aerosols of the surface active material. Visibility characteristics and droplet data were obtained for both seeded and control fogs. Cetyl alcohol stabilized fogs significantly during compressions and periods of constant pressure. This observation was substantiated by measurements of visibility, drop size, drop concentration, and liquid water content of the fogs. The cetyl alcohol, a linear molecule, formed tightly packed surface films around the fog drops, which inhibited the transport of water molecules across the water-drop-air interface and thereby appreciably retarded the evaporation rate of the fog droplets. Oleyl alcohol, a nonlinear molecule, formed fluid surface films that did not retard evaporation or alter fog visibility. The data suggest that droplet growth on treated nuclei can be retarded but that under the controlled conditions of these experiments fog formation was not significantly altered by the chemical seeding. It is concluded that additional experiments designed to limit natural fog formation with evaporation retardants are not warranted. In the natural atmosphere the release of organic compounds from human activity may cause inadvertent cloud and fog stabilization; the determination of the chemistry and physics of this process represents an important and relatively unexplored area of environmental inquiry.