EFFECTS OF AEROSOL-PARTICLES ON THE MICROPHYSICS OF COASTAL STRATIFORM CLOUDS

Aerosol particles can act as cloud condensation nuclei and thereby inf luence the number and size of droplets in clouds. Consequently, anthro pogenic particles have the potential to influence global climate by in creasing cloud albedo and decreasing precipitation efficiencies. Enhan ced cloud reflectances associated with increases in particle number ha ve been observed, but our understanding of these interactions has been hindered by incomplete empirical studies and models of limited scope. In this study, aerosol and droplet size distributions were measured o n 13 research flights in stratiform clouds within 300 km west of the n orthern California coast. The chemical composition of the droplet solu te was also assessed. Microphysical and chemical properties indicated that most of the clouds were influenced by pollution from the North Am erican continent, but pristine marine clouds were sampled on one fligh t during westerly flow conditions. Data from this flight and another, representing a pristine and polluted environment, were compared with h igh-resolution satellite observations. In the polluted case, particle and droplet number concentrations decreased, mean droplet size increas ed, and satellite-derived reflectance at 3.7 mu m decreased with incre asing distance from the northern California urban region. Relative to the unpolluted stratiform cloud, the polluted cloud had, on average, a sulfate concentration that was higher by an order of magnitude, dropl et number concentrations higher by a factor of 6, droplet sizes smalle r by a factor of 2, and 3.7-mu m reflectance that was higher by a fact or of 2. However, no significant difference in the visible reflectance was detected between the two cases, probably a result of differences in liquid water path.