Cloud Droplet Residual Particle Microphysics in Marine Stratocumulus Clouds Observed during the Monterey Area Ship Track Experiment

Abstract

The effect of marine boundary layer pollution level (as determined by the aerosol particle number concentration) on the size distribution of aerosol particles that formed cloud droplets in marine stratiform clouds is examined. In situ measurements of cloud droplet residual particles with a counterflow virtual impactor during the Monterey Area Ship Track experiment are presented. The variation in residual particle size distribution and number concentration in clouds that formed in marine boundary layers with different pollution levels are discussed. The measurements show a constant shape of the residual particle size distributions for the variety of pollution levels encountered, even though the aerosol number concentration varied by up to a factor of 6. The measurements indicate that particles smaller than 0.1-?m radius controlled cloud droplet number concentrations in the clouds investigated. From literature values of the maximum supersaturations encountered in stratocumulus clouds, one can estimate that the lower size limit of droplet-forming nuclei would only in extreme cases be lower than 0.01-?m radius. As a conservative estimate, the range of particle size controlling cloud droplet number concentrations in the encountered clouds was between 0.01 and 0.1 ?m; in typical cases the lower size limit would be somewhat larger. The measurements also indicate that cloud droplet solute mass was determined by particles between circa 0.1 and 1.2 ?m.