The size distribution and chemical composition of cloud droplet residual particles in marine stratocumulus clouds observed during the mast experiment

Abstract

The chapter discusses the size distribution and chemical composition of cloud droplet residual particles in marine stratocumulus clouds observed during the mast experiment. Measurements of cloud droplet residual particle microphysics and chemistry were made in marine stratocumulus clouds off the central California coast during the Monterey area ship tracks (MAST) experiment in June, 1994. Typically, 80% or more of the residual particles were below 0.1 μm radius. While the mode size of residual particles was slightly higher in polluted clouds than in clean ones, the droplet population in the two cases was determined by the submicrometer aerosol. One explanation for the shift towards larger sizes in the residual particle size distributions for polluted versus clean clouds is a reduction of the peak supersaturation in the polluted clouds because of an increase in the rate at which water vapor is taken up by the larger number of particles present in the polluted clouds. Single particle analysis for samples taken in a clean marine boundary layer showed the residual particles larger than 0.1 μm radius to have been composed of sea salt and a combination of sea salt and sulfate. This composition is typical of the background marine boundary layer aerosol. Residual particles containing exclusively organic material, and also organic material plus CI were observed in a sample from a moderately polluted cloud. These clusters accounted for 182 of the 500 particles analyzed, indicating that organic particles were a substantial fraction of the cloud condensation nuclei in these particular clouds.