Aerosol particles and clouds: which particles form cloud droplets?

Abstract

Measurements of cloud droplet residuals, which represent the cloud droplet nuclei (CDN) that formed cloud droplets, were made in ambient clouds with a 1-min time resolution. Only a weak relationship was found between the CDN number and volume concentrations, because the particles dominating the two concentrations resided in different size ranges. A comparison between the total particle size distribution and the size distribution of the CDN showed that only a small fraction of the total number of particles at a given size (smaller than 0.2 μmm diameter) formed cloud droplets. Among the CDN, however, 75% of the number of particles were smaller than 0.2 μmm diameter. Concurrent measurements showed that hygroscopic particles of the same size and larger remained in the interstitial air. The same feature was observed over longer time periods on a 1-min basis. Suggested hypotheses to explain why only a few of the smaller hygroscopic particles formed cloud droplets while larger particles remained in the interstitial air are that the growth of the droplet could have been influenced by the composition of individual particles and/or that entrainment introduced hygroscopic particles in the interstitial air.