Individual particles and droplets in continentally influenced stratocumulus: A case study over the Sea of Japan

Aerosol particles were collected with an aircraft-borne sampler above, within and below a stratocumulus over the Sea of Japan on December 3, 2000, when continental influence was expected. Particles in the range of 0.3–7 μm and cloud droplets in the range of 0.3–10 μm were captured and they were identified individually upon their elemental composition and morphologies from the analyses by using electron microscopes and an energy dispersive X-ray spectrometer. Interstitial particles and droplets coexisted in the cloud and their number ratio was about 3:2 in the detected ranges. Spherical particles that were inferred to be mainly composed of sulfate, nitrate and ammonium occupied more than 95% of the interstitial particles. Similar particles were also majorities in the above- and below-cloud air, where their percents to total particles were 93% and 74%, respectively. Acidic sulfate particles were rarely found in the below-, in- and even above-cloud air. The nuclei of 90% cloud droplets had similar composition and size distribution to the spherical particles in the scope of our analysis. These results suggest that the continentally influenced stratocumulus was characterized by neutralized nuclei and interstitial particles. Sea-salt particles were detected mainly in the below-cloud air. A few droplets with sea-salt nuclei were detected there and in the cloud. Such droplets were usually larger than those with spherical nuclei but their number fraction to total droplets in the cloud (about 8%) was much smaller than the spherical nuclei to total droplet fraction, indicating that sea-salt particles produced larger droplets but they constituted only a small number fraction of droplets in the cloud.