Vertical distribution of cloud condensation nuclei concentrations and their effect on microphysical properties of clouds over the sea near the southwest islands of Japan

Aircraft observations were performed over the sea near the southwest islands of Japan under Asian Atmospheric Particulate Environmental Change Experiment 2/Asian Pacific Regional Aerosol Characterization Experiment (APEX-E2/ACE-Asia) project during the period of 16-28 April 2001. The polluted air mass from east Asia was associated with very high concentrations of SO2 (1-10 ppb) and aerosol particles (3000-5000 cm(-3)) in the marine boundary layer. The cloud condensation nuclei (CCN) concentration at 0.3% supersaturation was as high as 800-2000 cm(-3) during the penetrations of air pollutants from east Asia. The correlation coefficient between SO2 and aerosol particles was significant in such polluted atmosphere. Concentration of CCN (N-CCN) was linearly related to concentration of aerosol particles (N-AP) according to N-CCN similar to 0.75N(AP). The ratio of CCN to aerosol condensation nuclei particle concentrations was lower than 0.3 in the relatively clean maritime atmosphere, but it was as high as similar to 0.5 in the continentally influenced atmosphere in the boundary layer. These results indicated that the influence of anthropogenic pollutants from east Asia increased the contribution percentage of aerosol particles to CCN in the polluted atmosphere over the observation area. The observational results also indicated that a mean cloud droplet concentration (N-C) in the continentally influenced clouds was similar to 2 times as much as N-C in the relatively clean maritime clouds. The slope of log-log relationship between N-C and N-CCN was similar to 0.39. This study strongly suggests that high CCN concentration formed many cloud droplets and decreased their effective radius at similar liquid water content under the outflow of air pollutants from east Asia.