Physical Properties of Background Aerosols and Cloud Condensation Nuclei Measured in Kochi City in June 2010 and Its Implication for Planned and Inadvertent Cloud Modification

Background (BG) aerosol particles (APs) acting as cloud condensation nuclei (CCN) and/or ice nucleating particles (INPs) influence short-range precipitation forecasts and climate change projections by modulating cloud and precipitation microphysical structures and influence the effects of cloud seeding on precipitation enhancement. However, data on the CCN and INP capabilities of BG APs are limited in terms of geographical locations and time. To investigate the characteristics of BG APs, we conducted ground-based measurements of BG AP and CCN in Kochi City, Japan, in June 2010. Comparisons with previously published data on AP and CCN concentrations in East Asia showed that the mean concentrations of APs and CCN at the observation site were considerably affected by air pollution. Our findings also suggest that during the observation period, even air masses from the Pacific Ocean were considerably affected by air pollution in East Asia, including Japan. Moreover, aircraft-measured AP and CCN concentrations in the boundary layer were comparable to those measured concurrently at the surface observation site, although the horizontal positions of the ground- and aircraft-based measurements were not identical; the size distributions of the APs were similar. These results suggest that ground-based measurements represent APs and CCN in the boundary layer, where the air is ingested by clouds. Numerical simulations with a detailed bin microphysics parcel model showed that cloud droplet number concentrations, based on meteorological conditions and aerosol characteristics expected near the observation site environments, would range from 500 droplets cm−3 to 1,500 droplets cm−3. These concentrations were consistent with aircraft measurements. These values are higher than the threshold concentration of ∼ 500 droplets cm−3 in clouds suitable for hygroscopic seeding, as suggested by previous studies. Therefore, this area is considered to be suitable for rain enhancement by hygroscopic seeding.