Air Mass Dependence of Aerosol Size Distributions and CCN Activation Spectra over Kochi prefecture in Warm Season

Aerosol particles acting as cloud condensation nuclei (CCN) and ice nuclei (IN) determine the microphysical structures of cloud and precipitation, and affect a short-range precipitation forecast and climate change projection. Also an efficiency of hygroscopic seeding is dependent upon the characteristics of background CCN as well as physico-chemical properties of seeding particles and cloud types. Therefore we investigated the physico-chemical properties and CCN ability of background aerosols as a part of Japanese Cloud Seeding Experiments for Precipitation Augmentation. We made about 30 flights over Kochi prefecture during three 3-week IOPs in early summer of 2008, 2009 and 2010. 72-hr HYSLIT back trajectory analysis shows that 20 flights were made in air masses originated from the East Asia coastal region, and 10 flights were in air masses from the Pacific Ocean region. The aerosol size distributions and CCN spectra in the Pacific Ocean region air masses showed that their shapes were similar to those in the East Asia coastal region air masses, but total number concentrations of aerosol particles and CCN number concentrations were about 1/2 of those in the continental/polluted air masses from the East Asia coastal region. These concentrations were much higher than typical values in maritime air masses, but were close to typical values in continental air masses. In warm season, air masses originated from the Pacific Ocean region came from not far away, and were stagnant around there because of low wind speeds due to gentle pressure gradients. Therefore the maritime air masses we observed were very much influenced by pollution from Japan itself and big cities and industrial areas in the East Asia.