| Summary: | To investigate interactions between aerosols and clouds, the size and number concentrations of the cloud condensation nuclei (CCN) and the cloud droplets (CDs) were measured at the summit of Mt. Fuji (altitude 3776 m), Japan. The CCN number concentrations (<i>N</i><sub>CCN</sub>) are significantly higher in continental air masses than in air masses from the Pacific Ocean. The hygroscopicity parameter <i>κ</i> did not change much for different air mass origins, indicating that aerosol particles in the free troposphere are well mixed. Based on the CD number concentrations (<i>N</i><sub>CD</sub>), the degree of supersaturation in the ambient air during the cloud-shrouded period was estimated to be 0.15% (25th percentile) to 0.44% (75th percentile). To evaluate factors influencing the <i>N</i><sub>CD</sub>, measured <i>N</i><sub>CD</sub> were compared to ones calculated based on the Köhler theory using aerosol number size distributions, <i>κ</i>, and the degree of supersaturation. The results showed that <i>N</i><sub>CD</sub> could not be reproduced satisfyingly when the mean number size distribution or the mean effective supersaturation were used for the calculation. This study highlights the importance of obtaining information about the degree of supersaturation to predict <i>N</i><sub>CD</sub> in the atmosphere.
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