In several runs, condensation nuclei were intentionally injected into the airflow in the lowest horizontal tunnel and the water droplet size distribution was observed. The injection was carried out by spraying a water solution of NaCl by using a humidification system (AKIMist D, Ikeuchi Co. Ltd.) settled at about 30 m from the bottom of the vertical shaft, which was composed of a solution tank, a spray nozzle, and an air compressor. This system can yield droplets of which the mean droplet size was 10 µ m or less. During the 30 m travel in the horizontal tunnel, the solution droplets released from the atomizer dried up leaving fine salt particles capable of acting as condensation nuclei. Figure shows the NaCl particle size distribution observed 10 m away from the atomizer set . The sizes of the salt particles flowing into the shaft were considered to be around 0.1 µ m since aerosol particles of such sizes have the minimum deposition velocity. An auxiliary blower was used to diffuse the sprayed droplets uniformly. The concentration of NaCl solution was 200 g/L, and the spray rate was 2.0 L/hr with an airflow rate of 36 N-L/min.