Abstract
An automatic CCN (Cloud Condensation Nucleus) counter has been developed by utilizing a thermal gradient diffusion chamber (Naruse, 1978). The main features of the system are automatic sampling, measurement and recording of CCN concentration with the capability of direct calibration by the photographic method. The concentration of droplets in an illuminated volume was determined by the measurement of the peak intensity scattered at an angle of 45°. The system verified the linear dependence of scattered light on droplet number concentration. The lower limit of supersaturation used in the CCN counter was assessed experimentally, together with the maximum concentration of CCN measured accurately with the counter.
Measurements of CCN as a function of supersaturation were carried out with the CCN counter at Tsukuba, which is located in a rural area 50 km apart from Tokyo. Results based on hourly measurements of the CCN spectra were used for the investigation of their seasonal variation, the relation between CCN concentration and aerosol particle concentrations in the Aitken and the large size ranges, and influence of wind and relative humidity on CCN concentration. Difference in the CCN spectra between summer and winter was discussed in relation to the solubility of CCN.
