Calibration of a photoelectric cloud condensation nucleus counter

Calibrations were performed for the photodetector output of a static thermal-gradient diffusion chamber by comparing the detector signals with visual counts of the number of droplets developing in the chamber. The calibrations cover the supersaturation range 0.3% to 1 %. For each supersaturation, the CCN concentration was found to be proportional to the detector output. During the activation and growth of droplets, the time of peak signal corresponds well to the time at which droplet count is a maximum within the sample volume. The use of an average output signal over a period bracketing the mean position of the peak is a slightly better measure of droplet concentrations than the peak value. The calibration equations here derived are specific to the CCN counter used to obtain the data. Also, the calibration constants can be expected to be dependent on the type of aerosol which serve as CCN; the limited data so for available show only weak variations in the constants.