Simultaneous measurements of aerosol size distribution and the supersaturation spectrum of cloud condensation nuclei (CCN) made at the Second International Workshop on Condensation and Ice Nuclei are used to investigate the relationship of CCN supersaturation spectra to aerosol size distribution and composition. The measured CCN spectra are compared with those calculated from the aerosol size distributions for a wide range of values of particle solubility. For each of three natural aerosol samples considered, the shape of the calculated CCN spectra is shown to be quite insensitive to aerosol solubility and to agree quite well with the shape of the measured spectrum. These results tend to confirm the conclusion of Junge and McLaren that the shape of CCN supersaturation spectra is primarily determined by aerosol size distribution rather than by aerosol composition. The concentration of CCN is, however, critically dependent an aerosol composition.
An expression is derived for the mean size of CCN active at a given supersaturation as a function of particle solubility and the slope of the CCN supersaturation spectrum. From this equation it is shown that Twomey's measurements of the mean size of CCN active at 0.35 and 0.75% supersaturation imply a mean CCN composition of greater than 70% soluble material, assuming the soluble constituent of CCN to be ammonium sulfate. A comparison of the measured aerosol size distributions with those inferred from the CCN supersaturation spectra on the assumption that the CCN component of the aerosols has a mean solubility of 70% shows that only about 35–40% of the aerosol population in the size range 0.03–0.1 μm diameter is active at supersaturations between 0,2 and 1.0%. The fraction of soluble material in the natural aerosol samples used at the Workshop is estimated to be between 15 and 35%.