CLOUD CONDENSATION NUCLEI AS A SOURCE OF ICE-FORMING NUCLEI IN CLOUDS

For the first time it has been found that sorption ice-forming nuclei (IFN) are produced in a cloud when cloud droplets are evaporating. Ice crystals form by vapor diffusion at the moment when a fraction of the residues of evaporated droplets (newly formed aerosol particles) become IFN. In this study, the concentration of sorption IFN varied from 0 to 6.4 x 10(3) m-3 at -4.4-degrees-C, and it was as high as 7.0 x 10(3) m-3 at -20-degrees-C. The fraction of evaporated cloud droplets producing a transient population of sorption IFN was estimated between 10(-5) and 10(-4). The concentration of aerosol particles active as IFN operative by condensation-followed-by-freezing was found to be between hundreds and 1.5 x 10(4) m-3 at temperatures of -6 and -20-degrees-C. Evidence now exists that only one measuring technique of concentrations of IFN does not suffice to explain concentrations of ice particles in natural clouds.