Diffusion charging of ultrafine aerosol particles by positive helium, argon, and nitrogen ions

The unipolar charging phenomena of ultrafine aerosol particles less than 0.1 μm in diameter by positive gas molecule ions produced by α-ray irradiation are experimentally evaluated in helium, argon, nitrogen, and helium-argon mixture gases. The dependencies of the diffusion charging process of particles on ion species of positive helium, argon, and nitrogen ions are quantitatively well explained by a numerical solution of the population balance equations with the use of Fuchs' combination probability between particles and ions [N. A. Fuchs, Geofis. Pura Appl. 56, 185 (1963)]. The quantity of charge on particles decreases with an increase of ion mass in the order of the helium, nitrogen, and argon ions. The particle charging in a helium-argon mixture is determined by the ion concentration of both species.