On the theory of charging of aerosol particles by unipolar ions in the absence of an applied electric field

Abstract

Abstract The charging of aerosol particles by unipolar ions in a gaseous medium and in the absence of an applied electric field has been analyzed from the standpoint of the kinetic theory and compared with theories based upon the macroscopic diffusion of ions and experimental data. It is shown that the quasi-steady charging rate can be expressed by the simple equation, d γ d τ = e −γ , where γ is a dimensionless particle charge and τ is a dimensionless particle charging time. The particle charging rate is shown to be unaffected by the mean free path of the ions, and hence by pressure. The theoretical equation shows good agreement with the experimental data when the mean thermal speed of the ions is taken as 1.18 × 104 cm./sec., which corresponds to a molecular weight of 460 for the corona ions used in the charging experiments. It is also shown that the theory based upon the solution of the continuous macroscopic diffusion equation for ions cannot be applied to the electrical charging of aerosol particles in the micron and submicron size range because of the low concentration of ions normally encountered and the essentially discontinuous nature of the charging process.