Unipolar and bipolar diffusion charging of ultrafine aerosol particles

Abstract

Unipolar and bipolar diffusion charging of monodisperse aerosol particles of 4–100 nm in diameter has been studied experimentally and theoretically. Particles are charged by unipolar or bipolar ions produced by α rays from Am-241. They are enlarged by condensation of DBP vapor on them using the particle size magnifier and the enlarged particles are admitted into an observation cell having parallel d.c. electrodes to distinguish charged particles from uncharged particles by TV camera. In the case of unipolar charging, the ratios of charged particles to total particles at various values of the ion concentration and charging time are found in quite good agreement with the solution of equations based on the Fuchs' formula for the combination probability of ion with particle and introducing ion properties reported in previous papers. From these results, the necessary conditions to bring uncharged particles to a desired charge distribution are clarified for various particle sizes. In the case of bipolar charging, the number ratios of charged particles to total particles in a steady-state are again found in quite good agreement with the solution of basic equations similar to those of unipolar charging using the Fuchs' theory for the combination probability and the same ion properties as those used for unipolar charging. The ratio of positively to negatively charged particles is found to be approximately 0.35:0.65, which is explained by the difference in physical properties between positive and negative ions.