Bipolar diffusion charging of spheres and agglomerate aerosol particles

The effect of particle shape on the diffusion charging of aerosols was investigated. The charging-equivalent sphere diameter d_(QE), found previously to be much larger than the mobility diameter dₘ, may be related to the uncharged fraction of particles leaving a bipolar diffusion charger. This fraction was measured for three types of particles classified by electrical mobility: polystyrene latex (PSL) spheres, ammonium sulfate spheres and TiO₂ agglomerates of 10–20 nm primary particles. The uncharged fraction was ∼5% lower for the agglomerates than for spheres with the same mobility, for size range 100 < dₘ < 800 nm. This implies that d_(QE) ∼ 1.1 dm for the agglomerates, which is a smaller difference between d_(QE) and dₘ than reported by previous studies, but is consistent with the theoretical predictions (Laframboise and Chang, 1977, J. Aerosol Sci.8, 331–338).