Bipolar charging of ultrafine particles in the size range below 10 nm

Bipolar diffusion charging of aerosol particles in the size range of 2.3–10 nm was studied experimentally. Charging probability for WOx nanoparticles as a function of particle size was measured using an 241Am aerosol neutralizer for aerosol charging and a tandem DMA (Reischl type) with a VIE-06 Faraday cup electrometer for aerosol measurement. In spite of small deviations in the predictions from the theory of N. Fuchs on bipolar aerosol charging at 2–4 nm range, the theory was seen to be valid. Correction of the deviations may be performed by adjusting the free parameters in the theory (ion mass and mobility). As a rule of thumb, the charging probability of 2 nm particles was seen to be around 0.6% for negative and around 0.4% for positive particles.