Unipolar Diffusion Charging of Ultrafine Aerosols

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Abstract
Experimental data are reported for unipolar diffusion charging of NaCl and Ag aerosols in the 0.004- to 0.075-μm-diameter range. Monodisperse, uncharged particles were exposed to unipolar positive ions produced by a corona discharge. The nt product was varied between 3 × 106 and 1 × 107 (ions/cm3) (s), where n is the unipolar ion concentration and t is the charging time. The resulting aerosol charged fraction was measured by using a single-particle-counting condensation nucleus counter in conjunction with an electrostatic condenser. From the measured charged fraction and the known charging parameters, the combination coefficient between the neutral particles and the positive ions are obtained. Measurements are then compared with the available charging theories. It is found that the theory of Marlow and Brock best predicts charging rates in the ultrafine particle size range. Above 10 nm, the data approach the theory of Fuchs consistent with the experimental observation of Adachi et al.