Unipolar diffusion charging of aerosol particles in the transition regime

This paper presents experimental results of unipolar diffusion charging of monodisperse combustion aerosols in the size range of 10-300 nm. A Hewitt-type corona charger was designed for the purpose of the investigation and tested in a Tandem-DMA experimental configuration with particles produced by a combustion aerosol generator (CAG). Measurements of the mean charge and the actual charge distribution of the monodisperse samples have been performed, while the apparatus allowed for the investigation of the effect of pre-existing charge of the aerosols. Results are compared with Fuchs' limiting-sphere theory and good agreement is found in most cases. Calculations with the birth-and-death method, using combination coefficients determined by the limiting-sphere theory, show that a Gaussian-type distribution fits well the experimental results. It is shown that pre-existing charge on the particles, as long as it is lower than the mean charge dictated by the operating conditions (Nit product) of the charger, does not affect the final charging state of the aerosol. Using the classification column of a differential mobility spectrometer (DMS) we performed similar measurements at sub-atmospheric pressures. Variation of the pressure in the range of 250-1000 mbar appears to have an effect on the average number of charges acquired by the particles, although the differences among the measurements lie within the uncertainty levels of the experimental arrangement.