Bipolar charge distribution of a soft X-ray diffusion charger

The conditioning of the aerosol particle population into a bipolar charge equilibrium is an essential prerequisite to calculate the particle number size distribution using mobility particle size spectrometers. This is commonly realized by diffusion charging of bipolar air ions generated by e.g. a 85Kr source. Because of strict legal regulations on radioactive sources in several countries, soft-X-ray (SXR) appears as a suitable alternative. However, multiple measurements showed a systematical and significant difference between the particle charge distribution delivered by a radioactive source and an SXR charger, respectively. In this investigation, a calibrated particle charge distribution, suitable for the SXR chargers, was calculated based on the Fuchs model. An approximation analogous to the commonly used Wiedensohler approximation formula (Wiedensohler, 1988) was computed. The use of the new SXR approximation of the bipolar charge equilibrium for the inversion of an electrical mobility distribution to a particle number size distribution improves the comparability of these results, compared to measurements involving a 85Kr charger or to bipolar chargers using radioactive material in general. A systematic error in case of using the SXR charger could be eliminated and hence the root mean square deviation could be reduced from 13% using the common parameters for both charger types to 7% using the new SXR approximation for the SXR bipolar charger.