Reducing Multiple Charging of Submicron Aerosols in a Corona Diffusion Charger

Abstract

An existing corona diffusion charger (Büscher et al. 1994) is optimized in order to reduce multiple charging of submicron particles. A positive-zero rectangular wave voltage is applied to the inner electrode and the ion concentration in the charging region is controlled by varying the duty cycle of the pulse voltage, i.e., the fraction of time during which the voltage applied to the electrode is positive. The performance of the corona charger is assessed experimentally and theoretically for monodisperse aerosols with particle sizes in the range of 50 to 250 nm and number concentrations below 10 ⁵ cm⁻³. Optimal corona voltage and pulse voltage values of +4.5 kV and +5 V are identified. A duty cycle about 10% resulted in multiple charging levels comparable to positively charged particles in bipolar chargers, with higher fractions of singly charged particles. Multiple charging can be lowered by reducing further the duty cycle. When using the corona ionizer for charging of polydisperse aerosols, operational corona voltage and pulse voltage values must be adjusted in dependency of the aerosol particle number concentration. The corona charger is suitable for size selection of submicron particles by differential mobility analysis (DMA).

Acknowledgments

The authors thank Prof. Heinz Burtscher and Christian Peineke for valuable discussions and experimental support. Also to Dr. Manuel Alonso for providing the TSI 3076 atomizer used for aerosol generation as well as for his advice with the theoretical model. This work was funded by the Spanish Ministry of Education and Science (Grant DPI2001-0689).