An existing corona diffusion charger(Biischer 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 polydis-perse 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).
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