Size-resolved chemical composition analysis of ions produced by a dielectric barrier discharge bipolar charger

Abstract

Accurate prediction of aerosol charge distribution is crucial for aerosol size distribution measurements using electrical mobility spectrometers. The charge distribution of widely used bipolar diffusion aerosol charging is affected by the electrical mobility and mass of ions. In this study, we developed and evaluated a concentric cylindrical double dielectric barrier discharge (DBD) bipolar charger, and investigated the impact of measurement conditions on electrical mobility and mass of charging ions. The size-resolved chemical composition of ions produced by the DBD charger was analyzed using a high-resolution half-mini differential mobility analyzer coupled to an atmospheric pressure interface time-of-flight mass spectrometer. The effects of the discharge gas, carrier gas and relative humidity (RH) on ion properties were evaluated. Our results show that both discharge gas and carrier gas influenced the chemical composition of ions. The detected high-abundance ions were mainly originated from impurities in the carrier and discharge gases, or compounds used when manufacturing the system components. The ion mobility distribution varied with the type of carrier gas and its relative humidity, but was not sensitive to discharge gas or its flowrate. The measured charge distribution using the DBD charger was in a good agreement with Wiedensohler's approximation (Wiedensohler, 1988), and the theoretically predicted charge distribution, calculated from the measured ion properties, was also consistent with the experimental results. Only minor variations with a relative uncertainty of 12.1% and 9.5% for positive and negative particles, respectively, in singly charged particle fractions were expected among different measurement conditions. Despite a higher uncertainty likely introduced by using ambient air as the carrier gas, our work indicates that the newly developed DBD charger has the potential to be used as a bipolar charger under typical laboratory and ambient measurement conditions.