Technique for Assessing the Electrical Charge Levels of Aerosols

Abstract

A relatively low-cost and easy-to-use method for estimating the charge level on aerosols has been developed. It uses the properties of an electrostatically enhanced (electret) filter combined with an optical particle counter to obtain size-dependent charge levels of workplace aerosols. The optical particle counter is calibrated to give a “filtration equivalent” particle size. For the size range of the present measurements, this is similar to geometric size. The aerosol concentration is measured before and after neutralization to determine a penetration ratio that can be approximately correlated with particle electrical mobility. For a particle of a given size, the penetration ratio increases with increasing particle charge level. The method was calibrated with monodisperse methylene blue particles charged to a known level. A polydisperse CaCO3 aerosol, characterized as to size-dependent charge levels, also was measured. Finally, the charge level on a copy-machine toner dust was measured to simulate a highly charged workplace aerosol. The method is limited to the size range of 0.1 µm to 0.7 µm by the characteristics of the electret filter. Electrical mobilities ranging from 0.01 to 1 cm2/statV sec can be measured. The charge level on one particle size in a size distribution is proportional to the charge on other sizes, and can thus be an indicator of charge level of the overall aerosol. Although of limited size-range capability, the method can serve as an indicator of the importance of aerosol charge for sampling or for health effects.