Characteristics of aerosol charge neutralizers for highly charged particles

Aerosol charge neutralizers containing various radioactive sources have been used to apply an equilibrium charge distribution to aerosols of unknown charge distribution. However, the performance of aerosol charge neutralizers is not well known especially for highly charged particles. In this paper, we discuss some issues related to the neutralizers' low performance in the measurement of highly charged aerosols, especially with high flow rates. Characteristics of two aerosol charge neutralizers for charging singly charged monodisperse particles and highly-charged polydisperse particles were investigated. One neutralizer had a radioactive source of Kr-85 (beta source, 2 mCi) and the other Po-210 (alpha source, 0.5 mCi). The air flow rate of each aerosol charge neutralizer was varied from 0.2 to 3.0 1 min(-1). The results for the Kr-85 aerosol charge neutralizer at an air flow rate of 0.3 1 mm(-1) agree well with the empirical curve (Journal of Aerosol Science 19 (1988) 387), but deviate from the empirical curve at air flow rates of 0.6, 1.0, and 1.5 1 min(-1). Unlike the Kr-85 aerosol charge neutralizer, the effect of air flow rate was insignificant for the Po-210 aerosol charge neutralizer. For highly charged polydisperse particles penetrating the Kr-85 aerosol charge neutralizer, the non-equilibrium character also appeared as the air flow rate of the aerosol neutralizer increased. The shift of size distribution was observed most severely at 1.0 1 min(-1) for both negatively and positively charged particles. Measurements by a scanning mobility particle sizer (SMPS) system showed that the low neutralization efficiency of the Kr-85 aerosol charge neutralizer could distort the particle size and number concentration in measurements of highly charged particles. However, the efficiency was insensitive to the air flow rate for the Po-210 aerosol charge neutralizer. (C) 2004 Elsevier Ltd. All rights reserved.