In this paper an aerosol charger that largely avoids the production of multiply charged particles in the 0.1-1.0 μm diameter range is described. The input aerosol is first passed through an electrostatic condenser toremove all charged particles and ions. The remaining neutral aerosol then flows into a 23-cm-long, 2.1-cm inner diameter cylindrical tube; the inner surface of this tube is uniformly coated with 0.09 μCi63 Ni, a 0.067 MeVβ-emitter with a half-life of 92 years. At typical airflow rates of 0.2-1.0 lpm, this low-activity source of ionizing radiation produces bipolar ion concentrations ranging from 1 × 104 to 9 × 104 ion/cm3, which is much lower than levels required to bring the aerosol to Boltzmann charge equilibrium. At a flow rate of 1.0 lpm, particles smaller than about 1.0 μm typically interact with no more than one ion en route through the charger. Therefore, particles at the charger exit are mostly either neutral or singly charged. Charge distributions of initially-neutral mono-disperse polystyrene latex particles were measured at the exit from the charger for particle diameters ranging in size from 0.09 to 1.09 μm. It was found that, at an airflow rate of 1.0 lpm and particle size 1.09 μm, the ratios of singly, doubly, and triply charged to total positively charged concentrations were 0.75, 0.19, and 0.06 respectively; particles with more than three charges were not detected. In contrast, the analogous charge ratio at Boltzmann equilibrium is 0.28 (+ 1), 0.24 (+ 2), 0.19 (+ 3), 0.13 (+ 4), 0.08 (+ 5), 0.05 (+ 6), and 0.7 (+ 02).
Bibliographical noteFunding Information:
This research was supported by National Science Foundation grant ATM-8515490.