Enhanced Charging of Aerosol Particles

Bipolar charging introduces ions of both polarities into an aerosol stream, which interact with particles and charge them through a combination of electrostatic and diffusion forces. When ion concentrations are sufficiently high and their interaction times with particles are sufficiently long, charging rates reach a steady state. Under these conditions, the resulting charge distribution becomes independent of ion and particle concentrations and of their interaction time, which is governed by the flow rate and charging volume. This charging mechanism has been utilized for aerosol particles since at least 1940 [1] and forms the basis for the most common form of aerosol measurements using the Mobility Particle Size Spectrometer (MPSS) [2].

Despite more than 86 years of use, advances in bipolar charging continue to emerge. Asymmetries in ion mobilities and concentrations between positive and negative ions, quantified by the ion conductivity ratio (γ), shift the charge distribution away from steady state due to preferential diffusion losses of one ion polarity downstream of the charging region [3]. This deviation, as well as additional deviations introduced by the temporal response of the charger [4], are not considered in conventional bipolar charging models and measurements. These insights have enabled steady-state charging at higher flow rates using low-activity ion sources [5], despite their lower ion concentration (Ni) and shorter (albeit more consistent) interaction time (t) — commonly referred to as Nit product. Achieving this charging performance from low-activity sources reduces regulatory burdens, safety considerations, and costs relative to traditional high-activity sources, such as Kr-85 and Am-241.

This work takes aerosol charging one step further by varying the ion conductivity ratio (γ) to control particle charge, spanning from unipolar negative to bipolar to unipolar positive charging, using a low-activity source that generates bipolar ions. This approach enables precise and stable control of particle charging and can be applied to both existing and emerging aerosol measurement techniques.

References
[1] Lissowski, P. (1940). Das Laden von Aerosolteilchen in einer bipolaren Ionenatmosphare. Acta physicochimica URSS, 13.
[2] Flagan, R.C. (1998). History of electrical aerosol measurements. Aerosol Sci. and Technol., 28(4), 301–380.
[3] Nishida, R.T., Johnson, T. J., and Olfert, J.S., (2025). Aerosol Sci. and Technol., 59(5), 499-520.
[4] Nishida, R.T., Woo, M., Johnson, T. J., Scott, S., Smallwood, G. and Olfert, J. (2026). Residence Time Distributions in Bipolar Charge Conditioners. Submitted to Aerosol Sci. and Technol.
[5] Payne, S., Symonds, J., Johnson, T.J. and Nishida, R.T. (2026). The Advantages of Classifying Particles by Aerodynamic Diameter for Filtration Measurements. In Preparation.