AAAR 31st Annual Conference
October 8-12, 2012
Hyatt Regency Minneapolis
Minneapolis, Minnesota, USA
Abstract View
Bipolar Neutralization of Spherical Particles <23 nm using Radioactive, X-ray and AC Corona Methods
JACOB SWANSON, Jean de La Verpilliere, Adam M Boies, University of Cambridge
Abstract Number: 761 Working Group: Instrumentation and Methods
Abstract Many methods of particle size and concentration measurement rely on the knowledge of the distribution of charges such particles have acquired after a charging or neutralization process. Variations in neutralization techniques may lead to different charging characteristics and other parameters such as particle size, shape, composition, concentration, and ambient air properties impact the charging process.
In a typical neutralizer, particles pass through a cloud of ions and acquire a steady state distribution of charges that is slightly asymmetrical. Many techniques are available to generate these ions but this work focuses on radioactive sources (10 and 2 mCi 85Kr, 3077 TSI, Inc), X-rays (3087 Soft X-ray Charger, TSI, Inc), and corona discharge (1090 Electrical Ionizer, MSP Corp). These methods differ in the way they ionize the surrounding gas. For example, the de-facto standard, 85Kr, emits highly energetic beta particles. On the other hand, the x-ray charger generates energy x-rays (<9 keV) and the electrical ionizer uses corona discharge operated with AC voltage to generate positive and negative ions.
The objective of this work is to better quantify the impact of these neutralization techniques and aerosol composition on the neutralization process. A series of experiments were conducted using solid silver particles <23 nm. For all experiments, the four chargers were operated using a realistic aerosol flowrate of 1.5 L/min. Carrier gas composition was varied to examine the sensitivity of charging to aerosol chemistry. Carrier gases included dry nitrogen, humidified nitrogen, humidified air, humidified air with sulfur dioxide, and dry argon. For each neutralizer and carrier gas condition the ion mobility distribution, particle charged and neutral fractions, and the particle size distribution were measured. Overall, results obtained from these measurements provide further insight into the nature of particle charging for very small particles.