10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Aerosol Charger Characterization using the Aerodynamic Aerosol Classifier
TYLER J. JOHNSON, Robert Nishida, Martin Irwin, Jonathan Symonds, Jason S. Olfert, Adam M Boies, University of Cambridge
Abstract Number: 229 Working Group: Instrumentation
Abstract Electrostatic classification is a common method used to characterize submicron particles. However these instruments require the aerosol charging characteristics to be known, which directly affects their measurement accuracy (Leppä, Mui, Grantz, & Flagan, 2017). Previous charging characterization studies have largely utilized tandem differential mobility analyzers (DMA) with the aerosol sample passing through the charger of interest between the DMAs. The upstream DMA remains at a constant setpoint, while the downstream DMA resolves each charge-state by stepping through the particle mobility domain of the charged, mobility size-selected aerosol. This experimental setup must be designed and operated to limit the upstream DMA classifying multiple-charged particles and biasing the charging fraction results. Furthermore, due to only a portion of the particles being charged to the desired state, the concentrations measured after being charged and classified twice may be low. This limits the range of particle sizes that can be studied from the same aerosol source, as well as the number of multiple-charged states that can be resolved.
To overcome these challenges, this study used an Aerodynamic Aerosol Classifier (AAC) to characterize different aerosol chargers. The AAC uses a sheath flow at a set rotational speed to select nanoparticles based on their aerodynamic diameter, a property independent of particle charge. This operating principle selects a mono-disperse aerosol source (Tavakoli, Symonds, & Olfert, 2014), making the AAC well-suited for aerosol charger characterization applications.
A BGI Collison nebulizer was used to atomize DOS (bis-2-ethylhexyl sebacate) and produce a poly-dispersed aerosol. The AAC was set at a constant rotational and sheath flow to select a discrete aerodynamic diameter range from the poly-dispersed aerosol. These size-selected particles were then passed through the charger of interest. As a mono-disperse source, each particle charge-state produced a different electrical mobility which was resolved using a DMA stepping through the mobility domain of the AAC classified particles. This methodology was used to measure the negative, neutral, and positive DOS charging fractions at three different particle sizes generated from four different aerosol chargers: a TSI 308701 x-ray neutralizer, two TSI Krypton-85 radioactive neutralizers (one brand new and the other aged for approximately one half-life) and a Cambustion Unipolar Diffusion Aerosol Charger (UDAC). Due to the high transmission efficiency of the AAC relative to the DMA (Johnson, Irwin, Symonds, Olfert, & Boies, 2017), up to 13 charge fractions (6 positive, 6 negative and the neutral) produced from the neutralizers were resolved at each particle size. These results allowed comparison between the chargers as well as against current particle charging theory.
Bibliography
Johnson, T. J., Irwin, M., Symonds, J. P. R., Olfert, J. S., & Boies, A. (2017). Measuring Aerosol Size Distributions with the Aerodynamic Aerosol Classifier. Aerosol Science and Technology, Submitted(Nov. 3, 2017).
Leppä, J., Mui, W., Grantz, A. M., & Flagan, R. C. (2017). Charge distribution uncertainty in differential mobility analysis of aerosols. Aerosol Science and Technology, 51(10), 1168–1189. https://doi.org/10.1080/02786826.2017.1341039
Tavakoli, F., Symonds, J. P. R., & Olfert, J. S. (2014). Generation of a Monodisperse Size-Classified Aerosol Independent of Particle Charge. Aerosol Science and Technology, 48(3), i–iv. https://doi.org/10.1080/02786826.2013.877121