AAAR 37th Annual Conference October 14 - October 18, 2019 Oregon Convention Center Portland, Oregon, USA
Abstract View
Accelerated Size Distribution Measurements using a Scanning Aerodynamic Aerosol Classifier
TYLER J. JOHNSON, Jonathan Symonds, Jason S. Olfert, Adam M Boies, University of Cambridge
Abstract Number: 210 Working Group: Instrumentation and Methods
Abstract The Aerodynamic Aerosol Classifier (AAC) selects particles based on their relaxation time by passing the aerosol between spinning concentric cylinders with a sheath flow. Johnson et al. (2018) demonstrated that the aerodynamic size distribution of the aerosol is measured by stepping the AAC setpoint and quantifying the classified particle number concentration at each steady-state condition. However, this approach is similar to stepping a Differential Mobility Analyzer (DMA) in a Differential Mobility Particle Sizer (DMPS) system. In either sizing system, the stepping procedure requires long times to achieve sufficient size spectral resolutions. This limitation was overcome in the DMPS by Wang and Flagan (1990) developing the Scanning Mobility Particle Sizer (SMPS).
This study follows a similar methodology and demonstrates size distribution measurements are accelerated by continuously changing rather than stepping the AAC classifier speed. By varying the speed following an exponential function, the change in centrifugal force a particle experiences during its classifier residence time is independent of its inlet time, and the scanning AAC transfer function converges to the same shape as produced during steady-state operation. The high agreement between aerodynamic size distributions of three different aerosol sources (DOS, salt and soot) independently measured by stepping and scanning the AAC validates this new methodology. For instance, the Count Median Diameter (CMD), Geometric Standard Deviation (GSD) and total number concentration of the DOS size distribution measured by the stepping and scanning AAC agreed within 0.55%, 0.12% and 1.05% at low flow, respectively.
Bibliography [1] Johnson, T. J., Irwin, M., Symonds, J. P. R., Olfert, J. S., & Boies, A. M. (2018). Measuring aerosol size distributions with the aerodynamic aerosol classifier. Aerosol Science and Technology, 52(6), 1–11. https://doi.org/10.1080/02786826.2018.1440063. [2] Wang, S. C., & Flagan, R. C. (1990). Scanning Electrical Mobility Spectrometer. Aerosol Science and Technology, 13(2), 230–240. Retrieved from http://dx.doi.org/10.1080/02786829008959441.