AAAR 36th Annual Conference October 16 - October 20, 2017 Raleigh Convention Center Raleigh, North Carolina, USA
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Experimental Validation of the AAC Transfer Function and Data Inversion
TYLER J. JOHNSON, Martin Irwin, Jonathan Symonds, Jason S. Olfert, Adam M Boies, University of Cambridge
Abstract Number: 62 Working Group: Instrumentation and Methods
Abstract The Aerodynamic Aerosol Classifier (AAC) is a novel instrument that selects nanoparticles based on their aerodynamic diameter by generating known drag and centrifugal forces using a controlled sheath flow and rotational speed. Particles with aerodynamic diameters larger than the AAC setpoint impact the outer surface of the classifier, whilst particles of smaller aerodynamic diameter remain in the sheath flow. Particles of the setpoint aerodynamic diameter pass through the classifier, and thus the AAC generates a monodispersed aerosol (Tavakoli, Symonds, & Olfert, 2014) classifying particles independent of their charge state (i.e. no multiple-charge artefacts).
To quantify an aerosol’s aerodynamic size distribution accurately, a tandem AAC (TAAC) setup was used to experimentally characterize the AAC transfer function. The upstream AAC (AAC1) was set at a constant setpoint and selected one aerodynamic diameter from a laboratory generated poly-dispersed aerosol, while the downstream AAC (AAC2) stepped through the aerodynamic domain of these classified particles. The final particle number concentration was recorded as a function of AAC2 setpoint. To capture particle behaviour not represented by the idealized AAC transfer function, such as particle diffusion and losses, a transmission efficiency factor and transfer function width factor were applied within the triangular transfer function. These factors were determined by minimizing the squared difference between the TAAC experimental data and theoretical transfer function convolution.
To determine the particle aerodynamic size distribution from the raw measurements collected by the AAC stepping through discrete setpoints, an inversion was developed (incorporating the previously determined transmission and width factors). This inversion was validated by measuring and comparing the same poly-dispersed aerosol with an AAC and Scanning Mobility Particle Sizer (SMPS) in parallel.
Bibliography 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.