THE BEHAVIOR OF REGULAR-SHAPED NONSPHERICAL PARTICLES IN A TSI AERODYNAMIC PARTICLE SIZER

The TSI Aerodynamic Particle Sizer (APS) is known to underestimate the size of liquid droplets larger than a few micrometers aerodynamic diameter. It is believed that the cause is shape related, since the liquid spheres will be distorted as the droplets pass through the region of strong acceleration in the tapered nozzle prior to size classification by a time-of-flight technique. The influence of particle shape on APS performance has been investigated using a range of monodiperse, regular-shaped and non-porous solid particles in the size range from about 6 to 14-mu-m aerodynamic diameter. These particles were first size-classified under Stokesian conditions using a Timbrell aerosol spectrometer, so that their true aerodynamic diameters could be determined. The size-classified particles were collected on glass slides, and a technique was developed to resuspend them for further size analysis in an APS. Deviations from true aerodynamic diameter were caused by the ultra-Stokesian environment as the particles were accelerated in the APS and this behaviour was assessed as a function of size. The dynamic shape factor of these particles was almost constant with varying size and was determined to be 1.19 +/- 0.06. Their true aerodynamic diameters were underestimated by an average of 25% when using the APS.