Size Shifts in Measurements of Droplets with the Aerodynamic Particle Sizer and the Aerosizer

Abstract

Observations of the size of liquid droplets using the Aerodynamic Particle Sizer (APS) and the Aerosizer indicated that the measured size was significantly different from the aerodynamic diameter as calculated by measuring droplet settling velocity. The size shifts appeared to be caused by droplet distortion in the detector flow field for the Aerosizer. However, for the APS, droplet sizing was further affected by droplet impaction on the upstream side of the focusing nozzle. It is suggested that liquid accumulated in and constricted the nozzle, resulting in a particle velocity increase at the sensor. The size shift can occur with the deposition of < 0.5 μ L liquid onto the nozzle; the size shift can occur in 1–10 minutes even at concentrations of 1000 particles/L; and the size shift can disappear after cessation of liquid aerosol sampling. CFD calculations provided information about the amount of velocity increase at the APS sensor for a selected constriction. Both solid and liquid particles are affected ...