Impactor designed to increase mass output rate of nanoparticles from a pneumatic nebulizer

A modular impactor was designed to remove large droplets from aerosols generated by a pneumatic nebulizer, the Six-Jet Atomizer from TSI Inc. (Shoreview, MN), with the aim of generating dry nanoparticles. Three interchangeable nozzle heads were designed to provide droplet cutoff diameters of 0.5, 1, and 2 μm at an air flow rate of 8.3×10−4 m3 s−1 (50 L min−1), which corresponds to all six jets of the nebulizer operated at 25 °C and an air pressure of 241 kPa (35 psi). The collection and output characteristics of the 0.5 μm impactor were evaluated from dry particle size distributions produced by nebulizing an aqueous solution with a NaCl mass fraction of 1% both with and without the impactor present. The impactor characteristic cutoff curve was sharp (impactor geometric standard deviation, GSDimp=1.15–1.19) with a 50% cutoff diameter d50 that ranged from 0.48 μm at 3.0×10−4 m3 s−1 to 0.74 μm at 11.7×10−4 m3 s−1. The rate of dry NaCl particle generation ranged from 0.5 to 5 g s−1 (0.04 to 0.4 g day−1) with mass median diameters MMDp=80–123 nm and geometric standard deviations GSDp=1.6–1.8 (depending on flow rate). Anomalous negative impactor efficiencies were observed at flow rates >8.3×10−4 m3 s−1 for 100 to 400 nm droplets and at all flow rates for droplets smaller than 100 nm. This phenomenon will be investigated further as a way to increase the generation rate of nanoparticles. A step-by-step procedure is presented for the selection of an appropriate impactor design and operating flow rate for a desired maximum aerosol particle size.