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 mu m at an air flow rate of 8.3 x 10(-4) m(3) s(-1) (50 L min(-) (1)), which corresponds to all six jets of the nebulizer operated at 25 degrees C and an air pressure of 241 kPa (35 psi). The collection and output characteristics of the 0.5 mu m 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, GSD(imp) = 1.15-1.19) with a 50% cutoff diameter d(50) that ranged from 0.48 mu m at 3.0 x 10(-4) m(3) s(-1) to 0.74 mu m at 11.7 x 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 MMD(p) = 80-123 nm and geometric standard deviations GSD(p) = 1.6-1.8 (depending on flow rate). Anomalous negative impactor efficiencies were observed at flow rates > 8.3 x 10(-4) m(3) 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. (c) 2009 Elsevier Ltd All rights reserved.