Multi-nozzle impactors are widely used for sampling aerosol particles under low Reynolds number conditions without large pressure drop, as compared to single-nozzle impactors at the same flow rate. The objective of this study was to determine the optimal nozzle array for collecting PM2.5 with a flow rate of 50 l/min. Twenty nozzles of the same size with different nozzle positions were designed to produce five different models. Each nozzle plate was evaluated using monodisperse particles. Particle collection efficiencies were measured for particles between 1 and 5 mum, by counting particle penetration through the individual test stages. Cutoff diameter at the 50% collection efficiency was calculated using both linear and non-linear regression techniques. In addition, particle deposition patterns for each model were assessed visually. (C) 2002 Elsevier Science Ltd. All rights reserved.