An aerodynamic lens consisting of several thin plate orifices has been widely used to focus 30-300 nm nanoparticles. However, there is a big challenge in focusing sub-30 nm particles in air mainly due to flow instability in the lens. Here, we propose a new design of the aerodynamic lens consisting of three converging-diverging orifices to resolve the challenge. Through a series of numerical simulation from single- to multi-lens, we determined a specific design for sub-optimal focusing of 5-100nm aerosol particles while keeping the air flow stable and free of shock formation. The final multi-lens assembly was revealed to be quite successful. (C) 2009 Elsevier Ltd. All rights reserved.