Particle aerodynamic diameter measurement using an aerosizer (a time-of-flight (TOF) particle size measurement instrument) requires assuming the density of particle being measured. In this paper, a relationship between TOF of spherical particles with different densities through three laser beams, and the lumped parameter, Log[d(ae)C(D)(-1/2)], is found. This allows the effect of density in particle aerodynamic diameter measurement to be minimized. (C) 2003 Elsevier Science Ltd. All rights reserved.