Aerosol Science 34 (2003) 501–506 www.elsevier.com/locate/jaerosci Minimizing the eect of density in determination of particle aerodynamic diameter using a time of ight instrument Ali K. Oskouie a; b; * , Kenneth E. Noll a , Hwa-Chi Wang c a Department of Chemical and Environmental Engineering, Illinois Institute of Technology, Perlstein Hall, 10 W. 33rd Street, Chicago, IL 60616, USA b Metropolitan Water Reclamation District of Greater Chicago Chicago, USA c American Air Liquide, Countryside, IL, USA Received 30 July 2002; received in revised form 28 October 2002; accepted 29 October 2002 Abstract Particle aerodynamic diameter measurement using an aerosizer (a time-of-ight (TOF) particle size mea- surement instrument) requires assuming the density of particle being measured. In this paper, a relationship between TOF of spherical particles with dierent densities through three laser beams, and the lumped param- eter, Log[d ae C -1=2 D ], is found. This allows the eect of density in particle aerodynamic diameter measurement to be minimized. ? 2003 Elsevier Science Ltd. All rights reserved. Keywords: Particle characterization; Time-of-ight; Particle density 1. Introduction In time-of-ight (TOF) particle size measurement devices the velocity of a particle upon passage through two laser beams is measured, and correlated with the aerodynamic size of the particles (Cheng, Barr, Marshall, & Mitchell, 1993). Particles passing through the sonic nozzle system expe- rience resistance (drag force) due to the relative velocity between the gas and particles. The drag force acting on particles depends on the particles’ mass, density and shape. Particles lag behind the gas stream depending on the magnitude of the drag force they experience in the ow eld. In a 2-beam TOF this lag is related to the aerodynamic diameter, particle density and shape factor. Extension of the numerical analysis for a 2-beam TOF system to a 3-beam system is discussed by Oskouie et al. (2002). In the present study the particle trajectory through a 3-beam system is * Corresponding author. Tel.: +1-312-567-3023; fax: +1-312-567-8874. E-mail address: chee@iit.edu (A.K. Oskouie). 0021-8502/03/$ - see front matter ? 2003 Elsevier Science Ltd. All rights reserved. PII:S0021-8502(02)00175-1