About the validiy of the aerodynamic diameter concept in the submicron size range.

To conduct deposition studies in the human lung and in filter beds consisting of glass beads, studies have been carried out within the particle size range of 0. 08 to 2. 5 mu m in order to find practical limits for the aerodynamic, intermediate and thermodynamic domain of particle deposition. For the identification of the transport mechanisms (diffusion, sedimentation) their different time-dependency has been used. This paper combines the effect of diffusion and gravitational settling on particle transport in order to indicate the range of validity of the aerodynamic diameter concept. Diffusional and gravitational transport are examined. It is clear that an aerodynamic diameter obtained for instance from inertial or centrifugal spectrometry below 1 mu m is no longer a suitable size parameter to describe deposition of submicron particles in the human lung or in filter beds under low flow conditions. As long as diffusion and sedimentation are the main mechanisms the best way for data presentation is to use either the geometric or the aerodynamic diameter with the particle density as a parameter