The hypothesis of this thesis is that the time-of-flight method of determining an estimate of the aerodynamic diameter of aerosol particles is fundamentally flawed when applied to non-spherical and/or non-unit density particles. Such a particle-sizing system, the TSI Aerodynamic Particle Sizer, is challenged with solid, non-spherical particles of known aerodynamic diameter to assess the influence of particle shape on instrument response. The aerodynamic diameter of the non-spherical particles is also determined under gravitational settling. Deposits that had been size-separated are resuspended for aerodynamic sizing by the APS. The experimental study is supplemented by a theoretical investigation of the relative effects of particle density and shape on APS-measured diameters. This is achieved through the development of a computational routine to calculate the trajectories of particles of various densities and shapes through the APS nozzle and sensing zone. The results of these calculations are compared with the experimentally-measured APS performance. The consequences for the traceability and accuracy of data measured using this technique are assessed in the light of the outcome of both aspects of the study.