On the effective density of non-spherical particles as derived from combined measurements of aerodynamic and mobility equivalent size

Effective densities derived from combined mobility and aerodynamic sizing provide a valuable tool for the characterization of non-spherical particles. Different effective densities have been introduced depending on the primary measurement parameters (mass, mobility and/or aerodynamic size) and the flow regime (transition, free-molecular). Here we explore the relationship between these effective densities, their physical interpretation and their dependence on particle shape, density and various equivalent diameters. We also provide an overview over the wide range of practical implications of the effective density concept with a particular focus on the characterization of particles with irregular or even unknown shape using commercially available instruments such as DMA, SMPS, FMPS, ELPI, APS, TEOM and multi-stage impactors. Finally, we identify new perspectives for particle characterization by extending the effective density concept into the free-molecular regime and by suggesting a triple-instrument approach for on-line determination of both particle density and shape as well as the dynamic shape factor for different flow regimes.