From Agglomerates of Spheres to Irregularly Shaped Particles: Determination of Dynamic Shape Factors from Measurements of Mobility and Vacuum Aerodynamic Diameters

Abstract

With the advent of advanced real-time aerosol instrumentation, it has become possible to simultaneously measure individual particle mobility and vacuum aerodynamic diameters. This paper presents an experimental exploration of the effect of particle shape on the relationship between mobility and vacuum aerodynamic diameters. We make measurements on systems of three types: (1) Agglomerates of spheres, for which the density and the volume are known; (2) Ammonium sulfate, sodium chloride, succinic acid and lauric acid irregularly shaped particles of known density; and (3) Internally mixed particles, containing organics and ammonium sulfate, of unknown density and shape. For agglomerates of spheres we observe and quantify alignment effects in the Differential Mobility Analyzer (DMA), an important consequence of which is that mobility diameter of aspherical particles can be a function of DMA operating voltages. We report the first measurements of the dynamic shape factors (DSFs) in free molecular regime. We report the first experimental determination of DSF for ammonium sulfate particles, for which we find DSF to increase from 1.03 to 1.07 as particle mobility diameter increases from 160 nm to 500 nm. Three types of NaCl particles were generated and characterized: nearly spherical particles with DSF of ∼ 1.02; cubic with DSF that increases from 1.06 to 1.17 as particle mobility diameter increases; and compact agglomerates with DSF 1.3–1.4. Organic particles were found to be nearly spherical. The data suggest that addition of organics to ammonium sulfate particles lowers their DSF.