Evaluation of DMA size selection of dry dispersed mineral dust particles

Mineral dust particles play a significant role in the Earth's radiative balance via direct interaction with solar radiation and indirectly through their ability to initiate cloud formation. Many field and laboratory studies utilize a differential mobility analyzer (DMA) for particle size selection. Here we evaluate the use of a DMA to size-segregate dry dispersed mineral dust particles. We examine the post-DMA size distribution using four different techniques: a scanning mobility particle sizer (SMPS) for mobility sizing, an optical particle sizer (OPS) for optical sizing, the Particle Analysis by Laser Mass Spectrometry (PALMS) instrument for vacuum aerodynamic sizing, and electron microscopy (EM) for geometric sizing. While the SMPS measured a narrow mobility size distribution at the DMA-selected diameter, the OPS, PALMS, and EM in most cases showed broader distributions and a smaller mode size than that selected by the DMA. These techniques also observed supermicrometer particles, often extending beyond the upper size limit of a typical SMPS scan. Complicating analysis, particle shape factor (χ) was observed to be a function of mobility size, ranging from 1.3 at 500 nm to 3.1 at 1000 nm. We conclude that mobility size selection of mineral dust particles using a DMA most often does not yield particles of the desired physical size or surface area. We suggest that attempts to size-select from a broad distribution of non-spherical particles require an independent measurement downstream of the DMA to verify the actual selected size.