AAAR 31st Annual Conference
October 8-12, 2012
Hyatt Regency Minneapolis
Minneapolis, Minnesota, USA
Abstract View
The Effect of Shape on the Electrical Mobility “Diameter” of Mineral Dust
SWARUP CHINA, Kristopher Bunker, Claudio Mazzoleni, Alexander Kostinski, Will Cantrell, Michigan Technological University
Abstract Number: 344 Working Group: Aerosol Physics
Abstract While a spherical particle's electrical mobility is unambiguous, this is not the case for irregularly shaped aerosols such as mineral dusts. For such particles, the Stokes drag depends on the orientation of the particle in the flow, which in turn, depends on the placement of an attached electric charge. What are the quantitative implications of such shape and orientation dependence for mobility?
To that end, we have compared the sizes of Arizona Test Dust and kaolinite derived from electrical mobility measurements with those derived from scanning electron microscope (SEM) images. Specifically, we select the size of the dust with a differential mobility analyzer (DMA), then collect the dust on a Nucleopore filter, which is then examined with the SEM. The discrepancy between the sizes (and distribution of sizes) derived from the DMA and SEM is striking. The projected Area Equivalent Diameter, Daeq, as derived from the SEM images, is as much as a factor of three greater than the electrical mobility diameter for the sizes examined for both dusts. The factor itself is size dependent, decreasing with increasing size of the dust. The distribution of sizes seen in SEM images for a given electrical mobility diameter is also quite broad, extending up to a factor of seven (i.e. particles with a Daeq seven times greater than the electrical mobility diameter comprise a non-negligible fraction of the particles deposited to the filter). Like the ratio of the diameters derived from the two methods, the spread of the SEM-derived size distribution diminishes with increasing size of the dust. Furthermore, kaolinite, which has a plate-like structure and tends to be less compact than the Arizona Test Dust, exhibits a larger discrepancy.