Bidimensional Charge Distribution of Compact and Fractal Aerosols in a Unipolar Charger

NISHAN SAPKOTA, Timothy Sipkens, Rym Mehri, Steven Rogak, Joel Corbin, University of British Columbia

     Abstract Number: 572
     Working Group: Instrumentation and Methods

Abstract
Charging aerosols unipolarly is often an ideal choice for particle detection and signal enhancement in many aerosol applications. A good understanding of charging behavior is, therefore, necessary for interpreting the data accurately. However, measuring the charge distributions from a unipolar charger is a difficult task, especially for higher ion concentrations and larger particle sizes. As the number of elementary charges on the particles increases, the electrical mobility for adjacent charges becomes indistinguishable requiring a complicated data inversion. To further resolve it, recent studies have proposed a new analysis method, interpreting the measurement of charging efficiencies as bidimensional distributions. In this study, we apply an adaptation of this method while characterizing a unipolar charger for aerosol particle aerosols of different shapes– spheres (Santovac® polyphenyl ether), cubes (sodium chloride), lacey aggregates (soot from an inverted burner), and compact aggregates (compaction using oleic acid). Aerosols of these morphologies were conditioned and size-selected using both a differential mobility analyzer (DMA) and an Aerodynamic Aerosol Classifier (AAC), then passed through the candidate charger for final characterization using a second DMA column. The use of an AAC ensured that there were no multiply charged artifacts in the classified particles passed through the first DMA and ensured that the particles had a known mobility diameter. The unipolar charger used in this study was a diffusion charger operated with two ion concentration-time products (Nit) of 5e12 and 3e13 ion s/m3 and a fixed flow rate. Computed charge fractions agree well with previous literature and demonstrate an increase in the level of charge on fractal aggregates.