Measuring the Charging Efficiency of Unipolar Charger in Electrical Low-Pressure Impactor for Soot Aggregates

NISHAN SAPKOTA, Timothy Sipkens, Mang Guan, Steven Rogak, University of British Columbia

     Abstract Number: 283
     Working Group: Instrumentation and Methods

Abstract
The average number of charges on soot aggregates after unipolar charging is much higher than for compact aerosols of the same drag-equivalent diameter. This makes it difficult to interpret measurements from many aerosol instruments, including the Electrical Low Pressure Impactor (ELPI+) which is often used to measure combustion aerosols known to have a fractal-like structure. Previous efforts to determine the charging-equivalent diameter of soot have used soot aggregates classified by a Differential Mobility Analyzer (DMA). However, the presence of multiply charged particles introduces uncertainty, partly because the multiple charged fraction from the classifier is itself influenced by particle morphology. To largely eliminate multiple charging, we classify soot first with an Aerodynamic Aerosol Classifier (AAC) and then a Differential Mobility Analyzer (DMA). Soot was produced from a laminar inverted flame with ethylene or propane as the fuel. Although the soot produced by the two flames had substantially different effective densities, the increased charge on the soot was very similar and for both fuels. The charge carried by the particles is a function of the details of the ELPI+ charger, including penetration and ion current, but the charging of the soot can be universally correlated with the mobility diameter of a sphere (d_m) picking up the same charge i.e. charging-equivalent diameter (d_qe). We found the relation to be d_qe = 1.59d_m^1.12, which is expected to be a useful approximation for other instruments using unipolar charging.