A Semi-Analytical Model of the Scanning Transfer Function for Radial-Flow Differential Mobility Analyzers
STAVROS AMANATIDIS, Yuanlong Huang, Richard Flagan, Aerosol Dynamics Inc.
Abstract Number: 542
Working Group: Instrumentation and Methods
Abstract
Differential mobility analysis (DMA) is a well‑established measurement technique that provides size‑classification of aerosol particles according to their electrical mobility. In the DMA, charged particles in the incoming flow are separated as they pass through an electric field between two electrodes which is varied to enable classification of different particle mobilities.
A common DMA application is its use as part of a Scanning Electrical Mobility Spectrometer, wherein the DMA electric field is varied in a continuous exponential voltage ramp and the output aerosol concentration is detected by a particle counter. The measured response is then reduced to size distribution data by an inversion routine that requires input information of the various instrument parameters. A key parameter in this process is the DMA transfer function, the particle transmission probability through the DMA as a function of input particle mobility and instrument operating parameters. While an analytical model of the transfer function for radial-flow DMAs has been previously described in a steady-state applied electric field (“static” mode), no model is currently available for operating the instrument in “scanning” voltage mode.
In this work, we present a semi-analytical model of the scanning transfer function for radial-flow DMAs. The model is derived for fully‑developed laminar flow and takes as input the physical dimensions of the classifier geometry, operating flow rates, scanning voltage rate, as well as the input particle mobility. We demonstrate the characteristic particle trajectories in the DMA and how those vary with scanning parameters. Using the geometry characteristics of an actual radial-flow DMA, the transfer functions predicted by the model are compared against finite element modeling results across a range of operating parameters, for both increasing (upscans) and decreasing (downscans) voltage ramps, as well as against the “static” transfer function model.