Theoretical and experimental validation of the aerosol electrical mobility separator electrometer

In this work, we introduce and validate a novel device, the Mobility Separator Electrometer (MSE). The MSE classifies particles by their electrical mobility, separating particles into two electrometer outlets. Automatic adjustment of the voltage set-point to achieve equal electrometer currents enables autonomous measurement of the aerosol population’s average electrical mobility. The transfer function of the device was experimentally validated, demonstrating good agreement with the theory once calibrated for an effective length. The power of the MSE occurs when integrating it in a tandem system involving a Unipolar Diffusion Charger, an upstream classifier such as a Centrifugal Particle Mass Analyzer (CPMA) or Aerodynamic Aerosol Classifier (AAC) and a Condensation Particle Counter (CPC). These tandem combinations provide a measurement of mass, aerodynamic diameter and mobility diameter, enabling the measurement of particle effective density. Using numerical simulations, we show the effective density of various aerosols can be determined within 5% using the aforementioned tandem systems over an extremely wide size range (30 nm $<d_m<$ 3000 nm) if the effective density is assumed to be single-valued. Further analysis of two-dimensional distributions, which better reflect soot aerosols, indicate errors of up to 80%. We further probe the model, demonstrating that the unipolar charger has to operate with a high concentration-time product ($n_{i}t$ $\ge$ $5\times 1013$ s m$-3$) to mitigate the effect of uncharged particles. Experimental measurements of the effective density of using a UDAC-AAC-MSE-CPC system Dioctyl Sebacate and Ammonium Sulfate were within 13% of the bulk density of each material.