Aerosol Filtration with Mobility-Classified Particles: Role of Multiply Charged Particles in Skewing Penetration Measurements

Filter media exhibit strong particle-size dependent capture characteristics that have a complicated dependence to filter properties and operating conditions. As simulation-based determination of the filtration characteristics of real media is very challenging, accurate experimental approaches are critically necessary. Conventionally, filter penetration measurements were made using test particles extracted from a differential mobility analyzer (DMA) and measuring the ratio of particle counts downstream of the filter to that upstream. Size-dependent filter penetration can be directly determined from these measurements if the DMA was operated at high resolution and the contribution of multiply charged particles was negligible. The accuracy of this well-established approach has, however, never been fully assessed. In this study, the influence of multiply charged particles to the filter efficiency measurements is determined using a system-modeling approach. The model results suggest that the contribution of multiply charged particles could significantly influence penetration results when the mode size of the test particles entering the DMA or the most penetrating particle size (MPPS) of the test filter are larger than ∼100 nm. Under such conditions, even if multiply charged particles constitute a small fraction of the test aerosol, they can significantly skew filter penetration results. For accurate calculation of filter penetration, two approaches are proposed: an improved filter test protocol and a new multiple-charge correction approach for calculation of size-dependent filter penetration from mobility-based measurements. The validity of the proposed approaches is established with experimental data.

Copyright 2015 American Association for Aerosol Research