Chapter twelve - Filtration of ambient aerosols

Abstract

Using a portable filter test setup, filtration using nanofiber filters on real aerosols from mixed vehicular emission and ambient pollutants has been studied. The size of aerosols being monitored is between 10 and 433nm. Although some of the aerosols, such as diesel particulate matter are known to have fractal dimensions, comparison using the equivalent aerodynamic diameter provides a good interpretation of the filtration behavior. Different parameters, such as aerosol size, face velocity, and fiber basis weight, have been investigated in the study. A new dimensionless parameter, the specific filter resistance γ, is introduced. γ measures the viscous drag on flow through the fibers in the filter with a given fiber basis weight (γ has also been independently derived from the dimensional analysis in Appendix E). Therefore a low value of γ would be more favorable for air filtration. In the test examples, γ is approximately unity. Through multilayering/multimodule nanofiber configuration using a nanofiber filter electrospun from polyvinyl alcohol, it has been demonstrated that it is possible to improve the filtration efficiency by increasing the basis weight of nanofibers while maintaining both constant quality factor (QF) and γ. On the other hand, the traditional approach of increasing basis weight of fibers in a single-layer nanofiber configuration would only lead to reduction in QF and increasing γ. As real aerosols from mixed vehicular emission and ambient pollutants are typically less than 200nm with Peclet number less than 10, diffusion plays an important role in the capture of these tiny aerosols by a nanofiber filter. The single-fiber efficiency due to diffusion based on testing has been closely examined, and it compares quite well with the diffusion correlation prediction. Despite this agreement, there is increasing deviation of the test results as compared to the theoretical prediction, especially at a large Peclet number (greater than 10) and at high face velocity greater than 0.07m/s. This may be attributed to unavoidable complicated aerosol–aerosol interaction for polydispersed aerosols challenging the filter, especially at a high velocity. In general, the behavior of ambient aerosols is very similar to that of controlled laboratory using solid sodium chloride aerosols lending assurance of laboratory development in simulating real situations. In the second part of the chapter, ambient aerosols have been used to simulate SARS-CoV-2 virus (100nm) transmission by air which is an important issue during the COVID-19 pandemic. We have demonstrated that nearly 90% capture of these airborne virus can be achieved with a 6-module electrostatic charged PVDF filter. This demonstrates very importantly that dielectrophoretic capture (charge induction followed by electrostatic charge attraction) with charged nanofiber filter also works for ambient aerosols. The pressure drop of the filter at 5.3cm/s can be quite low at 26Pa. Comparing with the NaCl aerosol test using the same filter, the ambient aerosol efficiency can be 6%–24% lower depending on the number of modules deployed in the multimodule electret filter, with the difference being less with more modules in the multimodule filter. Thus, the ambient aerosol is more difficult to filter due to interaction of aerosols of different sizes. By increasing the number of modules, the multimodule approach follows nearly an iso-QF curve; both high efficiencies and QF can be reached. We have obtained similarly favorable comparison for the 50-55nm aerosol and the 300-nm aerosols which simulate the smallest and largest size of airborne SARS-CoV-2 virus (with carrier), respectively. The ambient aerosol test results have been compared to those using monodispersed sodium chloride aerosols filter tests using the same test filters in the laboratory under controlled condition, with similar findings, yet the sodium chloride aerosol capture efficiency is slightly higher by a few percentages (absolute).