## 3 Results and Discussion
### Particle FE Performance over Time  
To determine the overall PN FE as a function of time, the particle number size distributions were integrated to obtain the total PN measured at both pre- and post-filter locations at various time points, which were then compared to determine how PN FE changed over time. The total accumulated particle number concentration introduced at the pre-filter location at various time points in an experiment was scaled based upon the on-roadway rush-hour exposure references from UTES, assuming 3 h of driving per day and ignoring weekend and weekday driving pattern [47; 48]. Particles generated from the miniCAST contain a higher percentage of BC than ambient particles. Therefore, scaling the experimental time using BC mass results in more than double the time coverage. Since the deposited BC mass on the cabin filter still contributes to the overall deposited mass on the filter, which will have an impact on FE, scaling the time with BC mass gives a more realistic scenario. After scaling, a typical experiment in this study represents about 300-800 days of filter usage. Figure 8 summarizes the change in PN FE over time. Data are color coded based on the filter categories identified from the size-dependent FE. Results in Figure 8 show consistent behavior for different filters within the same group. Group I filters represent typical nominal filters, showing relatively flat FE behavior over the course of about 2 years of filter usage. Since observations in Figure 5 show that FE for different sizes did not change dramatically over the course of the experiment, the results in Figure 8 suggest that the relative FE for different sizes for Group I filters are stable over 2 years of usage in real life. Generally, FE for Group I filters varied from 30% to 40%. The formation of dendrites was expected to improve FE over time [27], but observations for Group I filters did not show this behavior. Fibrous filters typically have very low packing density in the range of 1-15% [19] to permit low pressure drop. Therefore, this also means that these filters typically have rather high filter capacity, and the amount of PM generated from this experiment obviously was not enough to significantly change any microstructure of the filter fiber to illustrate the improved FE caused by the formation of dendrites. Group II filters had much higher FE (>60%) at the new state when compared to the nominal filters (FE of 30-40%). The high FE was due to the presence of the electrostatic charges on the filter fibers, which creates additional electrostatic attraction to help remove PM. The charges on the fibers typically do not dissipate over time and could be retained over years under favorable conditions. However, the charges could be lost for a number of reasons, such as high temperature, high humidity, or the deposition of organic liquid aerosol (e.g., oil or di(2-ethylhexyl)phthalate). The charges on the fibers can be neutralized when exposed to ionizing radiation, such as fine charged particles. The accumulation of dust on the fiber surface can mask the charges, creating a shadow effect and removing the benefit of electrically charged fibers [18; 19; 38; 39]. It is beyond the scope and limitations of this study to determine the exact reason for the reduction of FE for Group II filters over time. However, since the FE of Group II filters showed a gradual reduction and approach to the typical FE of Group I filters, it is consistent with the expectation that FE reduction could be due to the accumulation of dust. It is also possible that some of the BC particles generated from the miniCAST could carry charges, which could remove charges from the fibers. Once the charges from the filter fibers are neutralized or shadowed, the electrostatic filters lose their advantage and become a nominal filter and thus have comparable FE. This study suggests that the advantages of the electrostatic filters can be maintained for about 2 years of usage. Group III filters are the underperforming filters.