## 2 Materials and Methods  
### Setup and Measurement Protocol  
#### 2.1.1 Characterization of Technical Solutions  
The layout of the experimental setup employed for the characterization of the two technical solutions is shown in Figure 1. Following the regulatory requirements, soot-like particles, produced by a spark graphite generator (DNP 3000 from Palas, Karlsruhe, Germany) were employed [28]. Thermal treatment was performed by passing through an AVL catalytic stripper (CS) operating at 350 \({}^{\circ}\)C [29]. To precisely characterize the performance of the two technical solutions it is important to produce monodisperse particles of well-defined charge state. This was achieved by means of employing two differential mobility analyzers (DMA) in tandem as described in Appendix A. Both of them were model 3081 DMAs from TSI (MN, Shoreview, MS, USA) and were equipped with a 10 mCi \({}^{85}\)Kr neutralizer (model TSI 3077A). A third neutralizer of the same type was also employed in selected tests to neutralize particles exiting the tandem DMA setup. Number concentrations were quantified with a TSI 3775 CPC having a nominal 50% detection efficiency at 4 nm, while a TSI 3068B electrometer was used for the measurement of the total charge of the transmitted particles. The performance of the EF is characterized by the penetration of charged particles as a function of their size. For the experimental determination of the EF penetration, singly charged, monodisperse particles were produced with the tandem DMA setup shown in Figure 1a. The particles were sampled through the EF at the nominal flow rate of the AVL sensor (2 L/min), and their concentration was measured with the TSI 3775 CPC. Since the CPC operated at a flow rate of 0.3 L/min, the excess 1.7 L/min was extracted via a mass flow controller set at 1.7 L/min. The percentage drop in the concentration resulting when switching on the EF corresponds to the fraction of single charged particles removed by the EF. Sequential measurements were performed at each size evaluated, with the EF alternatively switched on and off and the concentrations recorded for 60 s. The ratio of the average concentrations at the different states was used to calculate particle losses and therefore penetration through the EF. In the specific tests, the DMAs operated at a sheath flow of 16 L/min, leading in a sheath over sample ratio of 8:1 that is within the recommendation of greater than 7:1 for aerosol instrument calibration [30]. The main performance characteristic of a corona charger is its charging efficiency, defined as the average number of charges acquired per particle as a function of particle size. This includes also any particle losses inside the corona. Experimental quantification of the charging efficiency is typically performed by parallel measurement of the concentration of neutralized monodisperse particles entering the corona and the total charge carried by the particles exiting the corona [31]. Figure 1b shows the instrument layout employed for the characterization of the various corona chargers evaluated. The characterization was performed with both neutral, singly and doubly charged monodisperse particles. Single charged particles were produced by operating the two DMAs at the same voltage, while double charged particles were extracted by operating the first DMA at double the voltage of the second (as described in Appendix A). Neutralized particles were produced by means of passing the extracted single charged particles through a third TSI 3077A neutralizer. The concentration of the produced particles was measured with the TSI 3775 CPC, while the total charge acquired by the particles in the corona was measured with the TSI 3068B electrometer connected at the outlet of the charger. The operating flow of the electrometer (and thus the charger) was set at the default flowrate of the AVL sensor (2 L/min). The sheath flow of the two DMAs was increased to 18 L/min to maintain the sheath over sample flow ratio above the recommended 7:1.