## 3 Results and Discussion
### Generated Aerosols Conductance Measurements  
In order to prove the role of the synergy between ionic fraction and EC in the determination of the electrical conductance of deposited aerosols, conductance measurements on laboratory generated aerosol samples were performed. First of all, 7 pure saline aerosol samples were generated, collected on PTFE filters and exposed to humidification and dehumidification cycles in the AEC; they were: AS, AN, AC, SC, SS, WI-mix and SU-mix. Their mass loadings ranged from 116.6 to 857.7 \(\mu\)g cm\({}^{-2}\) (concentrations refer to the range of the whole ensemble of component and mix) and were at least comparable or even significantly higher than the surficial mass distributions of the atmospheric samples presented in previous sections. In addition, SMPS data (Figure S5, Supplementary Materials) allowed the computation of deposit porosity (\(\varepsilon=0.9618\pm 0.0024\), Equations (1) and (2)) which was in keeping with data reported in [PERSON] et al. [91] and [PERSON] et al. [90]. From \(\varepsilon\), the deposit thickness was determined (from 16.0 to 125.6 \(\mu\)m, Equation (3)). Despite that their mass loading and layer thickness were considerable, none of the them showed any electrical signal when humidity raised inside the chamber. It was observed the formation of little droplets of saline solution over the surface of the PTFE filter that were visible to the naked eye at high RH values (Figure 5a,c). This kind of behavior was attributed to the hydrophobic nature of the substrate and it results in the formation a discontinuous medium and in the lack of any conductance signal between the electrodes. This first result is in keeping with those obtained on ambient samples (previous sections); moreover, as the filtering surface was chosen as a surrogate of hydrophobic surfaces used in electronic/energy applications, these results highlight the need for a deeper investigation of aerosol deposits in critical electronic/energy applications. Therefore, electrical conductance measurements were also conducted on pure laboratory generated EC as well and repeated eight times. Their porosity was 0.9569 \(\pm\) 0.0001, in agreement with diesel soot porosity (0.953-0.961) found by [PERSON] et al. [97]. The deposited EC thickness ranged from 0.3 to 2.6 \(\mu\)m. As mentioned in the method section, the measured electrical conductance at 30% RH for the eight EC samples was in the 20-180 \(\mu\)S range despite they had a surficial mass distribution of 3 to 20 \(\mu\)g cm\({}^{-2}\), at least of one order of magnitude lower than the mass loading for pure saline aerosol samples. As the main aim of the present work is to investigate the role that EC, in synergy with hygroscopic saline species, has in the formation of electrical bridging phenomena promoted by deposited aerosol particles on hydrophobic surfaces, the EC conductance was measured at 30% RH only as its conductance in function of RH was beyond the scope of the present work. At a later stage, mixed saline and EC samples were prepared; the average concentration of particles into the generation chambers were and \(8.9\pm 1.7\times 10^{4}\) and \(4.5\times 10^{3}\pm 7.6\times 10^{2}\) cm\({}^{-3}\), respectively. The ionic content was then quantified by ion chromatography and it resulted on average of \(86.6\pm 5.0\%\) on the overall deposited mass (\(402.3\pm 140.9\)\(\mu\)g cm\({}^{-2}\)). This data is close to that of ambient PM\({}_{2.5}\) samples in which the ionic fraction accounted for \(73.3\pm 12.0\%\) with respect to the ions+EC mass loading; most important, the highest ions/EC ratio (i.e., less EC) in the laboratory generated aerosol represent a worst scenario to test the crucial role in bridging effects. The possible presence of other contaminant ionic species from the previous generation experiment was also assessed via ion chromatography. This kind of impurities was quantified to be on average the \(1.5\pm 1.6\%\) of the overall sampled mass.