## 4 Conclusions  
Conductance measurements, carried out during humidity cycles in an Aerosol Exposure Chamber (AEC), were performed on ambient aerosol samples collected on PTFE filters as surrogate for hydrophobic surfaces used in electronic/energy application. The samples mass surficial densities, their chemical composition and the conductance in function of relative humidity were investigated. It was pointed out that the measured percentage ionic content was statistically higher for the non-electrical conductive samples, despite the overall aerosol mass was higher for electrically conductive samples. Therefore, the presence of components, other than the ionic ones, were necessary to determine aerosol conductance was investigated. EC and OC content were also measured allowing to demonstrate the presence of a synergy between the ionic and the carbonaceous fractions of atmospheric aerosol to promote electrical bridging phenomenon of particles deposited on hydrophobic substrates. A further proof of this kind of interaction was obtained using laboratory generated aerosol samples (saline only, EC only and mixed EC saline) that were exposed to humidity cycles while measuring their electrical properties. For none of the pure saline aerosol samples any conductance signal was detected, while it was possible for all the mixed saline and EC samples, confirming that a synergy between different species in the aerosol samples is essential for the formation of a conductive medium on a hydrophobic surfaces. When EC was added to the saline components, it helped the formation of an electrical bridge thanks to its conductive nature, As in literature the hygroscopic components in aerosols are believed to be the major responsible for electrical/energy failures, the results presented in this work demonstrated that other aerosol conductive species, such as EC, and their interaction with the hygroscopic components have a role in the formation of electrical bridging phenomena that can be potentially dangerous for energy applications. Thus, any cleaning protocol (e.g., for high power level insulators) able to remove not only the hydrophilic compounds but only the hydrophobic one should be adopted in energy applications. Figure 8: Electrical conductance (G) measurements during a humidity cycle (RH) in the AEC for WI-mix and SU-mix aerosol mixed with EC. **Supplementary Materials:** The following are available online at [[http://www.mdpi.com/2076-3417/10/16/5559/s1](http://www.mdpi.com/2076-3417/10/16/5559/s1)]([http://www.mdpi.com/2076-3417/10/16/5559/s1](http://www.mdpi.com/2076-3417/10/16/5559/s1)), Figure S1: Guillotine cutter for filters, Figure S2: Conductance and RH equilibration (steps of 30 min) within the AEC on a PM2.5 sample, Figure S3: Conductance for each RH steps at 0 s, 180 s, 1200 s and 1800 s for the sample showed in Figure S1, Figure S4: Conductance cycles repeated two times on the same PM2.5 sample, Figure S5: Normalized number size distribution of generated salts, WI-mix, SU-mix and soot particles, Figure S6: Active sampling spot size of a PTFE filter, Figure S7: Stereomicroscope image (Leica Wild M20, 64x enlargement) for a PM2.5 MI-TS ambient sample, Figure S8: Correlation between the conductance value at the DRRI overcoming with the product (i.e. synergic effect) between the number of equivalent of inorganic ions and the mass of EC on filters both ambient (panel a) and laboratory generated (panel b). Note that the mass of EC cannot be transformed into moles as EC is not a well-defined molecule characterized by its own molecular weight, Table S1: Raw data of PM2.5 ambient samples with sampling location (from Milano Torre Sarca, MI-TS, and Oasi Bine, OB), season, date, mass loading, surface spot size, electrical detection (DET*), electrode distance (ED*), conductance (maximum detected value, G*) and chemical data, Table S2: Raw data of laboratory generated PM samples with generation chemistry, mass loading, surface spot size, electrical detection (DET*), electrode distance (ED*), conductance (maximum detected value, G*).