## 2 Materials and Methods
### Aerosol Sampling  
The aerosol samples were collected choosing the PM2.5 as a reference standard. This choice is supported by previous studies [18; 20; 33; 68] in which it has been demonstrated that this fine aerosol fraction is capable of getting into a data center despite the presence of an industrial filtering system (MERV13). Moreover, as it has been shown in other studies [19; 69] PM\({}_{2.5}\) represents an important fraction of the aerosol involved in the deposition mechanism occurring on vertical surfaces such as the high voltage transmission lines insulators. In this respect, fine aerosol particles (i.e., PM\({}_{2.5}\)) have a long residence time and are able to reach high altitudes as PM vertical profiles in the Po Valley demonstrated [6; 70; 71]. Thus, PM\({}_{2.5}\) particles are also able to reach the electric line insulators making the choice of PM\({}_{2.5}\) reliable for the purpose of the present study. PM\({}_{2.5}\) samples were collected in accordance with the EN-14907 standard on both PTFE (\(\mathcal{O}\) = 47 mm, PTFE, PALL R2P)047, 2.0 \(\upmu\)m porosity with a retention of 1 and 2 \(\upmu\)m particle size of 99.99% and 3 \(\upmu\)m particle size of 99,79%) and quartz fiber filters (\(\mathcal{O}\) = 47 mm, Whatman QM-A, 2.2 \(\upmu\)m nominal porosity, 99.999% filtration efficiency) for 24 h with a FAI-Hydra Dual Channel Low-Volume-Sampler (2.3 m3 h-1 flow rate). PM\({}_{2.5}\) samples were collected at the Milan 'Torre Sarca' sampling site (MI-TS, 45\({}^{\circ}\)31'19\" N, 9\({}^{\circ}\)12'46\" E) and at the rural site of 'Oasi Le Bine' (OB, 45\({}^{\circ}\)8'17.24\" N 10\({}^{\circ}\)26'10.99\" E) from 2005 to 2009. Figure 1 presents the location of the two PM\({}_{2.5}\) sampling sites (MI, urban and OB, rural), which are characterized by a different chemical composition (Section 3.2) and are located in the Po Valley (a European atmospheric pollution hot spot) [71; 72; 73; 74] due to the frequent stable atmospheric conditions which favors pollutant accumulation. A full description of these sampling sites and the characterization of aerosol properties (chemical composition, vertical profiles, sources and toxicity) for both MI-TS and OB sites was published by [PERSON] et al. [7; 70; 73], [PERSON] et al. [67], [PERSON] et al. [75]. Here we just underline that all the collected samples were stored at \(-\)18 \({}^{\circ}\)C in dark conditions in the Filters Bank of the University of Milano Bicocca. This Filters Bank has the purpose of storing PM samples in safe conditions for later analysis and investigations on new aspects of the research on aerosols. For the aims of this work, comparing aerosol samples from different sampling sites was important, since their different mean chemical compositions result in different electrical behaviors (Section 3.2). The number of samples considered in this work together the season of sampling are detailed in Section 3.2 and Supplementary Material (Table S1). PTFE filters were chosen because they are among the most common membranes for aerosol collection, thanks to their low blank values for ions they are a suitable matrix for ion chromatography analysis [76] (Section 2.2); in addition, previous studies [8, 18, 19, 77] demonstrated that hydrophobic supports are the most suitable for aerosol-water interaction measurements (e.g., via conductance measurements). In addition, their hydrophobicity makes them a good surrogate [66] for hydrophobic surfaces, e.g., epoxy-based composites that constitute PCBs and circuit component encapsulates [78] and for glass/ceramic insulator material, making them a good candidate for the present study. Quartz fiber filters were used to determined elemental and organic carbon as detailed in Section 2.2. PTFE filters were cut exactly in two halves, so that it was possible to perform ion chromatography analysis (Section 2.2) and conductance measurements in the AEC (Section 2.3) on two equal portions of the same sample. In this respect, the homogeneity of PM samples on filters was assessed in a previous study [79].