## 3 Results and Discussion
### Measured Conductance
These results are very important as the failure threshold usually set for PCBs is \(10^{6}\) Ohms (1 \(\upmu\)S) [35, 93], one order of magnitude lower than the average increase in conductance associated with the deliquescence of the water-soluble components: \(8.56\pm 5.76\)\(\upmu\)S. The failure threshold indicates the minimum resistance (maximum conductance) beyond which electrical bridging between two neighboring plates can cause failure; thus, if the DRH promoted by atmospheric aerosols deposited on PCBs is reached it can represent an actual danger for them. These values also represent a hazard for insulators of high power lines. In addition, \(\Delta\)Gdeliquescence \(<1\)\(\upmu\)S was measured for just the 16% of the samples and for just one sample over the entire dataset the measured Gmax was below this threshold level. This means that for the large majority of the samples the 1 \(\upmu\)S threshold is exceeded either during the deliquescence process or because of the subsequent water absorption by the deposited aerosol particles indicating that these processes can be potentially dangerous for PCBs or electrical insulators contaminated with deposited hygroscopic aerosol. With respect to insulators, it is necessary to recall that dry aerosol deposition rates in the Po Valley were previously measured. In this respect, at MI-TS, [PERSON] et al. [66] determined (using a new developed deposition box; details in the reference above) an aerosol deposition rate on different surfaces ranging from 30 to 70 \(\upmu\)g cm\({}^{-2}\)month\({}^{-1}\); this implies that the mass distribution of the analyzed filters (collected using an active sampler) would correspond to a surface contamination obtained in -1-4 months, for deposition only, e.g., on high power supply insulators in the Po Valley. In this respect, the passive PM deposition can affect the surface characteristics with positive feedback on the deposition rate itself. As reported in [PERSON] et al. [66], if passive deposition rate data for different surfaces are considered in function of the exposure time a high linear correlation can be found showing that the \begin{table}
\begin{tabular}{c c c c} \hline \hline  & \(\Delta\)G\({}_{\text{deliquescence}}\) & \(\Delta\)G\({}_{\text{crystallization}}\) & \(\mathbf{G_{max}}\) \\  & **(\(\upmu\)S)** & **(\(\upmu\)S)** & **(\(\upmu\)S)** \\ \hline Minimum & 0.35 & 0.04 & 0.57 \\ Maximum & 54.04 & 10.01 & 127.77 \\ Average & 8.56 & 2.55 & 35.54 \\ Cl99\% & 5.76 & 1.35 & _13.58_ \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Highest, lowest and average measured values for \(\Delta\)Gdeliquescence, \(\Delta\)G\({}_{\text{crystallization}}\) and maximum measured conductance (\(\upmu\)S) for the subset of samples (42) with measured DRH and CRH. Figure 3: Electrical conductance (G) measurements during a humidity cycle (RH) in the Aerosol Exposure Chamber (AEC) of MI-TS winter sample collected in March 2009 (24 h sampling). Circles indicate the measured electrical conductance (blue: during humidification, red: during dehumidification). Dotted lines indicate the conductivity curves gradient, dG/dRH. Deliquescence relative humidity (DRH) and crystallization relative humidity (CRH) values are individuated in correspondence to the maximum gradient point for humidification (blu) and dehumidification (red) curves, respectively. Data for the present figure are extracted from [PERSON] et al. [18]. passive deposition rate increase with the exposure time due to a surface roughness increase. However, deposition on insulators follows a different mechanism with respect to filter sampling. The wind speed and direction, insulator shape and material and insulator coating will affect the deposit and its amount. Thus, the filter results cannot be directly transferred to insulators avoiding the aforementioned terms. Thus, future activity will be focused on sampling both on real insulators both on insulators embedded in the Deposition box described in [PERSON] et al. [66].