## 3 Results and Discussion
### Deposit Conductivity  
All the aforementioned results have important implications for the conductivity that can be present on each insulator in the different MRs. In order to measure the conductivity of the aerosol, calcium-sodium tempered glass specimens (with conductive silver bands at 1 cm) contaminated with the same aerosol composition of each MR were exposed to RH cycles in the AEC as detailed in Section 2.5. An increase in the electrical conductivity during the humidification phase was observed, reaching its maximum at 90% RH; conversely, during the dehumidification phase, between DRH and CRH, the conductivity gradually decreases as water evaporates until crystallization occurs. This is shown in Figure 10, which indicates the 25 th, 50 th, and 75 thpercentile of the conductivity for the three MRs at six relevant points: starting and ending points of the deliquescence process (DRH\({}_{\text{s}}\), DRH\({}_{\text{e}}\)), 80% RH, 90% RH, and starting and ending points of the crystallization process (CRH\({}_{\text{s}}\), CRH\({}_{\text{e}}\)). The conductivity starts to grow at the beginning of the humidification phase, reaching relevant values at the end of the process; it is noteworthy that the 25 th, 50 th, and 75 th percentile values at the DRH\({}_{\text{e}}\) for MR1 were 1.533, 2.388, and 2.919 \(\upmu\)s cm\({}^{-1}\); in MR2, the statistical indices were 1.640, 2.352, and 4.562 \(\upmu\)s cm\({}^{-1}\); while in MR3, they were 0.203, 0.297, and 0.375 \(\upmu\)s cm\({}^{-1}\). The highest values of conductivity were reached when RH > 80% because, after the deposit activation at DRH\({}_{\text{s}}\) below 80%, further condensation occurs, favoring the formation of a continuous layer of electrolyte solution. In particular, the 25 th, 50 th, and 75 th percentiles at 80%RH for MR1 were 2.251, 3.505, and 4.285 \(\upmu\)s cm\({}^{-1}\); for MR2, the same index values were 3.249, 4.662, and 9.041 \(\upmu\)s cm\({}^{-1}\); while for MR3, they were 0.686, 1.003, and 1.267 \(\upmu\)s cm\({}^{-1}\). The maximum values were reached at 90% RH. The statistical index (25 th, 50 th, and 75 th percentiles) in MR1 were 4.055, 6.314, and 7.720 \(\upmu\)s cm\({}^{-1}\); in MR2, they were 3.717, 5.332, and 10.342 \(\upmu\)s cm\({}^{-1}\); while in MR3, they were 1.101, 1.608, and 2.032 \(\upmu\)s cm\({}^{-1}\). During the dehumidification path, a progressive reduction in conductivity is observed, reaching at the end of the crystallization process values similar to those measured at DRHs. In particular, at the CRHe in MR1, the 25 th, 50 th, and 75 th percentiles were 0.806, 1.255, and 1.535 \(\upmu\)s cm\({}^{-1}\); in MR2, the same index values were 0.384, 0.552, and 1.070 \(\upmu\)s cm\({}^{-1}\); while in MR3, they were 0.0010, 0.0014, and 0.0018 \(\upmu\)s cm\({}^{-1}\). As reported in Section 1, atmospheric pollution is among the main natural causes of flashover accidents that threaten the stability of the electrical system [5] because of its conductive properties, which lead to the formation of a conductive layer on the surface, causing leakage current flow [6]. The conductivity can be affected by both chemical composition (which directly affects the DRH and CRH values) and the amount of the deposit on the insulator surface: MR1 and MR2 showed the highest values, while MR3 the lowest, relating to the mass concentration (mg cm\({}^{-2}\)) of aerosol on insulators of the three MRs. The highest values of conductivity (at 90% RH) were recorded for MR1, which presented also the highest mass concentration of aerosol (50 th percentile: 0.106 mg cm\({}^{-2}\)), while the lowest conductivity was recorded for MR3, which showed the lowest mass concentration (50 th percentile: 0.019 mg cm\({}^{-2}\)). MR2 showed a conductivity value between MR1 and MR3, related to the mass concentration, which also ranged between MR1 and MR3 (50 th percentile: 0.033 mg cm\({}^{-2}\)).