## 4 Conclusions  
In this paper, the chemical determination of deposit aerosol mass on insulators of the national power lines was investigated for the first time in Italy. The results of ion chromatography analysis carried out on the ESDD solutions obtained from the sixth, seventh, and eighth campaigns revealed that the total composition is mainly made up of inorganic ions: in particular, the water-soluble fraction mainly consists of Na\({}^{+}\), K\({}^{+}\), Ca\({}^{2+}\), Mg\({}^{2+}\), Cl\({}^{-}\), NO\({}_{3}\), and SO\({}_{4}\)\({}^{2-}\), while the concentrations of NH\({}_{4}\)\({}^{+}\), F\({}^{-}\), and PO\({}_{4}\)\({}^{3-}\) were negligible. The remaining part of the total composition, analyzed by thermos-optical and X-ray techniques, consisted of carbonaceous (black carbon and, especially, water-insoluble organic matter) and crustal material, such as Al, Fe, K, Ca, and Mg. The obtained results highlighted the role of the inorganic ions, which could form a conductive layer on the insulators' surface when wetted and cause a sharp phase transition of the deposited material depending on the ambient relative humidity; the consequent conductivity can lead to flashover discharges. The humidograms obtained from exposition cycles in AEC showed greater DRH values in Sardina and Sicily (MR1) that decreased going north till the Po Valley Area (MR3). The results underlined that, in Italy, depending on the temporal behavior of the atmospheric RH, the deposit on insulators can be hydrated or not in different conditions in the three MRs. The conductivity measurements evidenced a growing conductivity in parallel with the increase in mass of the deposit, while during the dehumidification phase (between DRH and CRH), the conductivity gradually decreased as water evaporated until crystallization occurred. The obtained results can be of interest in the prediction of flashover risks on glass (or porcelain) insulators with a twofold advantage: alerts to the dispatching center highlighting lines or stations at risk of flashovers and alerts for preventive maintenance actions, for example, switching from glass cap-and-pin insulators to pre-coated glass cap-and-pin insulators to reduce the risk of flashover. **Supplementary Materials:** The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app132312788/s1](https://www.mdpi.com/article/10.3390/app132312788/s1)]([https://www.mdpi.com/article/10.3390/app132312788/s1](https://www.mdpi.com/article/10.3390/app132312788/s1)), Table S1: Specifications of the scheduled time of the sampling campaign from 2016 to 2019 in Italy; Table S2: Ion composition, expressed in \(\upmu\)g/mL of the aerosol samples collected during the 6 th sampling campaign; Table S3: Ion composition, expressed in \(\upmu\)g/mL of the blank filters of the 6 th sampling campaign; Table S4: Ion composition expressed in \(\upmu\)g/mL of the aerosol samples collected during the 7 th sampling campaign; Table S5: Ion composition expressed in \(\upmu\)g/mL of the aerosol samples collected during the 8 th sampling campaign; Table S6: Measured values (\(\pm\)95% confidence interval) of the elements found on filter via energy dispersive X-rays micro-fluorescence analysis (ED-\(\upmu\)-XRF). The label \"nd\" indicates \"not detected\" values; Table S7: Equations for the calculation of the sea-salt and non-sea-salt contribution of the inorganic ions. 0.562 represents the _w/w_ Na\({}^{+}\)/Ca\({}^{2+}\) ratio in the crust [52] and 0.038 is the Ca\({}^{2+}\)/Na\({}^{+}\)_w/w_ ratio in seawater [53]. Similarly, the ss-SO\({}_{4}\)\({}^{2-}\) fraction was calculated from the ss-Na+ using 0.252 SO\({}_{4}\)\({}^{2-}\)/Na\({}^{+}\)_w/w_ ratio in seawater [52]. The same applies to the ss-K\({}^{+}\), ss-Mg\({}^{2+}\) and ss-Cl\({}^{-}\) fractions, which were calculated from ss-Na\({}^{+}\) using 0.037 K\({}^{+}\)/Na\({}^{+}\)_w/w_ ratio in seawater, 0.119 Mg\({}^{2+}\)/Na\({}^{+}\)_w/w_ ratio in seawater, and 1.795 Cl\({}^{-}\)/Na\({}^{+}\)_w/w_ ratio in seawater respectively [52].