## 3 Results and Discussion
### Chemical Composition  
#### 3.2.1 Chemical Composition of ESDD  
Using the methodology described in Section 2.3, the concentrations of inorganic ions were determined for the ESDD solutions obtained from the sixth, seventh, and eighth sampling campaigns. In addition to them, seven blank field samples were analyzed, which were blanks of the sponging procedure to test the accuracy of the analytical results. Table 2 shows the relative contribution of each ion to the total (more details of the inorganic ionic composition are shown in Supplementary Material, Table S2-S5 and in Figure S4). The results underlined a non-negligible contribution of Na\({}^{+}\) and Cl\({}^{-}\) for all three MRs. In particular, the highest values were found in the islands (MR1) while a decrease in their values can be observed moving toward MR2 and MR3, in agreement with the trend of the total ESDD. For MR1, the mean values (\(\pm\) 95% confidence interval) of Na\({}^{+}\) and Cl\({}^{-}\) were 12.51 \(\pm\) 3.30 \(\mu\)g cm\({}^{-2}\) and 20.82 \(\pm\) 5.89 \(\mu\)g cm\({}^{-2}\). For MR2 and MR3, Na\({}^{+}\) concentration was only 38.5 \(\pm\) 12.7% and 13.7 \(\pm\) 4.4% of the values measured in MR1; for Cl\({}^{-}\), the same ratios were 38.4 \(\pm\) 13.9% and 11.3 \(\pm\) 4.5%, respectively. Furthermore, large concentrations of Ca\({}^{2+}\) and SO\({}_{4}\)\({}^{2-}\) were observed, following the same trend as the previous ions: MR1 showed the highest values, while MR3 was the lowest. The measured values of Ca\({}^{2+}\) and SO\({}_{4}\)\({}^{2-}\) for MR1 were 13.47 \(\pm\) 3.65 \(\mu\)g cm\({}^{-2}\) and 25.27 \(\pm\) 11.38 \(\mu\)g cm\({}^{-2}\); the same values for MR3 were 1.89 \(\pm\) 0.34 \(\mu\)g cm\({}^{-2}\) and 1.21 \(\pm\) 0.19 \(\mu\)g cm\({}^{-2}\). MR2 showed values between MR1 and MR3. Considering the concentration of NO\({}_{3}\), it was the second highest ion, after Cl\({}^{-}\), for MR2 and MR3. It is noteworthy that the sulfate within the overall sulfate plus nitrate [SO\({}_{4}\)\({}^{2-}\)/(SO\({}_{4}\)\({}^{2-}\) + NO\({}_{3}\)\({}^{-}\))] [37] exhibits a decrease from MR1 to MR3. The respective ratios for the three macroregions were 0.82 \(\pm\) 0.48 \(\mu\)g cm\({}^{-2}\), 0.43 \(\pm\) 0.10 \(\mu\)g cm\({}^{-2}\), and 0.31 \(\pm\) 0.07 \(\mu\)g cm\({}^{-2}\). This parameter plays an important role in influencing the behavior of DRH as elucidated in Section 3.4. Lastly, the contents of NH\({}_{4}\)\({}^{+}\), F\({}^{-}\) and PO\({}_{4}\)\({}^{3-}\) were negligible, accounting for about 1.0 \(\pm\) 0.5% of the total ionic fraction. As mentioned above, a large section of the ESDD composition was represented by sodium chloride; especially, the contribution of Na\({}^{+}\) and Cl\({}^{-}\) was prevalent in MR1 and MR2 due to the geographical position of the regions included in these macrosectors. A progressive reduction in the contribution of sulfates moving from MR1 to MR3 is observed. \begin{table}
\begin{tabular}{l c c c c} \hline  & **MR1** & **MR2** & **MR3** & **Blanks** \\ \hline Na\({}^{+}\) & 12.51 \(\pm\) 3.30 & 4.82 \(\pm\) 0.95 & 1.71 \(\pm\) 0.31 & 0.29 \(\pm\) 0.11 \\ NH\({}_{4}\)\({}^{+}\) & n.d. & 0.16 \(\pm\) 0.29 & \(<\)0.01 & 0.50 \(\pm\) 0.60 \\ K\({}^{+}\) & 1.88 \(\pm\) 0.86 & 0.73 \(\pm\) 0.17 & 0.33 \(\pm\) 0.05 & 0.30 \(\pm\) 0.21 \\ Mg\({}^{2+}\) & 1.72 \(\pm\) 0.41 & 0.72 Vice versa, it is possible to observe an increase in the contribution of nitrates in keeping with the chemical composition in this region [2]. A significant contribution is represented by Ca\({}^{2+}\), which was higher for the eighth campaign, especially for MR3. Some research has already been devoted to the chemical analysis of natural contamination collected on insulator surfaces. Previous research indicated that the pollution constituents of outdoor insulators are various. The aforementioned data, for example, are in line with chemical analysis carried out by [PERSON] et al. [38] and [PERSON] et al.