## 3 Results and Discussion
### Chemical Composition
#### 3.2.3 The Role of Sea-Salt and Non-Sea-Salt Sources  
The results obtained and reported in Figure 6 for all the MRs highlight the presence of a significant contribution of both soluble ion tracers of a sea-spray origin (e.g., Na\({}^{+}\), Cl\({}^{-}\)) and of the crustal fraction. Therefore, a preliminary source apportionment was carried out to differentiate the ionic component accordingly by separating the species originating from sea spray and those that do not. The ionic components are in fact responsible for both the hygroscopic (Section 3.3) and electrical (Section 3.4) behavior of the aerosol deposit onto the insulators. Thus, in order to quantify the role of sea spray, Na\({}^{+}\), K\({}^{+}\), Mg\({}^{2+}\), Ca\({}^{2+}\), SO\({}_{4}\)\({}^{2-}\), and Cl\({}^{-}\) (which can originate both from sea salt, crustal, and other sources) were apportioned between sea-salt (ss) and non-sea-salt (nss) fractions using a previously developed and validated approach [41; 42; 43; 44], which is presented in the Supplementary Materials (Table S7). Table 3 and Figure 8 show the results obtained for all the three MRs. From a quantitative point of view (Table 3), the absolute deposit density values (expressed in \(\upmu\)g cm\({}^{-2}\)) of the contribution of both ss and nss sources are higher in MR1, followed by MR2 and then MR3. Particularly, the ss fraction in MR1 (18.19 \(\pm\) 5.31 \(\upmu\)g cm\({}^{-2}\)) was twice that in MR2 (9.59 \(\pm\) 1.79 \(\upmu\)g cm\({}^{-2}\)) and eight times higher compared to MR3 (2.54 \(\pm\) 0.58 \(\upmu\)g cm\({}^{-2}\)). The results related to the soluble ion amounts due to sea-spray emission can be explained considering the peculiar geographical features of Italy, characterized by islands (MR1) and a peninsula facing both toward the sea and with important inland areas (e.g., the Po Valley belonging to MR3) as detailed in Section 2.1. It is noteworthy that the water-soluble ions of nss origin also decreased from MR1 toward MR3. In fact, Figure 7 highlights that the majority of Na\({}^{+}\), Ca\({}^{2+}\), Cl\({}^{-}\), and SO\({}_{4}\)\({}^{2-}\) did not originate strictly from sea salt, but from other sources (e.g., crustal fraction) which affected the overall chemical composition of the investigated deposits for the three MRs. In particular, as shown in Figure 7, the proportion of ss-Na and ss-Cl appear to be lower in MR1 (7.5 \(\pm\) 3.6% and 13.3 \(\pm\) 6.5%, respectively) compared to MR2 (13.8 \(\pm\) 4.2% and 24.6 \(\pm\) 7.6%, respectively), even though MR1 included the islands. On the contrary, the MR1 exhibits a higher contribution from the nss fraction compared to MR2 (75.96 \(\pm\) 21.5% and 55.47 \(\pm\) 7.1%, respectively), possibly attributed to both Saharan dust transport events (due to the proximity to Africa) and local dust resuspension. MR2 also exhibited a significant contribution from the nss fraction, but to a lesser extent than MR1. Indeed, a higher percentage contribution of Na\({}^{+}\) and Cl\({}^{-}\) can be observed in MR2 for this reason. This can be explained by the fact that these regions are farther from Africa and, therefore, are less prone to dust transport events from the Sahara Desert. These considerations are in keeping with literature figures reported by [45; 46], which show a frequency of dust events per year ranging between 21 and 34% in the southern region of Italy, 13 and 21% in central Italian regions, and 5 and 10% in the northern ones; moreover, due to its proximity to the source region, Sicily is the Italian region receiving the largest impact of desert dust transport events, with a maximum number of desert dust days per year (over 150) being comparable to the climatological mean values derived in the Canary Islands (Spain), and in Sardinia, approximately three-quarters of the Saharan dust is deposited with precipitation [47]. These literature data agree with those reported in Figure 7, showing a marked north-to-south positive nss gradient, in agreement with the results reported by [48]. In addition to this, [PERSON] et al.