## 3 Results and Discussion  
### ESDD and NSDD Evaluation  
The variational range of ESDD and NSDD, expressed in mg cm\({}^{-2}\), of the monitoring campaign carried out from 2016 to 2019 is shown in Table 1. These values represent the minimum and maximum surface deposit of the soluble and non-soluble fraction accumulated on the total insulator surface (1585.33 cm\({}^{2}\)) that can be found from regular measurements over a two-year period in Italy. The minimum values of ESDD recorded during the whole campaign range from 0.007 to 0.029 mg cm\({}^{-2}\) for MR1, from 0.005 to 0.014 mg cm\({}^{-2}\) for MR2, and from 0.004 to 0.006 mg cm\({}^{-2}\) for MR3; while the maximum values range from 0.088 to 0.227 mg cm\({}^{-2}\), from 0.052 to 0.236 mg cm\({}^{-2}\), and from 0.020 to 0.084 mg cm\({}^{-2}\) for MR1, MR2, and MR3, respectively. The corresponding minimum values of NSDD vary from 0.003 to 0.012 mg cm\({}^{-2}\) for MR1, from 0.002 to 0.006 mg cm\({}^{-2}\) for MR2, and from 0.001 to 0.004 mg cm\({}^{-2}\) for MR3; conversely, the maximum variation of NSDD was 0.044-0.312 mg cm\({}^{-2}\), 0.025-0.116 mg cm\({}^{-2}\), and 0.014-0.110 mg cm\({}^{-2}\) for the three MRs. It emerges that the highest values, for both ESDD and NSDD, were mostly associated with MR1 (Sardinia and Sicily), markedly characterized by marine pollution and possible transport of material from the close African Sahara Desert. Indeed, the 25 th, 50 th, and 75 th percentile values of ESDD and NSDD, respectively, for Sardinia and Sicily, were 0.032, 0.051, 0.125 mg cm\({}^{-2}\) and 0.034, 0.063, 0.100 mg cm\({}^{-2}\) for ESDD, and 0.019, 0.046, 0.109 mg cm\({}^{-2}\) and 0.018, 0.033, 0.080 mg cm\({}^{-2}\) for NSDD. By contrast, both ESDD and NSDD decrease going northeast toward MR2 and MR3, which presented the lowest values, underlying the reliability of macroregion definitions on the basis of the geographical features of Italy (the statistical indexes and the 25 th, 50 th, and 75 th percentile values for the three MRs of ESDD and NSDD are shown in Supplementary Materials, Figure S3). The aforementioned data are in line with observed ESDD and NSDD deposition in Shantou, China, where [PERSON] et al. [33] found values between 0.032 and 0.083 mg cm\({}^{-2}\) and between 0.026 and0.3046 mg cm\({}^{-2}\), respectively, for ESDD and NSDD for a two-year-long natural pollution deposit on 220 kV lines; a similar comparison can be made with the ESDD values measured in the inland area of Japan [34] for an exposure period of more than one and a half years: their maximum values ranged between 0.0032 and 0.0052 mg cm\({}^{-2}\) and 0.0116-0.0126 mg cm\({}^{-2}\), respectively, on top and bottom insulator surfaces [34]. Referring to these values, Italy shows the lowest hazard compared to the Shantou area and an analogous one compared to the inland area of Japan. The level of pollution could be affected by different factors, such the meteorological conditions and the geographical environment. For example, [PERSON] et al. [35] measured the pollution class, using ANSI52-3 insulators for exposure periods of two, six, and twelve months, considering five substations in Barranquilla, Colombia, and they found that rainfall affects the insulator's surface contamination markedly. [PERSON] et al. [36] carried out a multi-year sampling campaign on a similar number of samples in five different areas (i.e., the coastal region, the highway region, the suburb region, the mountain region, and the urban region), and they found that the geographical environment has an impact on the pollution: the contamination of the coastal region is the heaviest and the mountain region the lightest, in keeping with the aforementioned results from Italy.