## 2 Materials and Methods
### Deposit Collection, ESDD, and NSDD Measurements  
The IEC/TS 60815-1 guideline was taken into account as a procedure reference for sampling and ESDD and NSDD determination.  
The sampling procedure was based on the swabbing technique, in accordance with the aforementioned guideline. In this respect, a plastic container of 1 was filled with distilled water (conductivity <0.001 S/m), and a sponge was immersed in the water. Therefore, before sampling, the sponge was cleaned with the same distilled water; then, the pollutant deposit layer was wiped off from the area of the insulator surface with the squeezed sponge. The sponge with collected aerosol was then put back into the container, and the aerosol was dispersed into the water solution by shaking and squeezing the sponge in the water. The quantity of water did not change before and after the sampling activity.  
After the sampling, the 60815-1 guidelines [4] were also followed for the determination of both the ESDD and NSDD values.  
The ESDD determination is based on conductivity measurements that, together with the temperature of the water containing the dispersed aerosol, were recorded. The measurements were carried out after stirring the water solution. A short stirring time, e.g., a few minutes, is required for high-solubility pollutants. The low-solubility pollutants generally require longer stirring time, e.g., 30-40 min.  
The conductivity correction is based on Equations (1) and (2); Equations (3) and (4) were used for determining the ESDD on the insulator surface [4]:  
\[\sigma_{20}=\sigma\theta[\text{1-b(@-20)}] \tag{1}\]  
\[\text{b}=-3.200\times 10^{-8}\theta^{3}+1.032\times 10^{-5}\theta^{2}-8.272\times 1 0^{-4}\theta+3.544\times 10^{-2} \tag{2}\]  
\[\text{S}_{\text{a}}=(5.7\sigma_{20})^{1.03} \tag{3}\]  
\[\text{ESDD}=\text{S}_{\text{a}}\cdot\text{V}/\text{A} \tag{4}\]  
where  
* \(\theta\) is the solution temperature (\({}^{\circ}\)C);
* \(\sigma_{\theta}\) is the volume conductivity at a temperature of \(\theta\)\({}^{\circ}\)C (S/m);
* b is the factor depending on the temperature of \(\theta\);
* \(\sigma_{20}\) is the volume conductivity at a temperature of 20 \({}^{\circ}\)C (S/m);
* S\({}_{\text{a}}\) is the salinity expressed in kg/m\({}^{3}\);
* ESDD is the equivalent salt deposit density (mg/cm\({}^{2}\));
* V is the volume of distilled water (cm\({}^{3}\)); and
* A is the area of the insulator surface for collecting atmospheric aerosol (cm\({}^{2}\)).  
After measuring the ESDD, the water containing the collected aerosol was filtered out by using a funnel and pre-dried and weighed glass filter paper (grade GF/A 1.6 \(\upmu\)m from the first to the seventh campaign) and quartz filter (grade QZ/A 1.6 \(\upmu\)m) for the final 8 th sampling campaign; this was carried out to improve the knowledge of the chemical composition of the NSDD by thermically analyzing it, thus requiring quartz filters for their stability up to 900 \({}^{\circ}\)C (see Section 2.4 for further details). The filters retaining the non-soluble aerosol fraction were dried (40 \({}^{\circ}\)C for 3 h) and then weighed (Precisa Gravimetrics AG, Dietikon, Switzerland, mod. 204A, f.s. 240 g, resolution 0.1 mg).  
The NSDD was then determined following Equation (5) [4]:  
\[\text{NSDD}=1000\ (\text{W}_{\text{f}}-\text{W}_{\text{i}})/\text{A} \tag{5}\]  
where  
NSDD is the non-soluble material deposit density (mg/cm\({}^{2}\));  
\(\text{W}_{\text{f}}\) is the weight of the filter paper containing non-soluble material under dry conditions (after the filtering, the filter was dried at 40 \({}^{\circ}\)C for 3 h) (g);  
\(\text{W}_{\text{i}}\) is the initial weight of the filter paper under dry conditions (g); and  
A is the area of the insulator surface for collecting pollutants (cm\({}^{2}\)).