This report describes the experimental activities aimed at understanding the phenomenon of surface contamination of insulators and its subsequent model reconstruction. The experimental activities were carried out partly in the laboratory and partly in the field.

The laboratory experimental activities focused on the study of the humidification/dehumidification processes and consequent liquid/solid phase transition of the compounds present in the deposit as the temperature and relative humidity change, in order to reproduce their conductive properties. To this end, an Aerosol Exposure Chamber (AEC) was configured to study the behavior of a synthetic aerosol deposited on a sample specimen under controlled environmental conditions.

In this first phase of the project, the synthetic aerosol consisted of simpler mixtures than the composition observed on the insulators to allow for a more effective verification of functionality. Experimental tests in AEC thus provided an initial comparison between the experimental data and the ISORROPIA modeling code (tested for inclusion in the RSE operational chain). Of the tests performed, the comparison of the experimental wetting curve with the reproduced model curve was satisfactory for potassium sulfate. For magnesium sulfate, magnesium chloride, calcium nitrate and the salt mix (NaCl+MgSO4) mix, the model showed agreement with the experimental curves only in the final stages of hygroscopic growth. The result confirmed the potential of the thermodynamic model, but at the same time highlighted the need to introduce some corrections.

Field activities also continued at the ContIsola station, the experimental facility dedicated to characterizing the temporal evolution of the aerosol contamination process under different exposure conditions. The ContIsola station, which has been active since 2020, was renovated during 2022 by placing the hardened glass chains, with six hood and pin glass insulators, on an aluminum platform that raises them 2 meters above the ground.

A second vertical chain and a second deposition box with hardened glass insulators coated with an anti-spill coating were then installed on the platform. The analysis of the ESDD and NSDD parameters, obtained from the 2022 samples and used by the regulations to quantify the level of contamination of the isolators, showed a substantial agreement both with the measurements made at the previous station site and with the Terna 2016-2018 measurements. This confirms the representativeness of ContIsola for the study of the phenomenon.

At the same time, laboratory activities continued for the characterization of the chemical composition of the sampled deposit. This characterization provides an essential contribution to the understanding of the sources contributing to the contamination and thus to the proper modeling of the phenomenon. The analysis of the ionic fraction, which determines the ESDD, showed that the main contribution is due to Ammonium, Calcium, Sodium, Potassium, Nitrate, and Sulfate. As for metals, the largest contribution is associated with Silicon and Zinc; although metal contamination never exceeds 1 milligram per liter. On the other hand, the analysis of the carbonaceous fraction, which contributes to the NSDD term, made it possible to quantify the contribution of elemental carbon (EC) and organic carbon (OC) to the non-soluble fraction of the deposit. The result obtained showed that the OC/EC ratio of the deposit is much higher than the corresponding ratio in the atmosphere, thus suggesting the likely presence of accretion and depletion processes in the deposit that affect the two carbon fractions differently.

Finally, a new annual measurement campaign has been planned, similar to the one conducted in collaboration with TERNA in the years 2016-2018, which involved 66 sites and started in spring 2023; deposits are being sampled on a quarterly basis and are subjected to chemical composition analysis with the same procedures adopted in the laboratory activities conducted in ContIsola. The results of the campaign will allow updating and extending the knowledge on the characteristics of the contaminant deposit over the entire national territory. In addition, during this campaign at a reference site, meteorological and air quality measurements will be added to the analysis of the deposit on isolator (67th site), sampled by sponging technique. These measurements will provide a complete characterization, not only of the contaminant deposition, but also of the characteristics of the atmospheric aerosol from which the deposition is generated and the meteorological variables that drive the process itself. The information collected will then be used to verify and update the modeling system.