###### Abstract  
The reliability of the national power grid is a key issue in modern society. Atmospheric aerosols are the main cause of the reduction in the performance of insulators and the increase in the possibility of flashovers, resulting in power line failures. Under high ambient humidity, the water-soluble compounds of atmospheric aerosols collected on the insulators' surface can dissociate in ions and form a conductive layer, which may lead to flashover events. With a view to investigating the processes that drive these phenomena, the chemical composition of aerosol deposits on insulators in Italy was determined by ion chromatography analysis and thermos-optical and X-ray techniques. In addition, a synthetic aerosol with the same analyzed chemical composition was generated in a laboratory and deposited on PTFE filters and glass specimens allowing us to determine the delipausecence and crystallization relative humidity and the conductive effect in an aerosol exposure chamber. The results evidenced the presence of a hazardous inorganic ion layer, which generates a sharp phase transition of the aerosol deposit as a function of the ambient relative humidity; this layer poses a dangerous threat to the reliability of the power grid, increasing the probability of flashover events where the conductive layer facilitates the flow of electrical current across the insulator surface, potentially causing power outages or damage to the power lines. a 2023 13 2023 11 October 2023 15 November 20  
deposited. The last factor, combined with atmospheric meteorological conditions, plays a relevant role in failures [5]. It has been reported that high humidity conditions cause insulator outages and consequently network outages [1]. In fact, it is well known that insulators remain quiescent when dry even if highly polluted. Conversely, when the insulator surface becomes moist because of rain, fog, or dew, these factors may lead to the formation of a conductive layer on the surface and cause leakage of current flow [6] due to a reduction in its dielectric strength. The leakage current density is non-uniform over the insulating surface, and, in some areas of the surface, sufficient heat is developed, leading to the formation of dry bands. Consequently, voltage redistribution along the insulating surface increases the stress across the dry bands, resulting in partial arcs. When the surface resistance is sufficiently low, the partial arcs will elongate along the insulator profile, and may eventually cause a full insulator flashover [7]. As aforementioned, atmospheric pollution is among the main natural causes of flashover accidents, which endanger the safety and stability of the power system due to its accumulation on the surface of insulators that operate in the outdoor environment [5]. However, the term atmospheric pollution is rather vague and refers to the ensemble of atmospheric aerosols that can deposit on surfaces [8]. The deposition process by itself is a very complex phenomenon as it is, first of all, a function of particle size, shape, and chemical composition, which turns into the particle aerodynamic properties influencing the capability of any particle to diffuse or settle in atmospheric conditions [9]. Moreover, any force (other than gravity) acting on any particle (such as the wind drag force, and thermophoretic and electrophoretic forces) is an important variable that can affect the particle deposition over the insulators [10]. Finally, even the properties of the surface over which the deposition phenomena occur are fundamental [8]. In addition to this, the gas-particle partitioning [9] can continuously act, affecting the deposit's chemical characteristics over time [8].