###### Abstract
All the aforementioned processes affect both the amount and the chemical characteristics of the deposit onto an insulator; as a result, any deposit can be composed of internal mixing of soluble and non-soluble compounds (water-soluble ions, organic matter, black carbon, metals, dust, and crustal material) [2; 9] caused by natural (e.g., sea spray and dust sources) or anthropogenic origins [11]. As a remarkable fraction (>40%) of an atmospheric aerosol is composed of water-soluble compounds [12; 13], a direct consequence is its hygroscopicity, such as in saline deposit layers, which causes an aerosol to pass into a liquid phase [14; 15; 16]. This phenomenon happens when soluble pollution species dissociate in ions, acquiring conductive characteristics. The consequence is a disruptive discharge over the insulator surface [2; 17; 18], which finally turns into line outages with possible not negligible financial consequences for the grid operator. With respect to this, the physical state (wet or dry) of atmospheric aerosols is controlled by the synergy between the aerosol deliquescence and the crystallization relative humidity (DRH and CRH) and the history of relative humidity (RH) of the ambient air. In fact, the DRH and CRH determine whether aerosol particles are solid or liquid (i.e., their soluble components are in solution) depending on the variation of the relative humidity (RH) of the surrounding atmosphere. Particularly, starting from an RH < DRH, the aerosol is dry until RH reaches DRH; from this point, the aerosol spontaneously absorbs water, producing a saturated aqueous solution. Any other RH increase leads to further condensation of water (hygroscopic growth). Conversely, a reduction in RH (starting from a value above DRH) leads to evaporation until the CRH is reached, promoting aerosol crystallization and bringing the aerosol to a dry state again. The aforementioned cycle is known as the hysteresis cycle of the aerosol [2], and a direct consequence of it is that in order to understand if an aerosol particle is \"wet\" or \"dry\" and the effect on the insulators, it is fundamentally important to know the RH history together with the DRH and CRH values. The latter values (DRH and CRH) are a function of the chemical composition of the aerosol itself, making the aerosol chemistry a key parameter to understand and investigate its effect on insulators. In fact, as the aerosol chemical composition is subjected to a high variability in space and time, the DRH and CRH will also vary greatly. In the past, most of the studies investigated the DRH and CRH of single aerosol components (i.e., (NH\({}_{4}\))\({}_{2}\)SO\({}_{4}\), NaCl, and NH\({}_{4}\)NO\({}_{3}\)), but a growing number of papers report on the DRH and CRH of aerosol systems with higher chemical complexity [13; 19]. Concerning the DRH, [PERSON] et al. [20] and [PERSON] et al. [21] studied a mixture of NaCl-NaNO\({}_{3}\) particles; [PERSON] and [PERSON] [22] studied the phase transitions of systems formed by (NH\({}_{4}\))\({}_{2}\)SO\({}_{4}\) and carboxylic acids, evidencing that the organic acids affect the DRH of the system, leading to a gradual dissolution of the solid particles resulting in an early water uptake for a complex system of ammonium. As a result [9], the DRH of the complex chemical aerosol system can be lower than that of each single chemical compound. Concerning the CRH, [PERSON] et al. [23] studied the effect of black carbon in the CRH of the NH\({}_{4}\)NO\({}_{3}\)-(NH\({}_{4}\))\({}_{2}\)SO\({}_{4}\) system while [PERSON] and [PERSON] [24] investigated the heterogeneous crystallization of a -(NH\({}_{4}\))\({}_{2}\)SO\({}_{4}\) aerosol with inclusions of metal oxides, such as TiO\({}_{2}\), Al\({}_{2}\)O\({}_{3}\), and ZrO\({}_{2}\), showing that these insoluble inclusions increased the CRH of the complex aerosol system. With respect to this, a step forward useful for ambient particle DRH and CRH determination is represented by the use of mass and electrical techniques used on ambient aerosol samples [19].