## 1 Introduction  
The physical state (wet or dry) of atmospheric aerosols determines their physical-chemical properties responsible for their behavior in various environmental processes, such as atmospheric corrosion and bridging phenomena [1; 2; 3; 4; 5], interaction with solar radiation and satellite applications [6; 7; 8; 9; 10; 11] as well as heterogeneous reactivity on the aerosol's surface [12]. The critical points of the phase transition of an atmospheric particle are the deliquescence and the crystallization relative humidity (DRH and CRH). DRH and CRH determine whether in certain atmospheric conditions aerosol particles are solid or liquid (i.e., their soluble components are in solution) in function of the variation of the relative humidity (RH) of the surrounding atmosphere. Particularly, starting from a RH < DRH, the aerosol is dry until RH reaches DRH; from this point the aerosol absorbs water producing a saturated aqueous solution. Any other RH increase leads to a continuous 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 the aerosol crystallization and bringing again the aerosol to a dry state [13; 14; 15; 16]. The DRH and the CRH depend on the chemical composition of the aerosol itself and on the ambient temperature [11; 13; 14; 17]. The aforementioned cycle is known as hysteresis cycle of the aerosol [14; 18; 19]. Among many fields of interest, the aerosol hydration state is relevant for energy applications related to energy distribution and electronic reliability [18; 20; 21]. They concern both outdoor and indoor environment. In the first case, an example of outdoor exposed components is represented by insulators of high voltage (HV) transmission lines. Transmission lines are used for transmission of electric power and their continuous operation is essential for the reliability of power grid. Insulators are a relevant part of the system, because of their role in isolation of conductors from the tower and mechanical support for the line. Insulator performance is commonly affected by several factors: material, shape, degradation and pollution [22; 23]. The last one, combined to atmospheric conditions, plays a relevant role in failures: hygroscopic aerosol particles, such as saline deposits layers, mostly cause flashover during their water-soluble state [24; 25; 26]. In fact, it is well known that an insulating surface, placed between two electrodes and moistened with atmospheric agents (dew, rain, fog, etc. ), reduces its dielectric strength [27]. This phenomenon increases when soluble pollution species dissociate in ions, acquiring conductive characteristics. The consequence is a disruptive discharge over insulator surface [28; 29; 30] which finally turns into energy distribution failures and financial consequences for the energy distributor. On the other hand, aerosols conductivity (in function of RH) also represents a central issue in indoor contexts. Hereinafter, the term \"indoor\" is used in its broader meaning describing any confined environment, ranging from an Information Technology room to a case (of any electronic device) installed outdoors. With respect to this, atmospheric aerosols can penetrate indoor [20; 31] and when hydrated, could represent a potential danger for the indoor installed electronic components [32; 33; 34; 35], since they can induce three main effects on electronics [36]. First of all, electrochemical corrosion can occur and the impact of the deposited aerosols can be different if ionic species are in the solid state or are dissolved [1; 4; 5; 37]. In addition, particles deposited on electronics can have mechanical effects, such as heat accumulation on electronic circuitry, or electrical effects. For example, electrical bridging phenomena could be caused by particles deposited between components that would normally be electrically insulated in a printed circuit [38; 39].