## 2 Materials and Methods
### DRH, CRH, and Related Electric Effect Determination  
Both the DRH and CRH and the related phase transitions of the aerosol deposit of each macroregion (MR1, MR2, and MR3) were identified by means of gravimetric hygroscopic determination together with measurements of the corresponding electrical effect in terms of conductance. All these measurements required two steps: (1) the sample preparation and deposition and (2) the use of a specifically designed aerosol exposure chamber (AEC). In order to investigate the phase transition of the aerosol deposit in each macroregion, pure saline aerosols were generated by means of the Aerosol Generator ATM 220 (Topas GmbH) from aqueous solutions. Some salts were investigated, namely: ammonium sulfate (AS: (NH4)2 SO4, VWR International S.r.l. ), ammonium nitrate (AN: NH4 NO3, VWR International S.r.l. ), sodium chloride (SC: NaCl, Sigma-Aldrich), sodium sulfate (SS: Na2 SO4, Sigma-Aldrich), sodium nitrate (SN: NaNO3, Laboratoriumdiscounter), calcium sulfate dihydrate (CS: CaSO4 2H2O, VWR International S.r.l. ), calcium nitrate tetrahydrate (CN: Ca(NO3)2 4H2O, Laboratoriumdiscounter), potassium dihydrogen phosphate (KDP: KH2 PO4, Laboratoriumdiscounter), potassium sulfate anhydrous (PS: K2 SO4, VWR International S.r.l. ), and magnesium chloride hexahydrate (MC: MgCl2 6H2O, VWR International S.r.l.). The starting solution was prepared at 10,000 ppm, with a composition similar to those of each macroregion (the theoretical and the real composition for MR1, MR2, and MR3 are shown in Supplementary Materials, Figure S2). Saline aerosol was generated into a mini-AEC (50 L volume, TEFLON(r) FEP type 200 A, thickness 50 \(\upmu\)m), as shown in the schematic in Figure 3 following the methodology validated and reported in [2]. The generated aerosol was collected on a PTFE filter (\(\O\) = 47 mm, porosity 2 \(\upmu\)m) by means of a Mega System pump with a flux of 20 L/min. The PTFE filters (after aerosol collection) were used for the gravimetric DRH-CRH determination (see below). In addition, in order to carry out the conductivity measurements, a specific calcium-sodium tempered glass specimen (5 \(\times\) 5 cm) simulating the insulator surface was used as a standard for the Italian system. The glass specimen presents two conductive silver bands with a width of 0.4 cm and a length of 3 cm, placed 1 cm apart, as reported in Figure 4.  
Even for the glass specimen, a saline mix, prepared according to the ionic composition resulting from the analysis of the deposit on the insulator surface, was deposited upon the calcium-sodium samples. This saline mix was deposited via a rotating impactor as reported in [32] with an amount of kaolin proportional to the NSDD mass to simulate the presence of the crustal material (black carbon was negligible in the chemical analysis results; Section 3.3). Both the PTFE filters and the glass specimen behavior in relation to the RH were analyzed in the AEC [13; 19]. The AEC is a sealed 1 m\({}^{3}\) glass chamber in which the samples are exposed to humidification and dehumidification cycles, from 20 to 90% RH and vice versa, with RH steps of 2%. The RH is varied inside the chamber by phasing ultrapure water  
Figure 4: The glass specimen (**a**), its scheme (**b**), and its appearance after aerosol deposition (**c**). Figure 3: Outline scheme of the mixed saline generation and sampling system (1: The atomizer; 2: Outlet; 3: HEPA filter; 4: Manometer; 5: Pressure regulator; 6: Inlet; 7: Safety valve). vapor (Milli-Q\({}^{\circledS}\); 18.2 M\(\Omega\) \(\cdot\) cm at 25 \({}^{\circ}\)C) into the AEC during humidification; conversely, during dehumidification, pre-filtered pure air was phased in to reduce the RH (Aria Zero\({}^{\circledS}\), Sapio, Monza, Italy). In order to carry out the gravimetric measures, a precision analytical balance (Sartorius SE-2F; accuracy \(\pm\)0.1 \(\upmu\)g) was placed into the AEC to record the rising (falling) mass measurements for each PTFE-sampled filter while varying the relative humidity.