## 3 Results and Discussion
### Measured Conductance  
Starting with ambient measurements, 62 samples were characterized and exposed to RH cycles in the AEC for the determination of their DRHs and CRHs together with their conductance. 50 of them were collected at the urban site of MI-TS and 12 at the rural site OB (sampling activity in OB was conducted on a shorter period compared to MI-TS). The mass distributions on filter for MI-TS samples varied from 74.8 \(\upmu\)g cm\({}^{-2}\) to 496.8 \(\upmu\)g cm\({}^{-2}\), while the samples from the OB site presented surficial mass distributions from 100.0 \(\upmu\)g cm\({}^{-2}\) to 184.4 \(\upmu\)g cm\({}^{-2}\). These are typical values for the Po Valley, which is characterized by seasonally modulated pollution [72]. As previously reported in [PERSON] et al. [18] a first estimation of the minimum aerosol loading to observe an electric conduction gave the value of 127 \(\pm\) 7.8 \(\upmu\)g cm\({}^{-2}\). In this respect, considering the whole ensemble of the investigated PM2.5 samples, a subset of 42 samples gave a detectable electrical signal in function of RH, representing 68% of the collected samples. Thus, in order to deepen the critical factors of the aerosol chemical composition affecting the aerosol conductivity (in an energy application perspective; next section), here below conductance measurements carried out on PM2.5 samples in the AEC are first investigated focusing on the electric signal behavior in function of RH changes. Figure 3 shows a typical electrical conductance measurement on a PM\({}_{2.5}\) sample as a function of RH applied in the AEC during both humidification and dehumidification. DRH and CRH values together with the deliquescence and crystallization regions can be individuated according to the conductance derivative [18] (Section 2.3). As a case study, a winter sample was chosen (PM2.5, 264.8 \(\upmu\)g cm\({}^{-2}\) on filter, collected March 2009, MI-TS site, 24 h sampling). The humidification and dehumidification curves show a well-rendered hysteresis cycle, in agreement with literature studies [11; 13; 14]. In facts, on the basis of the maximum gradient method (Section 2.3) the DRH was individuated at 56.6% RH and the CRH at 47.3% RH. The deliquescence region was between 53.3% and 61.3% RH and the corresponding increase in conductance (\(\Delta\)G\({}_{\text{deliquescence}}\)) in this range was 18.04 \(\upmu\)S. The crystallization region is located between 48.1% and 45.9% RH and it is characterized by an 8.84 \(\upmu\)S conductance drop (\(\Delta\)G\({}_{\text{crystallization}}\)). The maximum conductance value reached by this case study sample was 21.63 \(\upmu\)S at 70.7% RH. As aforementioned, considering the whole ensemble of the investigated PM\({}_{2.5}\) samples, a detectable electrical signal in function of RH was observed on 68% of the collected samples. The highest and lowest measured values of \(\Delta\)G\({}_{\text{deliquescence}}\), \(\Delta\)G\({}_{\text{crystallization}}\) and maximum conductance (for the subset of samples which gave an electrical signal in function of RH) are reported in Table 1. \(\Delta\)G\({}_{\text{deliquescence}}\) is always higher than \(\Delta\)G\({}_{\text{crystallization}}\) (average values of 11.20 \(\pm\) 7.43 and 2.55 \(\pm\) 1.35 \(\upmu\)S, respectively), simply because the crystallization occurs at supersaturated conditions: during the dehumidification, between the DRH and the CRH the measured conductance gradually decreases as water evaporates until the actual crystallization occurs, and therefore \(\Delta\)G\({}_{\text{deliquescence}}\)\(>\)\(\Delta\)G\({}_{\text{crystallization}}\).