## 3 Results and Discussion
### PM2.5 Samples Chemical Composition and Conductance Measurements  
Conductance measurements carried out on 62 samples collected at MI-TS and OB showed that an electrical signal was detected on 42 samples (68%). Thus, the aim of this section is to investigate and compare the PM\({}_{2.5}\) samples chemical composition with relationship to the measured conductance signal. Table 2 reports a comparison between the 36 samples in which a conductance signal was detected and the 20 ones in which it was not-detected; Table 2 refers to the whole dataset (both MI-TS and OB sampling sites). First of all, the average PM\({}_{2.5}\) mass concentrations on filter were on average higher for detected samples (\(226.5\pm 36.0\)\(\mu\)g cm\({}^{-2}\)) than for not-detected ones (\(128.4\pm 16.7\)\(\mu\)g cm\({}^{-2}\)) which are close to the \(127\pm 7.8\)\(\mu\)g cm\({}^{-2}\) threshold limit; the t-student test confirms that for the two sets of samples these means are statistically different. In order to understand if it is just the overall amount of depositedaerosol that determines its electrical conductance or also its chemical composition, the same comparison was carried out first for the ionic fraction. The inorganic ionic fraction average masses need to be considered first, because when charged species pass in solution they form a conductive medium (Introduction section). The data reported in Table 2 show that they did not statistically differ for detected and not-detected samples and they turn out to be 88.2 \(\pm\) 25.0 \(\mathrm{\SIUnitSymbolMicro\meter}\) cm\({}^{-2}\) and 57.6 \(\pm\) 14.9 \(\mathrm{\SIUnitSymbolMicro\meter}\) cm\({}^{-2}\), respectively. In addition, a quite surprising result can be obtained comparing mass/mass percentages (wt%) for the ionic inorganic fractions, since the order of their values reverse and appears greater for not-detected samples (43.7 \(\pm\) 5.6%) than for samples with detected conductance (37.1 \(\pm\) 5.6%) even not statistically different. If the samples with the highest ionic surficial content are those that are not conductive, it can be supposed that the presence of saline hygroscopic compounds is not the only necessary condition to create a continuous conductive path between particles deposited on a hydrophobic substrate (such as PTFE) and therefore to provoke electrical bridging phenomena. In this respect it is worth noticing that the components other than inorganic ions (\"other components\") were statistically different between the two subsets reported in Table 2: 138.3 \(\pm\) 20.1 \(\mathrm{\SIUnitSymbolMicro\meter}\) cm\({}^{-2}\) for samples that showed an electrical signal and just 70.9 \(\pm\) 10.4 \(\mathrm{\SIUnitSymbolMicro\meter}\) cm\({}^{-2}\) for those that did not. The same happened considering the wt% of these components other than inorganic ions: 62.9 \(\pm\) 5.6 and 56.3 \(\pm\) 5.6%, respectively. For this reason, the hypothesis that conductive EC plays a role in this process was investigated. Therefore, the elemental carbon (EC) and organic carbon (OC) content in the 62 samples in the dataset, previously determined via TOT analysis [88], was considered. The data are reported in Table 3 as surficial masses for EC and OC together with inorganic ions and their wt%, respectively. The most relevant result is related to EC: as envisaged, the mean surficial distribution for detected samples was statistically higher (26.4 \(\pm\) 4.1 \(\mathrm{\SIUnitSymbolMicro\meter}\) cm\({}^{-2}\)) and more than double than for not detected ones (12.0 \(\pm\) 4.1 \(\mathrm{\SIUnitSymbolMicro\meter}\) cm\({}^{-2}\)). The mean OC masses are another relevant factor, since many organic compounds have a well-known hygroscopic behavior that can promote an early water uptake even prior DRH (see previous section) and can therefore have a role in the formation of potentially dangerous conductive solutions on PCB surfaces.