## 3 Results and Discussion
### Measured Conductance
For what concerns the present work, whatever the deposit structure, the results presented in the following Sections 3.2 and 3.3 demonstrate the need to account for EC hydrophobic presence and not only for inorganic ions. Coming back to the electrical signal on filters, besides the electrical conductance increase caused by deliquescence, it is also worth noting that during the humidification phase the electrical conductance of the samples is always activated before the deliquescence range. For the case study sample in Figure 3, the first electrical signal is detected at 48.3% RH and the initial measured conductance is 0.04 \(\mu\)S. This early activation of the electrical signal before the deliquescence was likely caused by an early water uptake of water by hygroscopic aerosol components [94]. This is in line with the findings by [PERSON] et al. [8], who characterized the water uptake of MI-TS PM\({}_{2.5}\) samples as a function of the surrounding RH by means of a gravimetric method. They detected early water uptake of water before the deliquescence region that led to hygroscopic growth factor values higher that 1, even before deliquescence has occurred. After the activation point in Figure 3, the conductance gradually slightly increases up to 0.83 \(\mu\)S at 53.3% of RH, which is the starting point of the deliquescence region previously individuated. Most important, for the 55% of the samples the measured conductance was already higher than 1 \(\mu\)S at the initial point of the deliquescence region. These results indicate that the hygroscopic components are not the only potentially dangerous components in aerosol, but other conductive species could be involved in the formation of electrical bridging phenomena too, also at RH values below the DRH of the water-soluble components. Understanding what causes the activation in these conductance measurements is therefore crucial to better comprehend all the factors involved in the determination of the electrical conductance properties of aerosols. In this respect, a carboxylic acid quantification for MI-TS PM\({}_{2.5}\) samples during the same period was previously performed [88] showing concentrations of \(389\pm 133\) ng m\({}^{-3}\) (1.1 \(\pm\) 0.4% of PM\({}_{2.5}\)). As reported by both [PERSON] and [PERSON] [95] and [PERSON] et al. [96], the role of carboxylic acids (malonic acid, glutaratic acid and succinic acid) can affect the water uptake close to the deliquescence leading to a gradual dissolution of the solid particles. Thus, water soluble organic material can play an important role in the early water uptake. In addition to the aforementioned considerations, a second important aspect is that no electrical signal was detected for 20 out of the 62 considered samples. The existence of a minimum aerosol loading for the conductance to be detected with this experimental setup was yet discussed above and highlights the need to investigate what gives rise to it in order to understand what determines the electrical conductance properties of aerosol samples. Therefore, in the next section, the 'quantity' (surficial loading) and the 'quality' (chemical composition) of the considered aerosol samples are investigated.