## 3 Results and Discussion
### Generated Aerosols Conductance Measurements
Finally, the distribution of saline particles in soot deposits was investigated using a stereomicroscope (Leica Wild M420, 64\(\times\) enlargement; Figure 5b,d). The distribution appeared quite homogeneous (the EC particles embedded the white salt crystals) and close to ambient PM\({}_{2.5}\) samples (Figure S7, Supplementary Materials). All the aforementioned data highlight the reliability of the laboratory generated samples as a proxy of the ambient ones. The mixed saline and EC samples were exposed to the same humidity cycles in the AEC as the atmospheric aerosol samples. Contrary to the pure saline samples, an electrical conductance signal was measured for all the mixed saline and soot samples. Most important, the electrical signal sharply  
Figure 5: (**a**) Pure saline deposit after deliquescence onto a PTFE (Teflon)filter and (**b**) stereomicroscope image (Leica Wild M420, 64x enlargement) for a mixed saline and ES sample. (**c**) and (**d**) are the same as (**a**) and (**b**) with a higher magnification. increased at an RH corresponding with the theoretical DRH of each component [11; 14] (R\({}^{2}\) = 0.996, slope = 1.101; not shown) as reported in Figure 6.  
Therefore, the presence of EC is essential for the creation of a conductive medium in these laboratories generated samples. The reason for this is schematized in Figure 7, which represents the portion of aerosol deposited on PTFE filters between a pair of electrodes in the AEC filter housings. When a pure saline aerosol is deposited on a hydrophobic surface like PTFE and the deliquescence of its components occurs, tiny isolated droplets of an electrolytic solution are formed, thus preventing the creation of a continuous conductive path on the surface of the sample (Figure 7a), even with a very high surficial mass distribution on filter. When soot is added to the saline components, it helps the formation of an electrical bridge thanks to its conductive nature (Figure 7b); in this case, an electrical signal is measured between the two electrodes because a conductive continuum is formed between them. It noteworthy that the scheme reported in Figure 7 should be theoretically applied in 3-dimensions on a multi-layered deposit but with great care due to the complex shape and composition of particles. However, the mixed saline and EC samples were characterized by an average porosity of 0.9611 \(\pm\) 0.0020 and deposit thickness of \(55.8\pm 22.0\)\(\upmu\)m. As the porosity represents the ratio between the volume of void in the deposit and the total volume of the cake (the volume occupied by the material plus that of the void inside the deposited cake), the aforementioned porosity data could explain the crucial role of EC in acting as a bridge for electrical conductance. Thus, new studies are needed to deepen this topic in 3D especially in function of cake porosity. Some pictures of the experiment also confirmed the synergic behavior between EC and ionic component. The first ones were taken on a pure saline samples in the AEC (Figure 5a,c) showing isolated droplets, while the second ones (Figure 5b,d) show mixed saline and soot samples). In Figure 5b,d a mixed ammonium sulphate and soot sample is shown with a visual surficial distribution of the sample essentially uniform: the EC particles embedded the white salt crystals allowing conductance to occur at the DRH. Note again the similarity with stereomicroscope (Leica Wild M420, 64\(\times\) enlargement) image (Figure S7, Supplementary Materials) for a MI-TS typical PM\({}_{2.5}\) sample. Figure 6: Theoretical and measured DRH for ammonium sulfate (AS), ammonium nitrate (AN), ammonium chloride (AC), sodium chloride (SC), sodium sulfate (SS) mixed with laboratory generated EC. When winter and summer Po Valley composition (Section 2.4; WI-mix and SU-mix) are mixed with EC, a conductance signal, in keeping with the expected DRH, is measured. Results are reported in Figure 8. WI-mix sample showed a sharp conductance increase of 1.98 \(\upmu\)S RH\({}^{-1}\) (in keeping with previous results in [PERSON] et al.