## 3 Results and Discussion
### PM2.5 Samples Chemical Composition and Conductance Measurements
In a similar way to what has been pointed out for EC, detected samples result richer in OC than those that did not show any electrical conductance response, and their mean OC contents (52.4 \(\pm\) 8.6 \(\mathrm{\SIUnitSymbolMicro\meter}\) cm\({}^{-2}\) and 25.8\(\pm\)3.8 \(\mathrm{\SIUnitSymbolMicro\meter}\) cm\({}^{-2}\), respectively) were statistically different. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & & \multicolumn{3}{c}{**Mass Concentration (\(\mathrm{\SIUnitSymbolMicro\meter}\) cm\({}^{-2}\))**} & \multicolumn{2}{c}{**wt\%**} \\ \hline  & \(\mathbf{n}\) & & **PM** & **Ionic** & **Other** & **Ionic** & **Other** \\  & & & **Fraction** & **Components** & **Fraction** & **Other** \\ \hline
**Samples with** & & **Mean** & **226.5** & **88.2** & **138.3** & **37.1\%** & **62.9\%** \\
**conductivity** & **42** & _CI99\%_ & _36.0_ & _25.0_ & _20.1_ & _5.6\%_ & _5.6\%_ \\
**Samples without** & **20** & **Mean** & **128.4** & **57.6** & **70.9** & **43.7\%** & **56.3\%** \\
**conductivity** & **CI99\%_ & _16.7_ & _14.9_ & _10.4_ & _5.6\%_ & _5.6\%_ \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The mean mass surficial concentrations for the overall PM\({}_{2.5}\) content, the inorganic ionic fraction (calculated as the sum of F-, CI-, NO3-, SO4-2-, Na+, NH4+, K+, Ca2+, Mg2+), the other components (calculated as a difference between the previous two quantities) are presented. Weight percentages (wt%) for the inorganic ionic and the other components are reported, as well. Average values are reported alongside the corresponding 99% confidence intervals. The whole dataset is split into detected and not-detected samples, in relation to the conductivity measurements in the AEC. \(n\) indicates the number of samples in each subset. This broad discrepancy observed for the estimated carbonaceous fractions coupled with the observation that the average ionic percentage content was higher in non-detected samples represents a first important indication that a synergy between these different chemical components is necessary to determine the electrical conductance of aerosols. As reported in Table S1 (Supplementary Materials), winter samples were always detected due to high concentrations of PM\({}_{2.5}\), EC and ions related to the atmospheric stable conditions characteristics of this period [72]. Furthermore, if the entire dataset is split between samples collected at the urban site (MI-TS) and at the rural site (OB), other proof of a synergy between the ionic fraction and elemental carbon can be found (Figure 4). The statistically higher PM\({}_{2.5}\) surficial mass for detected samples (that was found for the entire dataset) is also observed once samples from the two different sampling sites are separated. The MI-TS samples present a mean PM\({}_{2.5}\) loading of 230.5 \(\pm\) 38.4 \(\mu\)g cm\({}^{-2}\) for detected ones and 126.7 \(\pm\) 27.2  
Figure 4: Average surficial mass concentrations (\(\mu\)g cm\({}^{-2}\)) for MI-TS (grey) and OB (green) sample data for the quantified inorganic ionic fraction, EC and OC. Data are reported separately for samples that showed a conductivity and for those where it was not-detected. \begin{table}
\begin{tabular}{c c c c c c c c} \hline  & & \multicolumn{4}{c}{**Mass Concentration**} & \multicolumn{2}{c}{**wt\%**} \\  & & \multicolumn{4}{c}{**(\(\mu\)g cm\({}^{-2}\))**} & & \\ \hline  & **n** & & **Ionic** & **EC** & **OC** & **Ionic** & **EC** & **OC** \\  & & & **Fraction** & & & **Fraction** & & \\ \hline
**Samples with** & **42** & **Mean** & **88.2** & **26.4** & **52.4** & **37.1\%** & **8.4\%** & **17.0\%** \\
**conductivity** & **Cl99\%** & _25.0_ & _4.1_ & _8.6_ & 5.6\% & 1.7\% & 3.4\% \\
**Samples without** & **Mean** & **57.6** & **12.0** & **25.8** & **43.7\%** & **5.2\%** & **11.3\%** \\
**conductivity** & **Cl99\%** & _14.9_ & _4.1_ & _3.8_ & 5.6\% & 1.9\% & 2.2\% \\ \hline \end{tabular}
\end{table}