## 2 Materials and Methods
### NSDD Characterization  
NSDD collected over filters (as detailed in Section 2.2) was first analyzed for the carbon content. As stated in Section 1, it is known that BC could potentially decrease the resistivity, making its determination important for the deposit characterization. Therefore, the analysis of the concentration of the Total Carbon (TC) and black carbon (BC) fractions was carried out using a total carbon analyzer (Model TCA08 Total Carbon Aerosol Analyzer, Magee Scientific) coupled to an optical transmissometer (\"SootScanTM\" Model OT21 Optical Transmissometer, Aerosol d.o.o.) on the filters of the 8 th sampling campaign; as reported below, the TC determination is based on combustion at 700 \({}^{\circ}\)C, leading to the opportunity to perform it only on the quartz fiber filters of the last sampling campaign. The BC fraction was measured by means of the optical transmissometer as reported by [PERSON] et al. [30]. It uses a non-destructive method by measuring the attenuation of transmitted light through each sample (and compared to an analogous measurement relating to a blank filter) at a wavelength of 370 \(\mathrm{nm}\) (UV) and 880 \(\mathrm{nm}\) (IR). The attenuation is experimentally determined as follows (Equation (6)):  
\[\mathrm{ATN}=100\times\mathrm{In}(\mathrm{I}_{0}/\mathrm{I}) \tag{6}\]  
where  
ATN is the attenuation;  
\(\mathrm{I}_{0}\) is the radiation transmitted by the blank filter; and  
\(\mathrm{I}\) is the radiation transmitted by the sample. Due to the high concentration of crustal material on the deposit (see Sections 2.5 and 3.3), a correction of the measured ATN was necessary in order to obtain the ATN value only due to the presence of BC (ATN\({}_{\mathrm{BC}}\)) and not to the presence of other absorbing material (e.g., crustal iron oxides). Thus, the final ATN\({}_{\mathrm{BC}}\) value was calculated as follows (Equation (7)):  
\[\mathrm{ATN}_{\mathrm{BC}}=\mathrm{ATN}_{\mathrm{pre-TCA}}-\mathrm{ATN}_{ \mathrm{post-TCA}} \tag{7}\]  
where ATN\({}_{\mathrm{pre-TCA}}\) is the ATN value measured before TCA analysis and ATN\({}_{\mathrm{post-TCA}}\) is the ATN value measured after TCA analysis when all the carbonaceous material is removed. An example of an NSDD filter is shown in Supplementary Materials, Figure S1. The BC mass concentration was then calculated following Equation (8) based on the attenuation coefficient (\(\mathrm{b}_{\mathrm{atm}}\), \(\mathrm{M}\mathrm{m}^{-1}\)) of the deposit on the filter surface [30]:  
\[\mathrm{BC}=(\mathrm{A}/\mathrm{V}\times\mathrm{ATN}_{\mathrm{BC}})/100\sigma_{ \mathrm{atm}} \tag{8}\]  
where  
A is the exposed surface of the filter (\(\mathrm{m}^{2}\)) equal to 0.00096 \(\mathrm{m}^{2}\). This value is the result of the average value of different filter diameters measured by means of a caliber (35.77 \(\mathrm{mm}\)). \(\mathrm{V}\) is the volume of analyzed fluid (\(\mathrm{m}^{3}\)). \(\mathrm{ATN}_{\mathrm{BC}}\) is the attenuation related to the BC fraction only. \(\sigma_{\mathrm{atm}}\) is the mass attenuation cross section of BC (10 \(\mathrm{m}^{2}\) g\({}^{-1}\)). The TC fraction was measured by means of a total carbon analyzer as aforementioned. It uses a destructive method based on the combustion of all carbonaceous compounds deposited on the filter. The combustion produces carbon dioxide, which is detected by a Li-Cor detector inside the TCA, which has the task of integrating the peaks of CO2 and providing the amount of TC on the filter. For this analysis, a 12 mm spot was derived from each filter, and the obtained TC was obtained with a proportion of the total filter area. Lastly, the water-insoluble organic matter (WINSOM) and unknown fraction (UF) were determined.