## 2 Materials and Methods
### Laboratory Generated EC and Saline Aerosols  
In order to investigate the role of EC in promoting the aerosol conductance, different aerosol types (soot only, saline only, externally mixed saline and soot) were generated in the laboratory. First of all, soot particles were generated from an acetylene non-premixed diffusion flame. Acetylene was produced from a controlled reaction between ultrapure water and calcium carbide. The generated acetylene was gurgled in ultrapure water in order to prevent any particle or solid contaminant to reach the flame. In addition, the gurgler avoids any possible backfire to the acetylene generation unit. The flame was constantly kept at a height of 5 cm and the generated soot was sampled by means of a glass cone placed above the flame itself for 20 s and injected into a 50 L PTFE chamber. As the aim of the paper is to investigate the role that the EC has (in synergy with hygroscopic saline species) on the aerosol conductance, the chemical-physical features of the generated soot were characterized. First of all, the generated soot was analyzed with a Thermal Optical Transmission method (TOT, Sunset Laboratory inc.; NIOSH 5040 procedure) to quantify the elemental and organic carbon (EC and OC) content (Section 2.2); the measured EC percentage on the total carbon content (TC) was determined on five soot samples: it was on average \(95.2\pm 2.5\%\). This indicates a low content of organic matter and is in agreement with what found in literature for acetylene flame soot [84; 85] reaching the highest level compared to industrial soot generators [86]. Too deepen the chemical investigation, the same samples were analyzed with gas chromatography (Agilent 6850, mass spectrometer 5973) according to the method described by [PERSON] et al. [87] for the quantification of polycyclic aromatic hydrocarbons (PAHs) and n-alkanes (C20-C32) as organic carbon matter, in order to verify the low content of organic species found with TOT measurements. On average, \(104\pm 62\) pg of PAHs were quantified per \(\upmu\)g of soot mass while the content of n-alkanes was \(305\pm 184\) pg \(\upmu\)g\({}^{-1}\). If these amounts are compared with the average concentrations of PAHs and n-alkanes measured in atmospheric aerosols at the MI-TS site (PAHs from 100 to 600 pg \(\upmu\)g\({}^{-1}\), n-alkanes in the 800-3000 pg \(\upmu\)g\({}^{-1}\) range, annual ranges; [88]) the PAHs and n-alkanes content in the laboratory generated soot samples is either much lower with what found in atmospheric aerosols. In addition to its chemical characterization, the number size distribution of the generated soot was determined by means of an SMPS 3936 (Scanning Mobility Particle Size Spectrometer, TSI Inc.) and it was found to be bimodal, with the two maximums at 68 nm and 250 nm, respectively (Figure S5, Supplementary Material). Finally, the generated EC conductance was also measured within the AEC in dry conditions (30% RH) for eight soot samples (3 to 20 \(\upmu\)g cm\({}^{-2}\) deposited on a PTFE filter). The measured conductance was in the 20-180 \(\upmu\)S range at 30% RH, which is comparable to what measured for the soot from a gas turbine engine by [PERSON] et al. [63]. In order to investigate the synergy between EC and inorganic ions, pure saline aerosols were generated by means of the Aerosol Generator ATM 220 (Topas GmbH) from aqueous solutions. Four salts were investigated, namely: ammonium sulfate (AS: (NH4)2 SO4), ammonium nitrate (AN: NH4 NO3), ammonium chloride (AC: NH4 Cl), sodium chloride (SC: NaCl) and sodium sulfate (SS: Na2 SO4). Among them, special attention was given to (NH4)2 SO4 and NH4 NO3, since they are among the most abundant inorganic species found in the Po Valley atmospheric aerosols; in this respect, as their relative abundances reflect their Winter and Summer concentrations in the Po Valley [7; 88], winter-like aerosol (WI-mix) and summer-like saline (SU-mix) aerosol were also generated. They were composed of 24% (NH4)2 SO4 + 76% NH4 NO3 (WI-mix) and of 86% (NH4)2 SO4 + 14% NH4 NO3 (SU-mix).