## 2 Materials and Methods
### Laboratory Generated EC and Saline Aerosols
The starting solutions were prepared at a concentration of \(2.5\times 10^{4}\) ppm, in order to achieve satisfactory particles concentrations according to the manufacturer specifications for the Aerosol Generator. The concentration was always the same for all solutions, and when a mix is used, the reported concentration was the total of all components. The number size distribution of the generatedaerosol was characterized with the TSI SMPS, too (monomodal, 50-80 nm range peak, Figure S5, Supplementary Material). First of all, pure saline samples were generated and they were exposed to humidification and dehumidification cycles in the AEC (Section 2.3). At a later stage, EC and saline aerosols were generated simultaneously into two different Sampling Chambers (50 L volume each, TEFLON(r) FEP type 200 A, thickness 50 \(\upmu\)m), as shown in the schematic in Figure 2. All the generated aerosols were collected on PTFE filters (\(\O\) = 47 mm, 2 \(\upmu\) porosity) by means of a pump (Leland Legacy, SKC, 15 L/min flow). In order to obtain well externally-mixed generated aerosol samples, the two flows from the two Sampling Chambers were conveyed through a three-ways glass connector, which couples the two sampling flows before they get to the filtering PTFE membrane. In order to avoid contamination between two consecutive aerosol generation experiments, the two Sampling Chambers were cleaned with a pure air flow for five times their volume. SMPS data were collected at the inlet of the PTFE filter holder to be representative of the real size distribution deposited onto the PTFE filters (average residence time of particles into the chambers: 6.7 min). Finally, the cake deposits were characterized as reported in literature [89; 90; 91] from SMPS data determining first the a-dimensionless Peclet number (\(Pe\)) and the following cake porosity (\(\epsilon\)) as detailed in [PERSON] et al. [91]:  
\[Pe=\frac{Ufda_{gg}}{D} \tag{1}\]  
\[\varepsilon=\frac{1+0.44 Pe}{1.019+0.46 Pe} \tag{2}\]  
where \(Uf\) is the filtration face velocity, \(d_{gg}\), the aggregated or agglomerated particle size (SMPS data, Figure S5, Supplementary Materials) and \(D\), the particle diffusion coefficient. In the present case, even the PTFE filters were of 47 mm diameter, the PMP ring reduced the active size spot to a 39 mm diameter (11.94 cm\({}^{2}\); Figure S6, Supplementary Materials); thus, considering a sampling flow rate of 15 l min\({}^{-1}\) this translate into a \(Uf\) of 0.21 m s\({}^{-1}\). \(D\) was calculated from \(d_{agg}\) according to the Stokes-Einstein equation [92]. The \(\varepsilon\) determined using the [PERSON] et al. [91] method enables to account for the non-linearity over a wide range of Pe. From the cake porosity and the cake mass per surface area (\(m_{s}\)) the deposit thickness \(\Delta Z\) can be computed as follows [90; 91]:  
\[\Delta Z=\frac{m_{s}}{(1\ -\ \varepsilon)\rho_{p}} \tag{3}\]  
where \(\rho_{p}\) is the material density. The aforementioned calculations were used to discuss the obtained results in Section 3.3. Similarly to the atmospheric aerosol samples, also these soot and saline aerosols were exposed to humidification and dehumidification cycles. The results of these measurements are presented and discussed  
Figure 2: Outline scheme of the mixed saline and soot aerosol generation and sampling system. in Section 3.3 while Table S2 (Supplementary Materials) reports the aerosol mass deposited and composition for all synthetic samples.