## 3 Results and Discussion
### Comparison with Reference Gravimetric Measurements  
Sampling devices are compared based on two quantitative parameters: the total mass collected during a defined monitoring period and the average daily and/or hourly concentrations. Extractive sampling devices allow both mass and concentration to be calculated if the flow rate is known. Only average concentration values can be used for an external sensing device with no defined flow rate. Given the significance of gravimetric measurement as a reference, the total mass calculated for ELPI+ was compared with the mass collected on the SKC filters and the results are presented in Table 2.  
The total mass on the filters was measured gravimetrically by weight difference with the blank filter. The relative percent error (RE%) was determined experimentally by performing three different measurements of the weight of each filter. One filter per environment was prepared (warehouse, outdoors, and garage). Unfortunately, some issues occurred with the SKC pump during the two-week sampling in the warehouse, so the filter was not used for the gravimetric analysis. Therefore, only samples collected in the last two days were considered. The total mass collected by a real-time sampling device can be calculated using Equation (1), where \(C\) is the average mass concentration of PM\({}_{2.5}\) for each interval \(\Delta t\), \(Q\) is the pump flow rate (10 L/min for the ELPI+), and \(\Delta t\) is the sampling time interval. The ELPI+ manufacturer provides an RE% value of 1% for PM\({}_{2.5}\) concentrations, which was considered as the estimated uncertainty, taking the other factors exactly.  
\[\sum C_{t}\ \left[\text{mg/m}^{3}\right]\times Q\ \left[\text{m}^{3}/\text{s} \right]\times\Delta t[\text{s}]=M\ \left[\text{mg}\right] \tag{1}\]  
The comparison between PMI and ELPI+ data highlighted a very good agreement, with the two values not significantly different. Although the comparison among average concentration values was rough, the one based on the total mass was more precise and allowed us to drive some hypotheses related to the particle size. Indeed, the cascade impactor values of PM\({}_{2.5}\) used for the previous calculations were obtained, summing up the data collected in the first 11 stages of the device and able to trap particles with aerodynamic diameters from a few nm up to 2.5 \(\upmu\)m. Since it is known that the smallest particles contribute less to the total mass, it is worth noting that this may be partly compensated by their very high number. In addition, it shall be considered that the conversion from number to mass concentration, used to extract ELPI+ data, assumes the unit density of all particles. On this basis, the agreement between PMI and ELPI+ data is even more satisfactory, and it also demonstrates the comparability between these two devices both  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Monitoring Site, Date, and** **Environmental Condition** & **Sampling Device** & **(PM\({}_{2.5}\)) Total Mass (\(\upmu\)g)** \\ \hline Outdoor 26 March 2018 & ELPI & 267 (1) \\  & PMI & 250 (2) \\  & ELPI & 277 (1) \\  & PMI & 270 (2) \\ \hline \hline \end{tabular}
\end{table}
Table 2: PM\({}_{2.5}\) total mass (expressed in \(\upmu\)g) calculated for the ELPI+ andPMI. Relative percent errors (%) are reported in round brackets.  
based on the impaction of particles on filter stages according to their aerodynamic diameter, including the ultrafine range. This was confirmed by SEM images reported in Figure 3, where micrometric-size spongy aggregates of nanoparticles attached to the smooth Teflon filter surface are clearly visible.