## 3 Results and Discussion
### Estimation of the Detection Limits of the Adopted Optical Devices  
A deeper understanding of the results obtained with the three real-time sampling devices needs to consider the differences based on the detection methods. The measurement range may be different mainly in terms of detection limits (LOD), which is the minimum dimension of the deterministic particles. The daily PM\({}_{2.5}\) concentration patterns of KORA and AirVisual Pro were compared with concentration patterns obtained by summing the PM concentration values associated with all the cascade impactor particle fractions covering the stages from 1 to 11. The efficiency curve of an ideal impactor stage is a step function that only allow particles larger than the cut-off diameter (D50%) to be collected by inertial impaction, whereas all particles smaller than D50% follow the flow and impact in lower stages. In practice, an un-ideal flow through the impactor nozzles determines a true collection efficiency to be S-shaped [46]. Additionally, due to particles bouncing and blowing off, some large particles will go to the lower stages than they should, and due to diffusion, ultrafine particles will stay in the upper stages. For this reason, the erroneous current signal derived by these factors can be significant, so we decided to consider only stages 1-11 for PM\({}_{2.5}\) determination  
Figure 6: Hourly average concentrations of PM\({}_{2.5}\) measured by KORA (blue circles) and AirVisual Pro (grey circles) with respect to ELIPI+. Ideal correlation is shown by the slope 1 line. The inset better evidences the correlation at the lower end of concentration range. by the cascade impactor when comparing data with the ones produced by AirVisual Pro and KORA. To understand which fractions were excluded from KORA and AirVisual Pro measurements and how this cut may have affected the sampling results, we first selected the garage location due to the uniformity of emission sources. The whole day concentration patterns are compared in Figure 7, based on average concentration per minute to have greater representativeness. Underestimation of KORA along the whole curve is clear, with an asymmetrical shift higher in the first part of the day. Similar behavior was observed for the AirVisual Pro, whose pattern was nearer to ELPI+ and moved away markedly in that part of the day. The emissions present in the sampled area during the morning included car exhaust, which is known to produce very fine particles [16]. On this basis, we guessed that both the optical devices were not able to detect the lower range of fine particles and thus the smallest fraction of the cascade impactor should not be included in the PM concentration sum for the comparison. Indeed, the dimensional range represented by the 11 stages was very wide, with lower granulometries up to 6 nm (collected on the filter stage F), well below the optically visible range. The hypothesis was that optical devices cut the particles with average diameters below 250-300 nm, so the ELPI+ stages from 8 to 11 (for simplicity called 8-11) were considered first. A preliminary graphical comparison showed a very good superposition of the KORA and 8-11 patterns throughout the whole day, whereas the agreement with AirVisual Pro was present only in the first two hours of sampling, and the 8-11 pattern was lower in the rest of the day (see Figure 7). For this reason, a new pattern also including fraction 7, called 7-11, was considered. In this case, the agreement was better for the rest of the day, but not  
Figure 7: PM\({}_{2.5}\) daily concentration patterns measured by AirVisual Pro, KORA, and ELPI+, the latter identified by the range of fractions (min–max) considered. for the first hours, and this may have been due to the type of emission present during this sampling time. Indeed, it is known that typical car exhaust, mainly occurring during the first morning hours, is mostly in the ultrafine range [16] and probably cannot be detected by the AirVisual Pro.