## 4 Discussion  
It is important to point out that this represents a pilot study and, for this reason, the investigated area in Milan was not so large and the four investigated locations not so distant from each other. Different highs from the ground level were chosen in order to study the effect of the distance from sources such as traffic. In fact, at the Botanical garden and ARPA station, the plants were placed at ground levels while at the Chem Dept and Bio Dept they were at an altitude of 6 meters (see Figure 1). A different behaviour was observed for Cr (Figure 3a) whose concentrations at 165 was higher at Botanical Garden and ARPA where the specimens were at ground level. Hence, it can be hypothesized that Cr is associated to some source linked to road dust where this element was identified at low level [48]. Figure 6: EDS analyses (**a**–**c**) acquired on single particles present on the leaves’ surfaces and evidenced with a red circles in (**a**–**c**). On the base of the results obtained and reported in Figure 4, it can be stated that the heavy metals considered in this study, with the exception of copper, come from atmospheric pollution. This is demonstrated thanks to the comparison with the not-exposed plant. It is worth noting that the metals penetrated inside the leaves [41; 42] since heavy metals were not detected on the surface by SEM-EDS analysis. Among heavy metals Zn, Mn and Cu are those present in the higher concentration in the aerosol particulate matter [24; 25]. Zn, Cu and Pb are associated with traffic emissions [25] while Mn together with Zn are due to industrial emissions [24; 25]. With respect to what is reported in [PERSON] et al. [25], a lower contribution of Pb should be expected since lead concentration in the atmosphere started to decrease since the introduction of the green gasoline. This justifies the low concentrations detected in _Chlorophytum comosum_ for this element. It is also worthwhile to note that Fe and Zn are micro elements naturally present in _Chlorophytum comosum_, as evidenced by [PERSON] et al. 2009 [44]. Indeed, Zn was also detected in the not-exposed plant confirming the presence of a natural background value, as shown in Figures 3b and 4a. This blank value determined in the not-exposed plant is about of the same order of Zn concentration detected in _Chlorophytum comosum_ for the different sites at 7 days (Figure 3b). Then, starting from 30 days, Zn increases reaching a maximum at 60 or at 128 days depending on the site. Indeed, the concentration at 165 is lower for all the sites and this can be explained taking into account that dilution of metals with plant growth can decreases the total metal concentration in older plants [41]. In the same period, during which _Chlorophytum comosum_ was exposed (i. e for 165 days from July to December 2018) data of atmospheric concentrations of heavy metals measured on the filters collected by ARPA Lombardia by means of gravimetric systems in the monitoring station of Milano Via Pascal, were considered (data were both downloaded from the ARPA Lombardia website [49] and also made available by the office responsible for the monitoring). All the other elements of interest for this study, i.e., referred to the same period, were in many cases below the detection limit or not available with the exception of Zn. As a consequence, a comparison between Zn concentrations detected in _Chlorophytum comosum_ leaves exposed at ARPA site were compared with the metal concentration detected in the aerosol PM (in particular PM10) for the same period. At this purpose Zn values detected on each filter (24 h of sampling) were added up in order to obtain the total concentrations corresponding to the different sampling periods considered for the plants. The results obtained are reported in Figure 7. It is worth noting how there is a good correlation up to 60 days.