## 3 Results  
To monitor the presence of the selected heavy metals (Cd, Co, Cr, Cu, Mn, Ni, Pb, and Zn) in the air of Milan, 16 plants were exposed in 4 different areas of the city and the leaves collected, as described in Table 1. For the samples collected at 7, 30, 60 and 128 day only Cr, Pb and Zn were measured while for the samples collected at 165 days all the 8 elements were quantified. In Figure 3a,b the trends observed for Cr and Zn for the four examined sites are shown. Zn is present in some cases in concentrations higher than 150 ppm while Cr is at maximum about 5 ppm. Pb concentrations (not shown in this case) reached a maximum at about 20 ppb. In some cases (missing data in Figure 3a,b) it was not possible to get a sample suitable for the analysis. Together with the values determined on the exposed samples, the concentrations detected for the not-exposed plant (see paragraph Methods and Materials), used as comparison, are shown. It is worth noting that this plant was analyzed after 165 days (corresponding to the maximum exposure period). As mentioned in the introduction, even if all the specimens should be similar, there is certain variability among the plants and the comparison with the reference is not easy to make. In the case of Cr, only the samples at 165 for ARPA and Bioscience Department are clearly higher, while the other cases are similar even if it cannot be excluded that the initial concentration in the not-exposed plant was slightly lower. As for Zn, the concentrations determined for the exposed plants are higher with only very few exceptions. In Figure 4a,b, the trends observed for all the elements for the four examined sites at 165 days are shown. Together with the heavy metals concentration determined for the exposed species, the values determined for the not-exposed plant are reported as well. With the only exception of copper, for which the concentration in the not-exposed plant is comparable with those determined for the exposed plants, for all the other elements _Chlorophytum comosum_ can be considered as a good bioindicator for heavy metals accumulation (in case of Cr surely at 165 days). The metals present with the higher concentration are Zn and Mn followed by Cd and Cr while Co, Ni and Pb are present in lower concentration (at maximum about 30 ppb for Ni and about 20 for Pb). Figure 2: The investigated area in Milan (**a**) with the indications of the sampling sites (**b**); in this case a represents ARPA station; b the Bioscience Department; c the Chemistry Department and d the Botanical Garden. Figure 3: Concentrations trends for Cr (**a**) and Zn (**b**) in the _Chlorophytum comosum_ analyzed samples in the different sites in Milan and for different exposure times (7, 30, 60, 128 and 165 days); the concentrations determined for the not-exposed plant have been added for both metals (reported in brown colour). In order to assess the presence of particles on the leaves' surfaces some measurements were carried out by SEM-EDS technique. As it is evident from Figure 5, the particles dimensions are quite variable ranging from some \(\upmu\)m to more than 100 \(\upmu\)m. Thanks to the comparison between BSE (back scattered electrons) and mix (BSE + secondary electrons) images, it is possible to better evidence the three-dimensionality of the surface. In fact, from mix images, it appears clearer if a particle is present on the surface (for example the larger particles in Figure 5a,b) or are embedded (as for the smaller spherical particles in Figure 5c,d). In Figure 5e,f a large particle present on the leave surface is shown. EDS analysis evidenced, on the leaves' surface, the presence of macro elements such as Ca, K and Na in accordance with what already reported in [PERSON] et al. (2014). Mg and Cl were also present. Most of the particles on the surface were silicates. Heavy metals were not highlighted probably because they are contained in the particles more embedded into the leave structure.