## 4 Discussion  
The results of bioaccumulation showed that Milan is quite homogeneously affected by high deposition loads of elements such as Cr, Cu, Fe, and Sb; some more spotted peaks emerged also for Pb. This outcome is consistent with other lichen biomonitoring studies in urban areas of Italy [37; 39; 40; 41; 42]. The results are indicative of a common origin of pollutants from non-exhaust sources of vehicular traffic, such as brake abrasion. As a matter of fact, the above elements are used as components of brake systems [43], and brake abrasion is reported as the main source of trace elements in PM [44]. This conclusion is supported by the results of EDS X-ray microanalysis, which pinpointed the massive presence of Fe, and the widespread occurrence of Zn, Cu and Ba, the latter usually being connected with emissions from car brakes and tyres [32; 45]. In addition, as shown by SEM analysis, Fe-rich particles are generally different from industrial spherical combustion particles, further indicating that their source are motor vehicles and more specifically, relying on their composition, break abrasion [32]. The range of the magnetic susceptibility values (7.8 to \(35.8\times 10^{-8}\) m\({}^{3}\)kg\({}^{-1}\)), when compared with previous studies on transplanted lichens, is indicative of a notable concentration of magnetic minerals and, consequently, of iron-rich bioaccumulated particles. Transplants of the lichen _Pseudovernia furfuracea_ exposed for 4 months in a complex and heavily polluted area, ranged from 7.1 to \(17.1\times 10^{-8}\) m\({}^{3}\)/kg\({}^{-1}\)[37]. Transplants of the same lichen species exposed for 2 months in an industrial area showed values in the range 0.4 to \(7.4\times 10^{-8}\) m\({}^{3}\)/kg\({}^{-1}\)[46]. Transplants of _Evernia prunastri_ exposed for 6 months around a cement production plant in Slovakia showed magnetic susceptibility ranging 1.3 to \(8.1\times 10^{-8}\) m\({}^{3}\)/kg\({}^{-1}\)[47]. Bags of _Parmotrema pilosum_ exposed to atmospheric pollutants over the course of 1 year increased from an initial mass-specific magnetic susceptibility value (mean \(\pm\) S.D.) of \(24.1\pm 5.0\times 10^{-8}\) m\({}^{3}\)kg\({}^{-1}\) to higher values up to \(51.2\pm 23.0\times 10^{-8}\) m\({}^{3}\)kg\({}^{-1}\)[48]. This dataset included three sites, two at metallurgical factories, and one influenced by vehicular emissions, where values increased by 100%. As emerged from the hysteresis loops, which are very similar and differ only for the concentration-dependent magnetic parameters, which follow the same trend of magnetic susceptibility, the variations of magnetic susceptibility are linked to different concentrations of magnetic particles similar for composition and grainsize. The coercivities range, the low saturation field and the SIRM/k values, spanning 2.4-7.1 kA/m, all suggest that magnetite-like minerals are the main magnetic carriers. In the Day Plot, the lichens transplanted in Milan have been compared to the previous points obtained for _Quercus__hex_ leaves sampled in Rome, Italy, PM filters and dusts arising from fuel exhausts and brakes' emissions, as discussed in [32]. The transplants markedly overlap the \"brake\" samples \begin{table}
\begin{tabular}{c c c c} \hline  & **R\({}^{2}\)** & **Cohen’s \(f^{2}\)** & **Effect Size** \\ \hline Al & 0.00–0.36 & 0.00 & small \\ As & 0.00–0.36 & 0.00 & small \\ Cd & 0.00–0.30 & 0.00 & small \\ Cr & 0.43–0.73 & 0.75 & large \\ Cu & 0.36–0.70 & 0.56 & large \\ Fe & 0.51–0.76 & 1.04 & large \\ Pb & 0.00–0.14 & 0.00 & small \\ Sb & 0.23–0.64 & 0.30 & medium \\ Zn & 0.00–0.41 & 0.00 & small \\ Ms & 0.66–0.82 & 1.94 & large \\ Mrs & 0.51–0.76 & 1.04 & large \\ \hline \end{tabular}
\end{table} 
Table 3: Ninety-five percent credible intervals of the R\({}^{2}\) values, Cohen’s \(f^{2}\) and magnitude of the effect size between magnetic susceptibility and chemical elements as well as other magnetic parameters.