## 4 Discussion
and fall at the lower right end of the cluster defined by the particles accumulated on leaves and filters, in a region of the plot falling in-between the theoretical trends for SD-MD and SD-SP pure magnetites, far from \"diesel\" and \"gasoline\" exhausts, which instead follow the theoretical trends for mixtures of single domain (SD) and multidomain (MD) magnetites. This result is at variance with the observations of [37] for combustion dusts accumulated by lichens exposed in an area subjected to arsons, where the magnetic mineralogy corresponded to magnetite-like minerals in PSD magnetic domain state/grain size, similarly to the diesel exhaust emissions in [32]. Also the SIRM/k values (\(5.0\pm 1.2\) kA/m) are compatible with the brake emissions (\(6.7\pm 2.5\) kA/m) and distinct from gasoline and diesel emissions (\(13.8\pm 6.3\) kA/m and \(14.5\pm 12.1\) kA/m, respectively) reviewed in [49]. Even FORC diagrams confirm the uniform prevalence, irrespective of the concentration, of viscous components of magnetization, closely resembling the diagrams for brakes and leaves reported by [32]. It is recalled that the critical magnetic grain size transitions, theoretically determined for equidimensional magnetite, are about 0.03 \(\upmu\)m for SP to SD, 0.08 \(\upmu\)m for SD to PSD, and 17 \(\upmu\)m for PSD to true MD [50]. Overall, it is not easy to attribute these magnetic features to SP or MD particles. In this sense, none of the magnetic properties diagnostic of the relevant presence of SP particles--e.g., the enhancement of magnetic susceptibility, very low SIRM/k values, asymmetry along the Bu axis in FORC diagrams--supported the presence of ultrafine magnetic particles, although the latter could be linked to the choice of a relatively high averaging time (300 ms) during the measurements. As a further difficulty, in traffic-related PM, the SP fraction may occur as coating of MD particles and is originated by localized stress in the oxidized outer shell surrounding the unoxidized core of magnetite-like grains; thus it cannot be considered as a direct proxy for the overall content of ultrafine <30 nm particles [17]. On the other hand, the presence of clusters of fine and ultrafine magnetic particles is clearly shown by SEM observations; thus, the \"intermediate\" position in the Day plot might depend both on the chemically-impure composition of the magnetite-like particles and on the coexistence of SP and MD granulometric fractions. The strength of the statistical associations between magnetic parameters and bioaccumulation of chemical elements corroborated the above conclusions that the main source of air pollution in Milan is non-exhaust emission from vehicular traffic, notably brake abrasions. The deposition of PM enriched in Fe, Cr, Cu, and Sb caused the bioaccumulation of these chemical elements and determined the magnetic susceptibility of lichen samples. According to their strong intercorrelations, k, Ms and Mrs are indicative of the same magnetic fraction; thus, magnetic susceptibility is a valid, representative and fast proxy for the concentration of ferromagnetic particles associated with trace metals in PM. Brake wear debris comprises primarily carbonaceous and metal-bearing components, including, besides Fe, heavy metals such as Cu, Sb, Sn, Cd, Cr, Pb, and Zn [43]. In [49], it was shown that Fe-bearing particles result from the friction between a brake pad and the cast iron brake disc; at brake pad temperatures below 200 \({}^{\circ}\)C, abrasive processes dominate, and wear particles >1 \(\upmu\)m are mostly generated. At higher operational temperatures (>190 \({}^{\circ}\)C), the concentration of nanoparticles (<100 nm) increases due to evaporation, condensation and aggregation processes, increasing by four to six orders of magnitude the emission rate of particles, thus constituting >90% of total brake dusts and constituting a major source of airborne magnetic nanoparticles.