###### Abstract  
The concentration of selected trace elements and the magnetic properties of samples of the lichen _Evernia prunastri_ exposed for 3 months in Milan (Italy) were investigated to test if magnetic properties can be used as a proxy for the bioaccumulation of chemical elements in airborne particulate matter. Magnetic analysis showed intense properties driven by magnetite-like minerals, leading to significant correlations between magnetic susceptibility and the concentration of Fe, Cr, Cu, and Sb. Selected magnetic particles were characterized by Scanning Electron Microscope and Energy Dispersion System microanalyses, and their composition, morphology and grain size supported their anthropogenic, non-exhaust origin. The overall combination of chemical, morphoscopic and magnetic analyses strongly suggested that brake abrasion from vehicles is the main source of the airborne particles accumulated by lichens. It is concluded that magnetic susceptibility is an excellent parameter for a simple, rapid and cost-effective characterization of atmospheric trace metal pollution using lichens.  
magnetic biomonitoring; particulate matter; lichen transplants; brake wear; urban air pollution 20202011 March 20202011 March 20202011 March 20202011 March 20202  
Among living organisms, lichens are well-known very sensitive bioindicators of air pollution, which can act as real biological sentinels [4]. Unlike higher plants, lacking roots, stomata and waxy cuticles, lichens are completely dependent on atmospheric wet and dry deposition for nutrients, and there is evidence that their elemental content do reflect bulk atmospheric deposition [5]. Lichens are especially useful in urban areas, where the complexity of the environment and the great variety of pollution sources make monitoring quite a difficult task [6]. In addition, lichen biomonitoring can be used in cases of environmental forensics and environmental justice [7; 8]. The use of lichen transplants, i.e., samples taken from a background area and exposed into the study area, has recently overcome the use of native samples due to the possibility of planning any sampling design, controlling exactly the exposure time, and comparing the results with pre-exposure values [9].  
Urban PM may have remarkable magnetic properties related to the content of anthropogenic iron oxides, mostly magnetite-like ferrimagnetic particles. Magnetic nanoparticles (NPs), formed by combustion and/or friction-derived, which are common in urban airborne PM, were recently found even in the human brain, where they can enter directly through the olfactory nerve [10]. Exposure to up to ~22 billion magnetic NPs/g of ventricular tissue appears to be directly associated with early and significant cardiac damage in children and young adults [11].  
Heavy metals are often associated to the magnetic fraction of PM, arising directly from the emission sources (e.g., brakes), or incorporated in the iron oxides' structure during combustion processes (fuel exhausts, industrial emissions). Therefore, the magnetic properties of PM often constitute original, cheap and fast proxies of the anthropogenic fraction of PM, with immediate health-related interest.  
Among biological media, plant leaves and lichens turned out to be extremely suitable for magnetic biomonitoring of air pollution in urban areas (for a review see [12]), shedding light on the bioaccumulation of anthropogenic dust and providing location-specific and time-integrated air quality information.  
In this study, a biomonitoring survey of air quality using lichen transplants was carried out in Milan (Italy), investigating the trace element content and the magnetic properties of exposed samples. A Bayesian statistical approach was used to test if magnetic properties can be used as a proxy for the bioaccumulation of chemical elements.