Magnetic Characterisation and Mapping of Urban Dust in Madrid, Spain.

Air quality and particulate loadings are important contemporary issues, and their characterisation is of great interest. A study of the magnetic properties of dust deposited on horizontal surfaces and tree leaves across the main urban area of Madrid, Spain, has been carried out. In both cases the magnetic signal is dominated by a similar, low coercivity component, which temperature-dependent susceptibility and remanence measurements identify as oxidised magnetite or maghaemite. Subtle coercivity variations are seen in the samples, showing both tree species and time dependencies. Evergreen (pine) needles exhibit higher coercivities than deciduous leaves, whilst the coercivity of deciduous leaves increases with time. This can be ascribed to the progressive oxidation of the magnetic phase. Isothermal remanence intensity and susceptibility decrease as the distance from traffic-bearing roads increases, indicating traffic as the main source of the magnetic signal. City-wide maps highlight magnetic "hot-spots" where the accumulation of magnetic material is more pronounced. Sample collection, measurement and map production are quick and easy and provide a useful tool for delineating time-averaged pollution loadings.