Mapping the sources of urban dust in a coastal environment by measuring magnetic parameters of Platanus hispanica leaves.

Tree leaves accumulate atmospheric particles on their surface, and a fraction of these particles exhibits magnetic properties that can be used to determine the spatial distribution of atmospheric dust in an urban area. This observation is exploited here to map sources of atmospheric pollutants in the coastal city of Vigo. The magnetic carriers on the leaves were iron oxide spherules (5-10 microm) and larger iron-bearing particles (typically 10-50 microm). The maximum values of magnetic remanence were very restricted in space and could be directly related to nearby polluting activities, such as road and railway traffic and shipyard and harbor activities; hence, our data represent main sources of urban dust in the city and not sinks. The concentrations of Fe, Zn, Ni, Pb, and Cu on the leaves showed a strong positive inter-correlation and also with the magnetic remanence of the leaves, suggesting common sources for these heavy metals and for the magnetic carriers. Mn and Cr did not show any significant correlation and their presence was linked to natural accumulations as micronutrients by the trees. The iron spherules and the heavy metal species identified here coincide with materials commonly found in sediments of the adjacent Ria de Vigo. The identification of the main sources of urban dust and heavy metals in the industrial and urban areas can, therefore, help in controlling these emissions to the Ria de Vigo.