Particle pollution – An environmental magnetism study using biocollectors located in northern Portugal

In this study the magnetic properties of tree leaves were measured in order to compare their capability to accumulate particles, to establish the relationship between magnetic properties and chemical data and to assess the particle pollution in selected locations in the cities of Braga, Porto, Valongo and Trancoso-Reboleiro, northern Portugal. In Porto, Braga and Valongo, leaves from the evergreen Nerium oleander were sampled each month during a year. N. oleander and deciduous Quercus spp. and Platanus spp. samples were collected in the same site in Porto, in order to determine the ability of these different leaves to accumulate particles. The leaves of deciduous Tilia spp. were collected in Porto and in a rural area (Trancoso-Reboleiro) so that a comparison could be established between them. The results indicated a contrast between the urban and the rural areas. The highest concentration of magnetic particles was found in the sampling site of Valongo and the lowest concentration in the sampling site of Trancoso-Reboleiro. In Porto, the results have shown that the Quercus leaves possessed the highest capability to accumulate particles even though it is a deciduous species. The IRM acquisition curves and the S-300 ratios found in the samples of the urban areas indicated the presence of magnetite-like structures. SIRM/χ ratio revealed particles whose dimensions ranged between 5 μm and 8 μm in urban areas. The chemical elements copper and iron have a significant positive correlation with χ and SIRM, which highlights the use of magnetic properties as a proxy for the concentration of these metals in atmospheric dust. The magnetic properties were interpreted taking into consideration the rainfall peaks and then compared with the PM10 concentration levels monitored in an air quality station in Porto. Our data corroborated that magnetic properties provide a fast and inexpensive tool to evaluate long-term urban pollution from anthropogenic origin, especially heavy traffic.