Intra-urban spatial variation of magnetic particles: Monitoring via leaf saturation isothermal remanent magnetisation (SIRM)

Motorised traffic generates large numbers of small-sized, magnetisable particulate pollutants in the urban environment. Exposure to these small particles has been associated with adverse effects on human health. Magnetic properties of these particles are, therefore, increasingly used for assessing environmental stress. Biomonitoring of magnetic particles accumulated on leaf surfaces may provide information on the concentration of, and exposure to, atmospheric particles at high spatial resolution. In this study, leaf saturation isothermal remanent magnetisation (SIRM) of three urban tree types (Carpinus betulus and Tilia sp. with hairy and non-hairy leaves) was measured at high spatial resolution in the city of Gent, Belgium, in June and September 2009. We compared leaf SIRM between land use classes with different urban habitat quality. In a multiple regression model, we tried to explain the spatial variability in leaf SIRM by tree species, sampling height, distance to the nearest road and its traffic intensity, tram frequency and a measure for regional traffic emissions (derived from traffic intensity of and the distance to the most important highways around and in the city in the main four wind directions). We found that the leaf SIRM was significantly influenced by tree species, distance to the nearest road and its traffic intensity and tram frequency. The SIRM significantly increased with increasing traffic intensity and tram frequency and by decreasing distance to the nearest road. It is concluded that leaf SIRM is a good bio-indicator for monitoring spatial variation of magnetic particles in urban environments.

► Tree leaf SIRM distinguished between sites with contrasting urban habitat quality.
► Leaf SIRM correlated positively with traffic and negatively with distance to road.
► Local traffic emission is the main magnetic PM source in urban environments.
► Inter-species calibration of leaf SIRM allows a high coverage of the study area.
► Leaf SIRM is a good indicator of the spatial variation of urban magnetic particles.