Magnetic susceptibility of road deposited sediments at a national scale--relation to population size and urban pollution.

Magnetic properties of road dusts from 26 urban sites in Bulgaria are studied. Temporal variations of magnetic susceptibility (χ) during eighteen months monitoring account for approximately 1/3rd of the mean annual values. Analysis of heavy metal contents and magnetic parameters for the fraction d < 63 μm reveal significant correlations (p < 0.05) between χ and Fe, Mn and PLI index. The highest negative correlation (R(2) = -0.84) is observed between the ratio ARM/χ and Pb content. It suggests that Pb is related to brake/tyre wear emissions, releasing larger particles and higher Pb during slow driving - braking. Bulk χ values of road dusts per city show significant correlation with population size and mean annual NO2 concentration on a log-normal scale. The results demonstrate the applicability of magnetic measurements of road dusts for estimation of mean NO2 levels at high spatial density, which is important for pollution modelling and health risk assessment.