Metals concentrations in road dust from high traffic and low traffic area: a size dependent comparison

Metals in road dust are increasingly investigated. The fine size fractions are of particular importance because of their higher mobility and the increased concentrations of pollutants. In this study, two average road dust samples from areas with high- and low-traffic density were investigated. Prior to chemical analysis, a combined sieving and air classification procedure was applied where the road dust samples were split into seven size fractions. The metal concentrations in the road dust from the low-traffic area were only up to twice the background concentrations. An exception was Cu where the concentration was 15 time higher. In the sample from the high-traffic area, the respective concentrations of Zn, Mn, Fe, Pb, Ba, Sb and Cu were 5.4, 3.2, 3.0, 2.7, 2.7, 2.5 and 1.2 times higher compared to the low-traffic area. The ratio of the concentration in high- and low-traffic road dust was 0.8–2.0 for the fine size fractions and 2.0–6.0 for the coarse size fractions for several metals, including traffic-related metals like Fe and Ba. An explanation for this surprising result might be the atmospheric transport of re-suspended particles from high-traffic to low-traffic areas, which is feasible only for smaller particles because of their low sedimentation velocity. However, for the highly traffic-related metals Pb and Zn, the ratio was more size independent. For Cu, the concentration ratio was fluctuating. Some particles with a high Cu content in the analysed samples might be the reason.