Spatial variation in vehicle-derived metal pollution identified by magnetic and elemental analysis of roadside tree leaves

Exposure to metal-rich particulate pollution is associated with adverse health outcomes. In particular, lead has recently been shown to be toxic in young children even at low levels previously considered 'safe'. Lead poisoning from vehicle pollution has been addressed internationally by removal of leaded petrol but toxic blood lead levels in children continue to be reported in urban areas, the source suggested to be resuspended roadside soil, enriched in lead due to previous leaded fuel usage. Here, we use paired geochemical and magnetic analyses of natural biomonitors - kerbside tree leaves - and of air sample filters to examine contemporary sources of particulate pollution, and show that co-associated, fine (< 1 mu m) lead- and iron-rich particles are emitted as vehicle-derived pollutants. Higher and strongly correlated lead, iron and magnetic remanence values were found closer to roads and on the road-proximal rather than road-distal sides of trees. Critically, highest pollutant values occurred on tree leaves next to uphill rather than downhill road lanes. The lead content of the leaf particulates was associated only with sub-micrometre, combustion-derived spherical particles. These results indicate that vehicle exhaust emissions, rather than resuspended soil dust, or tyre, brake or other vehicle wear are the major source of the lead, iron and magnetic loadings on roadside tree leaves. Analysis of leaves at different heights showed that leaf particulate lead and iron concentrations are highest at similar to 0.3 m (i.e. small child height) and at 1.5-2 m (adult head height) above ground level; monitoring station collectors placed at 3 m above the surface thus significantly under-estimate kerbside, near-surface lead concentrations. These results indicate that vulnerable groups, especially young children, continue to be exposed to fine, lead- and iron-rich, vehicle-derived particulates. (c) 2007 Elsevier Ltd. All rights reserved.