Barium and other trace metals as indicators of vehicle emissions

In the Siena urban environment, unwashed Q when leaves had a significantly higher Al, Ba, Cr, Fe, Mn, Na, Ni, Pb, Ti, and V content than samples from a neighbouring Mediterranean maquis. Significant relationships between element pairs indicated both the presence of metals emitted by motor vehicles and lithophilic elements, presumably from soil particles, resuspended by wind or turbulence generated by traffic. In order to minimize the effects of soil dust contamination and to seek clues to the metal sources, the estimated substrate contribution was subtracted from the raw concentrations of elements in Q. ilex leaves. Normalization for local soil composition confirmed that automotive emissions are the principal source of pollution in Siena. On the basis of a very significant relationship between normalized concentrations of Pb and Ba and the presence of the latter element in diesel and unleaded gasoline, Ba was recognized as a valuable tracer for vehicle emissions, in the place of Pb.