Metal emissions from road traffic and the influence of resuspension—results from two tunnel studies

Metal emissions from road traffic were studied in two heavily trafficked tunnels in Gothenburg, Sweden. Five runs were made in each tunnel, generally extending over several hours, during which air concentrations of metals in tunnel inlet and outlet, traffic flow and composition, and air ventilation were determined. Emission factors for Cu, Zn, Cd, Sb, Ba and Pb in TSP were derived from these data, mainly representing vehicle emissions. For these metals, results from the two tunnels were fairly consistent and the variability of the emission factors was generally within the experimental uncertainty. In contrast, concentrations of TSP, PM10 and a number of other metals differed widely between the two tunnels. This difference was caused by different magnitudes of resuspension, mainly reflecting that the two tunnels were investigated at different seasons and that one of the tunnels was newly cleaned. Aerosol concentrations of larger particles and of several metals are apparently dominated by resuspended matter, rather than by emissions from vehicles. The vehicle-derived metals (Cu, Zn, Cd, Sb, Ba and Pb) mainly derive from wear rather than from combustion; Cu, Ba and Sb are probably dominated by brake wear. There are also indications that heavy duty vehicles are stronger emitters of Ba and Sb, but not of Cu, than light duty vehicles.