The La/Ce ratio for the different particle sizes ranged from
0.485 to 0.496, approaching the typical value found in crustal
material (0.492). This suggests that the origin of these
transition metals is associated with soil resuspension. Pio
et al. (2013) observed, in the Marquês tunnel, decreasing La/Ce
ratios with decreasing particle sizes (La/Ce ratios from 0.27 to
0.15). The ratio obtained in Lisbon for PM2.5 (0.2) is similar to
those reported for tunnels in the US (Chellamet al., 2005). The
use of catalytic converters in gasoline and diesel vehicles
results in atmospheric fractionation of lanthanoid elements