The Dispersion of Lead from Motor Exhausts [and Discussion]

Lead is added to petrol as organic tetraethyl or tetramethyl lead but is emitted as inorganic oxides, sulphides, halides and carbonates. From vehicles cruising at high speed on motorways, the lead is emitted as very small (ca. 0.02 $\mu $m) discrete particles. In city streets, where there is also a background aerosol, coagulated chain-aggregate particles are found. The concentration of lead in air downwind of a road can be calculated theoretically, and measured concentrations agree fairly well. Within a few metres of the carriageways of motorways carrying heavy traffic, concentrations as high as 10 $\mu $g/m$^{3}$ are found, but the concentration falls rapidly with distance owing mainly to the upward diffusion of the plume. Measurements of the rate of deposition to surfaces near the M 4 motorway show that an appreciable, but not large, fraction of the lead is deposited within 100 m of the motorway. Near towns, the contribution from a given highway merges rapidly into the background due to emissions from other roads. The dispersion is countrywide and probably some cross-frontier transfer occurs.