Inorganic lead in air

By far the major proportion of vehicle-emitted lead in air is inorganic, and hence exists in the air in particulate form. The fate of particulate pollutants is highly dependent upon the particle size. 

Particles of 10 nm diameter are subject to significant sedimentation rates as a result of gravitational forces (the velocity may be estimated crudely by use of Stoke s Law). Consequently their atmospheric lifetime is severely limited by the gravitational settling process, and by impaction upon surfaces. Particles of 10 jum diameter are removed only relatively slowly from the atmosphere, those greater than ca. 0.3 /im by impaction processes and the smaller particles by diffusive deposition. The deposition flux may be estimated from the deposition velocity and some data are presented in Section 4.2.2. 

Estimates of the atmospheric lifetime of lead aerosol range typically between 7 and 30 days. This time is sufficient for transport over thousands of kilometres, and consequently even the most remote sites may experience pollution by inorganic lead. It is hence valuable to consider inorganic lead pollution under two headings roadside and urban (close to source) and rural (remote from source) pollution. 

In an urban area, there are two separate effects. In addition to the lead arising from a nearby road, an urban background of perhaps 0.5-1 jUg of lead exists arising from the network of roads in the area. The effect of one particular highway becomes difficult to discern beyond about 100 m from the carriageway, and the lead concentration is best described in terms of emissions within the whole urban area. 

Chamberlain and co-workers [7] have calculated concentrations of lead in air from area-wide sources. The concentration, C, near the ground is given by 

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