Lead is heavy, even in trace quantities and after being initially airborne. It will deposit in soil, stream beds and sewers, particularly near roadways or transit systems. You will also find platinum-group metals accumulated in the same areas due to persistent catalytic converter atomization of running vehicles. Disturbing the soils (wind, rain, driving, construction, etc) will redistribute the tiny lead particles into the atmosphere, despite their relative weight.
> The lead content in that study was measured in particles collected either at the roadside or at rooftop height. The chemical fingerprint closely matched that of road dust and top soils, suggesting that contaminated soil is acting as a reservoir for 20-year old lead pollution, which is continually returned to the atmosphere when disturbed. The fact that lead found at street and building height shared the same chemical signature suggests airborne lead pollution is fairly well mixed across London.
You still have measurable lead levels in the air. Essentially the rain and gravity brings it down to the ground - mostly concentrated near roadways - where it mixes with other dust and soil. From there it can then get cycled again into the air by agitation. It is unlike CFC gasses that tend to float up and are no longer present at ground level shortly after they stop being introduced.
In this regard you still have lead present in the air in varying degrees 20 years after it stopped being introduced. The report cites 2% of 1990 levels nearly 20 years after it was outlawed.
> The lead content in that study was measured in particles collected either at the roadside or at rooftop height. The chemical fingerprint closely matched that of road dust and top soils, suggesting that contaminated soil is acting as a reservoir for 20-year old lead pollution, which is continually returned to the atmosphere when disturbed. The fact that lead found at street and building height shared the same chemical signature suggests airborne lead pollution is fairly well mixed across London.