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Observation will tell you that those heavier than air gases do in fact tend to gravitate to lower levels.
On cold days, you can actually see the bluish haze of exhaust fumes gathering in hollows and city "canyons", building up and rising like a tideas the gas cloud heats up as the day warms.
[/QUOTE]Originally Posted by ENT
People rapidly die of CO and CO2 poisoning in sub-surface enclosures, as gravity rules. Note that those exhausted gases will in fact migrate to lower levels in the surrounding environment, such as drains, tunnels, canals and streams on either side of the highways, especially during cooler hours of the day or months of the year.
Heat islands are created in cities, during peak road use and mid day temperature peaks, causing exhaust pollution to rise, then descend again as the air cools at night.
A look at city AQIs will show the peak PM 2.5 and PM10 levels occurring at peak hours then dropping as midday temperatures rise, then increasing once more while their effects on the vulnerable often only felt some 6 hours, after peak production as the gases cool and descend in the evenings, when vehicular traffic decreases.
Besides the exhaust gases, asbestos and road surface debris form up to 50% of air pollution in cities.