When scientists recently looked at all the known sources of air pollution in the Los Angeles metro area (including of course automotive emissions) and compared that the the known quantities of pollutants in the air: it didn’t add up. Some source was missing. A team of researchers then proposed that the missing source was right underneath those missing cars. It was — and is — the asphalt of the roads themselves.
When roads are paved and the asphalt is poured, its temperature is 140°C, or 284°F. AT that time, a lot of semivolatile organic compounds escape into the air. Most motorists recognize this as the smell of freshly filled potholes. There is also some escape at lower temperatures. Some of these compounds escape during typical summer-daytime southern California temperatures.
In Pill Form:
The roads’ emissions form small bubbles (aerosols) that linger in the atmosphere, can be harmful when inhaled, and contribute to the broader urban smog problem. The researchers indicate that the molecules from this source over the course of a year constitute between 1,000 and 2,500 tons of particular pollution, enough the justify the consideration of other materials for road paving (and roofing).