It turns out that there is another serious source of air pollution in cities that is not usually mentioned - the emissions from roof and road asphalt, especially on hot and sunny days.
Yale University researchers found that asphalt emits a complex mixture of compounds, including hazardous pollutants. The emissions really increase as temperature increases, as well as exposure to solar radiation (sunlight).
The researchers estimate that urban areas are about 45%+ paved, and 20% are roofs. They stated that in a city such as Los Angeles, asphalt's potential to emit secondary organic aerosols is comparable to vehicles.
From Science Daily: Asphalt adds to air pollution, especially on hot, sunny days
Asphalt is a near-ubiquitous substance -- it's found in roads, on roofs and in driveways -- but its chemical emissions rarely figure into urban air quality management plans. ...continue reading "Asphalt In City Streets and Roofs Emits Air Pollutants"