If we wanted to pinpoint the main source of air pollution on roads, we would point at the vehicles. But there is another source of air pollution on roads, scientists say, and that source is the asphalt used in the roads themselves.

“Asphalt-based materials are abundant and a major nontraditional source of reactive organic compounds in urban areas, but their emissions are essentially absent from inventories,” a team of experts explain.

“At typical temperature and solar conditions simulating different life cycle stages (i.e., storage, paving, and use), common road and roofing asphalts produced complex mixtures of organic compounds, including hazardous pollutants,” they elucidate.

The scientists discovered this after examining the variables behind the notoriously bad air pollution in Los Angeles and its environs. They realized that some factors had not been taken into account in measuring levels of air pollution generated by transportation. “Asphalt was something that jumped out to us,” notes Drew Gentner, a chemical and environmental engineer at Yale University who led the research.

Asphalt contains various fairly volatile organic compounds that can greatly drive air pollution, Gentner says. During laboratory tests conducted by him and his colleagues it turned out that asphalt-covered pavement released harmful organic compounds in especially large quantities when it was heated to 140°C, which is the temperature used for paving roads.

Emissions were found to be lower at cooler temperatures but they remained considerable even at 60°C, a temperature to which asphalt normally heats up on hot sunny days in Los Angeles. Meanwhile, even moderate levels of sunlight can increase harmful chemical emissions in lower temperatures by as much as 300%, leading to large amounts of invisible aerosol particulars that can damage lungs.

In Southern California alone, the scientists estimate, between 1,000 and 2,500 tons of particulate air pollution could be released each year from asphalt used in roads and roofing, compared with between 900 and 1,400 tons from gasoline and diesel vehicles.

In other words, even as vehicles release large amounts of harmful particles from combustion and exhaust gases, the roads they traverse also contribute significantly to air pollution. How long asphalt releases harmful chemicals in large quantities over time is yet to be examined, but what is already clear is that we will need to factor pollution from asphalt into air pollution models, scientists stress.

“[E]mission factors and emissions estimates are so essential for understanding air quality and there are many missing sources that we need to get a better understanding of,” says Eri Saikawa, an environmental scientist at Emory University in Atlanta, Georgia. “There is still so much that we do not understand about secondary organic aerosols, and these studies are very important to push the field forward.”

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