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 A pothole being fixed in Southern California. 

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It’s not just cars that make pollution. It’s the roads they drive on, too

The smell of summer in Los Angeles, or any major city, is often tinged with asphalt. A freshly paved road or a new tar roof doesn’t just wrinkle your nose, however: A new study suggests fresh asphalt is a significant, yet overlooked, source of air pollution. In fact, the material’s contribution to one kind of particulate air pollution could rival or even exceed that of cars and trucks.

"It's a super cool paper," says Allen Robinson, an environmental engineer at Carnegie Mellon University who was not involved with the research. "Asphalt could be a big, important contributor" to air pollution, he says.

Air quality has improved over the past several decades in California and many other parts of the United States, largely because of cleaner exhaust from vehicles and power plants. Despite that, air pollution still contributes to many health problems—ranging from asthma to heart attacks. And many sources of air pollution continue to be a problem, from livestock emissions to volatile organic compounds from paints, cleaning products, and personal care products (especially those that contain fragrances, such as shampoo).

Yet, when scientists looked at all the known sources of air pollution in Los Angeles and the surrounding areas, they didn't add up. Some sources had not yet been identified.

"Asphalt was something that jumped out to us," says Drew Gentner, an environmental engineer at Yale University who led the new study. The material, made from crude oil or similar substances, contains the kinds of semivolatile organic compounds that lead to some types of air pollution. There's also a lot of it.

Gentner and colleagues gathered two types of fresh road asphalt and heated them in a laboratory furnace. They also tested new asphalt shingles and liquid asphalts used for roofing. They reasoned that new material should release more chemicals than older material, and they wanted to see how the emission rate changes as the fresh asphalt ages.

The greatest amount of semivolatile organic compounds escaped when the pavement was heated to 140°C, the temperature of road-paving, the team reports in Science Advances. Emissions fell as the asphalt cooled, but remained constant and significant at 60°C, a typical temperature for asphalt in Los Angeles during the summer, for the duration of the 3-day experiment. This suggests that asphalt could be a long-lasting source of pollution, Gentner says.

Sunshine was also important. Even moderate light caused a large increase in emissions regardless of temperature, although the reason is not clear. For road asphalt, emissions increased as much as 300% in moderate light. These emissions react to form tiny particles suspended in air, called aerosols, that are harmful when inhaled, and the results suggest asphalt contributes even more to this kind of air pollution in hot, sunny weather, Gentner says.  

The researchers estimated the annual emissions from new paving and roofing in parts of Southern California. They calculate that molecules released from asphalt could lead to between 1000 and 2500 tons of particulate air pollution—compared with just 900 to 1400 tons from gasoline and diesel vehicles. (Both sources pale in comparison to volatile chemical products, such as pesticides, coatings, adhesives, cleaning agents, and personal care products, which together contribute 4500 to 9500 tons of particulate pollution per year.)

It’s not necessarily the case that asphalt roads cause more total air pollution than cars, however. Gentner notes that vehicles also release harmful particles from combustion and exhaust gases that form ozone.

"This is really one of the first papers that makes a quantitative connection between these gases from asphalt and aerosol formation in urban air," says Joost de Gouw, an environmental chemist at the University of Colorado, Boulder, who was not involved with the work.

One remaining question is how long asphalt continues to release these molecules. Gentner says it will be important to keep measuring them, because the molecules are large compared to those in other products, such as solvents, and it takes longer for them to escape.

Even though the picture is incomplete, Robinson says it's already clear that asphalt belongs in official inventories of air pollution sources; these data sets are important for modelers who study air pollution and for updating air quality regulations. He's optimistic that companies will come up with ways to process and apply asphalt that would release fewer emissions.

De Gouw points out that other materials can be substituted for asphalt, including clay tiles for roofs and concrete slabs for roads. But even then there are environmental trade-offs: Producing concrete, for example, causes high emissions of greenhouse gases.