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As it heats up, asphalt releases chemical compounds that contribute to air pollution. And its emissions double as its temperature increases from 104 to 140 degrees Fahrenheit, researchers found.
Sunlight plays a key role in these asphalt emissions, with even moderate levels of sunshine tripling the release of air pollutants, according to the study published Sept. 2 in the journal Science Advances.
The problem is likely to only grow worse as global warming increases, said lead researcher Drew Gentner, an associate professor of chemical and environmental engineering at the Yale School of Engineering and Applied Science, in New Haven, Conn.
"Megacities are likely to see urban temperature increases driven by climate change and urban heat island effects, thus enhancing [asphalt's] relative impact on urban air quality over time," Gentner said. Urban heat islands are metropolitan areas that are a lot warmer than their rural surroundings because of human activity.
The United States typically goes through 27 million metric tons of petroleum-based liquid asphalt each year in paving, roofing and construction, the researchers said in background notes.
Paved surfaces and rooftops constitute a majority of the exposed surface of urban areas, they noted. At least 45% of urban areas are paved, and another 20% are covered by roofs.
In-use pavement usually gets as hot as between 117 and 153 degrees Fahrenheit in the summer, while roofs can reach 167 degrees, the study authors said.
As the major contributors to air pollution get cut back -- for example, through cleaner vehicle emissions -- passive pollution sources like these will have a growing influence on the air we breathe, said Peter DeCarlo, an associate professor and air pollution expert with Johns Hopkins University, in Baltimore.
"In doing that reduction, we are discovering these new sources that are now playing a more prominent role in our air pollution issues," DeCarlo said.
For the new study, Gentner and his colleagues gathered asphalt samples during road paving in New Haven and Pittsburgh, and from roofing products like asphalt shingles and sealant.
In the lab, the researchers exposed the samples to heat and sunlight in sealed chambers, and monitored the emissions coming off the asphalt.
The asphalt samples tended to produce more volatile organic compounds as they got hot, especially when exposed to sunlight, the investigators found.
Emissions doubled when the asphalt surface temperature increased from 104 to 140 degrees Fahrenheit -- temperatures that "occur in hot summertime conditions in many locations," Gentner said. They then climbed by an average of 70% for every additional increase of 68 degrees Fahrenheit, the findings showed.
"Temperature and solar radiation are key environmental factors that influence emissions, since they influence the evaporation and emission pathways related to solar exposure," Gentner said.
On their own, volatile organic compounds are harmful to humans and potentially carcinogenic, according to the U.S. National Institutes of Health.
They also contribute to urban smog, which forms when sunlight bakes volatile organic compounds combined with nitrogen oxides, DeCarlo explained.
"Summertime is when we have our biggest air pollution problems in urban areas," DeCarlo said. "That's when we have high ozone, it's when we have the highest particulate matter, so this as a source of particulate matter is just another potential problem."
Asphalt probably contributes most to air pollution when it's freshly laid, DeCarlo added.
During the paving process, asphalt is heated to as much as 248 to 320 degrees Fahrenheit, the researchers said.
"If you've ever been around people laying asphalt, you smell it. It's clear something is getting into the air when that happens," DeCarlo said. But asphalt likely continues to emit air pollutants even after it's aged, when sunlight bakes the material, he noted.
Switching to concrete for paving would help reduce emissions, he said, but concrete is not an ideal paving material in all locales.
Another possible solution might be the application of "cool pavement" technology, where colored sealants are applied to paved surfaces so they reflect more solar energy and become less likely to heat up, Gentner said.
Emissions might also vary with different asphalt application methods and different formulations of the paving product, Gentner suggested.
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