On Hot Days, Asphalt Can Release as Much Air Pollution as Cars

During heat waves, pavement can reach 140 degrees Fahrenheit, which unleashes harmful particles into the air

A digital Calvary Church sign in Woodland Hills displays the temperature at 3:18 p.m. on 8/18/2020 as 116 degrees Fahrenheit
The total emissions released increased as temperatures rose, doubling when temperatures went from 104 degrees to 140 degrees Fahrenheit Al Seib / Los Angeles Times

Extreme rising temperatures are known to increase the risk of environmental hazards like drought and wildfires. But new research published last week in Science Advances adds another danger to the list: extreme heat sends harmful emissions into the air courtesy of hot asphalt.

The study, published by a team of Yale researchers, reveals that asphalt is likely an overlooked but major source of hazardous pollutants being released into the air.

“A main finding is that asphalt-related products emit substantial and diverse mixtures of organic compounds into the air, with a strong dependence on temperature and other environmental conditions,” says Peeyush Khare, a Yale chemical and environmental engineer and the lead author of the study, in a statement.

To discover this hazard, researchers placed real-world samples of fresh road asphalt in a controlled furnace and heated the samples to temperatures from 104 to 392 degrees Fahrenheit. The total emissions released increased as temperatures rose, doubling when temperatures went from 104 degrees to 140 degrees Fahrenheit, reports Michael Marshall for New Scientist.

When released, emissions from asphalt react and create a type of aerosol that can transform into air pollutant particles known as PM2.5 that can be harmful when inhaled, explains Science magazine’s Erik Stokstad.

“This is really one of the first papers that makes a quantitative connection between these gases from asphalt and aerosol formation in urban air,” Joost de Guow, an environmental chemist at the University of Colorado, Boulder, tells Science magazine.

On a typical summer day in Los Angeles, asphalt can reach to 140 degrees Fahrenheit. At this threshold, the asphalt will release a steady and significant stream of emissions. Under these conditions, scientists predict hot asphalt could become a long-lasting source of pollution.

When the samples were exposed to UVA and UVB wavelengths over a period of 18 hours, scientists found that total emissions increased by nearly 300 percent.

“That’s important from the perspective of air quality, especially in hot, sunny summertime conditions,” Khare says in a statement.

Researchers estimated that in California’s South Coast Air Basin, the total amount of aerosols formed from hot asphalt emissions is comparable to those released by gas and diesel motor vehicles, reports Emma Newburger of CNBC.

But asphalt may actually contribute more emissions than gas-guzzling cars in some places. In southern California, vehicles contribute 900 to 1,400 tons of air pollution, while new paving and roofing in the region releases between 1,000 and 2,500 tons, according to Science.

In most cities, paved roads make up 45 percent and roofing make up 20 percent of the built environment. As cities expand and days get hotter, asphalt emissions could become an even greater fraction of pollutants being released into the air, according to the study.

In 2019, Kendra Pierre-Louis of the New York Times reported that longer heat waves are becoming more common, with the average number of heat waves since the 1960s tripling in 50 major American cities, she wrote.

“While emissions from some other sources might decrease in the future, the current consumption of asphalt materials and their emissions may remain similar or increase with elevated summertime urban temperatures driven by climate change and urban heat island effects, thus affecting their relative impact on urban air quality over time,” Drew Gentner, an environmental engineer at Yale and co-author of the study, tells CNBC.

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