Asphalt, also known as bitumen, is a major source of air pollution, especially in sunny and hot places. For one kind of harmful particulate pollution, asphalt emissions from roads and roofs may be a bigger problem than emissions from all petrol and diesel-powered vehicles.
Peeyush Khare at Yale University and his colleagues placed samples of asphalt into an enclosed furnace so they could study their emissions in detail. They subjected them to temperatures ranging from 40°C to 200°C. Total emissions doubled when the temperature rose from 40°C to 60°C, which are typical temperatures for asphalt on a Californian summer day.
The pollutants released were all carbon-based chemicals, often with 12 to 25 carbon atoms per molecule.
“Many of these compounds are conducive to condensing to form secondary organic aerosol after reacting in the atmosphere,” says co-author Drew Gentner, also of Yale University. This can, in turn, go on to form tiny particles called PM2.5, which are one of the most dangerous types of air pollution for human health.
Emissions could also be triggered by sunlight shining on the asphalt. Under controlled conditions, this led to a 300 per cent increase in emissions.
Pollution from vehicles is declining in many places, as petrol and diesel vehicles are replaced with electric ones.
But pollution from asphalt could actually increase, the researchers argue. That is because climate change is causing higher temperatures, which will trigger more emissions from asphalt. “Megacities are likely to see urban temperature increases driven by climate change and urban heat island effects,” says Gentner.
“We are not making policy recommendations,” says Gentner, emphasising that they need to understand how much pollution the asphalt emits over its lifetime and how it interacts with other sources of pollution. However, he points to ongoing research into “cool pavement coatings”, which are being studied as a way to reduce the excess heat in built-up areas. Such coatings might also reduce emissions by cooling the asphalt surface, he says.
Science Advances DOI: 10.1126/sciadv.abb9785