Asphalt causes air pollution on hot days, study finds
This unsuspecting source of chemical emissions has urgent implications for public health and will be exacerbated as the number of hot days increases each year.
According to a new study, it seems that asphalt, a material that blankets roads, roofs and driveways in many urban areas, is a significant source of air pollution, particularly on hot and sunny days.
Researchers from Yale University state that scientists have long known that asphalt is a source of air pollution, but it is often left out of urban air quality management plans. The volume of pollutants that are released by the material varies based on sunlight exposure and temperature, so the researchers investigated the conditions that exacerbated asphalt’s polluting capabilities.
Road asphalt samples were heated to temperatures ranging from 40-200°C and were exposed to different levels of solar radiation. Their trials revealed that the material released a “complex mixture of organic compounds that span a wide volatility range” including those that are considered to be “hazardous pollutants.”
Emissions from road asphalt doubled from 40°C to 60°C, which is considered to be a typical temperature that roads experience during the summer months when there are long periods of sunshine.
Once the researchers began exposing the asphalt samples to temperatures of 60°C and above, the emissions increased by 70 per cent on average per 20°C increase up to 140°C (the temperature that is reached when the asphalt is being paved).
Credit: Sammya Nig Ltd/ Wikimedia Commons (CC BY-SA 4.0)
The researchers note that the biggest jumps in emitted pollutants were larger with lower temperatures (40°C to 140°C) than at higher temperatures (e.g., 140°C to 200°C), which they say could be due to asphalt’s internal composition.
The trials also revealed that the asphalt samples heated to 60°C had a more steady release of emissions, whereas those at 140°C quickly decreased. The researchers say that this exhibits asphalt’s potential to be a “long-lasting source of emissions in real-world environmental conditions beyond the 3 days tested [in the study].”
Sunlight also has a profound effect on the volume of air pollution released by asphalt. Total emissions from this material rose by nearly 300 per cent after an 18 hour period of constant solar exposure, with sulphur-containing compounds showing the greatest increase of 700 per cent.
“The total emissions measured at the 18th hour were still 30 per cent higher than the first sample collected at the 0th hour without artificial sunlight, whereas in the temperature-only experiments, they had reduced to ~10 per cent of the initial emission factor after 18 hours of heating,” the researchers state.
Between 1948 and 2016, the average annual temperature has increased by 1.7ºC across Canada and 2.3ºC in northern regions of the country, as stated by Canada’s Changing Climate Report released in 2019. This report also stated that regions that currently experience hot days (defined as days with maximum temperature above 30ºC) could see 50 additional hot days by 2100 if our greenhouse gas emissions continue on business-as-usual.
The type and volume of air pollution released from asphalt vary based on the temperature and sunlight and the study highlights the potential for significant public health implications. Increasing rates of urbanization in North America and around the world are resulting in more green spaces being converted into asphalt surfaces, which could become a growing source of emissions as the climate warms and creates more hot days with each passing year.
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