Asphalt may be a significant, yet overlooked, source of air pollution in urban areas. In a new paper published on 2 September in Science Advances, scientists reveal that common road and roofing asphalts emit hazardous pollutants, especially, on hot sunny days (1).
A considerable proportion of air pollutant in cities is believed to come from sources other than fossil-fuel guzzling vehicles. Nonetheless, pollutants emitted by sources, such as roads and pavement, are rarely factored into urban air quality analyses.
Non-combustion-related sources of air pollution
Asphalt is a sticky, black, semi-solid form of petroleum, also known as bitumen, that is commonly used roads, pavement, and roofs. While emissions from asphalt during the manufacturing and laying down processes are well understood, less is known about long-term emissions from asphalt while it is simply just sitting there.
The asphalt industry claims that continuous emissions are minimal and that all potential emissions are released during the manufacturing process. However, the new study suggests that hazardous compounds continue to slowly diffuse, and emissions may significantly increase – by up to 300 per cent – on hot, sunny days.
According to the authors, asphalt roads are a significant source of air pollution not just after they are first laid down. Asphalt may continue to release a wide range of chemicals into the air long after.
“Asphalt-related products emit substantial and diverse mixtures of organic compounds into the air, with a strong dependence on temperature and other environmental conditions,” said lead author Peeyush Khare, a PhD candidate at Yale University.
Temperature and solar radiation increase asphalt emissions
The team of researchers from Yale University, Carnegie Mellon University, and the Max Planck Institute for Chemistry heated fresh asphalt to temperatures between 40 to 200 degrees Celcius and found that when the temperature increased from 40 to 60 degrees Celcius – levels often reached in summer – the total emissions doubled.
After a period of time, the emissions levelled out – but persisted, suggesting that in the real world, emissions from asphalt may continue long-term, especially during hot summer months. In addition, the authors found that solar radiation led to a significant jump in emissions.
Potential impact on health
The carbon-based chemicals that are released condense to form secondary organic aerosols, also known as particular matter, contributing to the number of PM2.5 (dangerous air pollutant particles smaller than 2.5 micrometres across) in the air. This type of air pollution is known to have significant impacts on public health (2).
Emissions from vehicles are decreasing in many parts of the world, however, the planet is warming due to climate change, therefore, the amount of air pollution from asphalt may increase. The authors say more research is still needed to need to determine how much pollution is emitted from asphalt emits over its lifespan.
(1) Peeyush Khare, Jo Machesky, Ricardo Soto, Megan He, Albert A. Presto, Drew R. Gentner. Asphalt-related emissions are a major missing nontraditional source of secondary organic aerosol precursors. Science Advances (2020) DOI: 10.1126/sciadv.abb9785
(2) Tuet, W.Y. et al. Inflammatory responses to secondary organic aerosols (SOA) generated from biogenic and anthropogenic precursors. Atmospheric Chemistry and Physics (2017). DOI: 10.5194/acp-17-11423-2017