It sounds hard to believe, but a new study in the journal Science Advances suggests that the roads themselves may actually release more harmful air pollutants than the cars that drive on them. The study authors reached this conclusion after analyzing the chemical compounds given off by asphalt under a range of different conditions, revealing that the substance releases high concentrations of certain particles, particularly on hot sunny days.
When we talk about air pollution, we’re often referring to PM2.5, which is the name given to tiny particles with a diameter of under 2.5 micrometers. Emissions such as volatile organic compounds (VOCs) and intermediate/semivolatile organic compounds (I/SVOCs) act as precursors to both PM2.5 and ozone, and are typically released by fuel combustion as well as numerous other chemical sources.
One such source is asphalt, which is most commonly used on road surfaces and in roofing. As such, it always exists in large quantities in major cities, leading the study authors to speculate that it may be a major contributor to air pollution in urban areas.
To investigate, they used a technique called high-resolution mass spectrometry to analyze the VOCs and I/SVOCs released by asphalt at different temperatures. Results showed that emissions increased by an order of magnitude when asphalt is heated to 140°C (284°F) – as it typically is when it is being applied onto new roads – in comparison to 60°C (140°F).
Furthermore, asphalt was found to release 300 times more of these compounds when exposed to solar radiation, even if temperatures were kept constant.
After determining the levels of air pollutants given off by asphalt, the researchers consulted emissions data from the Southern California Air Basin (SoCAB) in order to see how this compared with vehicle emissions.
“In total, the annual potential [air pollution] production from asphalt related sources in the SoCAB is greater than that of gasoline and diesel on-road motor vehicles combined,” write the authors.
This is especially likely to be true in hot, sunny climates, where high levels of exposure to solar radiation and elevated temperatures trigger asphalt to release more hazardous compounds into the air.
As the study authors point out, great efforts are currently being made to curb vehicle emissions, yet new roads continue to be built all the time. It may therefore be time to look for an alternative to asphalt if we want really want to improve air quality in urban environments.
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