Asphalt emits more pollutants than expected


The urban environment is characterized by asphalt. (Image: ablokhin / iStock)

Asphalt is almost ubiquitous in our cities. As a study has now shown, this can have a significant impact on air quality. Accordingly, when exposed to sunlight, the asphalt releases significantly more volatile organic substances than previously thought. These vapors are not only caused by the heating of the material, but also by the UV radiation, as the experiments showed. The organic suspended matter formed from these emissions can promote the formation of ozone and fine dust and, in sunny areas, even exceed emissions from car traffic.

You can find it on streets, in driveways and also on roofs: asphalt. This dark material, which softens when heated, consists of rock rubble mixed with the bitumen produced from crude oil – a tough mass of hydrocarbon chains of different lengths. These are not volatile when cold, but can give off volatile organic substances when heated, for example when spreading the hot asphalt. It has long been known that such volatile hydrocarbons promote the formation of ground-level ozone and secondary organic aerosols (SOA). The latter form part of the ultra-fine dust, the particles of which have a maximum diameter of up to 2.5 micrometers. “Given the large area that asphalt makes up in urban environments, this substance represents an important potential source of precursors to secondary organic suspended matter,” stated Peeyush Khare of Yale University in New Haven and his colleagues.

The warmer the asphalt, the greater the emissions

However, little research has been done to date on the extent to which air pollutants are exhaled from asphalt under normal, everyday conditions on streets or asphalt-covered flat roofs. In order to create more clarity, Khare and his team have now investigated the question of how high the emissions of road asphalt are under various conditions typical for its use on roads and roofs. In laboratory tests, they exposed asphalt samples to temperatures of 40 to 200 degrees Celsius in special test chambers. These temperatures correspond to those that can occur when the asphalt is spread, but also when the road is exposed to intense sunlight. They analyzed the organic substances released in the process using various mass spectrometers and gas chromatographs. In a further experiment, the researchers tested whether the UV radiation from sunlight can cause volatile substances to evaporate from the asphalt, even when it is not heated.

The measurements showed that the higher the temperature of the asphalt, the higher the evaporation of volatile hydrocarbons. When the temperature rises from 40 to 60 degrees – as often happens on a sunny summer day, for example – emissions doubled. On average, under these everyday summer conditions, they were around one gram per kilogram of asphalt. If the temperatures continued to rise, the emissions rose by 70 percent for every 20 degree step. The substances emitted included mostly various straight-chain and branched hydrocarbons, but also polycyclic aromatic hydrocarbons (PAHs). “They accounted for around ten percent of emissions and also included substances known to be harmful to health such as naphthalene, pyrene and fluoranthene,” report Khare and his colleagues. The proportion of these PAHs also increased with temperature.

UV radiation also promotes evaporation

“One of our main findings is that asphalt products emit substantial and diverse mixtures of organic compounds into the air, depending heavily on temperature and other environmental conditions,” says Khare. The latter also includes UV radiation on the asphalt, as the experiments revealed: even UV radiation comparable to that of a sunny day was sufficient to increase the evaporation from the asphalt by 300 percent, as the measurements showed. After five hours of irradiation, the emission levels stabilized at around five milligrams per minute and kilogram of asphalt. The asphalt used to cover flat roofs emits more organic pollutants than the street asphalt, but streets have the greater area share, as the researchers explain.

For their study, they determined how high the proportion of volatile hydrocarbons from the asphalt in the Los Angeles area is compared to the organic pollutants from car traffic. The result: “The total asphalt-related emission rates in the Southern California basin exceed those of traditional fossil fuel-based sources such as gasoline and diesel vehicles,” report Khare and his team. Especially for the formation of ultra-fine dust in cities, the asphalt could therefore play a much greater role than previously assumed.

Source: Peeyush Khare (Yale University, New Haven) et al., Science Advances, doi: 10.1126 / sciadv.abb9785

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