Cars and motorbikes are two major contributors to air pollution. But the roads they run on are equally responsible, especially in hot summer days. As per a study, asphalt used in road construction is becoming a major concern for its long-term effects on the environment.
Most of the modern roads, airport runways, racing tracks and roofing are constructed using asphalt or bitumen, a solid form of petroleum byproducts. Approximately, 122.5 million metric tons of asphalt is used in a year. But over a long period of time, the black, sticky and semi-solid substance releases "complex mixtures of organic compounds, including hazardous pollutants."
Heat Accelerates Asphalt Emission
A team from Yale University conducted the study in California where over the years air pollution has decreased with the use of modern methods. But the level is still a concern. When researchers looked at known pollutants — cars, paints, cleaning products among others — they were puzzled as the numbers didn't tally. Then they looked at another pollutant asphalt.
The team, led by Drew Gentner, then put it to a test. They looked at two types of commonly used asphalt — one for roads and another for roofing — and heated them in a laboratory. The study which was published in the journal Science Advances on September 2, found that when the asphalt was heated at 140° Celsius, the emission of toxic chemicals was at its highest.
As it comes in contact with heat, asphalt melts and the softening of the substance speeds up emissions by up to 300 percent. While it cooled, the pollution dropped but remained significant at 60° Celsius which is also the temperature of the road during summer.
Long Term Impact
The emission then reacts to the tiny particles in the air called aerosols and forms "secondary organic aerosol" that are smaller than 2.5 micrometers in diameter. It indicates a long-term effect on the environment. The researchers calculated that between 1,000 and 2,500 tons of such molecules are released by asphalt every year. Car and motorbike emission, on the other hand, contributes to somewhere between 900 and 1,400. Together, both add a significant number of pollutants.
"To explain these observations, we calculated the expected rate of steady emissions and it showed that the rate of continued emissions was determined by the time it takes for compounds to diffuse through the highly viscous asphalt mixture," said Gentner, Yale's associate professor of chemical and environmental engineering, adding that it was important to keep measuring the emission from the asphalt as the molecules released were large and took longer to escape.
Joost de Gouw, an environmental chemist at the University of Colorado told Sciencemag that the study would help use substitute materials such as concrete and clay tiles in the future. In many countries, concrete, recycled materials and industrial wastes are being used to construct roads as they are more durable than asphalt while also moderately environmental-friendly.