The asphalt used on roads can under certain conditions produce more hazardous pollution than the cars that drive on it, according to research carried out by scientists from Yale and Carnegie Mellon universities.
Asphalt is a petroleum-based component of coverings used for roads and building roofs, and is an abundant source of organic compounds, particularly in urban areas. And while the study looks particularly at large American cities, the findings will be of interest worldwide.
The problem is the creation of what are known as secondary organic aerosols (SOAs) which account for a large proportion of the aerosols – suspension of fine solid or liquid particles in air – in the lower part of the Earth’s atmosphere, including air pollutants.
In the case of asphalt, the production of SOAs is strongly affected by sunlight – up to a 300% increase in the case of road asphalt in even moderate sunlight. But the emissions from the roads themselves rarely are included in reporting of pollution levels.
Nevertheless the asphalt emissions of SOAs in an urban setting can exceed the emissions from motor vehicles, and add substantially to total pollution load. According to previous research, urban areas consist of over 45% paved areas and 20% roofs.
When subjected to temperatures between 40° and 60° – typical temperatures for the road surface in use in summer – emissions doubled. When subjected to the temperatures reached during application – during construction or resurfacing work – emissions increased by 70% for each increase of 20° to 140°.