Although asphalt (also known as bitumen) is a naturally occurring substance found in deposits within the earth, it is also a by-product of petroleum processing. Almost all the bitumen used for asphalt pavement and roofs comes from petroleum refining and consists of hydrocarbon compounds that remain at the upper end (600°C or 1100°F) of the vacuum-distillation process. Because asphalt cannot be vacuum distilled, it has long been assumed that installed asphalt pavement and roofs do not emit hydrocarbon pollutants. Yet, according to a new report in the journal Science Advances (September 2020), asphalt pavements and roofs do, in fact, emit significant quantities of hydrocarbons, including polycyclic aromatic hydrocarbons (PAHs), particularly in environmental conditions found on hot and sunny days. This is problematic, because some PAHs are mutagens, teratogens, carcinogens, or endocrine disruptors and therefore harmful to human health and the environment. Hydrocarbons may also act as precursors to air pollutants, because hydrocarbons can react with nitrogen oxides (NOx) and sunlight to form ozone, a major constituent of smog. See also: Air pollution; Aromatic hydrocarbon; Asphalt and asphaltite; Environmental toxicology; Nitrogen oxides; Ozone; Pavement; Petroleum; Petroleum processing and refining; Petroleum products; Roof construction; Smog
To study emissions from asphalt, researchers exposed samples of asphalt pavement and roofing materials in an environmental chamber to temperatures that these materials would be expected to experience on hot summer days, and to light at wavelengths and intensities corresponding to summer sunlight. On heat exposure, both asphalt pavement and roof materials emitted hydrocarbon pollutants. In addition, upon exposure to light, hydrocarbon emissions increased by 300 percent. According to the researchers, light-induced enhancement of hydrocarbon emissions is best explained as the photochemical breakdown of bitumen into smaller emittable compounds. See also: Environmental test; Heat; Light; Photochemistry; Solar radiation
The researchers acknowledged that emission of air pollutants from pavement and roofs is potentially problematic for large cities and urban areas, where such products are widely used. In the short term, there are no alternative pavements that can replace asphalt. Alternatives to asphalts roofs are available to those who can afford them. But asphalt roofs, comprising only 20 percent of roofs in cities, are the smaller fraction of the problem. For now, emissions from asphalt products can be understood as another part of the whole of Earth’s air pollutants.