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Efforts to reduce emissions from cities have been focused on transportation and building energy consumption. However, it is likely that asphalt emissions are being underestimated or even ignored, according to a 2020 study published in the Science Advances journal. The global demand for liquid asphalt is 122.5 million metric tons per year, and 27 million metric tons correspond to the USA.
The study analyzed how temperature and sunlight influence asphalt emissions, and their role in the formation of secondary organic aerosols (SOA). In turn, these substances are precursors to ozone, a respiratory irritant at ground level; and fine particulate matter, which has been linked to several health conditions.
- Asphalt-based construction materials are very common in urban environments, but their impact is often missing from emission inventories.
- In large US cities, traditional sources of volatile organic compounds (VOC) are being dominated by volatile chemical products (VCP) and non-combustion sources.
Lack of Data and Studies: Main Challenge to Understand Asphalt Emissions
Asphalt emission rates have not been studied and quantified in detail, but the Occupational Safety and Health Administration (OSHA) has gathered evidence during occupational exposure studies. These measurements have identified high concentrations of volatile organic compounds, particulate matter, and other air pollutants. Emission factors have already been quantified for asphalt production, covering specific pollutants, but they don’t account for emissions after the manufacturing process.
Many compounds emitted by asphalt products are not present during their formulation, and the manufacturing process actually removes substances that could be released as emissions. However, this does not account for degradation by-products, formed with the exposure to high temperatures and solar radiation. According to the Science Advances study, asphalt products used in road construction and roofing release complex mixtures of hazardous pollutants, under the temperature and sunlight conditions that are typical during their life cycle.
In the case of road asphalt, the study found an emissions increase of 300% with moderate exposure to sunshine. In urban settings, the estimated emissions of SOA precursors from asphalt are higher than those produced by motor vehicles. These emissions are concentrated during the sunniest and hottest periods of the year.
Effect of Temperature and Sunshine on Asphalt Emissions
The study analyzed how temperature and sunlight affect asphalt emissions. The following results were found when studying the relationship between temperature and emissions:
- The emissions factor for primary road asphalt doubled from 40°C to 60°C, which is a typical temperature range during summer.
- From 60°C to 140°C, which is a typical range of temperatures during storage and application, asphalt emissions increase by 70% for every 20°C.
- Hazardous compounds like anthracene, naphthalene and pyrene were identified among the emissions released.
The effect of solar radiation on asphalt emissions was even more drastic, and even moderate sunlight resulted in a 300% increase of total emissions. The largest increase was observed for sulfur compounds, which were 700% higher. Oxygen compounds increased by 400%, while hydrocarbons increased by 300%. These results were obtained by comparing two identical samples of road asphalt, with and without artificial sunlight.
In both the temperature-based and the sunlight-based experiments, emissions declined over time. However, the decrease was much faster in the temperature experiment.
- After 18 hours of heating, the emission factor had decreased to 10% of its initial value in the temperature experiment.
- However, the emission factor after 18 hours was still 30% above the initial value in the sunlight experiment.
The study also analyzed the effect of solar exposure on liquid asphalt used in roofing and pavement sealing. In this case, solar irradiance caused a 260% increase in asphalt emissions. Three common products were considered: asphalt shingles, asphalt sealand, and liquid roofing asphalt. Hydrocarbons represented the largest share of total emissions for all three products:
- 85% in asphalt shingles
- 80% in asphalt sealant
- 78% in liquid roofing asphalt
Sulfur and oxygen compounds accounted for the remaining emissions. The study also found that solvents increased total emissions and the formation potential of secondary organic aerosols.
Emissions from motor vehicles and other combustion sources are being addressed by regulations, and the adoption of electric vehicles can help reduce emissions from transportation. However, this also means that emissions from construction materials and nontraditional sources will now represent a larger percentage.According to Architecture 2030, the world will add 2.5 trillion square feet of buildings by 2060, to accommodate the growth of urban population. This can be compared with building the equivalent to New York City each month. Even if emissions from traditional sources are controlled, asphalt and other construction materials will continue to be used in larger amounts.