Asphalt is a near-ubiquitous substance—it’s found in roads, on roofs and in driveways—but its chemical emissions rarely figure into urban air quality management plans. A new study now finds that asphalt is a significant source of air pollutants in urban areas, especially on hot and sunny days.

Yale researchers found that common road and roofing asphalts produced complex mixtures of organic compounds, including hazardous pollutants, in a range of typical temperature and solar conditions. An open-access paper on their work is published in the journal Science Advances.

Asphalt’s life cycle and temperature-dependent emissions. (A) Different stages in asphalt’s life cycle with potential to emit reactive organic gases into the atmosphere. (B) Temperature dependence of total gas-phase emissions from asphalt, ranging from in-use (40 °C to 60 °C) and storage (80 °C to 140 °C) to paving and overheating (120 °C to 200c°C) temperature conditions (filled circles). The corresponding potential SOA that could be produced is also shown (hollow squares). The orange, green, and blue curves show the IVOC, SVOC, and aromatic fractions of the total emissions, respectively. The error bars indicate SD of the emission and SOA production factors, and the error bands indicate the SD in the volatility fractions. (C) Variation in the emission factors of select hazardous PAHs and total n-alkanes (C₁₀-C₃₂) with applied temperature. Khare et al.

Asphalt-based materials are abundant and a major nontraditional source of reactive organic compounds in urban areas, but their emissions are essentially absent from inventories. At typical temperature and solar conditions simulating different life cycle stages (i.e., storage, paving, and use), common road and roofing asphalts produced complex mixtures of organic compounds, including hazardous pollutants. Chemically speciated emission factors using high-resolution mass spectrometry reveal considerable oxygen and reduced sulfur content and the predominance of aromatic (~30%) and intermediate/semivolatile organic compounds (~85%), which together produce high overall secondary organic aerosol (SOA) yields.

Emissions rose markedly with moderate solar exposure (e.g., 300% for road asphalt) with greater SOA yields and sustained SOA production. On urban scales, annual estimates of asphalt-related SOA precursor emissions exceed those from motor vehicles and substantially increase existing estimates from noncombustion sources. Yet, their emissions and impacts will be concentrated during the hottest, sunniest periods with greater photochemical activity and SOA production.

—Khare et al.

Decades of research about and regulations of emissions from motor vehicles and other combustion-related sources have resulted in improved urban air quality. But recent studies show that as those efforts succeeded, numerous non-combustion-related sources have become important contributors of organic compounds. These can lead to secondary organic aerosol (SOA), a major contributor of PM_2.5—an important regulated air pollutant comprising particles smaller than 2.5 micrometers in diameter—that have significant effects on public health.

The researchers collected fresh asphalt and heated it to different temperatures.

A main finding is that asphalt-related products emit substantial and diverse mixtures of organic compounds into the air, with a strong dependence on temperature and other environmental conditions.

—Peeyush Khare, lead author of the study

After some time, the emissions at summer temperatures leveled out, but they persisted at a steady rate—suggesting there are long-term, continued emissions from asphalt in real-world conditions.

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.

—Dre Gentner, corresponding author

They also examined what happens when asphalt is exposed to moderate solar radiation and saw a significant jump in emissions—up to 300% for road asphalt—demonstrating that solar radiation, and not only temperature, can increase emissions.

Paved surfaces and roofs make up approximately 45% and 20% of surfaces in US cities, respectively. The researchers estimated the potential total emissions and formation of SOA in Los Angeles, a key city for urban air quality case studies.

Because of the types of compounds asphalt emits, its potential SOA formation is comparable to motor vehicle emissions in Los Angeles, the researchers said—implying that finding ways to make roads more environmentally friendly is as important as doing the same for cars and trucks.

Gentner noted, though, that the effect of asphalt emissions on ozone formation was minimal compared to that of motor vehicles and volatile chemicals in personal care and cleaning products—another key emerging source of reactive organic emissions that produces large quantities of SOA in urban areas.

Resources

• Peeyush Khare, Jo Machesky, Ricardo Soto, Megan He, Albert A. Presto, Drew R. Gentner (2020) “Asphalt-related emissions are a major missing nontraditional source of secondary organic aerosol precursors” Science Advances doi: 10.1126/sciadv.abb9785