VOC emissions from asphalt: Laboratory oxidation, ultrafine particle formation, and urban air quality implications

Asphalt-surfaced areas are increasingly recognized as non-combustion sources of non-methane volatile organic compounds (NMVOCs), yet their oxidative fate and particle-forming potential remain poorly constrained. We performed controlled chamber experiments (313 K) on a representative asphalt VOC mixture to compare daytime hydroxyl radical (OH) oxidation with nighttime nitrate radical (NO₃) oxidation under contrasting relative humidity (RH = 20% vs 80%). OH produced broad, non-selective depletion across the VOC spectrum, whereas NO₃ exhibited strong selectivity, rapidly consuming phenolics (catechol) and related oxygenated aromatics. Both oxidants formed abundant ultrafine particles (UFPs < 100 nm), with PM₁ yields of 11-14% (OH) and 16-41% (NO₃) with respect to total VOCs consumption observed in particle analysis, and disproportionate particle formation from low-volatility precursors, such as catechol. Higher RH delayed nucleation and suppressed selected gas-phase product yields, indicating humidity-dependent oxidation pathways and gas-particle partitioning. These results establish strong day-night contrasts in asphalt emission reactivity and identify low-volatility binder constituents, particularly phenolic oxygenates, as key particle precursors. Our results also signify that low volatility polar compounds strongly partition to the particle phase, and their elimination from asphalt binders would diminish secondary pollution formation, from asphalt pavements. Incorporating asphalt evaporation into urban emission inventories and reformulating binders to minimize such reactive species could reduce exposure to hazardous ultrafine particles and improve urban air quality.