Assessment of daytime hono emission source from asphalt surface to urban air

Existing studies suggest various potential daytime sources of atmospheric nitrous acid (HONO), including photolysis surface reactions and photo-enhanced NO₂ conversion on organic surfaces. However, the understanding of daytime HONO sources is still inadequate. In this study, we report the HONO formation on asphalt surfaces under various NO₂, VOCs (toluene and hexane), and UV irradiance conditions using a continuous flow chamber. Although no HONO formation was found without light exposure, the light threshold for HONO formation on the asphalt surface was very low, with a total UV (TUV) of 0.7 W m⁻² . HONO formation on the asphalt surface was linearly dependent on NO₂ up to 300 ppb in the presence of VOCs, but no HONO formation was observed with humified air and NO₂ . HONO production was saturated at high hydrocarbon concentrations and light intensities. The calculated first-order NO₂ conversion rate to HONO on the asphalt surface was 1.2 × 10⁻⁴ s⁻¹ . The observed mean HONO emission flux was 1.3 × 10⁹ molecules cm⁻² s⁻¹ with a similar range of those on other urban covered surfaces. The calculated vertical HONO profile using the measured HONO emission flux and 1-D steady state model revealed that the asphalt surface may account for 13% of daytime HONO in the elevated on-road pollutant concentrations in Seoul. However, we show that its HONO contribution could be much higher on real-life road surfaces directly exposed to much higher NO₂ emissions from vehicle exhaust.