Assessment of Daytime HONO Emission Source from Asphalt Surface to Urban Air

Abstract

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.