Heterogeneous Photochemical Conversion of NO₂ to HONO on the Humic Acid Surface under Simulated Sunlight

The poor understanding of HONO sources in the daytime highlights the importance of the heterogeneous photochemical reaction of NO₂ with aerosol or soil surfaces. The conversion of NO₂ to HONO on humic acid (HA) under simulated sunlight was investigated using a flow tube reactor at ambient pressure. The uptake coefficient (γ) of NO₂ linearly increased with irradiation intensity and HA mass in the range of 0–2.0 μg/cm², while it decreased with the NO₂ concentration. The HONO yield was found to be independent of irradiation intensity, HA mass, and NO₂ concentration. The temperature (278–308 K) had little influence on both γ and HONO yield. Additionally, γ increased continuously with relative humidity (RH, 7–70%), and a maximum HONO yield was observed at 40% RH. The heterogeneous photochemical reaction of NO₂ with HA was explained by the Langmuir–Hinshelwood mechanism.