Investigation of the Formation and Photolysis Characteristics of HONO and their Effect on O₃ Formation in the Daejeon and Seoul Atmosphere during Winter

Nitrous acid (HONO) can produce hydroxyl radical (OH radical) via photolysis during daytime. These OH radicals oxidize volatile organic compounds (VOCs), contributing to the formation of ozone (O₃) and secondary aerosols. In order to investigate the characteristics of HONO in the urban air, HONO and related gaseous compounds were measured in Daejeon, Korea during the winter 2022, and Seoul, Korea during the winter 2020. During the entire measurement period, HONO concentrations were measured to be 0.7±0.6 ppbv and 1.7±1.3 ppbv in Daejeon and Seoul, respectively. HONO showed a very good correlation with nitrogen dioxide (NO₂) with a R² of 0.78 in Daejeon whereas it showed good correlations with not only NO₂ but also Relative humidity (RH) with R² of 0.8 and 0.62, respectively in Seoul. These results implied that NO₂ is important for the nighttime HONO formation both Daejeon and Seoul during winter while RH is also important in Seoul. It was found that HONO concentrations started to decrease rapidly through photolysis as ultraviolet-A (UV-A) (320~400 nm) intensity increased. The satellite observations showed that high aerosol surface area over the Korean Peninsula during the study period was mainly caused by inflow of the long-range transported haze from the Asian continent. It was found that both high cloud cover and high aerosol surface area decreased UV-A intensity, resulting in less photolysis of HONO. Finally, O₃ was not actively produced due to the decreased UV-A intensity and less photolysis of HONO in Daejeon and Seoul during winter.