Nighttime N₂O₅ Chemistry in Fog-Influenced Urban Environments: Implications for NOₓ Removal and Aerosol Nitrate Formation

Dinitrogen pentoxide (N₂O₅) plays a central role in nighttime nitrogen oxide chemistry, acting as both a temporary NOx reservoir and an efficient pathway for permanent NOₓ removal through heterogeneous uptake on to aerosols. The nitrate radical (NO₃) serves as a key intermediate linking ozone and nitrogen dioxide to N₂O₅ formation and exists only under nighttime conditions, making it a critical component of nocturnal oxidation chemistry. In this study, we examine nighttime NO₃ and N₂O₅ behavior at two urban sites in Taiwan with contrasting fog frequencies, using surface observations of trace gases and meteorology together with satellite-based identification of fog and low cloud conditions. A box modeling framework was employed to estimate the nighttime evolution of NO₃ and N₂O₅ concentrations across a range of fog-likely and fog-free conditions. By comparing the nocturnal N₂O₅ chemistry across these conditions, we assess how the presence of liquid water modifies the partitioning of reactive nitrogen and the efficiency of nocturnal NOₓ loss. The analysis focuses on differences in inferred N₂O₅ abundance and persistence, with particular attention to conditions favorable for enhanced nitrate production. Model results indicate that efficient N₂O₅ loss to fog droplets shifts the NO₃ / NO₂ / N₂O₅ equilibrium, reinforcing N₂O₅ formation and accelerating reactive nitrogen removal from the gas phase. These findings underscore the importance of fog in regulating nighttime NOₓ chemistry and highlight N₂O₅ as a major pathway linking urban emissions to secondary aerosol nitrate formation.