Observation and Simulation of Vertically Resolved Nitrous Acid (HONO) in Autumn over Urban Beijing, China

As a key precursor of hydroxyl (OH) radicals, the budget of nitrous acid (HONO) at different altitudes has received extensive attention. In this study, vertically resolved observations of HONO, NO₂, O₃, and HCHO were conducted during an autumn field campaign in Beijing in 2019. The significant correlation between HONO and NO₂, along with variations in their ratios across different altitudes, underscores the importance of aerosol surface chemistry in HONO formation and its altitude-dependent behavior. To enhance the model performance, the heterogeneous conversion of NO₂ and its photochemical enhancement are incorporated into the 1D model. The simulations reveal that the nocturnal HONO production is dominated by the heterogeneous conversion of NO₂ both at the surface and aloft. During the daytime, ground-surface sources of HONO are mainly driven by nitrate photolysis and light-enhanced heterogeneous conversion of NO₂. Meanwhile, a large portion of the HONO generated at the surface is transported upwards through vertical mixing. In the higher atmosphere, HONO originates from vertical transport and in situ processes. As precursors of OH radicals, the observed concentrations of HONO, O₃, and HCHO exhibit distinct diurnal variations and vertical distribution patterns. HONO contributes to OH radical production predominantly during the early morning across all layers and it even becomes the main contributor throughout the daytime in the lowest layer near the ground, while O₃ and HCHO become more prominent towards midday, especially in the higher layers. These results will be beneficial for a deeper understanding of the atmospheric oxidation process within the urban boundary layer.