Nitrous Acid Budgets in Coastal Atmosphere: Insights into the Absence of a Daytime Marine Source

Abstract. Nitrous acid (HONO), a vital precursor of atmospheric hydroxyl radicals (OH), has been extensively investigated to understand its characteristics and formation mechanisms. However, discerning fundamental mechanisms across diverse environments remains challenging. This study utilizes measurements from Mount Lao, a coastal mountain in eastern China, and an observation-based chemical box model to examine HONO budgets and their subsequent impacts on atmospheric oxidizing capacity. The model incorporates additional HONO sources, including direct emissions, heterogeneous conversions of NO₂ on aerosol and ground surfaces, and particulate nitrate photolysis. The observed mean HONO concentration was 0.46 ± 0.37 ppbv. The updated model well reproduced daytime HONO concentrations during dust and photochemical pollution events. During dust events, daytime HONO formation was dominated by photo-enhanced heterogeneous reactions of NO₂ on aerosol surfaces (>50 %), whereas particulate nitrate photolysis (34 %) prevails during photochemical pollution events. Nevertheless, the model uncovers a significant unidentified marine HONO source in the “sea case”, with its HONO production rate reaching up to 0.70 ppbv h^–1 at noon. Without considering this unidentified source, an extraordinarily high photolysis coefficient of nitrate and/or heterogeneous uptake coefficient of NO₂ would be required to match observed HONO concentrations. This missing HONO source affected the peak O₃ production rate and OH radical concentration by 36 % and 28 %, respectively. Given the limited HONO observations data in coastal and marine settings, the unidentified HONO source may cause an underestimation of the atmosphere’s oxidizing capacity. This study highlights the necessity for further investigation of the role of HONO in atmospheric chemistry in coastal and marine environments.