Atmospheric Nitrous Acid in Contrasting Environments in North China

Nitrous acid (HONO) is a crucial precursor to hydroxyl radicals (OH) and plays significant roles in tropospheric photochemistry and atmospheric oxidation capacity. However, the quantitative understanding of HONO formation across diverse environments remains limited, especially in the marine atmosphere. In this study, we quantified HONO formation in contrasting environments: an urban site in the Yellow River Delta, a mountain site in North China, and two coastal sites in Qingdao with different distances from the sea using an updated observation-based model. The model reproduced HONO levels at the urban and mountain sites, showing that urban HONO formation was driven by nitrate photolysis (45%) and heterogeneous NO₂ conversion on the ground (22%), while nitrate photolysis (84%) dominated at the mountain site. In contrast, the model failed to reproduce daytime HONO levels at the two coastal sites, suggesting the presence of an unidentified marine HONO source with a peak production rate exceeding 0.4 ppbv h^-1 that was correlated to tidal change. At all four sites, HONO contributed significantly to daytime OH production (31-57%), with contributions of ∼50% at two coastal sites. Neglecting HONO chemistry leads to substantial underestimations of radical concentrations and ozone production, particularly at the coastal site where the marine-derived HONO source could contribute to ∼15% of net ozone production. This study provides a comprehensive analysis of HONO sources in contrasting environments and highlights critical knowledge gaps in marine atmospheric HONO chemistry.