Atmospheric Nitrous Acid (HONO) in Contrasting Environments

Nitrous acid (HONO) is a crucial precursor to hydroxyl radical (OH) in the atmosphere and significantly influences atmospheric photochemical processes. Despite extensive field observations, the quantitative understanding of HONO formation processes in varying environments remains elusive, particularly due to the divergent parameters used in different studies that have led to contradictory conclusions. This study measured HONO at four sites characterized by contrasting environments: an urban site in the Yellow River Delta, a mountain site in the North China Plain, and two coastal sites at varying distances from the sea in Qingdao. The HONO concentration at the urban site is the highest, with peaks occurring at night, consistent with previous observations in other urban areas, yet markedly different from the daytime HONO peaks observed at other three clean sites. Using an observation-based model incorporated with unified parametrizations, we identify the dominant HONO formation pathways in these sites. Results show that the model effectively reproduces HONO concentrations and diurnal variations at both urban and mountainous sites. In urban areas, HONO formation is primarily driven by the heterogeneous conversion of NO₂ and the photolysis of particulate nitrate, while in the polluted mountainous region, the photolysis of nitrate plays a more significant role, with vertical transport potentially contributing to morning HONO increases. However, the model could not replicate daytime HONO peaks at the two coastal sites, suggesting the presence of an unidentified or underestimated marine HONO source. At all four sites, HONO contributes significantly to atmospheric OH radicals and ozone production, and the missing marine source may affect evaluations of its influence in coastal regions. In summary, this study provides a comprehensive analysis of HONO sources in contrasting environments, underscoring the need for further observations, especially in clean marine/coastal atmospheres.