​Nitrate Photolysis as Major Summer Daytime Sources of ambient HONO in Southeastern Coastal China

Nitrous acid (HONO) is a key precursor of hydroxyl radicals (OH) in the atmosphere, yet its daytime sources are not fully understood. The photolysis of particulate nitrate (pNO3⁻) has been recognized as a potential HONO source, but its contribution exhibits notable spatial variability and remains debated. Based on summertime field observations from the coastal city of Xiamen, China, this study combined a Random Forest (RF) model with a chemical box model to investigate the role of pNO3⁻ photolysis in HONO formation. Our results identified pNO3⁻ photolysis as the most important factor explaining the daytime unknown HONO sources, surpassing photo-enhanced heterogeneous conversion of NO2. This photolytic process was found to be primarily constrained by pNO3⁻ concentrations rather than by UV intensity. By integrating a chemical pathway parameterization scheme with the RF results, we derived localized kinetic parameters for multiple heterogeneous HONO formation pathways. Implementing these mechanisms in the box model significantly improved the simulation of observed HONO levels and their diurnal trends. Model sensitivity analyses indicated that pNO3⁻ photolysis generated HONO at a rate of 1.04 ppb h-1, accounting for 31.9% of daytime HONO formation. This process subsequently contributed 14.9% to OH production and 13.9% to HO2 radical production. This study highlights the critical role of pNO3⁻ photolysis in daytime HONO formation and advances our understanding of atmospheric HONO budgets in coastal environments.