Surface nitrate photochemistry explains the long-standing missing source of nitrous acid

Abstract. Atmospheric nitrous acid (HONO) is the primary precursor to the hydroxyl radical (OH), yet state-of-the-art models systematically underestimate its ambient concentrations, pointing to missing sources. Laboratory experiments have implicated surface nitrate (sNO₃) within grime, a ubiquitous film on urban and natural surfaces, as a photochemical HONO source, but its atmospheric significance remains unconstrained. Here we develop a process-based kinetic parameterization for sNO₃ photolysis, constrained by controlled experiments and HONO flux measurements. We find that the HONO production rate is governed by the interplay of grime loading, grime chemical composition, sNO₃ photolysis frequency, and the branching ratio of HONO formation. After integrating this mechanism into the GEOS-Chem, the results show that sNO₃ photolysis produces HONO at rates of 20–60 pptv h^-1, explaining up to 85 % of the model-measurement mismatch. Our results identify sNO₃ photolysis as a considerable contributor in heavily polluted regions but a dominant source of HONO in clean environments, substantially affecting the atmospheric oxidative capacity across different chemical regimes.