Heterogeneous Photochemical Reaction between H ₂ O ₂ and NO as a Potential Source of Daytime HONO in the Atmosphere

H 2 O 2 is a vital atmospheric oxidant, while its photolysis in the troposphere is negligible in conventional view. For the first time, we find that gas-phase H 2 O 2 can undergo heterogeneous photolysis to produce OH radicals on the surfaces of particles such as SiO 2, Fe 2 O 3, and Na 2 SO 4 under solar irradiation, which further convert NO to produce HONO and NO 2 . It is demonstrated that acidity can inhibit the formation of HONO and promote the formation of NO 2 significantly. Moreover, the heterogeneous photolysis of H 2 O 2 is dependent on the reactivity of particles, with inert surfaces being more conducive to the generation of OH radicals. A MCM box model indicates that the HONO formation rate via the H 2 O 2 heterogeneous reaction can reach 0.10 ppb·h –1 at 9 a.m., and the relative contribution to the total daytime HONO production can account for approximately 5% at 1 p.m. Given the ubiquitous presence of SiO 2 in mineral dust particles, glass curtain walls, and terrestrial soils, these findings indicate that heterogeneous photolysis of H 2 O 2 on the particle surface could be a potential source of daytime HONO. Our research also underscores the importance of atmospheric interfacial chemistry and its contribution to unknown sources of atmospheric oxidizing capacity.