Significant but Overlooked: Atmospheric HONO Formation from Surface Ammonium Oxidation with Superoxide Radicals

Resolving the sources of HONO formation is an indispensable aspect in understanding the enhancement of atmospheric oxidation. However, the contributing sources of high HONO formation rate remain unclear during humid haze episodes. The photochemical conversion of surface nitrate (NO₃ ^-), considered as the dominant contributor to the daytime HONO generation, exhibits severe constraint under high relative humidity (RH) conditions. Unexpectedly, ammonium (NH₄ ⁺) on the surface of photoactive mineral dust shows a gradual acceleration of HONO generation with increasing RH under simulated solar irradiation, especially at high RH. This reversed observation stems from a change in the photochemical pathway for the HONO formation from NO₃ ^- and NH₄ ⁺. The photochemical conversion of surface NO₃ ^- is determined by photogenerated electrons (NO₃ ^-→NO₂→NO₂ ^-→HONO), while the superoxide radical (∙O₂ ^-) generated during photochemical reaction drives the surface NH₄ ⁺ to directly form HONO with the pathway (NH₄ ⁺∙+∙O₂ ^-→NO₂ ^- + H₂O→HONO). Under high RH conditions, oxygen molecules (O₂) have greatly better access to photogenerated electrons than NO₂, resulting in an interruption of the procedure from NO₂ to NO₂ ^- during NO₃ ^- conversion. Therefore, the favorably generated ∙O₂ ^- fuels the photochemical conversion of surface NH₄ ⁺ while inhibiting the conversion of NO₃ ^- to diurnal HONO formation. This work highlights the overlooked contribution of HONO formation from NH₄ ⁺, especially under high RH conditions, and advances the understanding of a renewed role for O₂ in atmospheric chemical processes.