Photolysis of iron-organic complexes regulates the HONO production from nitrate/organic acid/Fe(Ⅲ) mixtures: Roles of organic species, bulk acidity, and dissolved oxygen

Abstract. Despite extensive efforts to identify missing daytime HONO formation pathways, substantial discrepancies between observed and modeled HONO source strengths persist, highlighting the need to better constrain the environmental factors governing atmospheric HONO production. Here, we investigated Fe(Ⅲ)/Fe(Ⅱ) redox cycling and nitrate-to-N(Ⅲ) conversion in NaNO₃/organic acid/Fe(Ⅲ) mixtures, with emphasis on organic acid species, bulk-phase acidity, and dissolved oxygen availability. At pH 3.0, Fe(Ⅲ) increased HONO production rates (P_HONO) by 41.3 %, 126.2 %, and 78.2 % relative to the corresponding binary NaNO₃/succinic acid, NaNO₃/tartaric acid, and NaNO₃/citric acid systems, respectively, through Fe(Ⅱ) formation from Fe(Ⅲ)-carboxylate photolysis. In contrast, P_HONO decreased in the NaNO₃/oxalic acid/Fe(Ⅲ) and NaNO₃/malonic acid/Fe(Ⅲ) systems due to superoxide radical formation and available H⁺ consumption. At pH 1.6, P_HONO increased by approximately 1 order of magnitude in the NaNO₃/oxalic acid/Fe(Ⅲ) system but decreased by 88.4 % and 87.9 % in the NaNO₃/malonic acid/Fe(Ⅲ) and NaNO₃/succinic acid/Fe(Ⅲ) systems, respectively. Variations in dissolved oxygen availability further demonstrated that superoxide radical formation suppressed Fe(Ⅱ) accumulation, thereby modulating nitrate reduction and HONO production. These findings reveal that organic structure, bulk acidity, and dissolved oxygen jointly regulate Fe(Ⅲ)/Fe(Ⅱ) redox cycling and reactive nitrogen formation in multicomponent photochemical systems, providing a mechanistic basis for improving the representation of atmospheric HONO sources.