Formic Acid-Intensified Photoreduction of NOx on Iron Minerals Triggers Daytime HONO Formation through Active Hydrogen

Nitrous acid (HONO) is crucial in atmospheric chemistry as a precursor to morning peak hydroxyl radicals and significantly affects urban air quality by forming secondary pollutants, yet the mechanisms of its daytime formation is not fully understood. This study investigates the role of formic acid (HCOOH), a prevalent electron and proton donor, in the transformation of nitrogen oxides (NOx) and the formation of HONO on photoactive mineral dust. Exploiting hematite (Fe₂O₃) as an environmental indicator, we demonstrate that HCOOH significantly promotes the photoreduction of NO₂ to HONO, while suppressing nitrate accumulation. This occurs through the formation of a surface ≡Fe-OOCH complex, where sunlight activates the C-H bond to generate and transfer active hydrogen, directly converting NO₂ to HONO. Additionally, HCOOH can trigger the photolysis of nitrates as predeposited on Fe₂O₃, further increasing HONO production. These findings show that HCOOH-mediated photochemical reactions on iron minerals may contribute to elevated atmospheric HONO levels, highlighting a crucial pathway with broad effects on atmospheric chemistry and public health.