Formic Acid-Intensified\nPhotoreduction of NOx on Iron\nMinerals Triggers Daytime HONO Formation through Active Hydrogen

Nitrous acid (HONO) is crucial in atmospheric chemistry\nas a precursor\nto morning peak hydroxyl radicals and significantly affects urban\nair quality by forming secondary pollutants, yet the mechanisms of\nits daytime formation is not fully understood. This study investigates\nthe role of formic acid (HCOOH), a prevalent electron and proton donor,\nin the transformation of nitrogen oxides (NOx) and the formation of\nHONO on photoactive mineral dust. Exploiting hematite (Fe<sub>2</sub>O<sub>3</sub>) as an environmental indicator, we demonstrate that\nHCOOH significantly promotes the photoreduction of NO<sub>2</sub> to\nHONO, while suppressing nitrate accumulation. This occurs through\nthe formation of a surface Fe–OOCH complex, where sunlight\nactivates the C–H bond to generate and transfer active hydrogen,\ndirectly converting NO<sub>2</sub> to HONO. Additionally, HCOOH can\ntrigger the photolysis of nitrates as predeposited on Fe<sub>2</sub>O<sub>3</sub>, further increasing HONO production. These findings\nshow that HCOOH-mediated photochemical reactions on iron minerals\nmay contribute to elevated atmospheric HONO levels, highlighting a\ncrucial pathway with broad effects on atmospheric chemistry and public\nhealth.