Acid-Gated Interfacial Conversion of Aerosol Nitrate to Organic Nitrates Driven by Adsorbed Aldehydes

Abstract Particulate nitrate (NO3–) and organic nitrates (pON) represent important inorganic and organic reservoirs of reactive nitrogen, respectively, yet direct chemical conversion from NO3– to pON remains unresolved. Herein, using Born–Oppenheimer molecular dynamics (BOMD) simulations, we reveal the molecular mechanism underlying acid-gated inorganic-to-organic nitrate (IN-to-ON) conversion at the air–water and air–ice interfaces. Adsorption of aldehydes at the air–water interface, combined with acid-driven protonation, markedly enhances carbonyl electrophilicity, enabling nucleophilic addition of NO3– and yielding pON on a picosecond time scale. This interfacial mechanism exhibits broad chemical generality: it operates across typical anthropogenic acids (nitric and sulfuric acids, HNO3 and H2SO4), the marine-derived iodic acid (HIO3), and diverse carbonyl substrates ranging from C1–C3 aldehydes to the dicarbonyl glyoxal. Strikingly, analogous reactivity at the air–ice interface demonstrates that this chemistry extends beyond liquid aerosol water to ice particles in polar regions and the upper atmosphere. These findings establish a molecular-level bridge between aerosol NO3– and pON, identifying acid-mediated interfacial IN-to-ON processing as a potentially overlooked in situ formation pathway for pON. This interfacial chemistry may help explain the persistent underprediction of atmospheric pON burden and advance our understanding of global nitrogen chemical cycling.