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

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.