Rapid Formation of Nitric Sulfuric Ester at the Gas–Liquid Interface and Its Potential Contribution to Aerosol Growth

Owing to the large surface-to-volume ratio and unsaturated hydrogen-bonding network, interfacial water molecules can markedly accelerate many reactions in the gas phase. Although the gas-phase reaction between sulfur trioxide (SO₃) and nitric acid (HNO₃) has been shown to compete with the SO₃ + 2H₂O and OH + HNO₃ reactions at altitudes of 25-35 km, its interfacial mechanisms remain elusive. Herein, classical molecular dynamics (MD) simulations and Born-Oppenheimer molecular dynamics (BOMD) simulations were employed to elucidate the adsorption behavior and interfacial reactivity of SO₃ and HNO₃ at the gas-liquid interface. MD simulations reveal strong interfacial preference for both species, characterized by their lowest free energies and high aggregation probabilities. BOMD results show that the interfacial reaction between SO₃ and HNO₃ rapidly produces O₂NOSO₃⁻ on the picosecond time scale, a pathway significantly more favorable than the gas-phase reaction and competitive with SO₃ hydrolysis. Moreover, the resulting O₂NOSO₃⁻ exhibits stronger binding to SA-A clusters than other common species, thereby facilitating aerosol particle growth. This work provides a new insight into SO₃ consumption in HNO₃-polluted regions and the potential roles of sulfate compounds in aerosol growth.