Determination of the amine-catalyzed SO3 hydrolysis mechanism in the gas phase and at the air-water interface

New particle formation (NPF) involving amines in the atmosphere is considered an aggregation process, during which stable molecular clusters are formed from amines and sulfuric acid via hydrogen bond interaction. In this work, ab initio dynamics simulations of ammonium bisulfate formation from a series of amines, SO3, and H2O molecules were carried out in the gas phase and at the air-water interface. The results show that reactions between amines and hydrated SO3 molecules in the gas phase are barrierless or nearly barrierless processes. The reaction rate is related to the basicity of gas-phase amines—the stronger the basicity, the faster the reaction. Furthermore, SO3 hydrolysis catalyzed by amines occurs simultaneously with H2SO4-amine cluster formation. At the air-water interface, reactions between amines and SO3 involve multiple water molecules. The reaction center’s ring structure (amine–SO3–nH2O) promotes the transfer of protons in the water molecules. The formed ammonium cation (-RNH3+) and the bisulfate anion (HSO4−) are present and stable by means of hydrogen bond interaction. The cluster formation mechanism provides new insights into NPF involving amines, which may play an important role in the formation of aerosols in some heavily polluted areas — e.g., those with a high amine concentration.