Mechanistic Insight into the Reaction of Organic Acids with SO₃ at the Air–Water Interface

The gas-phase reaction of organic acids with SO₃ has been recognized as essential in promoting aerosol-particle formation. However, at the air–water interface, this reaction is much less understood. We performed systematic Born–Oppenheimer molecular dynamics (BOMD) simulations to study the reaction of various organic acids with SO₃ on a water droplet. The results show that with the involvement of interfacial water molecules, organic acids can react with SO₃ and form the ion pair of sulfuric-carboxylic anhydride and hydronium. This mechanism is in contrast to the gas-phase reaction mechanisms in which the organic acid either serves as a catalyst for the reaction between SO₃ and H₂O or reacts with SO₃ directly. The distinct reaction at the water surface has important atmospheric implications, for example, promoting water condensation, uptaking atmospheric condesation species, and incorporating “SO₄²⁻” into organic species in aerosol particles. Therefore, this reaction, typically occurring within a few picoseconds, provides another pathway towards aerosol formation.