Ion pair particles at the air-water interface.

Although the role of methanesulfonic acid (HMSA) in particle formation in the gas phase has been extensively studied, the details of the HMSA-induced ion pair particle formation at the air-water interface are yet to be examined. In this work, we have performed Born-Oppenheimer molecular dynamics simulations and density functional theory calculations to investigate the ion pair particle formation from HMSA and (R1)(R2)NH (for NH3, R1 = R2 = H; for CH3NH2, R1 = H and R2 = CH3; and for CH3NH2, R1 = R2 = CH3) at the air-water interface. The results show that, at the air-water interface, HMSA deprotonates within a few picoseconds and results in the formation of methanesulfonate ion (MSA-)⋅⋅H3O+ ion pair. However, this ion pair decomposes immediately, explaining why HMSA and water alone are not sufficient for forming stable particles in atmosphere. Interestingly, the particle formation from the gas-phase hydrogen-bonded complexes of HMSA with (R1)(R2)NH on the water droplet is observed with a few femtoseconds, suggesting a mechanism for the gas to particle conversion in aqueous environments. The reaction involves a direct proton transfer between HMSA and (R1)(R2)NH, and the resulting MSA-⋅⋅(R1)(R2)NH2+ complex is bound by one to four interfacial water molecules. The mechanistic insights gained from this study may serve as useful leads for understanding about the ion pair particle formation from other precursors in forested and polluted urban environments.