Mechanisms of Hydration Reactions of Typical Small α-dicarbonyls at the Interfaces of Ammonium-bearing Particles

Small α-dicarbonyls (SαDs) , as the important precursors of secondary organic aerosol (SOA) , significantly contribute to SOA formation through their interfacial chemical processes. However, the reaction mechanisms of SαDs at the interfaces and their roles in the formation of SOA remain uncertain. Hence, Born-Oppenheimer molecular dynamics simulation was performed to investigate the interfacial hydration reaction mechanisms of typical SαDs, including glyoxal (GL) and methylglyoxal (MG) , at the interfaces of ammonium chloride (NH₄Cl) and ammonium sulfate [(NH₄)₂SO₄] particles. The results show that the hydration reactions of the aldehyde groups of GL and MG at both interfaces proceed via three steps: (I) hydrogen transfer of the interfacial water molecule leading to the formation of the cationic intermediates, (II) the attack of the aldehyde carbon atom by the hydroxyl group of the interfacial water molecule forming the transition states (TS) , and (III) subsequent aldehyde-diol formation. Sulfate ion ( ${\mathrm{S}\mathrm{O}}_4^{2-}$) promotes the interfacial hydration reactions of the aldehyde groups of GL and MG. However, two steps, i.e., (I) TS formation and (II) ketone-diol formation exist for the hydration reaction of the ketone group of MG. The process is mediated by one and seven water molecules, respectively. Excess water molecules hinder the hydration reaction of the ketone group of MG at the interface of (NH₄)₂SO₄ particle. The diol products generated by the hydration reactions of GL and MG at the interfaces have the potential to come into the droplet interior and subsequently engage into oligomerization reactions, thereby contributing to the formation of SOA. The results provide an insight into the interfacial chemistry of GL and MG in different particles and their influence on the SOA formation.