Toward a molecular level understanding of heterogeneous processes at atmospheric aerosol surfaces: ozonolysis of maleic acid droplets

Dicarboxylic acids are an essential component of tropospheric aerosols emitted directly or formed in chemical processes. The physiochemical properties and heterogeneous oxidation of aerosol particles containing maleic acid (MA) have been investigated using a mixed quantum and classical approach. The multiphase reactions of primary ozonide formation between the gas phase, the particle interface, and its bulk strongly influence the reaction mechanism and rate coefficients. Based on snapshots issued taken from molecular dynamics simulations, the mechanism of MA + O₃ reaction is investigated in three different environments using ab initio method and density functional theory. The interfacial water molecules enhance the initial reaction step of MA + O₃, with a larger rate constant at the air-water interface than in the gas phase. By assuming the Langmuir-Hinshelwood behavior and comparing it with the bulk, the ozonolysis of maleic acid mainly occurs in the bulk, and O₃ diffusion in the bulk may be the limiting process.