Understanding the heterogeneous reactions and transformations between organic particles and gas–phase oxidants, such as ozone, within atmospheric aqueous phases is of crucial importance. However, investigating the aerosol/droplet phase presents significant challenges in distinguishing between interfacial and bulk scenarios in laboratory studies. In the present study, the physiochemical properties and heterogeneous ozone oxidation of multiphase reactions in a water droplet containing maleic acid (MA) are investigated by means of both classical and quantum methods. The relative occurrence of interfacial and bulk reactions is dependent on the bulk affinity of O₃ and the interfacial affinity of reacting MA, which is predominantly found in the bulk phase. Reaction energetics and rate coefficients have been evaluated in different environments (gas phase, at the particle interface, and in the bulk) using a multilevel approach. The presence of interfacial water molecules enhances the initial reaction step of MA + O₃, with a larger rate coefficient at the air–water interface than in the gas phase. By assuming a Langmuir–Hinshelwood behavior and comparing it with the bulk, the ozonolysis of maleic acid mainly occurs in the bulk with rate coefficients of the order of 10⁻¹⁹ and 10⁻¹⁶ cm³.molecule⁻¹. s⁻¹, respectively. The calculated rate coefficients are compared with reported values. The original method presented here quantifies the influence of a specific heterogeneous environment on the reaction rates taking into account explicit solvation.