Modeling and Parametrization of Size-Dependent Processes in Multiphase Aerosol Chemistry.

Many atmospherically relevant multiphase reactive systems exhibit size-dependent kinetics in laboratory studies, with apparent reaction rates increasing with decreasing droplet size, suggesting an important role for interfacial processes. Here, we present CHAI (CHemistry of Aerosol Interfaces), a physicochemical modeling framework that describes these reactive systems using an additive resistance approach, considering the various mass transfer and reaction processes taking place simultaneously in the gas phase, droplet bulk, and at the droplet surface, and their relative time scales. We demonstrate its applicability to modeling and developing parametrizations for several inorganic and organic oxidation processes in microdroplets and aerosols, including S-(IV) to S-(VI) conversion, through simulation of experimental data. CHAI is also used to reconcile single-droplet observations with chamber and flow tube experiments and is extended to parametrization of these processes for representation in large-scale atmospheric models. Our results show that neglecting the size dependence of aerosol reaction processes can lead to inaccuracies in atmospheric chemistry modeling, motivating the CHAI approach for modeling these multiphase reactive systems and further experimental studies.