Ozonolysis of Oleic Acid Aerosol Revisited: Multiphase Chemical Kinetics and Reaction Mechanisms

The chemical processing of organic aerosol particles is important for atmospheric chemistry, climate, and public health. The heterogeneous oxidation of oleic acid particles by ozone is one of the most frequently investigated model systems. The available kinetic data span a wide range of particle size and ozone concentration and are obtained with different experimental techniques including electrodynamic balance (EDB), optical tweezers, environmental chamber, and aerosol flow tube reactors using mass spectrometry and Raman spectroscopy as detection methods. Existing kinetic and mechanistic analyses, however, reveal systematic differences and inconsistencies that are a matter of ongoing debate. We developed and applied an inverse modeling approach using a kinetic multilayer model (KM-SUB) and Monte Carlo-based global optimization algorithms to 11 literature data sets and an additional new set of EDB data. We were able to reconcile most experimental data with consistent sets of multiphase chemical kinetic parameters. For a unique determination of these parameters, however, further experiments with simultaneous measurement of multiple observables at specific, insightful reaction conditions are required. We tested three different reaction mechanisms and conclude that secondary chemistry involving Criegee intermediates appears crucial to resolve the discrepancies found in earlier studies. Primary ozone chemistry occurs close to the particle surface and secondary reactions seem to dominate in the particle bulk, involving OH formation and radical chain reactions.

Automated summary

The chemical processing of organic aerosol particles plays a crucial role in atmospheric chemistry, climate, and public health. Heterogeneous oxidation of oleic acid particles by ozone is a commonly studied model system. However, there are systematic differences and inconsistencies in existing kinetic data and mechanistic analyses, which can be reconciled with consistent sets of multiphase chemical kinetic parameters through inverse modeling. Further experimental evidence is needed to uniquely determine these parameters and it is suggested that secondary reactions involving Criegee intermediates are essential for resolving discrepancies found in earlier studies.