Comment on acp-2021-919

Milsom and co-workers investigate the kinetics of the decay of oleic acid (pure and “lamellar”, by mixing with its sodium salt) by ozone. This system is a common proxy for heterogeneous chemistry of organic aerosol in general and cooking aerosol in particular. The study focuses on kinetic modelling and derives a set of parameters for compositiondependent diffusion of ozone and organics by inverse fitting of experimental data of double bond prevalence in the reaction system. The experimental data in this study is based on previous work using Raman spectroscopy in quartz capillaries (Milsom et al. 2021) and supplemented by a new data set. The authors find accumulation of reaction product close to the film surface, leading to formation of a surface crust, i.e. a region of decreased diffusivity. A remarkable result is the extraordinarily low ozone diffusivity in the organic matrix indicative of a semi-solid phase state of the “lamellar” oleic acid, even in the unreacted state.