Thickness-Dependent Oxidation Kinetics of Coated Films of a Self-Assembled Unsaturated Fatty Acid Aerosol Proxy with Evidence for Inert “Crust” Formation

This study focuses on the effect of surface film thickness on the ozone reaction kinetics of films of a self-assembled unsaturated fatty acid aerosol proxy coated inside quartz capillaries. It also reveals evidence for reaction stagnation and stopping for the thickest films, leaving a significant amount of unreacted material and suggesting that an inert product is formed during the course of the reaction. These findings have implications for the atmospheric lifetime of such a system.

The oleic acid-ozone reaction is used as the model system for heterogeneous oxidation reactions in organic aerosols. Major sources of oleic acid in the atmosphere include marine and cooking emissions. Oxidation of organic aerosols is known to affect Cloud Condensation Nuclei (CCN) generation and therefore cloud formation. It follows that factors affecting aerosol reactivity have an effect on cloud formation potential and therefore also on the climate.

In our experiments, oleic acid is mixed with its sodium salt (sodium oleate) to form a highly viscous self-assembled lamellar phase system observable using a synchrotron-based technique: Small Angle X-ray Scattering (SAXS). Here, we take advantage of intense synchrotron radiation to probe our coated capillary films. We use the observed decay of the self-assembled scattering peak as a function of time exposed to ozone. We have obtained ~50 kinetic decay parameters spanning a range of film thicknesses, showing a drastic increase in reaction kinetics with decreasing film thickness.

There is a linear relationship between increasing film thickness and amount of self-assembled material (reactant) remaining at the end of the reaction. This implies that a reaction product hinders further reactivity and that this product may take a while to form, explaining the occurrence only in thicker films.

Modelling studies will help us understand the mechanism behind these observations and to relate to a previously-postulated idea of an inert “crust” of products forming on the surface of this viscous aerosol proxy (Pfrang et al., Atmos. Chem. Phys., 2011, 11, 7343-7354).

In summary, we demonstrate thickness-dependent reaction kinetic parameters which vary significantly with film thickness, implying that the atmospheric lifetime for a film is sensitive to the film thickness. We present evidence for reaction stagnation by an as of yet unknown inert product. Kinetic modelling is ongoing in order to explain these findings.