Abstract/Summary

The following thesis describes the design and development of a pioneering reaction and analysis chamber for combining neutron reflectometry with simultaneous in situ Fourier transform infra-red reflection-absorption spectroscopy for studying the oxidation of mixed lipid monolayers at the air/water interface by gas-phase oxidants. The thesis then describes several bodies of work performed using the apparatus, each of which advances the field in a specific manner. The first body of work describes the oxidation of galactocerebroside (GCB) monolayers by NO3 • radicals (the key night-time atmospheric oxidant species), both as a pure monolayer and as part of a mixed monolayer with palmitic acid and palmitoleic acid. Galactocerebroside was chosen as a model system for glycolipids, which have been found to be a key component in sea-spray aerosol. NO3 • radical oxidation did not fully remove the GCB monolayer from the surface under any conditions, suggesting that such monolayers are likely to persist under night-time atmospheric conditions. The second body of work describes the ozonolysis of oleic acid monolayers at low temperatures. Oleic acid (OA) is a key component of anthropogenic aerosol and has been established as a reference system in this domain. The effect of cooling the system to near-freezing temperatures was selected for study as the real-life conditions in which these reactions take place are likely to be better modelled at such a temperature condition than at the room temperature conditions usually employed for such studies. A residual monolayer resistant to further oxidation persisted after OA ozonolysis at low temperatures, which could be built up by repeated deposition and ozonolysis, suggesting that the effects of an organic film, such as surface tension depression, can persist beyond the expected lifetime of the deposited film species itself. The third body of work describes the ozonolysis of OA as part of a mixed monolayer with stearic acid (SA). Real-life aerosol will be highly heterogeneous, and so the effect of co-deposited species on the reactivity and lifetime of reactive components such as OA is important to determine. The presence of SA in the system allowed the formation of a stable residual product monolayer from OA ozonolysis even at room temperature. Finally, possible future applications of the combined reaction and analysis chamber are developed by presenting preliminary data from a study of the oxidation of linoleic acid in a variety of mixtures and temperature conditions. The primary implication of this work is that unsaturated fatty acid emissions could affect the physical properties of water droplets in the atmosphere longer than implied by their atmospheric lifetimes.