Abstract/Summary

The organic fraction of aqueous atmospheric aerosols has received much recent attention due to the ability of fatty acids to partition to the air-water interface, thus altering the surface properties of the aerosol. With the complex surface films formed, there is a need for an efficient model system to understand key features. To further both our understanding of film formation processes and changes in films due to ozonolysis reactions within the atmosphere, this work employs coarse-grained molecular dynamics simulations of model surfactants at the air-water interface. Molecular dynamics simulations of surface films have not yet been widely used in atmospheric science; however, they can provide useful insight into sub-micrometre aerosols, giving a molecular level view of the structure and dynamics of the system. Thus, this work paves the way for future coarse-grained simulations in the field of atmospheric chemistry. In the simulations in this thesis, a planar interface is assumed, mimicking a cross-section through a large aerosol with low curvature and allowing direct comparison to experimental studies on Langmuir troughs. Based on structural data derived from allatom simulations, pressure-area isotherms were calculated from coarse-grained simulations of single component monolayers of oleic acid, stearic acid and palmitoleic acid, which were in good agreement with experimental isotherms. The phase behaviour of the main reaction products of the ozonolysis of both oleic acid and palmitoleic acid, assuming an analogue reaction pathway as for oleic acid, were determined. The simulations suggest that the only products remaining at the interface from these reactions are nonanoic acid and heptanoic acid. Pressure-area isotherms of mixed component films, which are more realistic proxies for the complex mixtures on real atmospheric aerosols, were then calculated. For systems below 10 % unsaturated material, ordered liquid-condensed phase and solid state films were formed. These may act as a transport barrier and hence supress the growth of aerosols in the atmosphere. For surface films containing more than 33 % of unsaturated material, disordered liquidexpanded films were formed. For such films, the higher the amount of unsaturated material, the higher the surface pressure of the system, and the lower the surface tension. It was shown that the ozonolysis of a surface film can lead to phase changes, hence influencing the growth potential of the aerosol, and highlighting the importance of a complete understanding of aged surface films in the atmosphere.