Competitive reaction pathways for functionalization and volatilization in the heterogeneous oxidation of coronene thin films by hydroxyl radicals and ozone.

X-ray photoelectron spectroscopy (XPS) is used to monitor the heterogeneous reaction of hydroxyl radicals (OH) and ozone with thin films (∼5 Å) of coronene. Detailed elemental and functional group analysis of the XPS spectra reveals that there is a competition between the addition of oxygenated functional groups (functionalization) and the loss of material (volatilization) to the gas phase. Measurements of the film thickness and elemental composition indicate that carbon loss is as important as the formation of new oxygenated functional groups in controlling how the oxygen-to-carbon ratio (O/C) of the coronene film evolves during the surface reaction. When the O/C ratio of the film is small (∼0.1) the addition of functional groups dominates changes in film thickness, while for more oxygenated films (O/C > 0.3) carbon loss is an increasingly important reaction pathway. Decomposition of the film occurs via the loss of both carbon and oxygen atoms when the O/C ratio of the film exceeds 0.5. These results imply that chemically reduced hydrocarbons, such as primary organic aerosol, age in the atmosphere by forming new oxygenated functional groups, in contrast to oxygenated secondary organic aerosol, which decompose by a heterogeneous loss of carbon and/or oxygen.