The air-water interface of atmospheric aqueous secondary organic aerosol (aqSOA) is an important environmental system that has far reaching impacts on the climate and overall environment. Many small atmospheric aldehydes and ketones that are known to contribute to aqSOA formation can further react in the aqueous phase to form hydrated species and oligomers. Due to the potential complexity of these systems, it can be difficult to isolate the effects of individual species. For these reactions, the molecular nature of many of these organics at aerosol interfaces is largely unknown. This information is vital since the presence solvated molecules at air-water interfaces, as well as the conformations and orientations of those molecules, can significantly affect aerosol properties. Hydroxyacetone (HA) is an abundant atmospheric ketone found in aqSOA and exists predominantly in its unhydrated monomeric form in the aqueous phase, making HA an excellent model system to address some of these uncertainties. This work examines hydroxyacetone (HA) at the planar air-water interface using vibrational sum-frequency spectroscopy (VSFS). VSFS is a surface sensitive technique that can provide information about the interfacial molecular populations and behaviors of ordered molecules. Using VSFS, in combination with Wilhelmy plate surface tensiometry and computational molecular dynamics simulations and density functional theory calculations, reveals a dynamic HA interface, with both depth and conformational dependent population and orientation changes between the bulk, surface, and subsurface. This work lays the groundwork and provides a control for future VSFS studies of more complex systems of organics at aqueous interfaces.

Revealing the complexities of hydroxyacetone at the air-water via vibrational sum frequency spectroscopy and computational methodologies

255th National Meeting (2018)

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