Modeling Local Aerosol Surface Environments: Clustering of Organic Acids, Water, and Ions

Pyruvic acid is an omnipresent compound in nature and is found both in the gas- phase and in the particle-phase of the atmosphere as well as in aqueous solution in the hydrosphere. Despite much literature on the photochemical degradation and stability of pyruvic acid in different chemical environments, the study of simultaneous interac- tions between gas-phase pyruvic acid or similar carboxylic acids with water and ions is not well-understood. Here, we present a study of micro-hydrated molecular clus- ters containing pyruvic acid and the structural analogous caboxylic acids lactic acid, propionic acid, and 2,2-dihydroxypropanoic acid by probing geometries, binding free energies, hydrate distributions, as well as their IR absorption spectra. We performed a meticulous configurational sampling protocol for the various hydrated clusters ranging from low level of theory to high level of theory to identify the lowest free energy struc- ture. We find that cluster geometries and especially their water structure are highly sensitive to the presence and character of ions. We show that the hydration of the studied organic acids is thermodynamically unfavorable in the gas-phase and ions are necessary for mediating interactions between organic acids and water thus stabilizing the clusters. Finally, we find a clear correlation between decreasing pyruvic acid car- boxylic O–H stretching frequencies, increasing intensity when adding more water to the clusters, and a correlation between increasing red-shifting of the O–H frequencies upon addition of ions to the clusters. The observations done in this study could pave the way to unravel the mechanisms behind the transitioning of organic acids from the gas-phase to the particle phase.