Modeling the Aerosol Aqueous Phase: Solvation of Pyruvic Acid Analogs and Na⁺, Cl^- Ions

Organic acids are important atmospheric compounds that affect the aerosol physicochemical properties and the formation of secondary organic aerosols (SOA) with implications for air quality and climate. Pyruvic acid (PA) is ubiquitous in the atmosphere, biosphere, and hydrosphere. While the pure gas-phase and aqueous-phase chemistry of PA has been extensively studied, its simultaneous interactions with water and ions in the particle phase remains elusive. Here, we present a study on the solvation of PA and its structurally similar analogs─lactic acid (LA), propionic acid (ProA), and 2,2-dihydroxypropionic acid (diol)─by probing geometries, solvation free energies, and infrared (IR) absorption spectra using quantum chemical methods. We performed a refinement of structures in the aqueous phase based on an elaborate configurational sampling scheme in the gas phase, which we have reported previously. The aqueous phase is modeled using explicit microhydration within an implicit polarizable continuum model. We find that the solvated organic acid clusters have a high conservation of geometry when transitioning from the gas phase to the aqueous particle phase, while the solvated ion-containing clusters show significantly larger structural rearrangements. Solvation of organic acids is found to be thermodynamically favorable in the aqueous particle phase─both with and without ions─unlike in the gas phase. Finally, in order to identify weakly bound clusters and guide future experiments, our IR absorption analysis shows that the harmonic frequencies of PA carboxylic O-H stretching of the microhydrated PA clusters are red-shifted in the spectrum in the aqueous phase compared to the gas phase. Conversely, we find no clear trends in the spectrum obtained with our qualitative approach for the O-H frequencies of the microhydrated ion-containing PA clusters.