Modeling Aerosol Particles with Quantum Chemistry

Organic acids are ubiquitous in the atmosphere and are important com-<br/>pounds that affect the physicochemical properties of aerosol particles and<br/>the formation of secondary organic aerosols. This entails that these species<br/>have decisive implications on air quality and the climate. While the pure gas-<br/>phase and aqueous-phase chemistry of organic monocarboxylic and dicar-<br/>boxylic acids have been extensively studied, their simultaneous interactions<br/>with water and ions remain elusive. Furthermore, these two phase states of<br/>organic acids have hitherto not been put into a formal atmospheric frame-<br/>work describing their gas-to-particle partitioning. In this work, I present<br/>investigations of the structural, thermodynamic, and spectroscopic proper-<br/>ties of various organic monocarboxylic and dicarboxylic acids. The acids<br/>studied are the monocarboxylic acids pyruvic acid (PA), lactic acid (LA),<br/>propionic acid (ProA), 2,2-dihydroxypropionic acid (diol), and the dicar-<br/>boxylic acids fumaric acid (FA) and maleic acid (MA) in the atmospheric<br/>gas phase and aqueous particle phase. The physicochemical properties of<br/>these species were calculated by employing a configurational sampling pro-<br/>tocol in the gas phase relying on quantum chemistry to obtain the low-<br/>est free-energy structure, while a structural refinement protocol involving<br/>a re-optimization of gas-phase DFT structures was employed to model the<br/>aqueous phase relying on an implicit solvation scheme. The results from<br/>the initial equilibrium geometry benchmark study showed that the ωB97X-<br/>D/6-31++G(d,p) functional/basis set combination performed the best for<br/>full geometry optimizations of molecular clusters. The GFN1-xTB method<br/>was found to be suitable for geometric pre-optimizations of molecular clus-<br/>ters. It was found that the cluster geometries of the solvated pyruvic acid<br/>clusters without ions present are well conserved when transitioning from the gas phase to the aqueous particle phase, but the pyruvate-containing clus-<br/>ters were less conserved. It was also found that it is thermodynamically<br/>unfavorable to hydrate the organic monocarboxylic and dicarboxylic acids<br/>in the gas phase without ions present. On the other hand, it was found that<br/>it is favorable to solvate the organic acids in aqueous solution without ions<br/>present. When introducing Na+ and Cl– ions into the systems, it was found<br/>that the cluster geometries of the organic monocarboxylic acids in the gas<br/>phase were perturbed significantly. It was found that it is both favorable<br/>to hydrate the organic acids in the gas phase and to solvate them when<br/>ions are present, which implies that ions are necessary for facilitating the<br/>condensation of organic acids onto aerosol surfaces and to drive the uptake<br/>of organic acids into the particles. The IR spectrum analysis of pyruvic<br/>acid revealed a correlation between decreasing carboxylic O–H frequency<br/>and increasing explicit cluster water content in both the gas phase and in<br/>the aqueous phase. Moreover, the gas-phase signals in the spectra of both<br/>pyruvic acid and fumaric/maleic acid were found to be redshifted upon sol-<br/>vation. In addition, a correlation between decreasing O–H frequency and<br/>cluster ion content was found in the gas-phase spectrum of pyruvic acid,<br/>while no clear trend was found in the aqueous-phase spectrum of pyruvic<br/>acid nor in the gas-phase and aqueous-phase spectra of fumaric/maleic acid<br/>when adding ions to the systems. The interfacial studies of Na+ and Cl–<br/>at the surface of aqueous nanodroplets revealed that Na+ is taken up by<br/>the droplet, while Cl– resides at the surface of the droplet. The study of<br/>the interaction properties of various ions with aqueous nanodroplets using<br/>configurational sampling showed that Na+, Mg2+, and Ca2+ are favorably<br/>taken up by the droplet, while F– , Cl– , Br– , I– , and K+ are found at the<br/>surface. Finally, the subsequent umbrella sampling simulations on the ion–<br/>water clusters were found to be in good agreement with the results from<br/>the configurational sampling and showed that Ca2+ and Mg2+ are favorably<br/>taken up by the droplet, while Na+, K+, F– , Cl– , Br– , and I– are found<br/>at the surface. These studies will altogether pave the way for the accurate<br/>modeling of aerosol particles, such as sea spray aerosols containing ions, and<br/>help shed light on their atmospheric implications.