Sulfate aerosols are important components of the climate model of Earth, as they provide a net cooling effect to the atmosphere through their ability to form cloud condensation nuclei and grow into clouds that reflect incoming radiation from the sun. However, the extent of this cooling effect is uncertain, necessitating computational studies into the atmospheric clusters that can grow into these aerosols. Thus, this study set out to understand the thermodynamics of ternary sulfuric acid clusters and their various low-energy conformations. We used a genetic algorithm to sample a large pool of various cluster conformations, and then optimized these clusters with the PM7 semi-empirical method. We then put these clusters through three subsequent rounds of optimization with increasingly tight criteria and robust basis sets, ending with a couple dozen clusters at the M08-HX/MG3S level of theory. Finally, after extracting thermodynamic corrections from the M08-HX/MG3S method, we used several forms of DLPNO-CCSD(T) single point energy calculations to obtain final electronic energies and combine these electronic energies with the thermal corrections to obtain values for Gibbs free energy for these structures. These single point energy calculations include complete basis set extrapolation with the 4-5 Polynomial and Riemann Zeta extrapolation schemes, and the explicitly correlated DLPNO-CCSD(T)-F12/cc-pVTZ-F12 method. In summary, we compared a handful of low energy clusters using the M08-HX/MG3S thermodynamic corrections with various electronic energy calculations. We discovered that of all our methods, the complete basis set extrapolation schemes, our most rigorous methods, predict the least favorable Gibbs free energy of formation for the global minimum. This formation energy is significantly more positive than past studies indicate, leading to potentially multiple orders of magnitude less ternary sulfuric acid cluster formation.

Computational study of ternary sulfuric acid clusters

ACS Fall 2021

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