Computational Exploration of the Ability of the 2-Methyltetrols Produced from Photooxidation of Isoprene to Form Prenucleation Complexes

A central question in the formation of secondary aerosols is whether organic molecules participate in the formation of prenucleation clusters or are they only adsorbed after formation of larger aerosols? The difficulty in understanding the role of organic molecules in aerosol formation is that there are very few studies of prenucleation clusters produced from various organics and sulfuric acid, so it is uncertain whether organic compounds form prenucleation clusters. Isoprene is the most abundant volatile biogenic organic compound (VOC) emitted into the atmosphere, accounting for about 70% of biogenic VOC emissions, excluding methane. Each year, approximately 600 teragrams of isoprene enter the atmosphere, primarily from natural sources like vegetation. This makes it a significant component of atmospheric organic molecules, much more prevalent than other VOCs emitted by plants or anthropogenic activities. Photooxidation of isoprene produces the diastereomeric tetrols, 2-methylthreitol and 2-methylerythritol, which contain four hydroxyl groups. We completed a comprehensive conformational search of both tetrols, and extensively explored the potential energy surfaces of these tetrols complexed with sulfuric acid and water. We report the vast ensemble of structures that are within 1 kcal/mol of the DLPNO-CCSD-(T)/CBS//ωB97X-D/6-31++G** minimum for each system. These high level Δ<i>G</i>° values for each system were used to estimate the concentrations of all the possible complexes from these molecules in the lower troposphere. At the upper limit of tetrol concentration, we find that the two diastereomers will bind to one to three water molecules in high concentrations. However, formation of sulfuric acid-tetrol-water complexes lead to lower concentrations, leading us to suggest that these tetrols are unlikely to be involved in the formation of prenucleation clusters that will lead to further aerosol growth. Researchers should continue the search for organic molecules that lead to prenucleation.