Formation of atmospheric molecular clusters from organic waste products and sulfuric acid molecules: a DFT study

Abstract

The interaction of one or two sulfuric acid molecules with the indole (C₈H₇N) molecule and with the skatole (C₉H₉N) molecule and two of its oxidation products (C₉H₉NO₂ and C₉H₉NO₃) has been investigated by means of computational methods at the quantum level. Gibbs free energies of formation have been calculated using the ωB97X-D exchange-correlation functional with three different basis sets to characterize the stability of the corresponding hetero-molecular clusters. The careful examination of the cluster geometries shows the key role of the hydrogen bonding in the stabilization of the critical nucleus that these organics can form with the sulfuric acid molecules. However, the thermodynamic results demonstrate that the interaction between indole/skatole and one or two sulfuric acid molecules remains quite weak in atmospheric conditions and may likely compete with the formation of sulfuric acid dimers. By contrast, the oxidized C₉H₉NO₃ and C₉H₉NO₂ species appear to be much better candidates for the formation of critical nuclei with sulfuric acid molecules than their parent compounds, indole or skatole. This supports the conclusions of recent experimental observations on the ability of molecules issued from organic waste products to form nuclei with sulfuric acid molecules and emphasizes the potential role of agricultural recycling of sewage sludge as an unaccounted source of nucleation precursors in the atmosphere.