A theoretical perspective on the structure and thermodynamics of secondary organic aerosols from toluene: molecular hierarchical synergistic effects

Abstract

Toluene is an important constituent of liquid fuel, and it contributes to the formation of secondary organic aerosols (SOAs) under photochemical conditions. However, the underlying mechanism of toluene SOAs is not fully understood. To address this issue, an atomistic molecular dynamics (MD) approach and density functional theory (DFT) are coupled to study the molecular chemistry of toluene SOAs by examining their structural characteristics and thermodynamic properties. Both MD and DFT methods are proven to be consistent with experimental results. Our results suggest that the molecular hierarchical synergistic effects majorly determine the formation of core–shell SOA nanoparticles. In the toluene photooxidation production, pyruvic acid acts as a “bridge” that promotes concentration of benzaldehyde and benzoic acid at the surface of H₂SO₄–H₂O clusters. Besides, the chemical composition and environmental temperature have a key role in the probability distribution and lifetime of hydrogen bonds of toluene SOAs, thus altering the formation energy barrier of gas-to-nanoparticle conversion. When compared with van der Waals interactions, electrostatic interactions are found to be the central driving force that yields stable toluene SOA nanoparticles. Our results reveal the molecular chemistry for the atmospheric aerosol and highlight the need to account for molecular hierarchical synergistic effects when assessing the atmospheric aerosol.