Computational Vibrational Sum Frequency Spectra of Formaldehyde and Hydroxymethanesulfonate at Aqueous Interfaces

The identity and arrangement of aqueous species at the interface of atmospheric aerosol impacts aerosol properties including albedo and propensity to uptake additional gas phase species. Formaldehyde and sulfur dioxide are two common atmospheric species that alter (individually and in concert) aqueous atmospheric aerosol interfaces. Vibrational sum frequency (VSF) spectroscopy studies of planar aqueous formaldehyde solution surfaces have shown alteration during exposure to sulfur dioxide gas. Additional changes were observed once exposure was ceased. The results suggested the formation of a new organic species, hydroxymethanesulfonate (HMS), which acts as a thermodynamic sink for aqueous sulfur dioxide and lead to acidification of the aerosol particles. The coherent nature of the VSF response and its strong dependence on surface species present and their orientations, however, made definite vibrational assignments for exact species notoriously difficult. The focus of this paper is on elucidating the species and orientations that give rise to specific experimentally derived spectral features through VSF spectra calculated using a combination of classical molecular dynamics (MD) and density functional theory (DFT). The results demonstrate that the most prevalent surface species for a formaldehyde containing aqueous solution is hydrated formaldehyde in the form of methylene glycol. Calculated VSF spectral frequencies for the proposed product HMS are in agreement with experiment. Furthermore, changes in the experimental spectra both during and after the flow of sulfur dioxide are consistent with HMS in different interfacial orientations.