Negative ion photoelectron spectroscopy reveals thermodynamic advantage of organic acids in facilitating formation of bisulfate ion clusters: Atmospheric implications

Recent lab and field measurements have indicated critical roles of organic acids in enhancing new atmospheric aerosol formation. Such findings have stimulated theoretical studies with the aim of understanding the interaction of organic acids with common aerosol nucleation precursors like bisulfate (HSO ₄^-). We report a combined negative ion photoelectron spectroscopic and theoretical investigation of molecular clusters formed by HSO₄^- with succinic acid (SUA, HO₂C(CH ₂)₂CO₂H), HSO₄^-(SUA) _n (n = 0-2), along with HSO₄^-(H ₂O)_n and HSO₄^-(H₂SO ₄)_n. It is found that one SUA molecule can stabilize HSO₄^- by ca. 39 kcal/mol, three times the corresponding value that one water molecule is capable of (ca. 13 kcal/mol). Molecular dynamics simulations and quantum chemical calculations reveal the most plausible structures of these clusters and attribute the stability of these clusters to the formation of strong hydrogen bonds. This work provides direct experimental evidence showing significant thermodynamic advantage by involving organic acid molecules to promote formation and growth in bisulfate clusters and aerosols.