Hydration of a sulfuric acid-oxalic acid complex: acid dissociation and its atmospheric implication

Oxalic acid (OA), one of the most common organic acids in the Earth's atmosphere, is expected to enhance the nucleation and growth of nanoparticles containing sulfuric acid (SA) and water (W); however, the details about the hydration of OA-SA are poorly understood, especially for the larger clusters with more water molecules. We have investigated the structural characteristics and thermodynamics of these clusters using density functional theory at the PW91PW91/6-311++G(3df, 3pd) level. The favorable free energies of formation and obvious concentrations of the OA-SA-W-n (n = 0-6) clusters at 298.15 K predict that oxalic acid can contribute to the aerosol nucleation process by binding to sulfuric acid and water until n = 6. There is strong temperature dependence for the complexes formation, and the energy order of these complexes is altered from 100 to 400 K, regardless of different cluster sizes or different isomers within the same cluster size. The lower temperature and higher relative humidity promote the formation of hydrates. Additionally, the investigation of acid dissociation predicts that several acid-dissociated models could coexist in the atmosphere, specifically when more water molecules are present. Fewer waters may be needed to cause the acid dissociation, as the relative acidity of the cluster increases, which plays a key role in forming relatively stable hydrated clusters of OA-SA. Finally, the Rayleigh scattering properties of OA-SA-W-n (n = 0-6) have been systematically investigated for the first time to further discuss its atmospheric implication.