Formation and properties of hydrogen-bonded complexes of common organic oxalic acid with atmospheric nucleation precursors

Nucleation in the Earth's atmosphere plays a vital role in the aerosol radiative forcing associated directly with the production of ultrafine particles responsible for adverse public health impacts and global climate changes. In the present work, the interaction of the common organic oxalic acid with the atmospheric nucleation precursors and trace ionic species has been investigated using the Density Functional Theory (DFT). A comprehensive study of the hydrogen-bonded complexes of oxalic acid with neutral monomers and ionic clusters of the sulfuric acid, water and ammonia, the key atmospheric nucleation precursors, has been carried out. We found that the bonding of the oxalic acid with the above-mentioned neutrals is moderately weak, and thus, the oxalic acid cannot stabilize the binary sulfuric acid-water and ternary sulfuric acid-ammonia clusters. The interaction of the oxalic acid with positive ionic species is, in contrast, strong and significantly enhances stability of the clusters formed over the ions. The hydration of positively charged (C2H2O4)(H3O+), (C2H2O4)(NH4+) and (C2H2O4)(H3O+)(H2SO4) clusters is also strong and close to that of the sulfuric acid, the key atmospheric nucleation precursor. These considerations lead us to a logical conclusion that the oxalic acid can catalyse the production of positively charged pre-nucleation clusters and, thus, its role in nucleation of positive ions in the Earth's troposphere should be studied in further details.