Interpreting activity in H₂O- H₂SO₄ binary nucleation

Sulfuric acid-water nucleation is thought to be a key atmospheric mechanism for forming new condensation nuclei. In earlier literature, measurements of sulfuric acid activity were interpreted as the total (monomer plus hydrate) concentration above solution. Due to recent reinterpretations, most literature values for H2 S O4 activity are thought to represent the number density of monomers. Based on this reinterpretation, the current work uses the most recent models of H2 O- H2 S O4 binary nucleation along with perturbation analyses to predict a decrease in critical cluster mole fraction, increase in critical cluster diameter, and orders of magnitude decrease in nucleation rate. Nucleation rate parameterizations available in the literature, however, give opposite trends. To resolve these discrepancies, nucleation rates were calculated for both interpretations of H2 S O4 activity and directly compared to the available parameterizations as well as the perturbation analysis. Results were in excellent agreement with older parameterizations that assumed H2 S O4 activity represents the total concentration and duplicated the predicted trends from the perturbation analysis, but differed by orders of magnitude from more recent parameterizations that assume H2 S O4 activity represents only the monomer. Comparison with experimental measurements available in the literature revealed that the calculations of the current work assuming aa represents the total concentration are most frequently in agreement with observations.