Ternary nucleation of H₂SO₄, NH₃, and H₂O in the atmosphere

Abstract

Classical theory of binary homogeneous nucleation is extended to the ternary system H₂SO₄-NH₃-H₂O. For NH₃ mixing ratios exceeding about 1 ppt, the presence of NH₃ enhances the binary H₂SO₄-H₂O nucleation rate by several orders of magnitude. The Gibbs free energies of formation of the critical H₂SO₄-NH₃-H₂O cluster, as calculated by two independent approaches, are in substantial agreement. The finding that the H₂SO₄-NH₃-H₂O ternary nucleation rate is independent of relative humidity over a large range of H₂SO₄ concentrations has wide atmospheric consequences. The limiting component for ternary H₂SO₄-NH₃-H₂O nucleation is, as in the binary H₂SO₄-H₂O case, H₂SO₄; however, the H₂SO₄ concentration needed to achieve significant nucleation rates is several orders of magnitude below that required in the binary case.