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

Classical theory of binary homogeneous nucleation is extended to the ternary system H 2 SO 4 ‐NH 3 ‐H 2 O. For NH 3 mixing ratios exceeding about 1 ppt, the presence of NH 3 enhances the binary H 2 SO 4 ‐H 2 O nucleation rate by several orders of magnitude. The Gibbs free energies of formation of the critical H 2 SO 4 ‐NH 3 ‐H 2 O cluster, as calculated by two independent approaches, are in substantial agreement. The finding that the H 2 SO 4 ‐NH 3 ‐H 2 O ternary nucleation rate is independent of relative humidity over a large range of H 2 SO 4 concentrations has wide atmospheric consequences. The limiting component for ternary H 2 SO 4 ‐NH 3 ‐H 2 O nucleation is, as in the binary H 2 SO 4 ‐H 2 O case, H 2 SO 4 ; however, the H 2 SO 4 concentration needed to achieve significant nucleation rates is several orders of magnitude below that required in the binary case.