A parameterization of ion‐induced nucleation of sulphuric acid and water for atmospheric conditions

This paper describes a five‐dimensional parameterization of ion‐induced nucleation (IIN) that covers the complete range of conditions relevant to the lower atmosphere. The parameters are (1) temperature T (190–300 K), (2) relative humidity RH (0.05–0.95), (3) number concentration of H 2 SO 4 (10 5 –10 8 cm −3 ), (4) first‐order loss of H 2 SO 4 to particles (0.00009–0.0245 s −1 ), and (5) ion source rate (2–50 ion pairs cm −3 s −1 ). The parameterization is based on a steady state version of the kinetic aerosol model Sulphuric Acid and Water Nucleation (SAWNUC) that uses experimentally measured thermodynamics for the ion clusters. Parameterized formulas are obtained for the following variables: (1) particle nucleation rate (cm −3 s −1 ), (2) H 2 SO 4 nucleation rate (cm −3 s −1 ), (3) number of H 2 SO 4 molecules in average nucleating cluster, (4) number of H 2 O molecules in average nucleating cluster, and (5) radius (nanometers) of average nucleating cluster. The parameterization generally reproduces the modeled nucleation rate to within an order of magnitude over the whole range of conditions, except when the nucleation rate is very low (<10 −6 cm −3 s −1 ), which corresponds to a rate of less than 0.1 particle d −1 cm −3 . This parameterization speeds up IIN calculations by a factor of ∼10 6 , as compared to the original SAWNUC model.