Quasi-unary homogeneous nucleation of $H2SO4‐H2O$

We show that the binary homogeneous nucleation (BHN) of $H2SO4‐H2O$ can be treated as quasi-unary nucleation of $H2SO4$ in equilibrium with $H2O$ vapor. A scheme to calculate the evaporation coefficient of $H2SO4$ molecules from $H2SO4‐H2O$ clusters is presented and a kinetic model to simulate the quasi-unary nucleation of $H2SO4‐H2O$ is developed. In the kinetic model, the growth and evaporation of sulfuric acid clusters of various sizes are explicitly simulated. The kinetic quasi-unary nucleation model does not have two well-recognized problems associated with the classical BHN theory (violation of the mass action law and mismatch of the cluster distribution for monomers) and is appropriate for the situations where the assumption of equilibrium cluster distribution is invalid. The nucleation rates predicted with our quasi-unary kinetic model are consistent with recent experimental nucleation experiments in all the cases studied, while the most recent version of the classical BHN model systematically overpredicts the nucleation rates. The hydration of sulfuric acid clusters, which is not considered in the classical model but is accounted for implicitly in our kinetic quasi-unary model, is likely to be one of physical mechanisms that lead to lower nucleation rates. Further investigation is needed to understand exactly what cause the difference between the kinetic quasi-unary model and the classical BHN model.