The Importance of Ion-Dipole Interaction in Ion-Clustering Thermodynamics and Ion-Mediated Nucleation

Ions generated by cosmic rays and radioactive materials are ubiquitous in the atmosphere. The possible involvement of ions in the formation of new particles in the atmosphere has received increased attention in recent years. The classical Kelvin-Thomson (CKT) equation, which was derived by Thomson about a century ago, has been the fundamental of classical ion-induced nucleation theory. In the CKT equation, the effect of charge on Gibbs free energy change for the formation of the cluster (or equilibrium vapor pressure over an ion cluster) is taken into account by considering the electrostatic potential energy of the cluster. Recently, we find that the interaction of dipole molecules with small ion clusters, which is neglected in CKT equation, is important in ion-clustering thermodynamics. A modified Kelvin-Thomson (MKT) equation containing a dipole-ion interaction term (in addition to the Thomson term) is derived. The predictions based on MKT equation are in much better agreement with experimental ion clustering thermodynamic data and experimental ion cluster mobility-equivalent diameters. On top of Thomson effect, the dipole-charge interaction effect further stabilizes the small charged clusters. All major molecules known to be involved in nucleation in the atmospheric (H2SO4, H2O, NH3, HNO3, acetone, etc.) have high dipole moment, and the dipole-ion interaction effect increases the importance of ions in mediating the formation of new particles in the atmosphere.