Deposition nucleation on mineral dust particles: a case against classical nucleation theory with the assumption of a single contact angle

Deposition nucleation on two mineral species, kaolinite and illite, was studied using a flow cell coupled to an optical microscope. The results show that the S-ice conditions when ice first nucleated, defined as the onset S-ice (S-ice,S-onset), is a strong function of the surface area available for nucleation, varying from 100% to 125% at temperatures between 242 and 239 K. The surface area dependent data could not be described accurately using classical nucleation theory and the assumption of a single contact angle (defined here as the single-alpha model). These results suggest that caution should be applied when using contact angles determined from S-ice,S-onset data and the single-alpha model. In contrast to the single-alpha model, the active site model, the deterministic model, and a model with a distribution of contact angles fit the data within experimental uncertainties. Parameters from the fits to the data are presented.