Homogeneous ice nucleation in adsorbed water films: A theoretical approach

Abstract. Ice nucleation plays a critical role in cloud formation and atmospheric processes, influencing precipitation and climate. In this study, we present a theoretical approach for describing homogeneous ice nucleation within adsorbed water films on insoluble substrates, and suggest that it may be a mechanism for deposition ice nucleation with non-porous ice nuclei that induce ice premelting. Our theory is based on the Frenkel-Halsey-Hill (FHH) adsorption model, which characterizes the substrate-adsorbate interaction, and the classical nucleation theory of homogeneous freezing, which describes the probability of ice formation. We use the theory to model the melting point, critical ice nucleus size, and nucleation rates as functions of adsorbed water film thickness and substrate properties. Our results indicate that the melting point depression can be as much as 5 K on hydrophilic substrates when the thickness of the water film is 1 nm. The onset temperature for homogeneous ice nucleation (235 K for cloud droplets) can shift 1–2 K lower in adsorbed films. At temperatures below 235 K, the humidity at which ice nucleation occurs is determined by the condition that the adsorbed water film must be thick enough to accommodate the critical ice nucleus. Comparisons of calculated relative humidity conditions with experimental ice nucleation data for silica particles show promising agreement, validating the FHH model as a framework for describing deposition ice nucleation in the atmosphere.