Abstract
Abstract. Cirrus clouds play a key role in the radiative and hydrological balance of the upper troposphere. Their correct representation in atmospheric models requires an understanding of the microscopic processes leading to ice nucleation. A key parameter in the theoretical description of ice nucleation is the activation energy, which controls the flux of water molecules from the bulk of the liquid to the solid during the early stages of ice formation. In most studies it is estimated by direct association with the bulk properties of water, typically viscosity and self-diffusivity. As the environment in the ice–liquid interface may differ from that of the bulk, this approach may introduce bias in calculated nucleation rates. In this work a theoretical model is proposed to describe the transfer of water molecules across the ice–liquid interface. Within this framework the activation energy naturally emerges from the combination of the energy required to break hydrogen bonds in the liquid, i.e., the bulk diffusion process, and the work dissipated from the molecular rearrangement of water molecules within the ice–liquid interface. The new expression is introduced into a generalized form of classical nucleation theory. Even though no nucleation rate measurements are used to fit any of the parameters of the theory the predicted nucleation rate is in good agreement with experimental results, even at temperature as low as 190 K, where it tends to be underestimated by most models. It is shown that the activation energy has a strong dependency on temperature and a weak dependency on water activity. Such dependencies are masked by thermodynamic effects at temperatures typical of homogeneous freezing of cloud droplets; however, they may affect the formation of ice in haze aerosol particles. The new model provides an independent estimation of the activation energy and the homogeneous ice nucleation rate, and it may help to improve the interpretation of experimental results and the development of parameterizations for cloud formation.
Highlights
Ice nucleation in cloud droplets and aerosol particles leads to cloud formation at low temperatures and promotes cloud glaciation and precipitation (Pruppacher and Klett, 1997)
This work advances a theoretical description of the process of the interface transfer of water molecules from the liquid phase to ice during the early stages of nucleation
Instead a theoretical approach is proposed where the interaction of several water molecules is required for interface transfer
Summary
Ice nucleation in cloud droplets and aerosol particles leads to cloud formation at low temperatures and promotes cloud glaciation and precipitation (Pruppacher and Klett, 1997). Modeling and experimental studies suggest a significant contribution of homogeneous freezing to the formation of clouds in the upper troposphere (Barahona and Nenes, 2011; Barahona et al, 2014; Gettelman et al, 2012; Jensen et al, 2013). The parameterization of ice nucleation is critical to the proper representation of clouds in atmospheric models and is mostly done using empirical formulations (e.g., Lohmann and Kärcher, 2002; Kärcher and Burkhardt, 2008; Barahona et al, 2010, 2014). The most common approach uses the so-called water activity criterion (Koop et al, 2000), where the homogeneous nucleation rate, Jhom, is parameterized in terms of the difference between the water activity, aw, and its equilibrium value, aw,eq The greatest advantage of the water activity criterion is that it is independent of the nature of the solute and facilitates the formulation of general parameterizations of ice nucleation (Barahona and Nenes, 2008; Kärcher and Lohmann, 2002; Liu and Penner, 2005).
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
