Jump to main content
Jump to site search

Volume 165, 2013
Previous Article Next Article

A water activity based model of heterogeneous ice nucleation kinetics for freezing of water and aqueous solution droplets

Author affiliations

Abstract

Immersion freezing of water and aqueous solutions by particles acting as ice nuclei (IN) is a common process of heterogeneous ice nucleation which occurs in many environments, especially in the atmosphere where it results in the glaciation of clouds. Here we experimentally show, using a variety of IN types suspended in various aqueous solutions, that immersion freezing temperatures and kinetics can be described solely by temperature, T, and solution water activity, aw, which is the ratio of the vapour pressure of the solution and the saturation water vapour pressure under the same conditions and, in equilibrium, equivalent to relative humidity (RH). This allows the freezing point and corresponding heterogeneous ice nucleation rate coefficient, Jhet, to be uniquely expressed by T and aw, a result we term the aw based immersion freezing model (ABIFM). This method is independent of the nature of the solute and accounts for several varying parameters, including cooling rate and IN surface area, while providing a holistic description of immersion freezing and allowing prediction of freezing temperatures, Jhet, frozen fractions, ice particle production rates and numbers. Our findings are based on experimental freezing data collected for various IN surface areas, A, and cooling rates, r, of droplets variously containing marine biogenic material, two soil humic acids, four mineral dusts, and one organic monolayer acting as IN. For all investigated IN types we demonstrate that droplet freezing temperatures increase as A increases. Similarly, droplet freezing temperatures increase as the cooling rate decreases. The log10(Jhet) values for the various IN types derived exclusively by T and aw, provide a complete description of the heterogeneous ice nucleation kinetics. Thus, the ABIFM can be applied over the entire range of T, RH, total particulate surface area, and cloud activation timescales typical of atmospheric conditions. Lastly, we demonstrate that ABIFM can be used to derive frozen fractions of droplets and ice particle production for atmospheric models of cirrus and mixed phase cloud conditions.

Back to tab navigation

Supplementary files

Publication details

The article was received on 08 Mar 2013, accepted on 23 Apr 2013 and first published on 24 Apr 2013


Article type: Paper
DOI: 10.1039/C3FD00035D
Citation: Faraday Discuss., 2013,165, 513-534

  •   Request permissions

    A water activity based model of heterogeneous ice nucleation kinetics for freezing of water and aqueous solution droplets

    D. A. Knopf and P. A. Alpert, Faraday Discuss., 2013, 165, 513
    DOI: 10.1039/C3FD00035D

Search articles by author

Spotlight

Advertisements