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Building a theory for heterogeneous ice nucleation

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Title: Building a theory for heterogeneous ice nucleation
Author(s): Ignatius, Karoliina
Contributor: University of Helsinki, Faculty of Science, Department of Physics
Language: English
Acceptance year: 2013
The ice crystals in the Earth's atmosphere have a considerable impact on cloud optical and radiative properties, such as reflectivity, cloud dynamics, chemical processes and initiation of precipitation and cloud lifetime. Thus, they directly affect the albedo of Earth which in turn is a significant factor concerning the climate change. The first step of ice formation, the phase transition from liquid to solid, is called ice nucleation. Ice has been identified to form in the atmosphere both homogeneously - without the presence of a foreign substance - and heterogeneously, i.e. induced by pre-existing surfaces. Heterogeneous ice nucleation is the primary pathway of ice formation in the atmosphere. Also homogeneous ice nucleation happens at the upper troposphere, but it requires very low temperatures, whereas in heterogeneous freezing the free energy needed for crystallization is lower and freezing can happen at higher temperatures, normally above -37 ◦C. In heterogeneous ice nucleation, there are four freezing mechanism called modes that describe the onset conditions for freezing. They are called deposition, immersion, condensation and contact modes. The seed particles on which ice forms in heterogeneous ice nucleation are called ice nuclei (IN). Typical IN particles include mineral dust, soot, metal, bacteria and other bioaerosols (pollen, fungal spores), humic-like substances, solid ammonium sulphate, organic acids and volcanic ash. This thesis is a literature survey on the current theoretical knowledge on heterogeneous ice nucleation. It has been long known that the classical nucleation theory (CNT), if employed with one constant contact angle does not reproduce the experimental results. This is due to the fundamental assumptions of CNT: the spherical form of the initial ice embryo having properties of the bulk ice crystal; uniform surfaces; equal nucleation probability for each particle; and stochastic freezing behaviour. To solve this challenge, several theoretical and empirical extensions of CNT have been derived: the use of a distribution of contact angles and active sites; integrating individual nucleation rates over measured size spectra of the IN; and using Ice Nuclei Active Surfaces Sites (INAS) density as a normalized metric from different experiments. As a result of this literature survey, three main lines of research on this field can be distinguished: (1) ice nuclei characterisation; (2) theoretical and empirical modelling of the heterogeneous ice nucleation scheme; (3) parameterising heterogeneous ice nucleation for cloud and climate models. These lines are not altogether separate, but intertwined: knowledge of e.g. the surface properties of the IN is essential for deriving theoretical formulations. Parameterisations, on the other hand, are very much needed in order to obtain knowledge about the indirect climatic effect of ice in the atmosphere.

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