Heterogeneous ice nucleation : laboratory freezing results and testing different schemes to describe ice nucleation in atmospheric models

Ice nucleation occurs throughout the atmosphere. Some atmospheric ice particles are formed through nucleation on insoluble atmospheric aerosols known as ice nuclei (IN). The abundance and chemical composition of these IN affect the properties of clouds and in turn the radiative balance of the Earth through the indirect effect of IN on climate. The indirect effect of IN on climate is one of the least understood topics in climate change. A better understanding of ice nucleation and better capabilities to parameterize ice nucleation are needed to improve the predictions of the effect of IN on climate. Using a temperature and humidity controlled flow cell coupled to an optical microscope, the ice nucleation properties of three different mineral dust particles are examined in two different freezing modes. Results showed that the freezing ability of supermicron dust particles is lower than that of submicron dust particles of the same type. These freezing results along with literature freezing results of nine biological aerosol particles are used to evaluate different schemes used to parameterize ice nucleation in atmospheric models. These schemes are evaluated based on the ability to reproduce the laboratory freezing results. It was found that a single parameter scheme based on classical nucleation theory was unable to reproduce the freezing results of all particles studied. However, more complex schemes were able to reproduce the freezing results. The results in this thesis can be used by atmospheric modellers to improve predictions of mixed-phase and ice clouds and climate change.Science, Faculty ofChemistry, Department ofGraduat