Kinetics and Mechanisms of Ice Nucleation in the Atmosphere

Ice clouds in the troposphere play a key role in the Earth’s climate system by strongly influencing the Earth’s radiative properties. These clouds form when ice nucleates on or in aerosol particles in the troposphere. Shown below are different mechanisms possible for ice nucleation from atmospheric aerosols.
Our research focuses on understanding these mechanisms and determining the kinetics of these processes. Results from this project should provide building blocks for accurately representing ice clouds in climate models. The end result, improved models, should provide better tools to investigate the impacts of increased levels of CO2 due to fossil fuel combustion upon the physical environment and to better understand and address climate change.

The effect of acid coatings on the ice nucleation properties of mineral dust
Mineral dust particles are abundant in the atmosphere, and both laboratory and field measurements have shown that mineral dust particles are effective ice nuclei. During their lifetime in the atmosphere, mineral dust particles can become coated with sulfuric acid. We are investigating if these coating effect the ice nucleation properties of these particles. This research should be useful when trying to determine if anthropogenic emissions of SO2 affect climate by influencing natural ice nuclei such as mineral dust. The research involves measuring the kinetics of ice nucleation in cloud chambers that simulate atmospheric conditions as well as sum frequency generation (an advance spectroscopic technique) and x-ray photoelectron spectroscopy to monitor surface modification of the mineral dust particles. The sum frequency generation measurements are being carried out in collaboration with Keng Chou (UBC).

Formation of cubic ice under atmospheric conditions
Under atmospheric conditions both cubic ice and hexagonal ice can be obtained. However, it is almost always assumed that hexagonal ice, the thermodynamically stable phase, is the only polymorph that forms under atmospheric conditions. Recently, we showed that when aqueous aerosols freeze homogeneously at temperatures less than 200 K they crystallize as cubic ice under atmospheric conditions. This may be of importance because the formation of cubic ice in the atmosphere may lead to a redistribution of water vapour in the upper troposphere and lower stratosphere, which is reported to affect polar stratospheric cloud formation, ozone destruction, and the Earth’s radiation budget. Future work will investigate the phase of ice formed by deposition and immersion freezing.

Ice nucleation in the arctic region
The goal of this project is to understand and predict the properties and formation conditions of atmospheric ice clouds and mixed-phase clouds (clouds that contain both ice particles and liquid water droplets) in the Arctic region. We will accomplish this goal by carrying out the following tasks:

(1) Through a series of focused laboratory studies, we will determine conditions at which ice nucleates on or in aerosol particles relevant for the Arctic region. Specifically, we will determine the conditions at which ice nucleates in water, aqueous ammonium sulfate, and aqueous sulfuric acid particles containing soot and mineral dust inclusions.

(2) From the laboratory data, we will determine the parameters necessary for improving the description of ice nucleation in atmospheric models.

The results from the laboratory experiments will be used to derive ice nucleation rates as well as relationships between the probability of freezing and temperature, ice saturation ratio, and size of the insoluble inclusions. These results will then be used to determine the critical energy of the ice embryo as a function of temperature and ice supersaturation for each aerosol type tested in the laboratory. This data will then be used as input for atmospheric models. The second task will involve a collaboration with Éric Girard of the University of Quebec at Montreal.

Theory to describe (and parameterize) ice nucleation
Classical Nucleation Theory is often used to describe and parameterize ice nucleation in atmospheric models. We are carrying out controlled laboratory studies to test the limitations of this theory.

The electrodynamic balance instrument

The conditions required for freezing will be measured using an electrodynamic balance. The electrodynamic balance (EDB) is a double-ring electrode configuration with two end cap electrodes (diagram at right). It is based on the configuration used by the Agnes group at Simon Fraser University (SFU). Single droplets are created using a commercial droplet generator, and the droplets are charged by induction. Charged particles can be trapped and suspended at the centre of the balance for hours to days. Light scattering will be used to determine if a particle crystallizes while suspended in the trap.


Optical microscopy

We are using an optical microscope coupled to a flow chamber operating at low temperatures to conduct ice nucleation studies. A CCD camera interfaced to a computer allows for the capture and storage of images during an ice nucleation experiment.

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