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Ice Crystal Model Aids Analysis of Ozone Depletion

When we think of ice, we think of something that makes drinks cold or causes slippery road conditions. But how molecules of ice interact in clouds and on the surface of the Earth affects weather, climate, and the global energy balance.

A KDI project team of chemists, physicists, and materials scientists at the University of Washington—with collaborators from the University of North Carolina, Pacific Northwest National Lab, the Supercomputer Computations Research Institute, and Los Alamos National Lab—set out to build a model that would help us understand these phenomena. The model they developed can simulate the interaction of water molecules under different conditions.

This new model is simple, yet produces very promising results. According to Fernando Vila, one of the investigators on this project, "We are currently studying the behavior of water molecules interacting on the surface of ice and are just starting to get results."

The investigators hope that these results will lead to a better understanding of the energy required for certain physical and chemical processes. For example, they expect the model to lead to a better understanding of how the ozone layer hole develops.

Ozone depletion occurs as a result of reactions in ice crystals that form in clouds that are very high in the atmosphere over Antarctica. Scientists used to think that ozone depletion was caused by a "single-step" process in which chlorofluorocarbons (CFCs) floating in the atmosphere were broken up by light. But breaking up CFCs takes a lot of energy, so scientists figured there must be something else going on.

Scientists now think that ozone depletion is a two-step process in which ice crystals act as a catalyst. They believe that as CFCs are floating in the atmosphere, they attach to the surface of ice crystals in the clouds. There, the ice helps break the bonds in the CFC molecules, forming an intermediate compound.

This intermediate compound accumulates in the ice crystals until summer comes. The light prompts the intermediate compounds to break up, releasing chlorine compounds that break the ozone molecules. As Fernando Vila explains, "Since the energy to break the intermediary is a lot less than the one you need to break the CFCs, the whole process is more efficient. That is why the size of the hole spikes so fast as soon as the summer comes: you have a reservoir of molecules ready to be broken and then start breaking the ozone."

The project team set out to find out more about the role played by ice crystals in ozone depletion. They plan to use their simulation model to learn how the molecules of the intermediate compound behave on the surface of the ice crystals. Vila says, "What happens is that the catalytic efficiency of the ice crystals depends on a series of parameters: how many molecules you have, if they interact with each other, what is the "shape" of the surface, if the molecules move away after they are broken down, if they are released back into the atmosphere or move inside the crystal, and so on. Those are the things you are interested in looking at."

With this new information about how these molecules behave, the researchers hope to get a fix on how much energy is needed to start the process of ozone depletion. As Vila explains, "What we want to do is to provide accurate energy values for as many processes as possible, in that way helping create the best possible global models." With these models, scientists hope to be able to test replacements for CFCs and determine which alternatives cause the least ozone damage.

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