Snowflakes' Shape Changed by Pollution

Understanding the structure of individual snowflakes and what can alter them could help scientists get a handle on future climate.

Every snowflake may be unique, but the shape it takes is not random. Pollution and other impurities in the air may contribute to whether the ice crystal forms a flat plate, a spiky needle, or another shape as it falls.

"We really don't understand why a snowflake has the shape that it has. We know really pathetically very little," said atmospheric chemist Paul Shepson of Purdue University in West Lafayette, Ind.

Snowflake science may remain elusive, but understanding how ice forms is important for understanding our future climate, Shepson said, because many atmospheric chemical reactions take place on the ice surface. Different shapes will have different surface areas, which will affect the rate of these reactions.

"The presence of ice and snow has a big influence on what is in the atmosphere." he said. Predictions that sea ice may be gone by the middle of the century could, therefore, cause changes to the atmosphere. "We'd like to be able to predict how that massive change in the surface of the Earth will change the composition of the atmosphere," he added.

To do that requires a fundamental understanding of snowflakes' lacy forms -- which so far is lacking.

"This has been known for about 70 years and no one has really figured out why yet," said physicist Kenneth Libbrecht of the California Institute of Technology in Pasadena.

In recent experiments published in Atmospheric Chemisty and Physics, Shepson and graduate student Travis Knepp grew tufts of snowflakes on a string inside a chamber in the laboratory and compared the shapes formed by snowflakes made from clean water with those made from water containing acetic acid.

The presence of acetic acid shifted the temperature zones in which the needles and plates appeared. The lower temperature transition to plates happened about 25 degrees colder in the presence of acetic acid than without it.

They used acetic acid as a simple test substance, but they expect other pollutants would have similar effects. "The bottom line is that we learned that the shape of snow crystals can be influenced by the presence of pollution," Shepson said.

Libbrecht has preliminary findings that suggest that impurities in air that stick to the snowflake's surface may be crucial in dictating the shapes that form at different temperatures.

He found that snowflakes grew 100 times faster under low pressure than under normal, atmospheric pressure.

"What I think is happening is that when I remove the air, I suck out the impurities," Libbrecht said. "The theory holds water. You can calculate how long it would take for impurities to build up on ice crystals in air. I have this crazy hypothesis that every snowflake ever grown is covered in gunk," Libbrecht said.

It's not just human-made gunk, he said, but the volatile compounds put out by plants. "One of the things I like about this impurity hypothesis is that one of the problems, for 50 years, is, how can just plain ice show all of this variation with temperature? How can you have so much variation with temperature if there's nothing there but ice? Maybe it's not just ice."

If the surface is covered with traces of impurities, "now, it's a whole different story. It doesn't mean it's the right story, but up until this, nobody had a story," Libbrecht said.

Libbrecht emphasizes that his evidence is preliminary, but he is setting up experiments to test the idea by growing crystals in scrupulously clean air where no impurities will be present.

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