Chemistry in the Clouds

Dana Bauer
January 01, 1997

Instead of traveling to Antarctica to study the ozone hole, why not bring Antarctica to Central Pennsylvania?

Penn State graduate students Ed Mereand, Ron MacTaylor, and John Gilligan do something like that every day, recreating the Antarctic atmosphere in the comfort of A. Welford Castleman's chemistry lab.

"We make clouds and we make them cold," Mereand explains. As cold as -225 degrees Fahrenheit, matching the frigid winter conditions in the stratosphere a half a dozen miles above the south pole. To do so, they use a cloud-making machine called VERA, for Variable Energy Reaction Apparatus.

sky and clouds in beaker

The machine, spread out over a lab table, is a mass of tanks and tubes, some wrapped in electrical tape and aluminum foil. "I know it looks like spaghetti," says Mereand, "but this is the bare bones." Since it was first conceived by atmospheric scientists at NOAA, the National Oceanic and Atmospheric Administration, variations on this type of machine have been used to make different kinds of ions for more than 30 years. Where Castleman's is unique, however, is that it makes water cluster ions: charged ice particles that mimic, on a minute scale, the shape and composition of polar stratospheric clouds. Actual cloud particles contain on the order of millions of molecules. VERA's clusters consist of three to 30 molecules.

The presence of polar stratospheric clouds is key to the ozone-depleting chemical reactions that take place above the pole.Each spring, the ozone concentration above Antarctica drops by about half. But the destruction actually begins in the dark winter months when polar winds circle Antarctica, forming what Mereand calls "a leaky beaker in the sky." Air trapped in the beaker becomes extremely cold during the night. Temperatures drop low enough to form clouds even in the very dry stratosphere. These clouds provide the necessary surface for a series of ion-molecule reactions.

First, inert chlorine compounds, mostly from chlorofluorocarbons (CFCs) released at ground level, photochemically react, releasing their chlorine. This, in the form of chlorine nitrate and hydrochloric acid, dissolves on the cloud surfaces to form chlorine ions. There, the ions react to form more active chlorine species. These active molecules don't destroy ozone immediately, but lie dormant until the polar night lifts and spring sunshine breaks them down into aggressive chlorine atoms.

Ordinarily, as Mereand explains, these atoms would be bound up by nitrogen compounds present in the atmosphere, as part of a natural atmospheric equilibrium, and returned to an inert state. Unfortunately, however, the nitrogen compounds are themselves trapped in an inactive form by the presence of the polar stratospheric clouds. The combined effect—immobilization of the nitrogen compounds and the overabundance of chlorine—disrupts the critical balance between ozone destruction and formation.

In the lab, miniature polar stratospheric clouds are created in a plexiglas tube called a "poor man's ion source." An electrical discharge zaps water molecules to form the charged clusters. Helium gas cools them down. Once cooled, the clusters flow into what looks like a thermos bottle. Gaseous chlorine and nitrogen compounds, pumped into the container through teflon tubes, collide with the water clusters and react, simulating the beaker in the Antarctic sky.

"We're interested in the reaction rates," explains MacTaylor—in how fast the reactions are taking place. Adds Castleman, "When cloud particles are present, these reaction rates change dramatically." Pinning down these reaction rates and showing how they change will help computer modellers make larger and more realistic representations of the upper atmosphere—and perhaps see how to fix the ozone hole.

Ron MacTaylor and John Gilligan are doctoral students in chemistry in the Eberly College of Science; Ed Mereand received his Ph.D. in chemistry in August 1996. Their advisor is A. Welford Castleman Jr., Ph.D., professor of chemistry, 128 Davey Lab, University Park, PA 16802; 814-865-7242. This project is funded by the Atmospheric Sciences Section of the National Science Foundation.

Last Updated January 01, 1997