May 17, 1996
University of Colorado at Boulder scientists have discovered new evidence that chemical reactions implicated in the production of ozone-gobbling chlorine atoms are occurring on the surface of ice particles high over Antarctica.
An exhaustive supercomputer modeling effort indicates hydrochloric acid — a key player in the release of chlorine atoms that destroy stratospheric ozone — is broken down on the surface of ice particles found in stratospheric clouds over the Earth’s poles, said chemistry and biochemistry Professor James Hynes. The ozone hole that forms over Antarctica each spring is believed caused by a cascade of chemical reactions that begin on the cloud particles at temperatures of minus 118 F.
While scientists have known that hydrochloric acid molecules which stick to the surface of the ice crystals in the clouds play a pivotal role in ozone depletion, the specific chemical processes have been the subject of much debate, said Hynes. Some scientists had speculated the ionization, or breakdown, of hydrochloric acid in the ice particles could only occur when the icy surfaces were in a liquid-like form.
But the computer modeling project by the CU-Boulder researchers indicates hydrochloric acid molecules trapped at the surface by the ice particles can be broken down while still in the solid ice phase. The ionization triggers a stream of complex chemical reactions resulting in the eventual release of chlorine atoms, which destroy huge quantities of ozone molecules by stripping away oxygen atoms.
A paper on the findings by postdoctoral research associate Brad Gertner of the CU-Boulder chemistry and biochemistry department and Hynes was published in the March 15 issue of Science.
“A primary question has been whether hydrochloric acid could act like an acid, that is, ionize, while in this solid ice phase,” said Hynes. “Our results give us confidence that it does act like an acid at the surface of these ice particles.”
The CU-Boulder research effort indicates the icy surfaces of the cloud particles are “an active player” in the ozone depletion process, said Hynes. “It’s almost as if the ice is acting like an enzyme for these chemical reactions,” he said.
The hydrochloric acid molecules on the polar cloud ice crystals originate from chlorofluorocarbons such as refrigerants and aerosol propellants released into the atmosphere by humans, said Gertner. The compounds, which can persist for decades, are stored in vast “reservoirs” in Earth’s upper atmosphere.
The ozone layer, which lies in a band from 12 to 15 miles above Earth’s surface, protects life by shielding it from the harmful effects of solar ultraviolet radiation. Scientists believe decreasing ozone levels may lead to increased cancer in humans, reduced agricultural production and damaging effects on ecosystems.
Scientists, who first detected decreases in Earth’s ozone layer in the late 1970s, eventually traced the problem back to the release of chlorine and bromine compounds on Earth. Ozone has been decreasing by 15 percent annually over the southern polar region and by 7 percent per decade in the northern polar regions, according to the World Meteorological Organization.
The size of the ozone hole over Antarctica has reached record proportions in recent years, opening up a region about the size of Europe in spring 1995.
The CU-Boulder researchers spent two years using supercomputers at the National Center for Atmospheric Research in Boulder to make their calculations. The study was funded by the National Science Foundation and the Petroleum Research Fund, administered by the American Chemical Society.
This study gives us a stepping stone for making predictions about what may be happening to other molecules in the stratosphere,” said Gertner.
Hydrofluoric acid, a product of industrial CFC substitutes which is considered far safer for the health of the atmosphere, also is known to adhere slightly to the ice crystals in polar stratospheric clouds, said Hynes. “But our opinion at this point is that hydrofluoric acid does not ionize like hydrochloric acid.”
International agreements beginning with the 1987 Montreal Protocol are helping to reduce CFC emissions around the world, said Gertner. But it can take some ozone-destroying chemicals six to eight years to reach the upper atmosphere, where they can destroy ozone molecules for another 20 to 25 years.
T hese are the kind of studies that help us eliminate the guesswork,” said Gertner. “We need to understand what is happening in the atmosphere so that we don’t repeat the mistake that was made with CFCs.”