Brian Dunbar
Headquarters, Washington, DC
(Phone: 202/358-1547)
December 19, 1994

Allen Kenitzer
Goddard Space Flight Center, Greenbelt, MD
(Phone: 301/286-8955)
RELEASE: 94-215


Three years of data from NASA's Upper Atmosphere Research Satellite (UARS) have provided conclusive evidence that human-made chlorine in the stratosphere is the cause of the Antarctic ozone hole.

UARS instruments have found chlorofluorocarbons (CFCs)--human-made products used in electronics and refrigeration systems--in the stratosphere. The satellite's global data set also has traced worldwide buildup of stratospheric fluorine gases corresponding to the breakdown of CFCs, according to NASA scientists.

For many years, scientists have warned that the widespread use of chlorofluorocarbons in refrigeration, spray cans and foam packaging was responsible for stratospheric ozone loss. The stratospheric ozone layer protects people, animals and plants from too much ultraviolet sunlight. The Antarctic ozone hole is a dramatic example of stratospheric ozone loss, which most scientists believe is a new phenomenon caused by the release of chlorine from human-made chlorofluorocarbons.

In the past few years, some debate has occurred over the origin of ozone-destroying chlorine. Sea spray and volcanic gases have been put forth as possible sources for chlorine reaching the stratosphere. The UARS data have ended that debate.

"These new results confirm our theories about CFCs," said Dr. Mark Schoeberl, UARS Project Scientist. "The detection of stratospheric fluorine gases, which are not natural, eliminates the possibility that chlorine from volcanic eruptions or some other natural source is responsible for the ozone hole." In addition to CFCs, UARS has detected hydrogen fluoride, a product of the chemical breakdown of CFCs, in the stratosphere.

"Hydrogen fluoride has no natural source, it is not produced by volcanic eruptions or salt spray," said Dr. Anne Douglass, UARS Deputy Project Scientist. "Furthermore, scientists can calculate how much chlorine in the stratosphere is man-made using the hydrogen fluoride data." This calculation shows that almost all of the chlorine in the stratosphere comes from human-made chlorofluorocarbons.

The UARS measurements of chlorofluorocarbons were made with the Cryogenic Limb Array Etalon Spectrometer, operated by Dr. Aiden Roche of Lockheed Palo Alto Research Laboratory. The hydrogen fluoride measurements were made with the Halogen Occultation Experiment, operated by Dr. James Russell of NASA's Langley Research Center, Hampton, VA.

Each year since 1979, the ozone layer thins dramatically over Antarctica. This sudden change in the ozone was first noticed by researchers in Antarctica and soon confirmed by NASA satellites. The unpredicted Antarctic ozone loss gave scientists a challenging puzzle. Aircraft observations in 1987 showed convincingly that the high concentrations of chlorine monoxide over Antarctica were destroying ozone in the lower stratosphere. Most scientists were convinced that a series of chemical reactions involving chlorine monoxide and ozone led to the formation of the ozone hole.

Two questions, however, remained: why was the change in the ozone layer taking place over Antarctica, and what was the source of the chlorine monoxide? Meteorologists long have known that the Antarctic stratosphere can be one of the coldest places on the planet. Air is so cold that wispy clouds can form even in the super-dry stratospheric air. These clouds, called polar stratospheric clouds, form in the dead of winter. Scientists believe that chemical reactions on the surface of the cloud crystals release chlorine from "reservoir" gases, which do not react with ozone. The chlorine reacts quickly with ozone to form chlorine monoxide. This reaction begins the catalytic cycle in which one chlorine atom can ultimately destroy many ozone molecules, leading to the polar ozone hole.

UARS has measured the winter build up of chlorine monoxide within the south and north polar regions every year since its launch. UARS has found that chlorine monoxide appears suddenly in the stratosphere after the formation of the polar stratospheric clouds. Infrared and microwave sensors on board UARS are able to track stratospheric clouds and the chemical changes they cause.

UARS measurements have confirmed that the chlorine monoxide can build up to extreme levels in the polar regions after polar stratospheric clouds appear. UARS data also have shown that the meteorology of the polar stratosphere prevents the chlorine monoxide from dispersing, thus increasing the ozone loss.

"We are getting daily polar maps of ozone-destroying chemicals," said Douglass. "These measurements are adding tremendously to our knowledge of the stratosphere."

The UARS data set also has provided a clearer picture of the overall chemistry of the stratosphere. UARS instruments have tracked the levels of chlorine "source" gases (CFCs), intermediate products (chlorine monoxide) and reservoir gases (hydrogen fluoride, hydrogen chloride and chlorine nitrate).

Under international treaties controlling the use of ozone-depleting chemicals, the amounts of CFCs in the atmosphere no longer are increasing. However, CFCs survive in the atmosphere for many years before being destroyed by ultraviolet light, and the ozone hole is expected to persist at current levels through this decade. (Their stability was one of their biggest assets when they were developed for industrial use in the 1930s.) Unless other conditions change, scientists expect the ozone hole to weaken and disappear in the 21st century.

UARS was the first satellite launched as part of NASA's Mission to Planet Earth, a comprehensive study of how the Earth's global environment changes, and how human activities contribute to that change. Mission to Planet Earth includes satellites, Space Shuttle instruments, aircraft research and ground teams. Goddard Space Flight Center, Greenbelt, MD, manages UARS for NASA's Office of Mission to Planet Earth, Washington, DC.

Note to Editors: Photographs, satellite imagery and a video, "Beyond the Clouds," describing the UARS mission, are available from NASA's Broadcast and Imaging Branch by faxing your request to 202/358-4333.