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A Season in the Life of the Ozone Hole As Seen by NASA’s TOMS instrument
Click to view 25 Years of TOMS
Each year, the Antarctic “Ozone hole,” which is actually a dramatic thinning of ozone, begins opening every July and shrinking in November. NASA’s Total Ozone Mapping Spectrometer (TOMS) continues to provide dramatic visual evidence of its annual growth and decay.
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| July 1, 2003 |
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August 31, 2003 |
The Antarctic ozone hole was first discovered in 1985 by Dr. Joseph Farman and his colleagues at the British Antarctic Survey. Dr. Farman used ground-based observations at Halley Bay station, Antarctica, of overhead ozone covering the period from 1958 to 1984 to show that ozone had decreased dramatically in October in comparison to the early years. This work was quickly followed by a satellite study by Dr. Richard Stolarski using the TOMS instrument aboard the Nimbus-7 satellite. The satellite study verified the results of the Farman paper, and showed that the ozone decrease covered nearly all of Antarctica. We now understand that these ozone losses over Antarctica result from man-made chlorine and bromine compounds and colder temperatures.
Temperatures cool down in the Antarctic stratosphere when darkness blankets the South Pole from March 21st to September 21st because the pole is tilted away from the Sun during that time. That period is called the “polar night.” By the end of May, and still under the cover of constant darkness of the polar night, temperatures become cold enough to form the thin polar stratospheric clouds (PSCs).
Once the clouds form,relatively stable chlorine compounds in the stratosphere react on the surfaces of the cloud particles to release chlorine into a more reactive form. The return of the Sun during September over Antarctica provides these chlorine and bromine compounds with the energy to rapidly destroy ozone.
By early July Antarctica lies in darkness, and the PSCs have converted all of the chlorine from non-reactive forms into forms that only require some of the Sun’s energy to spark ozone molecule breakdowns.
As the Sun appears over Antarctica in August, its energy triggers the chemical reactions that cause the chlorine and bromine molecules to break down the ozone molecules. In a complex chain of chemical reactions, the chlorine and bromine molecules convert ozone molecules into oxygen molecules, but regenerate themselves. Once regenerated, the chlorine and bromine can cause more ozone destruction. This destruction and regeneration cycle is known as a catalytic cycle. A single chlorine molecule can thus destroy about 1000 ozone molecules.
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| September 29, 2003 |
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November 03, 2003 |
Another factor in ozone depletion is weather systems of high and low pressure. Weather systems develop in the lower atmosphere and move into the stratosphere as they sweep eastward around Antarctica. These highs and lows are seen by the TOMS satellite because ozone moves over and around them as they follow along the jet stream that circles Antarctica. Viewed from above, the ozone “highs” and “lows” circle clockwise around Antarctica. The jet stream acts as a barrier to prevent the mixing of ozone-rich air from the mid-latitudes with the ozone-depleted air over Antarctica.
During the August and September period, the ozone hole becomes deeper as ozone is destroyed over Antarctica, and as ozone accumulates in the mid-latitudes.
By late-September, the ozone hole is at its maximum depth. In the middle of the ozone hole, the chlorine and bromine molecules have completely destroyed all of the ozone in the layer between 12 and 20 km (7.4 and 12.4 miles). There is, however, still some ozone above 20 km and below 12 km.
At the edge of the ozone hole, near the jet stream, temperatures are warmer, and PSCs are less frequent, so ozone loss is not quite as severe. Nevertheless, large ozone losses have also occurred near the jet stream. In addition, by late-September, temperatures have risen to the point that PSCs can no longer form. This warming shuts off the conversion of chlorine into reactive forms and chlorine begins to return to its non-reactive forms that cannot break down ozone molecules.
By mid-October, the ozone losses have completely stopped, and we’re left with a large region of low ozone levels over Antarctica. The strong jet stream begins to slow down, and temperatures continue to warm. At this point, ozone-rich mid-latitude air begins to push into the polar region and the ozone hole begins to mix with mid-latitude air. This mixing process is seen by TOMS in occasional streamers of ozone-depleted air coming off of the ozone hole, a gradual rise of the low ozone values over Antarctica, and a decrease of the size of the ozone hole.
By early December, the ozone hole has largely disappeared. In spite of the hole’s apparent disappearance, the ozone losses have a lasting effect by lowering ozone levels across the Southern Hemisphere. A simple analogy is to dip a cup of water out of a bucket. Water rushes in to fill the volume dipped out, and the water level drops in the bucket. Similarly, ozone from the mid-latitudes will fill and mix with the depleted layers of ozone over Antarctica, but ozone across the Southern Hemisphere will be reduced overall.
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| TOMS Earth Probe |
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Aura satellite |
After 25 years of monitoring ozone, the TOMS instrument observations will be turned over to its successor in 2004, NASA’s Earth Observing System (EOS) Aura satellite. Aura will study the Earth's ozone, air quality and climate and is scheduled for launch in early to mid 2004. This mission is designed exclusively to conduct research on the composition, chemistry and dynamics of the Earth's upper and lower atmosphere employing multiple instruments on a single satellite. EOS Aura is the third in a series of major Earth observing satellites to study the environment and climate change and is part of NASA's Earth Science Enterprise. Aura's chemistry measurements will also follow up on measurements which began with NASA'S Upper Atmospheric Research Satellite and continue the record of satellite ozone data collected from the TOMS missions.
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