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07 October 2011

Long Cold Spell Leads to First Arctic Ozone Hole

(Click on image for larger view.)

In March 2011, the Earth Observatory published images of the first Arctic ozone hole ever observed. The images came from daily observations made by the Ozone Monitoring Instrument on NASA’s Aura satellite. This week, a new paper in Nature uses a companion instrument on Aura, the Microwave Limb Sounder, to help describe why the ozone hole formed.
The Microwave Limb Sounder (MLS) looks through the edge of Earth’s atmosphere to measure gases, in this case ozone and chlorine monoxide, one of the most dominant ozone-destroying gases. These MLS images of ozone and chlorine monoxide concentrations were made with data collected on March 18, 2011. In both cases, high concentrations of the gas are dark, while low concentrations are light. The left image shows high ozone concentrations over most of the planet, but very low concentrations—the ozone hole—over the Arctic. The right image shows chlorine monoxide concentrations. In this case, the gas is concentrated over the Arctic in the region where the ozone hole developed.
The correlation between high chlorine monoxide and the ozone hole is no accident. Chlorine monoxide reacts with sunlight and ozone in a series of reactions that ultimately destroy ozone. What is unusual about the image is the concentration of chlorine monoxide. The gas forms naturally in the atmosphere only in very cold conditions where clouds or other particles are present in the atmosphere. Such conditions are extremely rare and are usually only found over Antarctica in the winter. In 2011, however, the Arctic also stayed cold long enough for ozone-destroying chlorine gases to build in the atmosphere.
The research published in Nature and led by Gloria Manney of NASA’s Jet Propulsion Laboratory, describes the conditions that allowed these high concentrations of chlorine monoxide to develop and destroy ozone high in the stratosphere. First, the atmosphere between 15 and 23 kilometers in altitude (part of the stratosphere) was persistently cold for more than 100 days, more than a month longer than in any previous year. Temperatures were cold enough that polar stratospheric clouds formed.
The clouds provided a surface for stable chlorine gases to turn into reactive gases like chlorine monoxide. In 2011, conditions in which such clouds could form were present in more of the atmosphere over the Arctic than ever seen before. The cold conditions lasted through the end of March.
The second circumstance that allowed chlorine monoxide to build up was a strong polar vortex. In the winter, polar winds isolate the Arctic. These winds, called the polar vortex, were stronger than normal and persisted from December through March. They prevented air over the Arctic from mixing with the rest of the atmosphere, so chlorine monoxide could build up.
Both the cold conditions and the strong isolating winds lasted through the end of March. By that time, enough sunlight had returned to the Arctic that the ozone-destroying reactions could take place. The difference between 2011 and other winters, says Manney, is that temperatures were low enough to produce ozone-destroying forms of chlorine for a long time. Colder winters have happened before, but the cold temperatures have never persisted for such a long time in the Arctic.
While it’s not clear why cold temperatures lasted so long in 2011, the fact that they did has some implications for the formation of an Arctic ozone hole in the future. If stratospheric temperatures drop as a result of climate change, then Arctic ozone holes may happen more frequently. On the other hand, the study authors note, if the Montreal Protocol hadn’t limited the concentration of ozone-depleting gases in the atmosphere, then an Arctic ozone hole would form regularly, even in relatively warm winters.
Ozone is made up of three oxygen molecules. Small amounts of the gas in the stratosphere absorb life-damaging ultraviolet light from the Sun. Ozone on the ground is toxic.
  1. References

  2. Buis, A. (2011, October 2). NASA leads study of unprecedented Arctic ozone loss. NASA Jet Propulsion Laboratory. Accessed October 5, 2011.
  3. Carlowicz, M. (2009, May 13). The world we avoided. NASA Earth Observatory. Accessed October 5, 2011.
  4. Manney, G.L., Santee, M.L., Rex, M., et al. (2011, October 2). Unprecedented Arctic ozone loss in 2011. Nature. Accessed October 5, 2011.
  5. NASA Goddard Space Flight Center. (2011, August 12). What is the ozone hole? Accessed October 5, 2011.
NASA image by Eric Nash and Robert Simmon with data from the Aura Microwave Limb Sounder team. Caption by Holli Riebeek.
Aura - MLS - NASA