Long Cold Spell Leads to First Arctic Ozone Hole

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.

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