Polar Ozone and Aerosol Measurement (POAM) is a satellite-borne instrument that monitors ozone in the Arctic and Antarctic stratosphere year-round. POAM was developed by NRL to measure the vertical distribution of ozone with a vertical resolution of one kilometer, obtaining 12 measurements each day around a circle of latitude in each hemisphere. The image shows a time series of POAM II measurements of ozone concentration in the lower stratosphere (altitudes between 15 and 35 kilometers). The period begins when POAM II started collecting atmospheric data in mid- October 1993 and ends in November 1996. During this time, POAM II observed all or part of the life cycle of four ozone holes over Antarctica.
The ozone hole is a severe destruction of ozone in the Antarctic stratosphere that begins in early September and ends in November, during the Antarctic spring. It is apparent from the picture that the destruction extends over a considerable vertical region, from below 15 kilometers to approximately 25 kilometers in altitude.
Conditions for the ozone hole are established early in the Antarctic fall and winter (May through August). During this time, temperatures become low enough to support the growth of Polar Stratospheric Clouds and a wind pattern called the Polar Vortex. The Polar Vortex has been shown to be like a laboratory beaker, containing the polar air and preventing it from mixing with the subpolar air. Chemical reactions on the surface of Polar Stratospheric Cloud particles transform chlorine into a form that can be activated by sunlight. When the Sun rises on the springtime Antarctic stratosphere, its light triggers a complex series of chemical reactions resulting in severe destruction of ozone inside the Polar Vortex. Later in the spring, the concentration of ozone in the polar region increases as the Polar Vortex disintegrates due to rising temperatures and altered winds, allowing undepleted nonpolar air to enter the polar region.