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FIGURE 8.11 (a) Antarctic measurements of ozone at Halley Station (75° south, 26° west) showing average concentrations in October, when ozone levels generally are lowest each year. Ozone concentrations are measured in Dobson units, in parts per billion, which represent the amount of ozone in a square-centimeter column through the atmosphere. Based on data from the British Antarctic Survey (http://www.nerc-bas.ac.uk/public/icd/jds/ozone/data/zoz5699.dat). (b) The ''ozone hole'' over Antarctica on 7 October 2000, showing the circumpolar concentrations of stratospheric ozone, which were minimal over the continent and lower part of South America and increasing toward the lower latitudes (continental perimeters are outlined). Based on ''Earth Probe'' satellite data collected with the total ozone mapping spectrometer (TOMS) from the Goddard Space Flight Center, National Aeronautics and Space Administration (http://toms/gsfc.nasa.gov/ozone/ozone01.html).

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