There are now many measurements and observations of the changes in ozone that occur over Antarctica. Such measurements come from ground based instruments at the Antarctica research stations, from aircraft during scientific missions and from satellites.
Ozone loss was first detected in the stratosphere over the Antarctic.Although mid-latitude and Arctic depletion has also been observed, the loss is most dramatic in the lower stratosphere over the Antarctica continent, where nearly all the ozone is destroyed over an area the size of Antarctica within a layer in the lower stratosphere that's many km thick.

Source:http://www.atm.ch.cam.ac.uk/tour/part2.html