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It is now widely accepted that depletion of stratospheric ozone in the polar regions is facilitated by the confinement of the air masses within the polar vortex, and by the heterogeneous chemical reactions occurring on polar stratospheric cloud (PSC) particles. Considerable uncertainty exists, however, with respect to the role of the various catalytic cycles of ozone removal and to the interactions between air masses of the polar stratosphere and middle latitudes.
The need for new experimental data, related to chemistry and transport issues, is particularly strong for the middle and high latitudes of the southern hemisphere. The altitudes of the low stratosphere are barely accessible with ground-based and satellite observations and, compared to the Arctic region, the southern hemisphere has been less sampled by the most recent measurement campaigns.
The objectives of the APE-GAIA campaign have been identified in the light of these considerations. The priority assigned in this mission to the study of chemistry, together with the new observation capabilities offered by remote sensing measurement techniques, have identified the beginning of the southern spring as the optimum period for observations. This transition period between the depletion phase (August-September) and the recovery phase (October-November) is preferable, compared to the activation phase (May-July), which is more suitable for microphysical observations (in particular, because of the higher probability of formation of PSCs).
In the depletion phase, key aspects are both the study of the most important catalytic cycles involved in ozone chemistry, as a function of latitude and altitude, and the possibility of extending the measurements to those compounds for which the observation data base is very limited (for example, HBr and HOBr in the bromine family). Similar interest exists for observations conducted at the beginning of the recovery phase, because of the analysis of the reconversion processes of the active chlorine species into the reservoir compounds ClONO2 and HCl.
Finally, another aim of the observations is to clarify the extent and altitude region of mixing of polar air masses with middle latitudes, in order to determine how much of the ozone losses in middle latitudes are due to transport or dilution effects of the vortex.
Observations will also be carried out during the return
transfer flight of the aircraft from South America to Europe.

The stratospheric ozone layer shields the
biosphere from UV radiation, capable of
damaging primary forms of life such as
marine phytoplankton and affecting
human beings directly though diseases
such as skin cancer.

Satellite measurements of the ozone column
over the Antarctic region obtained by GOME
(Global Ozone Monitoring Experiment) in
1996 and 1997. The maps highlight the ozone
losses at the beginning and end of the period
corresponding to the one covered by the
APE-GAIA campaign. (Courtesy of Deutsches
Zentrum für Luft- und Raumfahrt, DLR).