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2. The scientific objectives

The APE-GAIA campaign aimed at studying several key scientific issues related to chemistry and transport at the boundary of the southern polar vortex (Carli and Blom, 1997). The campaign took place from the operative base of Ushuaia, Argentina (lat 55° S, long 68° W) in the period from 15^th September to 15^th October 1999, thus allowing observations at the transition between the end of stratospheric ozone depletion and the beginning of the recovery phase of chlorine reservoir species. Flying southward, over the Antarctic Peninsula, up to the limit of its operative range (approximately 70° S) and at altitudes between 14 and 20 km (with occasional "dives" down to 10 km), the M55-Geophysica was able to reach the edge of the vortex and to penetrate inside the chemically perturbed region. The aircraft performed five scientific flights (for a total of 30 flight hours), during which the main targets of the mission were investigated using both remote-sensing and in-situ techniques.

In a detailed planning of the scientific objectives to be pursued during the Antarctic mission (Redaelli and Chipperfield, 1999), the following priorities were identified:

• to study the mixing out of the polar vortex as a function of altitude;
• to measure chemical composition of mid and high latitude lowermost stratosphere;
• to observe diurnal variation of chemical species at mid latitudes;
• to check whether chlorine activation by lee-waves can be predicted/ detected;
• to observe chlorine de-activation in the vortex;
• to measure ozone loss rate inside the polar vortex;
• to define and detect vortex edge and filaments.

Most of these goals were achieved, taking advantage of the profiling capabilities of the remote-sensing instruments, as well as of the information on chemical composition and physical properties of the air masses observed, respectively, by the in-situ chemistry package and by the microphysics payload.

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