Extension of THESEAO was a joint project between the Centre of Atmospheric Sciences at the university of Cambridge and the National Physical Laboratory.

Understanding the observed loss of ozone in the mid-latitude lower stratosphere was a central objective of the NERC UT/LS Ozone thematic programme. The challenge was to understand the causes of the observed small, long-term changes in ozone. In particular, chemically induced loss had to be separated from change due to transport. This was a much more difficult problem than in the Antarctic spring where ozone loss is very substantial, and where transport plays a relatively small role.

Ozone loss in the Arctic has been substantial in recent winters. However, the appearance of low ozone at a particular altitude does not necessarily imply that chemical destruction of ozone has occurred. Low ozone could instead have been transported, horizontally or vertically, from another location. To confirm the processes involved, coincident measurements of long-lived tracers are required.

Objectives
The overall scientific objective within this proposal was to contribute to the understanding of middle latitude ozone loss by making measurements of a number of important tracers of atmospheric motion and photochemistry, and by interpreting these measurements with state-of-the-art models of atmospheric chemistry and transport.

The particular aim of the proposal was to extend THESEO measurements of tracer and chemically active gases both spatially and temporally to provide more comprehensive coverage in the middle latitude low stratosphere and upper troposphere. The measurements were complement the similar EU funded measurements whose focus were primarily high and mid-latitudes in 1998/99. They were part of THESEO 2000 which was an extension of THESEO and which formed the basis of European collaboration with US SOLVE experiment which is studying Arctic ozone loss in the 1999/2000 winter.
Together, these measurements provide a unique data set for the study of chemistry and transport processes at mid latitudes. They studied annual transport through the middle latitude lower stratosphere and the processes of mixing with tropical and polar air.

Data were interpreted using 3D chemical transport models already developed at Cambridge. The large amount of data collected in polar middle and tropical latitudes during THESEO provided a unique opportunity for the new mid-latitude data to contribute to the understanding of middle latitude ozone decline and, in particular, to understanding the relative importance of in situ ozone loss and transport from other regions.

Balloon flights were made from Esrange Kiruna station during the 1999/2000 winter, with balloon payloads incorporating the same UK and European instruments deployed during THESEO.