The extratropical tropopause transition layer (ExTL) as seen from ACE

Characterizing the chemical composition of the upper troposphere/lower stratosphere (UTLS) is challenging because of the small vertical and horizontal length scales involved. Most existing knowledge of the spatio-temporal structure of chemical composition in the UTLS, and its relation to dynamical features such as the tropopause, stems from aircraft and balloon platforms. On the other hand, these measurements are necessarily restricted in space and time. The Atmospheric Chemistry Experiment is a Fourier transform spectrometer (ACE-FTS) on board the Canadian SCISAT satellite offering unprecedented accuracy and vertical resolution, allowing a new global perspective on trace gas distributions in the UTLS. We examine vertical profiles of O3, CO, and H2O relative to the local tropopause height, and show by comparison with previous aircraft measurements in the Northern Hemisphere the high potential of the ACE measurements in resolving the vertical structure of UTLS tracer distributions. We then use the ACE measurements to derive the thickness of the extratropical transition layer – i.e. the layer surrounding the extratropical tropopause which exhibits both tropospheric and stratospheric characteristics – using different tracer-tracer correlations, discuss interhemispheric differences and potential implications for chemistry.