SCIAMACHY is the atmospheric chemistry sensor on ENVISAT designed to determine the amounts of trace gases and aerosol in the lower troposphere, including the planetary boundary layer. From SCIAMACHY nadir and limb measurements tropospheric columns of O3, CO, NO2, BrO, CH4, H2O, SO2 and HCHO are retrieved. In cloud free regions, the tropospheric measurements of SCIAMACHY include the planetary boundary layer. In addition, surface spectral reflectance, aerosol and cloud parameters – cover and cloud top height – and the tropospheric radiative flux from 280-2380 nm will be retrieved. These data are required for studies of the oxidising capacity of the troposphere, photochemical O3 production and destruction, tropospheric pollution (biomass burning, industrial activities, aircraft), long range transport of pollutants as well as quantification of natural and human emissions.
The Tropopause Region
Exchange of gases and particles between the stratosphere and the troposphere is of importance for the chemical composition of both regions and the atmospheric energy budget in the case of water vapour (Holton et al. 1995). For example downward transport of stratospheric ozone is a source of tropospheric ozone which, as a precursor of OH radicals, to a large extent determines the oxidising power of the troposphere. In the opposite direction upward transport of the precursor molecules (e.g. H2O, CH4, CFCs) originating from the planetary boundary layer provides the feedstock for ozone-destroying HOx, NOx, BrOx and ClOx radicals. For example CH4 is emitted into the planetary boundary layer. Due to their long tropospheric lifetime molecules are transported to the stratosphere, where they are the dominant source of the ozone-destroying HOx radicals. An adequate knowledge of the processes that determine stratosphere-troposphere exchange and the distribution of trace gases, especially in the lower stratosphere, is required. Photo-chemically stable gases in the troposphere are useful as tracers for transport of tropospheric air into the stratosphere and for stratospheric dynamics, e.g. CH4 and H2O. Similarly, gases which have relatively high stratospheric but low tropospheric abundances such as O3, can be used as tracers for downward transport from the stratosphere.
For the investigation of stratosphere-troposphere exchange, SCIAMACHY measurements of the height resolved profiles of O3, H2O, CH4, BrO and NO2 as well as aerosol will be of primary significance. In addition, SCIAMACHY will deliver information on the thermodynamical phase of clouds, which are important for the water and energy budget especially in the tropical tropopause region. With these measurements investigations of the downward transport of stratospheric O3 and upward transport of important species (e.g. aerosol, CH4, H2O) become possible. In the neighbourhood of the tropopause the different measurement modes of SCIAMACHY will have different vertical and horizontal resolutions. The solar and lunar occultation mode yields measurements with a vertical resolution of 2.5 km and a horizontal resolution of 30 km across track and extending roughly 400 km along track. For the limb measurements the spatial resolution is approximately 3 km vertically and typically 240 km horizontally across track and extending roughly 400 km along track. Therefore studies of relatively small scale features such as tropopause folding at mid-latitudes requiring a high spatial resolution are unlikely to be unambiguously observed by SCIAMACHY. However larger scale stratosphere-troposphere exchange will be readily observed.