Atmospheric composition measurements by the Microwave Limb Sounder (MLS) on the Upper Atmosphere Research Satellite

The Microwave Limb Sounder (MLS) has monitored the composition of the stratosphere and lower mesosphere for more than three years since its launch on the Upper Atmosphere Research Satellite in September 1991. The MLS measures thermal emission from the atmospheric limb at millimetre wavelengths and is the first limb sounder to operate at these wavelengths from space. The primary measurements are of the concentrations of O/sub 3/, ClO and water vapour and of atmospheric temperature. Secondary products which have been produced include SO/sub 2/ from volcanic injections, HNO/sub 3/, and upper tropospheric water vapour. Vertical profiles of atmospheric concentrations are retrieved from the measured radiances. Daily global maps at a number of levels in the atmosphere are produced from these profiles. Major results to date are summarised. The depletion of ozone in the Antarctic spring is illustrated with maps for 1991, 92 and 93. Maps for the Arctic are compared with corresponding maps for the Antarctic. Significant decrease in gaseous HNO/sub 3/ has been observed in the Antarctic vortex. Water vapour fields used as tracers illustrate dynamical features of the winter polar vortices and, in the tropics, show a clear semi-annual oscillation in the upper stratosphere and a modulation of the annual cycle by the quasi-biennial cycle in the lower stratosphere. At low latitudes increased SO/sub 2/ was observed following the 1991 Pinatubo volcanic eruption.