The Microwave Limb Sounder (MLS) experiments obtain measurements of atmospheric composition, temperature and pressure by observations of millimeter and submillimeter-wavelength thermal emission as the instrument field-of-view is scanned through the atmospheric limb. Features of the measurement technique include the ability to provide global coverage on a daily basis at all times of day and night from an orbiting platform, to obtain reliable measurements even in the presence of dense cirrus and aerosol, and to measure many atmospheric gases as well as temperature and pressure. The composition measurements are relatively insensitive to uncertainties in atmospheric temperature. An accurate spectroscopic data base is available, and the instrument calibration is also very accurate and stable. The first MLS experiment in space, launched on the NASA Upper Atmosphere Research Satellite (UARS) in September 1991, was designed primarily to measure stratospheric profiles of CIO, 0 3 , H 2 0 and atmospheric pressure as a vertical reference. Global measurement of C1O, the dominant radical in chlorine destruction of ozone, was an especially important objective of UARS MI,S. All objectives of UARS MI.S have been accomplished, and additional geophysical products beyond those for which the experiment was designed have been obtained. A follow-on MLS experiment is being developed for NASA's Earth Observing System (EOS), and scheduled to be launched on the EOS CHEM platform in 2002. EOS MLS is designed for many stratospheric measurements, including HO X radicals which could not be measured by UARS because adequate technology was not available, and better and more extensive upper tropospheric and lower stratospheric measurements.

Validation of stratospheric ClO measurements by the Microwave Limb Sounder (MLS) on the Upper Atmosphere Research Satellite (UARS) is described. Credibility of the measurements is established by (1) the consistency of the measured ClO spectral emission line with the retrieved ClO profiles and (2) comparisons of ClO from MLS with that from correlative measurements by balloon‐based, ground‐based, and aircraft‐based instruments. Values of “noise” (random), “scaling” (multiplicative), and “bias” (additive) uncertainties are determined for the Version 3 data, the first version publicly released, and known artifacts in these data are identified. Comparisons with correlative measurements indicate agreement to within the combined uncertainties expected for MLS and the other measurements being compared. It is concluded that MLS Version 3 ClO data, with proper consideration of the uncertainties and “quality” parameters produced with these data, can be used for scientific analyses at retrieval surfaces between 46 and 1 hPa (approximately 20–50 km in height). Future work is planned to correct known problems in the data and improve their quality.

The UARS Microwave Limb Sounder (MLS) measures ClO (~25-45 km), O3 (~15-80 km), H2O (~15-85 km), and pressure (~30-60 km). ClO is the dominant form of chlorine which destroys ozone in the stratosphere; its measurement on a global scale, which will be done by the MLS, is essential for understanding and monitoring the depletion of stratospheric ozone by chlorine from industrial products. Simultaneous measurements of O3 and H2O provide additional important information on stratospheric ozone chemistry. The H2O and O3 measurements will be to higher altitudes than previously explored on a global basis. The pressure measurements provide the vertical reference for composition measurements. Secondary MLS measurement goals include H2O2, HNO3, temperature, and one component of wind in the mesosphere. Measurements are made by observing millimeter-wavelength spectral line thermal emission as the instrument field-of- view is vertically scanned through the atmospheric limb. All are made simultaneously and continuously, at all times of day and night. Prior development for the UARS MLS included an aircraft [1,2] and balloon [3,4,5] MLS. Development has also started on an MLS experiment for the future Earth observing system (Eos); the Eos MLS is described in a separate paper presented at this meeting.

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