The 2.5 THz heterodyne spectrometer THOMAS - Measurement of OH in the middle atmosphere and comparison with photochemical model results

The airborne 2.5 THz heterodyne spectrometer Terahertz OH Measurement Airborne Sounder (THOMAS) is introduced. Since the first THOMAS measurements in 1994/1995, the spectrometer was significantly improved by modification or replacement of individual components. The THOMAS instrumental setup and properties are presented together with a retrieval algorithm for atmospheric parameters based on a Phillips-Tikhonov regularization scheme. Furthermore, the results of a complete error assessment are given. In August 1997, during the second CRISTA/MAHRSI campaign (Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere/Middle Atmosphere High Resolution Spectrograph Investigation), OH observations were performed by THOMAS covering altitudes between about 30 and 90 km over a full diurnal cycle. Hydroxyl column densities derived from THOMAS measurements are presented and compared to photochemical model results. The model calculations using the standard HOx chemistry systematically show higher values by about 15 percent for the 40-90 km and 50-90 km OH columns. Moreover, a recently proposed change of an HOx chemistry reaction rate is included into the comparison which, for the same altitude intervals, yields OH column densities that are about 10 percent lower than the THOMAS measurements. (Author)