The ASUR sensor is a passive heterodyne radiometer working in the frequency range 605 - 662 GHz. It detects thermal emission from rotational lines of stratospheric constituents such as ClO, HCl, N₂O, O₃, HNO₃, CH₃Cl, H₂O, HO₂, HOCl and BrO. Using appropriate inversion techniques vertical profiles from 15 to over 50 km altitude can be retrieved with a vertical resolution of typically 6 km and 12 km in the lower and upper stratosphere, respectively.

The sub-mm radiometer ASUR will be used on board the NASA DC-8 aircraft to provide vertical profiles of stratospheric key constituents involved in ozone-photochemistry. Simultaneously, the Lidar on the DC-8 measures ozone and Aerosols and in particular PSC backscatter ratios. The combination of a microwave sensor and a LIDAR instrument on board the aircraft is unique as it opens the possibility to localize the area of heterogeneous chemistry where polar stratospheric clouds form and simultaneously to investigate the variability of key halogen species of importance in ozone depletion like ClO, HCl, HNO₃ and others.

Of particular interest are coordinated flights with large balloons or ground based instruments where our measurements will complement other sensors and additionally allow to cover areas in the vicinity not covered by the particular balloon or groundbased sensor.

Our specific objectives can be separated in two groups a) activities directly related to the campaign, and b) making use of all available data for improving our understanding of stratospheric photochemistry by comparison with model data and by improving models with measured sets of parameters.

Key objectives with respect to a) are:

• SAGE III validation. ASUR will measure with high accuracy profiles of a number of species for comparison with SAGE III.

• To study the fine structure on the vortex edge, so called "filaments" and "blobs".
• To study Chlorine activation and denitrification (HNO₃) in the presence of PSCs and/or Cirrus clouds.
• To study vertical transport and water vapor in the lower stratosphere.

With respect to objective b) our team has collaborated very successfully with modelers, namely Martyn Chipperfield (University of Cambridge, SLIMCAT model), and Frank Lefevre (Toulouse, REPROBUS model), who both developed a 3-D chemistry transport model. The ASUR Data have been compared to their model calculations and results have been published in several papers.

Because of the importance of model activities, we have developed our own box-model and a one dimensional model, both optimized for our own applications. Some key issues we would like to investigate during SOLVE are:

• Comparison of the measurements to 3D model results (validation) in close cooperation with experienced modelers as done previously.
• To compare modeled and measured diurnal variations of short-lived radicals such as ClO and HO₂ with the help of a 1D Photochemical model.
• Estimation of the OH concentration from a steady-state assumption of HOCl using a simple Photochemical model.
• Partitioning of stratospheric constituents with the aid of simultaneous measurements performed by ASUR and other remote instruments and in situ sensors available from various platforms during SOLVE which will give a nearly complete picture of Trace gases that are important in ozone loss cycles.

The ASUR hardware is a joint development by SRON, Groningen (Space Research Institute of the Netherlands), and the Institute of Environmental Physics at the University of Bremen. The AOS (Acousto Optical Spectrometer) has been developed by Observatoire de Meudon under an ESA/ESTEC contract.