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For the new NDSC (Network for the Detection of Stratospheric Change)-station Ny-Alesund (Spitsbergen) a novel type of a modular microwave radiometer has been developed and tested in a recent field campaign. The design is such that by changing a minimum of frequency dependent components (e.g. mixer, local oscillator) the receiver can operate at any frequency between 100 and 300 GHz. Therefore many constituents of interest in atmospheric chemistry can be investigated. The system is built for continuous operation at remote locations with little servicing necessary. It consists of a 142 GHz ozone radiometer equipped with a balanced mixer with planar diodes on a finline structure. A second unit with a 204 GHz ClO front-end with a whisker-contacted mixer is presently being tested. The mixer and first amplifier of both front-ends can be cooled to 15 K by a closed-cycle refrigerator. Quasi-optical elements are used to suppress standing waves and to achieve single-sideband operation. The sideband load is cooled to 65 K resulting in single-sideband receiver noise temperatures of 550 K at 142 GHz and 1000 K at 204 GHz. An acousto-optical spectrometer with 1 GHz bandwidth and 1.3 MHz resolution is used as spectrometer backend. Ozone measurements were performed at Ny-Alesund in the winters 1992/93 and 1993/94 from January to March, respectively. The measured spectra were inverted using the optimal estimation method. The influence of the assumed a priori profile and baseline effects on the derived volume mixing ratios have been investigated carefully. From this analysis the authors conclude that volume mixing ratios between 10 km and 60 km can be obtained with a vertical resolution of 9 km. For data validation the vertical ozone distributions obtained during a campaign (January-March 1993) are compared with data taken in the same time period by ozone sondes. From January to March 1994 the ClO front-end was also tested at Ny-Alesund. Preliminary Results from February 17-18 in the beginning of the daylight period suggest no excessive ClO above Ny-Alesund. >

Ground-based millimeter-wave measurements of stratospheric ozone at 142 GHz and chlorine monoxide at 204 GHz are performed at the Arctic NDSC station Ny-Alesund, Spitsbergen (79/spl deg/ N, 11/spl deg/ E). Baseline effects are removed by an empirical baseline model, derived from statistical analyses. The retrieved ozone profiles agree well with ozone sonde soundings and total ozone from the SAOZ instrument. A time-height cross section plot of the stratospheric ozone concentration in February 1994 over Ny-Alesund is included, showing strong variations within time periods of a few hours.