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A ground-based microwave radiometer dedicated to chlorine monoxide (ClO)measurements around 278GHz has been in operation from December 1993-June 1996 atthe Plateau de Bure, France (45° N, 5.9° E, 2500m altitude). Itbelongs to the international Network for the Detection of Stratospheric Change.A detailed study of both measurements and retrievalschemes has been undertaken. Although dedicated to the measurementsof ClO, simultaneous profiles of O3, ClO andNO2, together with information about the instrumental baseline,have been retrieved using the optimal estimation method. The verticalprofiles have beencompared with other ground-based microwave data, satellite-borne data and model results.Data quality shows: 1) the weak sensitivity of the instrumentthat obliges to make time averages over several hours; 2) the site locationwhere measurements of good opacities are possible for only a few days peryear; 3) the baseline undulation affecting all the spectra, an issue common toall the microwave instruments; 4) the slow drift of some componentsaffecting frequencies by 3-4MHz within a couple of months. Nevertheless,when temporally averaging data over a few days, ClO temporal variations(diurnal and over several weeks in winter 1995) from 35-50kmare consistent withmodel results and satellite data, particularly at the peak altitude around40km,although temporal coincidences are infrequent in winter 1995. In additionto ClO, it is possible to obtain O3 information from 30-60km whilst the instrument is not optimized at allfor this molecule. Retrievals of O3 are reasonable whencompared with model and another ground-based data set, although thelowermost layers are affected by the contamination of baseline remnants.Monthly-averaged diurnal variations of NO2 are detected at 40kmand appear in agreement with photochemical model results andsatellite zonally-averaged data, although the amplitude is weakerthan the other data sets. This NO2 result highlights the great potential of theretrieval scheme used

annales geophysicae

Quality assessment of ground-based microwave measurements of chlorine monoxide, ozone, and nitrogen dioxide from the NDSC radiometer at the Plateau de Bure