Detection of atmospheric chlorine-compounds with an airborne submillimeter receiver

The distribution of numerous upper atmospheric trace gases involved in the cycles of catalytic ozone depletion can be studied using passive submillimeter wave radiometry. The main intent was to investigate constituents of importance in the chlorine cycle, namely HCl and ClO. The authors instrument is based on all solid-state technology and has been optimized for the detection of thermal emission lines of these molecules. In order to avoid strong tropospheric water vapour absorption the radiometer has to be operated in a high flying aircraft (altitude ⩾10 km). The observation of HCl takes place at 625.9 GHz and ClO at 649.4 GHz. Emission spectra were analyzed over an instantaneous bandwidth of 1.2 GHz and a resolution of approx. 1.4 MHz in the line center. This system has been used very successfully in different campaigns. The retrievals of the volume mixing ratio profiles have been performed using the optimal estimation method (OEM). The error analysis shows the significance of model parameter uncertainties. Vertical profiles can be obtained over an altitude range of approx. 12 to 50 km