Investigation of pole-to-pole performances of spaceborne atmospheric chemistry sensors with the NDSC

Spaceborne atmospheric chemistry sensors provide unique access to the distr ibution and variation of the concentration of many trace species on the glo bal scale. However. since the measurements and the retrieval algorithms are sensitive to a variety of instrumental as well as atmospheric sources of e rror, they need to be validated carefully by correlative measurements. The quality control and validation of satellite measurements on the global scal e, as well as in the long term, is one of the goals of the Network for the Detection of Stratospheric Change (NDSC). Started in 1991, at the present t ime the NDSC includes five primary and two dozen complementary stations dis tributed from the Arctic to the Antarctic, comprising a variety of instrume nts such as UV-visible spectrometers, Fourier transform infrared spectromet ers, lidars, and millimeter-wave radiometers. After an overview of the main sources of uncertainty which could perturb th e measurements from space, and of the ground-based data provided by the NDS C for their validation, this paper will focus, as an example, on the measur ement of total ozone by Earth Probe Total Ozone Mapping Spectrometers (TOMS ), ADEOS TOMS and ERS-2 Global Ozone Monitoring Experiment (GOME) and their validations. The data recorded between summer 1996 and April 1997 by 16 Sy steme d'Analyse par observations zenithales (SAOZ)NV-visible spectrometers distributed over a range of latitudes from the Arctic to the Antarctic, and by Dobson and Brewer spectrophotometers operating at selected sites of the NDSC alpine and Antarctic stations, are used to investigate the solar zeni th angle (SZA) dependence, the dispersion, the time-dependent drift, and th e possible differences of sensitivity of the space-based sensors. Although the comparison demonstrates an excellent agreement to within +/-2%-4% betwe en all space- and ground-based instruments at northern middle latitudes, it also reveals significant systematic features, such as a SZA dependence wit h TOMS beyond 80 degrees, a seasonal SZA dependence with COME beyond 70 deg rees, a systematic bias of a few percent between satellite and SAOZ observa tions of low ozone columns in the southern Tropics, a difference in sensiti vity to ozone between the COME and ground-based sensors at high latitudes, and an interhemispheric difference of TOMS with the ground-based observatio ns.