VALIDATION OF NITROGEN-DIOXIDE MEASUREMENTS FROM THE IMPROVED STRATOSPHERIC AND MESOSPHERIC SOUNDER

Measurements of nitrogen dioxide (NO2) from the Improved Stratospheric and Mesospheric Sounder (ISAMS) on the Upper Atmosphere Research Sate llite (UARS) are assessed. Channel 5 of the instrument was dedicated t o observations of nitrogen dioxide and employed pressure-modulation an d wideband radiometry to make measurements at 6.2 mu m. This dual tech nique allows simultaneous determination of nitrogen dioxide mixing rat io and the aerosol extinction coefficient at this wavelength and there fore provides nitrogen dioxide data even in the presence of heavy aero sol loading. Approximately 180 days of data, in the period from Septem ber 1991 to July 1992, were obtained with, typically, over 2600 profil es per day for each retrieved species, covering an altitude range of 1 00-0.01 mbar. In this paper the version 10 data are assessed and a ful l error analysis is described. Comparisons with the Cryogenic Limb Arr ay Etalon Spectrometer (CLAES) on UARS and the Limb Infrared Monitor o f the Stratosphere (LIMS) on Nimbus 7 are also presented. It is conclu ded that the morphology of the retrieved ISAMS fields is robust and co nsistent with concurrent as well as previous infrared satellite measur ements. Random errors are estimated to be of the order of 10% for nigh ttime and 15% for daytime NO2 near the maxima of the distributions, an d systematic errors are estimated to be of a similar size. However, th ere remains an unresolved systematic difference of about a factor of 2 between ISAMS and CLAES. Both random and systematic errors are likely to be reduced in future versions of the processing.