Analysis of the eight-year trend in ozone depletion from empirical models of solar backscattered ultraviolet instrument degradation

Currently archived ozone data from the Solar Backscatter Ultraviolet (SBUV) spectrometer experiment on the Nimbus 7 satellite has been reported to show large global decreases in the amount of atmospheric ozone, both total content and as a function of altitude, for the period from 1978 to 1987. Analysis of atmospheric albedo data leading to these reported trends was based on empirical models of the SBUV spectrometer and diffuser plate degradation with time. Their combined degradation can be obtained from apparent decreases in measured solar irradiance at wavelengths where little or no long-term intensity changes are expected. The central problem in analyzing SBUV data is to separately specify the diffuser plate degradation [Husdon et al., 1988]. Ratios of radiance to solar irradiance used to obtain ozone amounts are proportional only to diffuser reflectivity and independent of any spectrometer degradation. Even observations of relatively short-term solar irradiance changes with time must be based on a model of diffuser plate degradation, since the SBUV data provide no internal means to uniquely specify diffuser plate reflectivity changes at ail wavelengths. A new class of explicitly empirical models have been developed that produce a better fit to all of the SBUV solar flux data. The models have a single free parameter to separately specify the diffuser plate and spectrometer degradation. This parameter must be within a narrow range to bring the calculated ozone trend into approximate agreement with data from the Dobson network, Solar Mesospheric Explorer (SME) and Stratospheric Aerosol Gas Experiment (SAGE) satellites, or with the different trends reported from the Umkehr ground stations. It is shown that outside sources of ozone data must be used to obtain a unique solution from SBUV radiance data within the precision necessary to determine the existence of a global annual ozone decrease. A correction factor is given as a function of time and altitude that brings the SBUV data into approximate agreement with SAGE, SME, and Dobson ozone trends. The total ozone results also agree with a recently developed wavelength pair justification method of internal calibration for the SBUV instrument. In our opinion, the currently archived SBUV ozone data should be used with caution for periods of analysis exceeding 1 year, since it is likely that the yearly decreases contained in the archived data are too large.