Accurate determination of total ozone using SBUV continuous spectral scan measurements

Abstract

Over the last 2 decades, satellite data have been used to monitor long‐term global changes in stratospheric ozone. In order to measure relatively small ozone trends on timescales of the order of a decade, degradation of instrument components must be accounted for, and accurate interinstrument calibration must be maintained. In this paper, we have used a self‐calibrating method to accurately retrieve total ozone from backscatter ultraviolet spectra in the wavelength range 310 to 340 nm. Using the information contained in this spectral region, we correct for time dependent and time‐independent wavelength errors and calibration errors as well as radiative transfer modeling errors. We use continuous spectral scan data from the Nimbus 7 solar backscatter ultraviolet (SBUV) instrument to retrieve total ozone at latitudes between 40°S and 40°N over the time period 1979 to 1986. These total column ozone retrievals are used to independently validate retrievals from the both Nimbus 7 total ozone mapping spectrometer (TOMS) and SBUV in discrete mode. TOMS and SBUV in discrete mode have better temporal and spatial coverage than SBUV in continuous scan mode and are two of the primary instruments used to derive long‐term global ozone trends. The time dependence of the difference between total ozone retrieved from SBUV continuous scan and that derived from TOMS and SBUV discrete mode is 0.6±0.3%/decade or less depending on latitude. The SBUV continuous scan radiances are modeled to an accuracy of ±0.3% (1σ). Unfortunately, an instrument problem terminated continuous scan mode observations after 1986 so that the later years of TOMS and SBUV discrete mode observations cannot be validated. The methods developed here may also be applied to other continuous spectral backscatter ultraviolet instruments in order to intercalibrate total ozone retrievals. This will be particularly important when temporal overlap of ozone monitoring instruments does not occur as in the case of the Nimbus 7 and Earth Probe TOMS instruments. In addition, the methods used here are applicable to ground‐based spectral ultraviolet measurements.