The Good, the Bad, and the Ugly: 25 Years of TOMS Optical Degradation

The long-term stability of the TOMS data products (total ozone, aerosols, reflectivity, and UVB) is determined primarily by the accuracy of time-dependent sensor calibrations. These calibrations include variations in components such as electronics and spectral registration. But none has had a greater influence on product uncertainty than optical degradation. Radiometry in the TOMS wavelength range, from 309 nm to 380 nm, is particularly susceptible to contaminants and their subsequent photo-polymerization. Each of the 4 TOMS instruments has exhibited a unique set of degradation characteristics, which resulted in distinctly different challenges to maintaining calibration. The Nimbus 7 TOMS (1978-1993) flight calibration system was inadequate for maintaining long-term calibration, so alternate methods were employed. The Meteor-3 spacecraft of the second TOMS (1991-1994) had a precessing orbit that introduced periods of large calibration uncertainty. And the scan mirror on the Earth Probe TOMS (1996-present) has degraded as much as 80%, resulting in complex, spatially and spectrally dependent changes. We present here a summary of TOMS degradation characteristics and their effect on calibration uncertainties. We expect the resulting uncertainty of the ozone trend, as measured by TOMS, to be in the 2%-3% percent range for the preceding 25 years.