2012-02-01: In-flight degradation correction of SCIAMACHY UV reflectances and Absorbing Aerosol Index

L.G. Tilstra, M. de Graaf, I. Aben, and P. Stammes

In this paper we study the close relationship between the radiometric calibration of a satellite instrument and the Absorbing Aerosol Index (AAI) derived from the observed Earth reflectance.
Instrument degradation of the SCIAMACHY instrument in the ultraviolet wavelength range is examined by analyzing time series of global means of the AAI, making use of the experience that the global mean should be more or less constant when instrument degradation is absent. The analysis reveals the magnitude of the (scan-angle dependent) instrument degradation of SCIAMACHY, and also shows that currently available correction techniques are not able to correct the instrument degradation in a sufficient manner. We therefore develop and introduce a new method for degradation correction, which is based on the analysis of the time evolution of the global mean reflectance. Seasonal variations in the global mean reflectance, which mainly result from seasonal variations in scattering geometry and global cloud coverage, are separated from the time series in order to isolate the instrument degradation. Finally, we apply the derived reflectance correction factors to the SCIAMACHY reflectances and calculate the AAI to find that the effects of instrument degradation are reduced to within the 0.1 index point level. The derived AAI is also compared with the AAI based on other correction techniques. The proposed in-flight reflectance degradation correction method performs best in all aspects.

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Time series of daily global mean residue calculated from SCIAMACHY observations as a function of time for the individual scan mirror positions. The colors indicate scan mirror positions, i.e., the position in the orbit swath. The time series cover the period from August 1, 2002 to September 30, 2010. Notice the increase in the global mean residue with time, and the growing scan-angle dependence. No correction for instrument degradation was applied to the data.
Global mean reflectance measured at 340 nm by SCIAMACHY, for each of its sixteen (IT = 0.25 s) scan mirror positions. To separate the time series graphically, an offset of (s ? 1)<=0.05 was added to each, where s is the scan mirror position as indicated. The gray color indicates the daily global mean reflectance, while the colored curves represent the 12-day average. Here the meaning of the colors is the same as in the figure above. The solid and dotted black curves are described in the text the dotted curve illustrates the effect of instrument degradation over the years.