SCIAMACHY\'s Absorbing Aerosol Index and the consequences of instrument degradation

In this paper we present the first results of long-term monitoring the Absorbing Aerosol Index (AAI) measured by the satellite instrument SCIAMACHY. We find a strong increase in the AAI with time, which is found to be caused completely by degradation of the optics. This we conclude from straightforward calculation of the effect of instrument degradation based on available degradation monitoring results. We only studied the scientific product (SC-AAI), not the operational level-2 product (L2-AAI). However, the conclusions we draw in this paper should also apply to the operational level-2 product. We find that the AAI is very sensitive to the absolute calibration of the Earth reflectance, and that a proper absolute calibration, and an accurate correction for the effects of instrument degradation, are essential for this particular product.