GOME-2 Absorbing Aerosol Index: Statistical analysis, comparison to GOME-1 and impact of instrument degradation

This paper studies the Absorbing Aerosol Index (AAI) from GOME-2 as produced by KNMI within the framework of the EUMETSAT Satellite Application Facility on Ozone and Atmospheric Chemistry Monitoring (O3M SAF). The AAI is an index capable of indicating the presence of UV-absorbing aerosols. These aerosols are mainly injected into the atmosphere by desert dust and biomass burning events, but they can also originate from forest fires and volcanic eruptions. We first introduce the GOME-2 AAI to the reader, and then present the results of a validation study in which we analyse distributions of the AAI measurements. We also study time series of global means of the GOME-1 AAI and compare these with similar time series of the GOME-2 AAI. The latter time series clearly prove that the GOME-2 AAI product is suffering more and more from the effects of instrument degradation. In particular, the impact that this instrument degradation has on the AAI product is found to be very dependent on the position of the internal scan mirror of the GOME-2 instrument.