Studying 15 years of Absorbing Aerosol Index (AAI) data
Tilstra, Lieuwe; De Graaf, Martin; Tuinder, Olaf; Stammes, Piet

In this study we combine Absorbing Aerosol Index (AAI) data from the instruments GOME-1, SCIAMACHY, and GOME-2. The AAI is calculated from the Earth reflectances measured at 340 and 380 nm; it can indicate the presence of UV-absorbing aerosols like desert dust or smoke. The AAI is capable of detecting aerosols over land and sea surfaces, and even in the presence of clouds when the absorbing aerosol layer overlies the clouds. As a result, it is a commonly used tool for aerosol screening.

We study time series of the AAI for a selection of regions to look for trends in aerosol presence. All three instruments are affected, in their own way, by instrument degradation. As the AAI product is very sensitive to the absolute calibration of the UV reflectance this complicates the analyses of the time series and trends therein.

We therefore developed a method to correct the measurements of the Earth reflectances for instrument degradation. This degradation correction method is based on studying time series of global means of the reflectances and isolating the instrument degradation from these time series. We show that this method works well.

The AAI time series of some of the studied biomass burning regions are compared with measurements of the tropospheric NO2 column. We find a good correlation between the AAI and the tropospheric NO2 column.