Aerosol Optical Depth Retrieval over Land Using Meteosat-8 Seviri Data

Geostationary sensors bear the potential to derive and analyze daily and seasonal trends of aerosol optical depth (AOD) from spatially homogeneous data. However, to date most AOD retrieval algorithms from geostationary sensors are limited to sea surfaces. In this study, a multi-temporal technique to retrieve AOD over land from the Spinning Enhanced Visible and Infra-Red Imager (SEVIRI) on-board the geostationary Meteosat-8 satellite is presented. The proposed method takes advantage of SEVIRI's high temporal resolution of 15 minutes and improved spatial and spectral resolution in comparison to its predecessors Meteosat-1 to 7. The crucial issue is the decomposition of the measured signal into its surface and atmospheric contributing parts. Assumed that surface characteristics remain constant during a certain time period, changes in observed reflectance can be assigned to varying atmospheric constituents and varying sun zenith and azimuth angles. Surface reflectance is estimated by the determination of the lowest reflectance within the observed time period for each satellite observation time of the day. Since clouds seriously affect the retrieval of AOD, clouds and cloud shadows are masked out and the data set is corrected for atmospheric gases prior to the AOD retrieval. Finally, AOD is calculated by means of the radiative transfer code SMAC. The study area covers Switzerland and its neighboring areas (45.0N-48.5N, 5.0E-12.0E). SEVIRI data for the entire month of August, 2004, acquired daily between 6:12 and 17:12 UTC are used in order to test the method. In comparison to sun-photometer measurements from the Aerosol Robotic Network (AERONET), first results show promising AOD values.