Validation of a modified AVHRR aerosol optical depth retrieval algorithm over Central Europe

The Advanced Very High Resolution Radiome- ter (AVHRR) carried on board the National Oceanic and At- mospheric Administration (NOAA) and the Meteorological Operational Satellite (MetOp) polar orbiting satellites is the only instrument offering more than 25 years of satellite data to analyse aerosols on a daily basis. The present study as- sessed a modified AVHRR aerosol optical depth a retrieval over land for Europe. The algorithm might also be applied to other parts of the world with similar surface character- istics like Europe, only the aerosol properties would have to be adapted to a new region. The initial approach used a relationship between Sun photometer measurements from the Aerosol Robotic Network (AERONET) and the satel- lite data to post-process the retrieved a. Herein a quasi- stand-alone procedure, which is more suitable for the pre- AERONET era, is presented. In addition, the estimation of surface reflectance, the aerosol model, and other processing steps have been adapted. The method's cross-platform ap- plicability was tested by validating a from NOAA-17 and NOAA-18 AVHRR at 15 AERONET sites in Central Eu- rope (40.5 N-50 N, 0 E-17 E) from August 2005 to De- cember 2007. Furthermore, the accuracy of the AVHRR re- trieval was related to products from two newer instruments, the Medium Resolution Imaging Spectrometer (MERIS) on board the Environmental Satellite (ENVISAT) and the Mod- erate Resolution Imaging Spectroradiometer (MODIS) on board Aqua/Terra. Considering the linear correlation coeffi- cient R, the AVHRR results were similar to those of MERIS with even lower root mean square error RMSE. Not sur- prisingly, MODIS, with its high spectral coverage, gave the highest R and lowest RMSE. Regarding monthly averaged a, the results were ambiguous. Focusing on small-scale structures, R was reduced for all sensors, whereas the RMSE solely for MERIS substantially increased. Regarding larger areas like Central Europe, the error statistics were similar to the individual match-ups. This was mainly explained with sampling issues. With the successful validation of AVHRR we are now able to concentrate on our large data archive dat- ing back to 1985. This is a unique opportunity for both cli- mate and air pollution studies over land surfaces.