Aerosol optical depth and land surface reflectance from multiangle AATSR measurements: global validation and intersensor comparisons

This paper presents the results and satellite intercomparisons for the retrieval of aerosol optical depth (AOD) and land surface bidirectional reflectance using the Multiangle Advanced Along-Track Scanning Radiometer (AATSR). The algorithm developed is based on inversion of a physical model of light scattering that requires no a priori knowledge of the land surface. The model is evaluated for a number of sites around the world to test its operation over a range of aerosol types and land covers including dark and bright surfaces. Validation is performed using Aerosol Robotic Network ground-based sun-photometer measurements and by intercomparison with independent estimates of AOD derived from spaceborne instruments including Multiangle Imaging Spectroradiometer (MISR), Moderate Resolution Imaging Spectroradiometer (MODIS), and Total Ozone Mapping Spectrometer (TOMS) aerosol products. Results show good agreement (Pearson's correlation coefficient r/sup 2/=0.70 for all sites combined) between the AATSR-derived estimates of AOD and the sun-photometer measurements. There is also a high correlation (r/sup 2/=0.84) between the AATSR- and MISR-derived AOD estimates, but the correlations of the AATSR-derived AOD with MODIS-derived AOD and TOMS aerosol index are lower. In addition, the ability of the sensor to discriminate between different aerosol types is evaluated. Moreover, the estimates of the aerosol properties are used for atmospheric correction of the top-of-atmosphere reflectance. The AATSR surface reflectances are compared with the MODIS bidirectional reflectance distribution function/Albedo and MISR surface products and are shown to correspond with root-mean-square errors of 0.03 and 0.06 or better, respectively. The retrieval method is applied on an image basis resulting in an image of surface reflectance and a separate map of AOD. A map of AOD at 550 nm covering the Sahel and southern Sahara region is presented to demonstrate operation at regional and potentially global scales.