The AATSR Aerosol Optical Depth (AOD)
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AATSR
The Advanced Along Track Scanning Radiometer (AATSR) on Envisat is a dual view imaging spectrometer with seven wavelengthbands, four in the visible and NIR (0.555, 0.659, 0.865, and 1.6 um) and three in the SWIR-TIR (3.7, 11, and 12 um). THe AATSR swath is 512km. Resolution at nadir view is 1x1 km2
The nadir view and the forward view at 55º incident angle to the surface allow for near-simultaneous observation of the same area on the Earth’s surface through two different atmospheric columns within a time interval of about 2 minutes.
AATSR algorithm
Aerosol retrieval algorithms have been developed for AATSR using the single view over ocean (SV) (Veefkind and De Leeuw, 1998; Robles-Gonzalez et al., 2006) and the double view over land (DV) (Veefkind et al., 1998; Robles-Gonzalez et al., 2000). This algorithm has been validated and compares favourably with both AERONET data (e.g., Veefkind et al., 1998; Robles-Gonzalez et al., 2000, 2006) and results from dedicated aerosol retrieval instruments such as MISR and MERIS (Kokhanovsky et al., 2007). From comparison with AERONET data, the accuracy of the AATSR algorithms over ocean has been determined as 0.03, and over land 0.05. The dual view capability offers the opportunity of improved cloud screening (De Leeuw et al., 2007).
Retrieval Methods
Two separate methods are used to retrieve aerosol optical properties. One method is used for retrieval over water and the other over land. Briefly: an aerosol model is constructed using two different aerosol types. The model is the weighted average of the two types. Reflectance at the top of the atmosphere (TOA) is computed using the model. The modelled reflectance is then compared with the reflectance measured by the AATSR device. The aerosol model that results in the minimum discrepancy between the modelled and measured TOA reflectance is used for the determination of aerosol optical properties.
Over land a special dual view algorithm is used for the compensation of the land reflectance. The surface reflectance for the forward and nadir view is assumed to be independent of the wavelength. First, the ratio of the forward and nadir reflectance is determined at 1600 nm. Then, this ratio is be used for land reflectance corrections at other wavelengths.
References
For detailed descriptions of the algorithms and other aspects concerning the
retrieval (e.g. ozone absorption and Rayleigh scattering) see
