IBAER Algorithm

Algorithm Specification

The IBAER method (Integrated Bremen AErosol Retrieval) is an algorithm for remote sensing of aerosols from MERIS data over land. The algorithm has been developed to monitor the aerosol optical thickness (proportional to the aerosol total loading), over most of part of the continents. The aerosol information is used in a second step to perform atmospheric corrections, using either the SMAC processor on a pixel-by-pixel basis or the UBAC processor, to derive the remotely sensed surface reflectance over the land.

The actual MERIS Level-2 product provides reflectance data with an incomplete atmospheric correction over land. The atmospheric correction is made for Rayleigh scattering only and the variable aerosol influence is not considered. Thus, an additional step of atmospheric correction for Level-2 data over land is required, considering the effect of the atmospheric aerosol.

For the retrieval of the AOT, the BAER method is used and modified for the use with MERIS Level-2 data. The original approach has been developed to retrieve AOT over land from SeaWiFS Level-1 data. It determines the spectral aerosol optical thickness (AOT) from nadir looking multi-wavelength radiometers. The method is based on the determination of the aerosol reflectance over dark surfaces, using the UV and short-wave-VIS range below the red-edge of the vegetation spectrum. This requires a proper separation of the variable surface effects, other atmospheric effects and aerosol effect.

For Level-2 data over land, the variability of the vegetation cover and the kind of the vegetation will be considered dynamically by means of a surface reflectance model tuned from the satellite scene self by the NDVI. The aerosol reflectance is obtained by removing the estimation of the surface effect. Three look-up-tables of the relationship between AOT - aerosol reflectance and the use of constraints enable the determination of the AOT for 7 MERIS channels in a spectral range of 0.412 - 0.670 μm. AOT is extrapolated, using Angstrøm power law with parameters estimated from the retrieved AOT. Others terms of radiative transfer (aerosol reflectance, total transmittance and hemispheric reflectance) are computed once the AOT known to correct the Top Of Aerosol reflectance from aerosol effect.

The look-up-tables choices are:

Once the aerosol optical thickness estimated, it is used as input of the atmospheric correction method (either SMAC or UBAC) to perform the aerosol correction and to provide the surface reflectance in the 13 MERIS channels.

BAER requires as input in addition to the measured top of aerosol reflectances in the 13 MERIS bands, the sun and view zenith angles, the relative azimuth and the atmospheric pressure.

Assumptions

It is important to take on considerations that the retrieval of aerosol optical thickness is not validated in case of heavy aerosol loading. The accuracy of the retrieval is also dependant on the presence of sub pixel clouds inside the pixel, which tends to decrease the spectral dependence of the aerosol optical thickness. These clouds are difficult to detect, particularly on the 1km MERIS Reduced resolution data.

Concerning the choice of the method used to correct atmospheric effects, it is important to remind that the radiative transfer is not solved in the same way in UBAC and SMAC method. UBAC does not consider the term of multiple scattering, which allows to take into account the environment effect in the modelling. This can lead in the case of very bright grounds, a slight underestimation of the surface reflectance.

Both methods require as input the aerosol characterisation as prior information of the retrieval. SMAC parameters available in IBAER processor have been optimised for continental aerosol type. And UBAC requires the choice of the aerosol type through the choice of the Aerosol phase function Look-Up Table in the interface panel. In cases of non continental or assimilated as continental aerosols like the three available LUT, the retrieval can be biased.

Limitations

  1. In cases of high surface reflectances (sparse vegetation in arid regions, desert ground, bright bare soil, or snow), the retrieval of the aerosol optical thickness fails. It is not recommended to plly the IBAER over such a region.
  2. The surface reflectance is related in both methods to an isotropically reflecting flat Lambert surface. Effects of bi-directional anisotropy and surface tilt are not considered.