The Processor for Aerosol Retrieval and Atmospheric Correction is the third of three main modules within the MERIS/(A)ATSR Synergy toolbox. Its purpose is to derive aerosol optical depths over land and/or ocean and (optionally) provide an atmospheric correction by using the derived AODs. The processor also makes use of quantities derived from the cloud screening module. Therefore, the input for the aerosol retrieval must be an output product from the cloud screening.
The user interface for the Aerosol Retrieval and Atmospheric Correction can be accessed by clicking the 'Aerosol Retrieval and Atmospheric Correction' tab in the 'MERIS/(A)ATSR Synergy Toolbox' dialog. (Figure 4.5):
The Aerosol Retrieval and Atmospheric Correction tab pane consists of the following components:
- 'Retrieve AODs over ocean': If this checkbox is selected, aerosol optical depths will be retrieved over ocean. The default is 'true'.
- 'Retrieve AODs over land': If this checkbox is selected, aerosol optical depths will be retrieved over land. The default is 'true'.
'Retrieve surface directional reflectances over land (time consuming!)': If this checkbox is selected, SDRs will be retrieved from rescaled AODs. This computation is performed over land only.
Due to the nature of the algorithm, the retrieval of surface directional reflectances requires much more computation time than the pure aerosol retrieval. If you are interested in aerosol quantities only, it is recommended to keep this option unselected. For the computation of surface directional reflectances on larger datasets, it is further recommended to create in advance subsets of the input data which just cover the regions of interest.
- 'N x N average for AOD retrieval': Text field to set the number of pixels to average over in the aerosol retrieval. The default number is '7', which corresponds to a square with 3 pixels to each side of a center pixel.
- 'Soil surface reflectance spectrum': Text field to set the filename containing the soil surface reflectance spectrum. The default is 'spec_soil.dat', this file is included in the auxiliary data (see Section 2.2) If the user wants to use a different file, it must be selected via the file chooser button right to the test field. Note that any user selected file must exactly follow the format of the default file.
- 'Vegetation surface reflectance spectrum': Text field to set the filename containing the vegetation surface reflectance spectrum. The default is 'veg_soil.dat', this file is included in the auxiliary data (seeSection 2.2) If the user wants to use a different file, it must be selected via the file chooser button right to the test field. Note that any user selected file must exactly follow the format of the default file.
'Use default land aerosol models (recommended)': If this radio button is selected, the default land aerosol models are used for aerosol retrieval. This set of models is included in the 'standard' auxiliary data set (seeSection 2.2). No further user action is required. This option is recommended not only for simplicity, but also because only a small number of models is used. For most purposes, this option will be sufficient. A large number of models (which can be specified with the option below) significantly increases computation time for the aerosol retrieval.
For more details on these aerosol models and underlying lookup tables see [3] and [6].
'Use specific land aerosol models (for advanced users)': If this radio button is selected, specific land aerosol models must be entered by a comma-separated list of model indices in the text field below. To use this option for aerosol retrieval, the 'full' auxiliary data set (seeSection 2.2) must be downloaded and installed.
For more details on these aerosol models and underlying lookup tables see [3] and [6].
'List of land aerosol models': Text field to set a comma separated list of the indices of the aerosol models to be used for the land aerosol retrieval (Figure 4.6). This text field is enabled only if the 'Use specific land aerosol models (for advanced users)' radio button is selected. The valid range of indices is [1,40]. In principle, up to all 40 models could be entered. However, to keep computation time in an acceptable frame, it is recommended to use as few models as possible.
For more details on these aerosol models and underlying lookup tables see [3] and [6].
