Location of aerosol optical depth file.
aerosol_tau_file file
The file must have two columns.
Column 1 is the altitude in km. The altitude grid must be exactly equal to
the altitude grid specified in the file atmosphere_file.
Column 2 is the aerosol optical depth of each layer.
To allow wavelength-dependent aerosol optical thickness please
use either aerosol_angstrom or aerosol_files.
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