In addition to H2O, O3 and CO2, libRadtran allows to overwrites the predefined total column concentration of trace gases from the selected atmospheric model in MODEL. The following trace gases are NO2, BrO, OCLO, HCHO, O4, SO2, CH4, N2O, CO and N2. All units are specified in Dobson Units (DU), and with the value -1 (default value) ALG uses the predefined gas concentration of from the selected atmospheric model.