QDOAS Analysis Windows properties : Molecules page
Molecules are characterized by their cross-section. They are represented by symbols previously defined in the Symbol page of the user interface. QDOAS needs these symbols for internal use : to create lists of available molecules for specific options (for example : in the Differential cross-sections column or in the Shift and stretch page) and to build cross-section files filters.
Right-click and select Insert, Remove or XS Filename options in the contextual menu respectively to insert a new molecule in the list, to delete an existing one or to modify the file associated to a cross-section. The Remove option is disabled for a symbol if another cross-section is orthogonalised to the selected one or if the selected symbol is used in the Shift and Stretch pages. Differential cross sections can be generated by orthogonalisation according to the definition of an orthogonal base formed with the component vectors (generally, a base of order 2) of the polynomial defined in the Polynomial page. Further details can be found in the Description of Algorithms of the QDOAS Software User Manual. Three options are available :
The latter case prevents from possible correlation between two cross-sections (for example, two O3 cross-sections measured at different temperatures typically to separation contributions from the stratosphere and the troposphere troposphere or two cross sections with absorption structures strongly correlated, eg. BrO and HCHO). The list of available cross-sections includes all cross-sections symbols defined in this page except the one selected. It is updated as cross-sections symbols are added in or removed from this page. Cross-sections should be defined on the grid of the control spectrum before the analysis. This column describes the action to perform on the selected cross-section :
Convolving cross-sections in real time is a comfortable option that avoid the pre-convolution of all cross-sections with respect to the selected reference spectrum and the calibration options. Usually, it is better to calculate vertical columns out of QDOAS using appropriate radiative transfer model (e.g. LIDORT, DISORT). Nevertheless, QDOAS gives the possibility to calculate vertical columns or to correct a cross-section using pre-calculated AMF through options proposed in the AMF column :
Additional option AMF Filename is proposed in the contextual menu to complete with the name of the file containing the AMF (see the requested file format according to the selected AMF option in the QDOAS Software User Manual). Control of the parameters to fit QDOAS minimises residuals of the DOAS equation using a Marquardt-Levenberg non-linear least-squares (NLLS) algorithm. The method implements a gradient-expansion algorithm, which is based on the iterative combination of a steepest-descent method (suitable for approaching the minimum from far away) and a linearisation of the fitting function.
The selection of the analysis method is made in the Analysis page of Projects Properties. Further details on the retrieval algorithms can be found in the Description of Algorithms section of the QDOAS Software User Manual For advanced users : it is possible to constrain the slant column density of a molecule to the value calculated in the previous analysis window : If the Cons SCD button is checked, the SCD Fit button is unchecked and the SCD Init value is 0., the slant column density of the selected molecule is not fitted but initialized to the value calculated in the previous analysis window if any. This column is active only if the Fits button is checked in the Display frame of the current dialog box. It allows selecting which cross-sections fits will be displayed. Note that the display of the fits of all spectral analysis windows could be disabled from the Display page of Projects Properties. This column is active only if a low-pass filter has been selected in the Filtering tab page of Projects Properties. It allows defining individually which cross-sections are to be filtered.
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