## 2 Materials and Methods
### Central Data Processing System
#### 2.3.1 General Workflow of the Processing System  
The processing chain is visualised by a workflow diagram in Figure 6. Circles represent files (yellow circles represent level-2 standard GEOMS format files, cyan circles represent master level-2 netCDF4 files, grey circles represent intermediate internal files and green incoming/archived level-1 files); dark blue pentagrams are processing steps (so-called modules) and (pink) parallelograms are decision stages. The workflow can be described as follows:  
1. A file submitter uploads a calibrated spectrum file to the FTP server. A CDL (network Common Data form Language, [[http://cfconventions.org/cf-conventions/cf-conventions.html](http://cfconventions.org/cf-conventions/cf-conventions.html)]([http://cfconventions.org/cf-conventions/cf-conventions.html](http://cfconventions.org/cf-conventions/cf-conventions.html)), accessed on 27 November 2024) representation of such a file can be found at [https://frm4 doas.aeronomie.be/](https://frm4 doas.aeronomie.be/), accessed on 27 November 2024. During the next year, this FTP server will be replaced by a Web-based Distributed Authoring and Versioning (WebDav) server. 2. A process is running every 15 min on the BIRA-IASB servers to check for new files on the FTP servers. If new file(s) have arrived, they are downloaded to BIRA-IASB file servers, and their names are written into a trigger list (TL.Level1.check) for further processing. 3. A process is running every 15 min to check the trigger list TL.L1.check. If a new entry is found, information about the instrument in question is extracted from the file name and the metadata of the file. With the help of this information, corresponding configuration files are loaded. The file is checked for consistency and archived as \"rejected\" if the check failed or otherwise as Level1.QAQC. This checking constitutes the first module, the LEVEL1.check. It consists of checking whether all compulsory variables and attributes are present and whether the units are suitable. However, the conversion from L0 to calibrated L1 files lies in the responsibility of the data submitter. Lastly, the name of the checked file is written into a new trigger list or in two trigger lists (TL_QDOAS.S and/or TL_QDOAS.T for stratospheric and tropospheric processing, respectively), depending on the configuration file loaded. For example, the configuration file for an instrument channel operating in the UV wavelength region will indicate to only write the file name of the file in the TL.QDOAS.T trigger list because only tropospheric processing will be performed. 4. (3a/b) A process is running every 15 min checking the trigger lists TL_QDOAS.S and TL_QDOAS.T. If there are new entries in either of the lists, corresponding configuration files are loaded, and the file is processed with the QDOAS software package [26], producing QDOAS.S and/or QDOAS.T output files. Details about the settings optimised for stratospheric (CONFIG.QDOAS.S) are summarised in Tables 3 and 4 (see Section 2.2.2). Directly after file production, a quality check is performed on the files and new QDOAS.S./T.QAQC files are created. Finally, the name(s) of the output file(s) are written into one or several of the following trigger lists: TL.TROPO.profiling (following the module TL.QDOAS.T.check), TL.O3 total.VCD and TL.NO2 strato.profiling (both following the module TL.QDOAS.S.check). 5. A process is running every 15 min checking the trigger list TL.Q3 total.VCD. If a new entry is found, the file is analysed by the O3.total.profiling module using the appropriate configuration files for the current instrument. A QAQC check on the quality of the processing is included in this process. Directly after producing what we call the \"master level-2 output file\" in CF-netCDF4 format, standard GEOMS files in hdf4 format are produced (indicated as process 4.1 in Figure 6). While this is not a separate module in the sense that it is handled via trigger lists, it can be called independently without the need to perform process 4 from the start. 6.