## 2 Materials and Methods
### Central Data Processing System
#### 2.3.1 General Workflow of the Processing System
A process is running every 15 min checking the trigger list TL.NO2 strato.profiling. If a new entry is found, the file is analysed by the stratospheric NO2 processing module using the appropriate configuration files for the current instrument. A quality check is performed at the end of the processing module. Directly after producing the corresponding master level-2 output file, standard GEOMS files in hdf4 format are produced (5.1 in Figure 6). As for the ozone module, this is not a stand-alone module but can still be processed independently if needed. 7. A process is running every 5 min checking the trigger list TL.TROPO.profiling. If a new entry is found, the file is copied to an HPC (high-performance computing) cluster, and corresponding configuration files are loaded and processed by the tropospheric processing module. A CDL representation of the master level-2 file can be found at [https://frm4 doas.aeronomie.be/](https://frm4 doas.aeronomie.be/), accessed on 27 November 2024. A quality check is included at the end of the tropospheric processing module. As for modules (4) and (5), this is directly followed by the creation of standard GEOMS files in hdf4 format. If more than 1 trace gas were analysed, as is usually the case for channels covering the UV wavelength region, a separate GEOMS file is produced for each trace gas. Currently, NO2 and HCHO are processed (6.1 and 6.2 in Figure 6), but only NO2 GEOMS files are passed to the NDACC server, see Section 2.3.8. While the checking intervals for Level-1.check, QDOAS, NO2.strato.profiling and O3.total.VCD are all equal to 15 min, they are slightly shifted to minimise the waiting time between processing. Each of these modules takes less than a couple of minutes to finish; hence, a shift of 2-3 min is sufficient. Likewise, the reduction in the interval for the tropo.profiling module is also motivated by optimising the total processing time. To ensure easy back-tracing of the data throughout the processing chain, there is a specific naming convention for the files. The naming of the submitted level-1 file follows this structure:  
ESA-FRM4 DOAS-L1-INSTITUTE-STATIONNAME-INSTRUMENTNUMBER-CHANNELUMBER-STARTDATE-STOPDATE-FILEVERSION.nc  
The entries are defined as indicated in Table 6.  
Each following processing step adds an entry after \"L1\", separated with a dot (.). Hence, the names for the quality-controlled level-1 files, QDOAS and quality-controlled QDOAS (both internal files only) are as follows:  
ESA-FRM4 DOAS-L1.QAQC-INSTITUTE-\(\ldots\)-FILEVERSION.nc, ESA-FRM4 DOAS-L1.QAQC.QDOAS-INSTITUTE-\(\ldots\)-FILEVERSION.nc and ESA-FRM4 DOAS-L1.QAQC.QDOAS.QAQC-INSTITUTE-\(\ldots\)-FILEVERSION.nc, respectively. Although the actual start and end time of measurements that are used during the total ozone processing, stratospheric NO2 profiling and tropospheric trace gas profiling \begin{table}
\begin{tabular}{p{142.3 pt} p{142.3 pt}} \hline \hline Institute & Name of the institute to which the instrument is affiliated. \\ \hline Stationname & Name of the station, usually a city/town name. \\ \hline Instrumentnumber & A specific 4-digit identifier given to each instrument submitting data to FRM4 DOAS. \\ \hline Channelnumber & A 1-digit number specifying the channel, usually for different spectrometers, but can also be used for different viewing directions for instruments that look in multiple viewing directions at the same time. \\ \hline Startdate (stopdate) & Time of the beginning (end) of the first (last) measurement included in the file in UT. \\ \hline Fileversion & Version of the file submitted, a 3-digit number preceded by “fv”. \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Field of the file name for the level-1 files. Figure 6: The workflow diagram of the FRM4 DOAS processing system. Circles represent files (green are level-1 files, grey are internal files, cyan are master netCDF4 level-2 files, orange are level-2 files in standard GEOMS hdf4 format), dark blue pentagons represent processing steps and magenta diamond-shapes are decision steps.