## 3 Results
### NDACC Rapid Delivery Demonstration Service  
As already explained in Section 2.3.8, the FRM4 DOAS system delivers daily total ozone as well as tropospheric NO\({}_{2}\) vertical profile level-2 GEOMS HDF files to the NDACC rapid delivery repository. Tropospheric formaldehyde and stratospheric NO\({}_{2}\) vertical profiles are not distributed yet, but they are processed together with the other products and internally monitored. The submission to the NDACC happens once per day at 15:30 UTC, and once catalogued on the NDACC, the data are further deposited on the ESA EVDC database by mirroring. The service started in December 2020 and, since then, has been operated without  
Figure 13: Comparison of MAX-DOAS and sun-photometer retrieved AOD during the CINDI-2 campaign (12–28 September 2016). The first row shows the comparison between medians and total and partial sun-photometer AOD; the second and third rows show all results for the UV and the vis aerosol retrievals. Other descriptions of Figure 11 apply. interruption. Most stations deliver data at a daily frequency, and where possible, the processing of the data has been extended back to July 2018, which corresponds to the official start of the demonstrational Central Data Processing System. At some stations, historical datasets are available back to early 2000. A full reprocessing of these data is among the planned activities when moving the system into a fully operational status. Table 9 provides an overview of the distribution of the retrieved data products at the different stations of the network. The product availability is mostly driven by instrument specifications, in particular the geometry of observation and the spectral range covered by the spectrometers available at the different sites. High-altitude sites are currently excluded from tropospheric NO\({}_{2}\) and HCHO processing due to the low concentration of these molecules at free-tropospheric altitudes, and the difficulty to interpret such measurements for satellite validation. Figures 14 and 15 give a visual overview of the status of the data processing for tropospheric NO\({}_{2}\) vertical profiles and for total ozone, respectively. Note that for double-channel instruments (i.e., those for which the UV and visible channels are reported in different files), we produce NO\({}_{2}\) GEOMS files both for the UV and for the visible channels. However, for those instruments that are part of the NDACC service (see Table 9), only the visible is submitted to the RD repository unless there is no visible channel at all. Due to limited resources, the FR4 DOAS system has so far been operated in a demonstrational mode, with an emphasis on rapid data delivery to the NDACC and ESA's EVDC data centres. In a subsequent step of the project, it is planned to extend the processing to historical data series available at the different stations listed in Table 9. Among them, 15 sites have time series extending over more than 10 years, and 7 feature data series covering more than 20 years of measurements. Such data records will expand the capabilities of the NDACC network to address long-term variability and trend studies as well as the validation of historical satellite data records.