## 3 Results
### Application to Sentinel-5 Precursor and GEMS Validation  
The FRM4 DOAS datasets have been used for the validation of tropospheric NO\({}_{2}\) and HCHO satellite datasets, including GOME-2 [56], S5p/TROPOMI [57] and GEMS. The HCHO datasets have also been used for model evaluation [58], and the NO\({}_{2}\) and HCHO data from the Kinshasa site have been studied in detail in [59]. An overview of the TROPOMI tropospheric NO\({}_{2}\) validation results using the FRM4 DOAS sites is given in Figures 17 and 18. Following the approach described in [57], only TROPOMI pixels containing the site are compared to the MAX-DOAS data. TROPOMI data are from the latest version of the operational processor (RPRO v2.4 and OFFL \(\geq\) 2.4, see [60]) and the MAX-DOAS data are all based on the FRM4 DOAS processing. In line with previous validation results (e.g., [57]), a systematic negative bias is observed in TROPOMI data at all sites, with values ranging between \(-11\%\) at Xianghe and \(-38\%\) at Kinshasa. It was shown in the literature that this negative bias can be reduced when the MAX-DOAS profiles and the TROPOMI vertical sensitivity are considered, see e.g., [16; 59]. Depending on the MAX-DOAS viewing direction (see results for Mainz in Figure 17), the bias can vary from around \(-15\%\) (Mainz_4 and Mainz_3) to \(-24\%\) (Mainz_1) and up to \(-35\%\) (Mainz_2) in the most polluted viewing direction. Advanced comparison techniques considering the MAX-DOAS field-of-view direction and the wavelength dependency of the NO\({}_{2}\) absorption can also be used to better consider the impact of horizontal inhomogeneities [16]. In the present case, the network median relative bias is \(-24\%\) (\(-1.32\)\(\times\)\(10^{15}\) molec/cm\({}^{2}\) in absolute), with a dispersion (IP68/2) of the station medians of 9.5% (\(3.99\times 10^{14}\)) and a spread (median of the stations' dispersions) of 27% (\(2.3\times\)\(10^{15}\)). TROPOMI NO\({}_{2}\) validation has been regularly performed within the ATM-MPC ESA project, relying on quarterly updates of validation reports (e.g., ROCVR 22, available on [[https://s5p-mpc-vdaf.aeronomic.be/](https://s5p-mpc-vdaf.aeronomic.be/)]([https://s5p-mpc-vdaf.aeronomic.be/](https://s5p-mpc-vdaf.aeronomic.be/)), accessed on 27 November 2024) and operational comparisons performed by the automated validation server AVS ([[https://mpc-vdaf-server.tropomi.eu/](https://mpc-vdaf-server.tropomi.eu/)]([https://mpc-vdaf-server.tropomi.eu/](https://mpc-vdaf-server.tropomi.eu/)), accessed on 27 November 2024). Both include comparisons with MAX-DOAS data, provided by each MAX-DOAS Principal Investigator (PI) to BIRA-IASB in the first case (through the NIDFORVAL project, N4V [57]), uploaded on the NDACC rapid delivery database and mirrored by EVDC in the second case. Figure 19 presents an overview of the validation results for the three treatments, focusing on the stations of Athens, Bremen, Cabauw, De Bilt, Kinshasa, Mainz, Uccle and Xianghe. One can see that for these stations, the results of the three treatments are consistent with each other although different choices were made in implementing the comparison (e.g., interpolation or averaging of the ground-based data, use of the station coordinates or the ground-based data air-masses location for the satellite pixel selection). An interesting feature is the reduction in the comparison spread when moving from individually processed MAX-DOAS data (as for N4V and AVS cases) to the harmonised FRM4 DOAS system. Once fully operational, the FRM4 DOAS data stream will become the reference data stream for the S5p validation. Figure 18: Mosaic plot of the monthly median relative biases for S5p tropospheric NO\({}_{2}\) v2.4 vs. MAX-DOAS based on the FRM4 DOAS processing. As in Figure 17, the stations are ordered from bottom to top by increasing median NO\({}_{2}\) MAX-DOAS VCD values (values given in brackets in units of \(10^{15}\) molec.cm\({}^{-2}\)).