## 3 Results
### FRM4 DOAS Validation and Performance Assessment
Comparing individual results, MAPA 1.0 and MMF tend to be lower than the other two MAPA versions on many days. Overall, the scatter of individual results is not insignificant but mostly of the order of 30%. Figure 11: Comparison of retrieved surface concentrations of HCHO (top) and NO\({}_{2}\) (bottom) with LP-DOAS observations during CINDI-2 (12–28 September 2016). The upper panel in each figure shows the comparison between LP-DOAS (black) and the median of the data (blue); the lower panel includes all individual values (represented in different colours). Results from invalid profiles are shown in washed-out colours. For NO\({}_{2}\), lidar data are included as grey crosses where available. The colorbar at the bottom of the figure specifies the cloud conditions. Green stands for good, orange for critical and red for bad conditions. For NO\({}_{2}\), there is more variability, and at least on some days, there is a clear underestimation of the surface NO\({}_{2}\) in the morning by the median of the MAX-DOAS retrievals, mainly driven by MAPA results. When looking at individual values, the scatter is large, partly more than 100%, and no clear pattern of which algorithm or instrument yields lower or higher values can be identified. The large variability is a surprise given the much stronger absorption signal when compared to HCHO, but the confinement of the NO\({}_{2}\) to a shallow layer close to the surface makes retrieval of surface concentrations from MAX-DOAS measurements difficult. As is the case for HCHO, there is no clear indication for better results under clear-sky conditions. In addition to surface concentrations, also vertical tropospheric columns of NO\({}_{2}\) can be compared to independent measurements, in this case, results from direct sun observations also performed during the campaign [50]. The latter are insensitive to the vertical profile of NO\({}_{2}\) and can therefore be considered as much more accurate than vertical columns retrieved from MAX-DOAS data. The results are shown in Figure 12. Surprisingly, there is a clear overestimation of the NO\({}_{2}\) column by the MAX-DOAS retrievals during the first days, which could be linked to the underestimation of the surface concentrations discussed above. Compared to the surface concentrations, the scatter in the vertical columns is much reduced, indicating that the column is a better-constrained retrieval quantity than the surface concentration. This supports the hypothesis that deviations in the NO\({}_{2}\) surface concentrations are a result of a shifting/smoothing of gas into higher layers. Also shown in Figure 12 is a comparison between individual HCHO columns and the median values. This comparison is less instructive, but there are unfortunately no independent HCHO columns available for comparison. As for NO\({}_{2}\), the scatter in the columns is much less than in the surface concentrations. During the cloudy parts of the campaign, the scatter increased, but most of these retrievals are flagged as invalid. For the aerosol retrievals, the results can be compared to AOD measurements from a CIMEL sun photometer located at the MAX-DOAS measurement site. This comparison is shown in Figure 13 for both wavelengths and two different ways to calculate the AOD, total AOD and partial AOD. The latter accounts for the fact that MAX-DOAS retrievals are sensitive only to the lowest part of the extinction profile and therefore combine the vertical extinction profile from the ceilometer with the averaging kernels of the measurements to compute that part of the sun-photometer observed AOD that is accessible to MAX-DOAS measurements. Details of this approach are given in [50]. Figure 12: Comparison of all retrieved tropospheric columns of NO\({}_{2}\) and results from direct sun measurements (top) and comparisons of HCHO median values with all individual retrievals (bottom). The grey-shaded area in the NO\({}_{2}\) panel indicates the uncertainty of the direct sun columns. Other descriptions of Figure 11 apply.