## 3 Results and Discussion
### Co\({}_{2}\): WRF-Chem vs. Observations

Table 5: Mean and one standard deviation of the enhancement values provided by the model framework and the observations for Summertime 2016 and Wintertime 2017. Figure 6: A comparison of WRF-based CO\({}_{2}\) enhancement values, including a component breakdown, compared against a measured enhancement, subtracting off the ERSA station. The left column is July 2016, the middle is Wintertime 2017, and the right is Springtime 2017. The top row is the OHP station at 100 m sampling height, the middle is the CAV station, and the bottom is the SME station. The enhancement values in the city (CAV, in the heart of Marseille) have much stronger anthropogenic CO\({}_{2}\) emissions compared to OHP. At CAV, the model indicates that the FF emissions signal is quite similar in winter and in summer, while the biospheric fluxes are five to ten times higher in summer than in winter, and are quite small in the latter period. Depending on wind speed and wind direction, peaks of several ppm can be seen in the observations (in the 20-30 ppm range in wintertime and 5-15 ppm in summertime), which is likely due to the contribution of FF emissions, as analyzed further below. The temporal variation pattern is quite different between the model and the observation, with a higher variability in the observations in both seasons. In wintertime, the modeled CO\({}_{2}\) signal mostly follows the FF emissions' temporal variations and the pattern is closer to the observations; however, the model framework seems to underestimate the contribution of FF emissions, as the modeled peaks only reach the 10-15 ppm range. In summertime, the modeled CO\({}_{2}\) signal results from a combination of biospheric and FF fluxes, but again underestimates the observed peaks. Thus, the model seems to underestimate the FF emissions at CAV in both seasons (Figure 6, middle left and middle central panels, respectively), but could also overestimate the biospheric fluxes in summertime or mix too much. This could partly arise from limitations in the modeling framework's accuracy and spatial and temporal resolution with regard to local urban vegetation. There is a need to establish a detailed database providing this information and the associated biogenic parameters required as inputs in C-TESSEL. Additional field work is needed to further address these questions, including: (i) the use of C\({}^{14}\) isotope analysis, potentially with CO measurements, to separate out biogenic and anthropogenic signals (e.g., [96]); (ii) the use of background measurements closer to the city, which would pick up much of the same vegetation signal advected by synoptic airmasses as well as the spatial variability of the background (e.g., [19; 97]); and (iii) monitoring of the boundary layer height to infer the correctness of the mixing in the model. The SME station, which is located where the edge of Marseille meets the coast, shows an observed mean enhancement in the summertime that, while almost zero, is still quite variable as a function of the wind conditions (Figure 6, bottom middle panel). The pattern is similar to that at CAV, but with less apparent influence from anthropogenic emissions evident in both the model and the observations. The model enhancement is more than 3 ppm lower than the observed value, which is compensated for by the higher background value of the model. The model mostly follows the variation in NEE fluxes, but is partly influenced by the FF fluxes. It underestimates the peaks seen in the observations and the observed variability. In the wintertime, the NEE fluxes are much lower than in the summertime, while the FF fluxes are quite similar in both seasons. The modeled enhancement is lower and less variable than the observed one, mostly because the model is not able to reproduce the amplitude of the observed peaks that reach 30 ppm and models them at about half of their observed values. This could be due to an underestimation of the contribution of FF emissions in the model, but might also be due to overly strong mixing.