## 3 Results and Discussion
### Back-Trajectory Footprints
#### 3.4.2 Sea Breeze Case Study: 23-24 July 2016  
In Figures 5 and 6, we focus on one particular event observed at SME in the summertime, corresponding to a large PBL height mismatch between 23 July 2016 at 16:00 UTC and 24 July 2016 at 16:00 UTC. This 24 h period includes an uncharacteristically significant mismatch in the PBL height for the simulations with and without UHI. The corresponding simulated total CO\({}_{2}\) value drops sharply, from 3.94 ppm to \(-\)3.94 ppm in the simulation including UHI and from 3.00 ppm to 1.08 ppm in the simulation without UHI, as evidenced in the lower-left panel of Figure 6. A zoomed-in look at this period for these figures is included as the middle and right-hand images along the bottom row of Figure 8.  
The corresponding CO\({}_{2}\) enhancements from WRF and those observed at the CAV station are shown in Figure 8 (middle panels), while the simulated PBL heights are shown on the right-hand side of the top row. To compare the wind conditions at these hours, the 12 h footprints corresponding to the measurements recorded at each of the OHP, CAV, and SME stations at these times are shown in the left-hand column of Figure 8. These figures are plotted on a log-scale because of the high density of released particles. The footprints for 23 July 2016 16:00 UTC are on the top and those from 24 July 2016 16:00 UTC are on the bottom. Looking at the footprints on 23 July 2016 at 16:00 UTC, it can be seen that the CO\({}_{2}\) enhancement values were higher and that the model captured this better at SME than at CAV. Using data from AtmoSud measured at the CAV site, the wind for the 16:00 UTC hour was 3.9 ms\({}^{-1}\) at 10\({}^{\circ}\), coming from the northwest between 3:00 and 16:00 UTC and from the northeast before that, which compares well with the corresponding footprint figure. In the course of the 12 h of the footprints from CAV and SME, the airmasses traveled for about 200 km, but the wind direction was not very stable (NW-NE) and CAV mostly received emissions from the Marseille agglomeration and Berre I'Etang, an industrialized area region near Berre Pond in the central part of the domain ([[https://cigale.atmosud.org/](https://cigale.atmosud.org/)]([https://cigale.atmosud.org/](https://cigale.atmosud.org/)), accessed on 7 May 2024). The middle panel shows that the observations captured an enhancement that the model does not reproduce. The enhancement is less pronounced at SME, and the model matches the observed enhancement reasonably well when using the UHI module. The footprints from 24 July 2016 at 16:00 UTC show some distinct differences from those of the previous day at the same time. In particular, we focus on the footprints from CAV and SME stations. The AtmoSud wind data at the CAV site at the 16:00 UTC hour was 4.5 ms\({}^{-1}\) at 260\({}^{\circ}\). This seems to indicate a higher wind speed; however, over the course of the 12 h of the footprints the mean hourly wind speed shifted between 0.6 ms\({}^{-1}\) and 6.0 ms\({}^{-1}\), compared to a range of 0.2 ms\({}^{-1}\) to 5.9 ms\({}^{-1}\). In effect, the wind speeds across the footprint periods between these two days were very similar. In contrast, the wind direction data for the footprints on 24 July was more stable, starting north and then heading west. The sources seem to have originated north of Montelimar in the Rhone Valley, after which the footprint passed over Fos/La Mede, where there are large industrial sites, then passed over the Bay of Marseille, which should not contain any major sources. The westerly wind observed at CAV and SME corresponds to typical summertime sea-breeze conditions, with a strong influence from the industrialized Rhone valley. Biogenic signals remain dominant in the observed CO\({}_{2}\) mixing ratio enhancements, originating from active cropland in the Rhone valley. Here, the model does a poorer job reproducing the observed enhancements at SME but a decent job at CAV.