## 1 Introduction
#### 2.2.2 Model CO\({}_{2}\) Flux Components
Fire estimates are obtained through the Fire Inventory from NCAR (FINN) version 1.5 [57], which provides global fire emissions estimates at \(\sim\)1 km\({}^{2}\) resolution. There are multiple speciation options; for this study, we chose the product based on the MOZART-4 chemical mechanism [63]. To create the flux maps for the WRF simulations, the emissions were assigned to the relevant WRF domain and scaled appropriately to suit the pixel size. For example, a fire burning in a space of approximately 1 km\({}^{2}\) would have its emissions scaled down by 1/9 when assigned to the domain with (3 km)\({}^{2}\) resolution. We calculated the ocean fluxes manually using the sea surface temperature (SST), surface pressure, and 10 m u and v wind speed component values provided in the ECMWF model files used to drive WRF. With these values, we incorporated measured pCO\({}_{2}\) values from the Bay of Marseille along with background atmospheric CO\({}_{2}\) mixing ratios measured at the ICOS station at Cap Corse. These were combined to solve the following formula:  
\[Flux = k\cdot K_{0}(pCO_{2\_water}-pCO_{2\_air}) \tag{2}\] \[= k\cdot(K_{0}\cdot pCO_{2\_water}-F\cdot\mathcal{X}CO_{2\_air})\]  
where \(pCO_{2\_water}\) is the measured partial pressure of CO\({}_{2}\) in the water, \(\mathcal{X}CO_{2\_air}\) is the measured dry air mole fraction of CO\({}_{2}\), \(K_{0}\) is the solubility, \(F\) is the solubility function, and \(k\) is the gas transfer velocity. For \(pCO_{2\_water}\), we used values measured in the Bay of Marseille during the same period [64]. For \(\mathcal{X}CO_{2\_air}\), we used the monthly mean observed CO\({}_{2}\) dry air mixing ratio values from ERSA, except for the Springtime 2017 period, when such measurements were not available. For this period, measurements from the SME site were used as a background instead (see Figure 1), a change which is expected to have a minimal impact on the resulting flux values [64], particularly considering how low these values are compared to the other CO\({}_{2}\) flux components. The calculation of the other parts of the oceanic flux equation is complex; here, we follow recommendations from [65; 66] and the detailed explanation in [67]. As with the other flux variables, a positive value here indicates that the oceanic reservoir is acting as a source, while a negative value indicates that it is acting as a sink. By using the six-hourly ECMWF maps for the u and v wind components, surface pressure, and SST to solve Equation (2), we obtained ocean flux maps for every 6 h. These were linearly interpolated to create hourly ocean flux maps and appropriately converted to units of mol/km\({}^{2}\)/h for use in our WRF-Chem simulations. Figure 2 includes example sea flux maps on the bottom row, where the left-hand map shows the fluxes for 3 July 2016 at 14:00 UTC, the middle map shows the fluxes for 3 February 2017 at 14:00 UTC, and the right-hand map shows the fluxes for 3 May 2017 at 14:00 UTC. We also maintained a tracer strictly to set the boundary conditions. This was achieved by providing blank (i.e., zero-emission) flux maps for the three domains. The boundary conditions of the outer ((9-km)\({}^{2}\)) domain were defined based on the output of a global CO\({}_{2}\) model. For this study, these boundary condition values were provided by the latest available output from CarbonTracker Europe [58; 59] (specifically, CTE2017-FT), which was originally an offshoot of the CarbonTracker CO\({}_{2}\) data assimilation system developed at the National Oceanic and Atmospheric Administration (NOAA) [68; 69]. This latest version of CarbonTracker Europe implements a gridded state vector for improved accuracy of ecosystem emission regional estimates [59]. Newer versions of the CarbonTracker Europe results have been used in studies of various regions and fields of research, including Amazon carbon balance (e.g., [70]), China/Asia CO\({}_{2}\) balance (e.g., [71]), global methane inversions (e.g., [72]), and more.