## 4 Conclusions  
In this study, the CO fluxes and concentrations of the four consecutive winter seasons in western Europe from 2017 to 2020 were estimated through the assimilation of the MOPV8J data [75; 76] by a regional CO flux inversion system. The analyses of the MOPV8J data used by the inversion system indicated that the mean averaging kernel row sums of the surface level was about 0.25, and the difference percentage of the surface-level retrievals relative to a priori CO-mixing ratios was 14.79%, which was similar to that of the other levels. The inverted CO fluxes of the four winter seasons were 6198.15 kilotons, 4939.72 kilotons, 4697.80 kilotons, and 5456.19 kilotons, respectively. Compared with the prior CO fluxes, the inverted CO fluxes of the four winter seasons increased by 95.53%, 52.27%, 37.16%, and 37.06%, respectively. Based on the assumption that the anthropogenic CO emissions accounted for the super majority of the winter CO fluxes in western Europe, the UNFCCC inventories [46] were used to evaluate the accuracy of the above inverted CO fluxes. The evaluation results indicate that the differences between the inverted CO fluxes and UNFCCC inventories [46] of the three winter seasons of 2017 to 2019 were 13.36%, \(-\)4.59%, and \(-\)4.76%, respectively. Considering the influence of boundary conditions on the uncertainty of the CO \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**CO Flux** & **Exp2017** & **Exp2018** & **Exp2019** & **Exp2020** \\ \hline Posterior & 28.45 & 21.79 & 20.19 & 19.86 \\ Prior & 14.55 & 14.31 & 14.72 & 14.49 \\ Difference & 13.90 & 7.48 & 5.47 & 5.37 \\ \hline \hline \end{tabular}
\end{table} 
Table 11: The mean posterior and prior CO fluxes of the four DA experiments. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Winter Season** & \multicolumn{2}{c}{**CO Emissions (Kilotons)**} & **Difference** \\
**of the Year** & **DA Experiment** & **From UNFCCC** & **(\%)** \\ \hline
2017 & 6198.15 & 5467.90 & 13.36 \\
2018 & 4939.72 & 5177.29 & \(-\)4.59 \\
2019 & 4697.80 & 4932.55 & \(-\)4.76 \\
2020 & 5456.19 & - & - \\ \hline \hline \end{tabular}
\end{table} 
Table 12: Comparison of CO emissions of the winter seasons in western Europe inverted by the DA experiments with the corresponding anthropogenic CO emissions extracted from the UNFCCC inventories. fluxes' inversion results, the evaluation results suggested that the above assumption was reasonable. Detailed surface-CO concentrations and XCO comparative analyses between the DA and SIM experimental results, the external CAM-Chem results [48], and the MOPV8] data [75; 76] were conducted. The comparative analysis results indicated that the DA and SIM experimental results of 2017 were obviously affected by the high boundary conditions. The CO concentrations results of the experiments were also evaluated by the ICOS CO concentration observations [47]. The evaluation results showed that the average MBEs and RMSEs of the DA experiments were better than those of the SIM experiments and the external CAM-Chem results [48]. The average MBE of the DA experiments was \(-\)22.43 ppb, and was 17.52-ppb and 14.84-ppb better than that of the SIM experiments and the external CAM-chem results [48], respectively. The average RMSE was 4.17-ppb and 13.03-ppb better than that of the SIM experiments and the external CAM-chem results [48], respectively. The comparison of the ICOS data [47] with the external CAM-Chem results [48] also confirms that the boundary conditions might be high in 2017.