## 3 Results and Discussion  
### CO Concentration Experimental Results  
The mean surface-CO concentrations (MSCC) and XCO distributions of the DA and SIM experimental results are shown in Figures 6 and 7, respectively. The MSCC and XCO of the four consecutive winter seasons in western Europe from 2017 to 2020 estimated by the DA, SIM, and SIM_anthro experiments are listed in Tables 4 and 5, respectively. The MSCC and XCO of the same periods in western Europe, extracted from MOPV8J [75,76] and the external CAM-Chem results [48], are also listed in the tables for reference. The CO concentrations and XCO extracted from the external CAM-Chem results were not used in all the experiments. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Source**} & \multicolumn{4}{c}{**Mean Surface CO Concentrations (ppb)**} \\ \cline{2-5}  & **Exp2017** & **Exp2018** & **Exp2019** & **Exp2020** \\ \hline DA experiment & 160.93 & 134.60 & 130.12 & 122.59 \\ SIM experiment & 130.12 & 122.72 & 123.48 & 116.43 \\ SIM\_anthro experiment & 129.20 & 122.22 & 122.52 & 115.67 \\ MOPV8J & 166.47 & 166.56 & 166.43 & 164.46 \\ External CAM-Chem & 126.84 & 111.51 & 112.06 & 104.23 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Mean surface-CO concentrations of winter seasons in western Europe form 2017 to 2020. Figure 6: Distributions of the mean posterior surface-CO concentrations and XCO of winter seasons in western Europe from 2017 to 2020, estimated by the DA experiments. (**a–d**) mean posterior surface-CO concentrations, (**e–h**) mean posterior XCO. As shown in Tables 4 and 5, the average differences of MSCC and XCO between the DA experiments and the MOPV8] data [75,76] were \(-28.92\) ppb and \(-12.82\) ppb, which were \(1.09\)-ppb and \(13.87\)-ppb better than those of between the SIM experiments and the MOPV8] data [75,76]. This was as expected; since the MOPV8] data were assimilated in the DA experiments, the DA experimental results should be closer to the MOPV8] data [75,76] than the SIM results. The differences between the DA, the SIM experimental results, and the MOPV8] data are shown in Figures 8 and 9, respectively. The comparison of surface-CO concentrations and XCO of the experimental results with the MOPV8] data [75,76] are listed in Tables S2 and S3, respectively. The differences between the DA and SIM experimental results are shown in Figure S1. From the MSCC of the SIM experimental results, shown in Figure 7, and the differences of the MSCC between the SIM experiments and the MOPV8] data [75,76], shown in Figure 9, the locations of London, Paris, Berlin, Rotterdam, Antwerp, Madrid, Rome, Naples, and other major western European cities can be clearly identified. From the same results and differences of the DA experiments shown in Figures 6 and 8, not only the above major cities can be identified, but also the urban agglomerations composed of cities in the Netherlands and Belgium, northern Italy, etc., are more clearly reflected. As shown in Figure 7, the MSCC distributions of the SIM experimental results were roughly similar; so were the differences of the MSCC between the SIM experiments and the MOPV8] data [75,76], as shown in Figure 9.  
Figure 7: Distributions of the mean surface-CO concentrations and XCO of winter seasons in western Europe from 2017 to 2020, simulated by the SIM experiments. (**a–d**) mean surface-CO concentrations, (**e–h**) mean XCO. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Source**} & \multicolumn{4}{c}{**Mean XCO (ppb)**} \\ \cline{2-5}  & **Exp2017** & **Exp2018** & **Exp2019** & **Exp2020** \\ \hline DA Experiment & 74.87 & 70.26 & 67.96 & 67.93 \\ SIM Experiment & 72.76 & 69.02 & 67.38 & 67.49 \\ SIM\_anthro & 72.68 & 68.97 & 67.28 & 67.43 \\ Experiment & 84.59 & 84.63 & 82.06 & 81.01 \\ External & 74.50 & 69.40 & 68.39 & 68.52 \\ CAM-Chem & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Mean XCO of winter seasons in western Europe from 2017 to 2020.