## 3 Results and Discussion
### CO Concentration Experimental Results
However, for the DA experiments, as shown in Figures 6 and 8, the MSCC distribution, the difference of MSCC relative to the MOPV8J data [75; 76], in the winter of 2017 were significantly different from those in the winter of the other three years. The DA experimental results for the winter season of 2017 showed high MSCC across a large region where Germany, the Czech Republic, Hungary, Poland, and other countries were located. Furthermore, as shown in Figures 6 and 7, both the DA and SIM experiments showed that the same region also had high mean XCO in the winter of 2017, making the mean XCO distribution results of the DA and SIM experiments for the winter season of 2017 significantly different from those in the other three years. The MSCC and XCO of the SIM experimental results depended on the prior CO fluxes and the initial and boundary conditions. As shown in Table 3, the maximum  
Figure 8: The differences between the DA experimental results and the MOPV8J data, (DA minus MOPV8J). (**–d**) difference of mean surface-CO concentrations, (**e–h**) difference of mean XCO. Figure 9: The differences between the SIM experimental results and the MOPV8J data, (SIM minus MOPV8J). (**–d**) difference of mean surface-CO concentrations, (**e–h**) difference of mean XCO. difference between the four prior CO fluxes was 2.86%. There were also no large differences between the prior CO flux distributions of the four winter seasons, as shown in Figure 3. Tables 4 and 5 also confirmed that there were no large differences between the MSCC and XCO of the MOPITT data over the four winter seasons. Therefore, the significant differences between the SIM experimental results for the winter season of 2017 and those for the other three winter seasons were likely due to the differences of the initial and boundary conditions. Actually, from the MSCC and XCO distributions of the external CAM-Chem results [48] shown in Figure 5, it is clear that the mean XCO distributions for the winter of 2017 were significantly different from those for the other three winter seasons, and the MSCC and mean XCO of the external CAM-Chem results [48], listed in Tables 4 and 5, also confirmed this significant difference. Although the external CAM-Chem results [48] were not used in all the experiments, they were derived from the same dataset as the initial and boundary conditions; therefore, there were strong correlations between them. Similarly, there were no significant differences between the MSCC and mean XCO distributions of the MOPV8] data [75,76] of the four winter seasons, as shown in Figure 4, and Tables 4 and 5 also confirmed that there were no large differences between the MSCC and XCO of the MOPITT data over the four winter seasons. Therefore, the significant differences between the DA experimental results for the winter season of 2017 and those for the other three winter seasons were also likely due to the differences of the boundary conditions. The MSCC and XCO results of the SIM_anthro experiments are shown in Figure 10. Since the only difference between the SIM and SIM_anthro experiments was that the prior CO fluxes for the SIM experiments were the sum of the prior anthropogenic, biogenic, and biomass-burning CO fluxes listed in Table 3, those for the SIM_anthro experiments were only the prior anthropogenic CO fluxes. Furthermore, the differences between the prior CO fluxes of the SIM and SIM_anthro experiments were less than 5.37%, as shown in Table 3. Therefore, it was not surprising that the results of the two experiments were close to each other; the differences of the MSCC and mean XCO results between the two experimental results were no more than than 0.97 ppb and 0.1 ppb, respectively. The differences between the SIM and SIM_anthro experimental results are shown in Figure S2. Figure 10: Distributions of the mean surface-CO concentrations and XCO of winter seasons in western Europe from 2017 to 2020, simulated by the SIM_anthro experiments. (**a–d**) mean surface-CO concentrations, (**e–h**) mean XCO.