## 3 Results and Discussion
### CO Concentration Experimental Results
#### 3.1.2 Evaluated by the ICOS CO Observations  
The number of the ICOS CO observations used in the evaluation are listed in Table 8. The CO concentrations measured at the highest observation point of the tall tower stations, which are considered closer to the background, were chosen to evaluate the experimental results. The boxplots of the DA, SIM experimental results, and CO concentrations from the external CAM-Chem results [48], and the MOPV8] data[75; 76] compared with the ICOS CO observations [47] are shown in Figure 13. The DA and SIM experimental results, the external CAM-Chem results [48], and the MOPV8] data [75; 76] were interpolated to the same location and the same pressure level or the same altitude of the ICOS observations [47] before evaluation. The mean and median CO concentration values in the boxplots are listed in Table 9.  
As can be seen from Table 9, the mean and median CO concentrations observed by the ICOS sites [47] in the winter of 2018 increased by 15.13 ppb and 8.30 ppb, respectively, compared to the same period in 2017. After this significant increase, the mean and median of ICOS CO concentration observations [47] had maintained a downward trend between \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Type**} & \multicolumn{2}{c}{**Exp2017**} & \multicolumn{2}{c}{**Exp2018**} & \multicolumn{2}{c}{**Exp2019**} & \multicolumn{2}{c}{**Exp2020**} \\ \cline{2-9}  & **MBE** & **RMSE** & **MBE** & **RMSE** & **MBE** & **RMSE** & **MBE** & **RMSE** \\ \hline SIM & 3.28 & 2.86 & 11.21 & 3.45 & 11.43 & 3.51 & 12.20 & 3.56 \\ DA & 33.84 & 5.89 & 20.06 & 4.50 & 14.71 & 3.94 & 14.53 & 3.89 \\ MOPV8] & 39.63 & 6.32 & 55.05 & 7.42 & 54.37 & 7.37 & 60.23 & 7.76 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Comparison of the surface-CO concentrations with the external CAM-Chem results. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline  & \multicolumn{2}{c}{**Exp2017**} & \multicolumn{2}{c}{**Exp2018**} & \multicolumn{2}{c}{**Exp2019**} & \multicolumn{2}{c}{**Exp2020**} \\ \cline{2-9}  & **MBE** & **RMSE** & **MBE** & **RMSE** & **MBE** & **RMSE** & **MBE** \\ \hline SIM & \(-\)1.73 & 1.46 & \(-\)0.38 & 1.10 & \(-\)1.00 & 1.22 & \(-\)1.04 & 1.26 \\ DA & 0.38 & 1.40 & 0.86 & 2.17 & \(-\)0.43 & 2.63 & \(-\)0.59 & 2.63 \\ MOPV8] & 11.12 & 3.34 & 17.69 & 4.21 & 16.13 & 4.02 & 14.56 & 3.82 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Comparison of the XCO with the external CAM-Chem results. the consecutive winter seasons of the following years. The variation trend in the external CAM-Chem results [48] was a straight downward trend. Compared with 2017, the mean and median CO concentrations from the external CAM-Chem results [48] in the winter of 2018 decreased significantly, by 27.93 ppb and 23.06 ppb, respectively. The comparison of these two trends suggested that the boundary conditions provided by the CAM-Chem datasets might be high in 2017. The evaluation results between the ICOS observations [47] and CO concentrations from the DA, SIM experiments, and the external CAM-Chem results [48] are listed in Table 10. All  
Figure 13: Boxplots of the CO concentrations from the DA, SIM experiments, and external CAM-Chem results, and the MOPV8J data compared with the ICOS observations. The lower and upper edges of the boxes represent the first (Q1) and third (Q3) quartiles, the lower and upper whiskers represent the minimum and maximum values within 1.5 times the interquartile range from Q1 and Q3, and solid lines and dots represent the median and mean values, respectively. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & **Exp2017** & **Exp2018** & **Exp2019** & **Exp2020** & **Total** \\ \hline Number of observations & 7585 & 18,893 & 25,461 & 30,631 & 82,570 \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Number of the ICOS CO observations used in the evaluation.