# XCO\({}_{2}\) Fusion Algorithm Based on Multi-Source Greenhouse Gas Satellites and CarbonTracker
## 4 Results and Analysis
### Accuracy Verification of XCO\({}_{2}\) Fusion Dataset with TCCON Data  
In order to verify the XCO\({}_{2}\) accuracy of the fusion dataset, this study selected the data of eight TCCON sites and compared them with the actual observations of OCO-2, GOSAT, CT and TC fusion data. Figure 4 shows the time series scatter diagram of half-monthly average XCO\({}_{2}\) data from 2017 to 2018 at the eight TCCON sites. At the same time, we calculated the difference between the measured concentrations of TCCON and the four datasets. Figures 5-7 show the MAE, RMSE, and \(R^{2}\) indicators of the verification results at each TCCON site. In terms of MAE (Figure 5), the average MAE of the TC fusion dataset at the eight sites is 0.6273 ppm, OCO-2 is 0.7628 ppm, GOSAT is 1.0029 ppm, and CT is 0.7306 ppm. In general, the fused XCO\({}_{2}\) values obtained by the TC algorithm are more consistent with that measured by TCCON ground sites. While the average MAE of GOSAT is the largest, which also proves the accuracy of weight allocation to some extent. In terms of RMSE (Table 6), the average RMSE of the TC fusion dataset is 0.7683 ppm, OCO-2 is 0.9898 ppm, GOSAT is 1.3557 ppm, and CT is 0.8601 ppm. At site OC, the RMSEs of the three input datasets are the largest, indicating that there are the largest deviations between the observed values and the measured values at this site, resulting in the high RMSE of the fused dataset. In terms of \(R^{2}\) (Table 7 and Figure 6), the \(R^{2}\) values of the XCO\({}_{2}\) fusion dataset are mostly greater than those of the three input datasets. The average \(R^{2}\) of the TC fusion dataset at the eight sites is 0.8279, OCO-2 is 0.7533, GOSAT is 0.6118, and CT is 0.7812. At site OC, the \(R^{2}\) of the TC fusion dataset is the smallest because it is affected by the accuracy of CT and GOSAT XCO\({}_{2}\) data. In addition, except for the sites JS and OC, the \(R^{2}\) of the TC fusion dataset in all datasets is the highest, and it reaches more than 0.9 at sites PA, Br, WG, and TK. At sites JS and OC, the \(R^{2}\) of the fusion dataset is slightly smaller than that of the CT and OCO-2. It shows that the fitting degree between the XCO\({}_{2}\) fusion dataset based on the TC algorithm and the observations of TCCON is good, which proves the feasibility of this method.  
\begin{table}
\begin{tabular}{c c c c c} \hline
**TCCON Site** & **TC/ppm** & **OCO-2/ppm** & **GOSAT/ppm** & **CT/ppm** \\ \hline PA & 0.68 & 1.235 & 1.501 & 0.829 \\ OC & 1.229 & 1.13 & 1.811 & 1.312 \\ IZ & 0.897 & 1.278 & 1.177 & 1.066 \\ BR & 0.668 & 0.94 & 1.001 & 0.963 \\ JS & 0.829 & 0.922 & 1.556 & 0.703 \\ WG & 0.404 & 0.631 & 0.779 & 0.396 \\ TK & 0.716 & 1.06 & 1.461 & 0.784 \\ DB & 0.723 & 0.722 & 1.56 & 0.828 \\ \hline \end{tabular}
\end{table}
Table 6: RMSE of TC Fusion XCO\({}_{2}\), OCO-2 XCO\({}_{2}\), GOSAT XCO\({}_{2}\), and CT XCO\({}_{2}\).  
\begin{table}
\begin{tabular}{c c c c} \hline
**Time** & **OCO-2/ppm** & **GOSAT/ppm** & **CT/ppm** \\ \hline
2017–2018 & 0.6346 & 0.7995 & 0.5273 \\
2018 & 0.6410 & 0.8211 & 0.5196 \\
2018.1 & 0.5815 & 0.6991 & 0.4636 \\ \hline \end{tabular}
\end{table}
Table 4: Random errors of OCO-2, GOSAT, and CT model.  
\begin{table}
\begin{tabular}{c c c c} \hline
**Time** & **OCO-2** & **GOSAT** & **CT** \\ \hline
2017–2018 & 0.3337 & 0.2648 & 0.4015 \\
2018 & 0.3318 & 0.2590 & 0.4093 \\
2018.1 & 0.3241 & 0.2695 & 0.4064 \\ \hline \end{tabular}
\end{table}
Table 5: Weights of OCO-2, GOSAT, and CT model.  
Figure 4: Scatter diagrams of time series of OCO-2, GOSAT, CT, and TC fusion XCO\({}_{2}\) data at the eight TCCON sites from 2017 to 2018. The black solid line represents the observations of the TCCON site, the purple point represents CT, the red point represents OCO-2, the blue point represents GOSAT, and the green point represents the fused XCO\({}_{2}\) based on the TC algorithm.  
Figure 5: MAE of TC fusion XCO\({}_{2}\), OCO-2 XCO\({}_{2}\), GOSAT XCO\({}_{2}\), and CT XCO\({}_{2}\).  
Figure 6: Correlation analysis between TCCON and fused XCO\({}_{2}\) data.