## 4 Measurement Estimation and Analysis
### Robustness Test
#### 4.4.2 The Analysis of the Regression Results of Eastern, Central, and Western Regions  
In China, there are distinct regional differences in economic development. In other words, the eastern region is significantly more developed than the central and western regions. Therefore, this paper researches the differences in how environmental self-purification capacity affects environmental pollution in different regions. As the results of the SDM model suggest, in eastern, central, and western regions, environmental regulation (Iner) passes the significance test of the direct effect and proves negative. The direct effects of population density (Inden) all prove negative, yet the central region does not pass the significance test. The coefficients of built-up area (lnbua) all prove negative, yet the eastern region does not pass the significance test. The economic development levels (Iney) all pass the significance test and prove negative. The contribution of the economic development level of eastern region to environmental pollution control proves significantly higher than these of the central and western regions. In the direct effect of industrial structure (Inin), the coefficients of eastern and central regions prove significantly positive, whilst that of the western region proves significantly negative. In general, eastern and western regions embody relatively high environmental governance capacity, which probably results from the geographical environment. The eastern region possesses the advantages of developed economy, high-quality employees, advanced technologies, and high-tech talents. The eastern region not only achieves rapid economic development but also accumulates rich experience in environmental governance, which furnishes technological support for environmental improvement and establishes a foundation for a high quality of economic growth. The western region basically boosts economic development via the support of national policies. Accordingly, the eastern region enhances environmental self-purification capacity by technological means, whereas the western region improves environmental self-purification capacity or environmental quality based on natural conditions. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & **(1) The Yangtze** & **(2) The Pearl** & \begin{tabular}{c} **(3) Beijing-** \\ **River Delta** \\ \end{tabular} & \
\begin{tabular}{c} **(4) Northeastern** \\ **Region** \\ \end{tabular} &
\begin{tabular}{c} **(5) Chengdu-Chongqing** \\ **Economic Circle** \\ \end{tabular} \\ \hline \multicolumn{7}{c}{**InAEP**} & **InAEP** & **InAEP** & **InAEP** & **InAEP** \\ \hline \multicolumn{7}{c}{LR\_Total} \\ \multicolumn{7}{c}{wnAPI} & \(10.035\)* & \(5.893\) & \(-19.463\)** ** & \(1.158\) & \(-19.533\) \\  & \((1.801)\) & \((0.863)\) & \((-3.143)\) & \((0.475)\) & \((-1.355)\) \\ \multicolumn{7}{c}{lner} & \(-1.361\)** ** & \(-0.548\) & \(-1.791\)** ** & \(-0.222\) & \(0.311\) \\  & \((-3.301)\) & \((-1.619)\) & \((-3.645)\) & \((-1.391)\) & \((0.605)\) \\ \multicolumn{7}{c}{lnden} & \(3.541\)** ** & \(0.057\) & \(-0.466\) & \(0.130\) & \(-0.303\) \\  & \((3.561)\) & \((0.074)\) & \((-0.546)\) & \((0.547)\) & \((-0.878)\) \\ \multicolumn{7}{c}{lmbua} & \(1.562\)* & \(-1.780\) & \(1.618\) & \(1.191\) & \(-2.789\)* \\  & \((1.690)\) & \((-0.925)\) & \((1.434)\) & \((1.419)\) & \((-1.702)\) \\  & \((1.205\)** ** & \(-1.136\) & \(-0.855\) & \(-2.842\)** ** & \(-0.142\) \\  & \((-3.383)\) & \((-1.442)\) & \((-0.957)\) & \((-6.987)\) & \((-0.207)\) \\  & \(\mathrm{lin}\) & \(12.605\)** & \(14.336\)** ** & \(5.284\)** & \(2.173\)** ** & \(-2.454\) \\  & \((4.961)\) & \((4.656)\) & \((3.432)\) & \((3.552)\) & \((-1.169)\) \\  & \(N\) & \(574\) & \(126\) & \(182\) & \(490\) & \(266\) \\  & \(R^{2}\) & \(0.537\) & \(0.654\) & \(0.783\) & \(0.443\) & \(0.209\) \\ \hline \hline \end{tabular}
\end{table} 
Table 10: The Regression Results of SDM Model Heterogeneity in Five Regions.