## 4 Measurement Estimation and Analysis  
### Estimation Results and Analysis Statistics of Spatial Correlation Effect  
To verify hypotheses 1 and 2, Table 2 shows the empirical results of spatial correlation effect of atmospheric self-purification capacity on environmental pollution by using SAR and SDM. As Table 2 suggests, the statistical test of the estimation value of spatial lag parameter of environmental pollution proves significant, indicating that environmental pollution has significant spatial correlation and spatial spillover phenomenon. Spatial correlation effect of atmospheric self-purification capacity on environmental pollution can be summarized as the total effect, the direct effect, and the indirect effect. The direct effect means the average impact of changes in atmospheric self-purification capacity on environmental pollution in one region, or the spillover effect of atmospheric self-purification capacity in one region. The indirect effect means the average impact of atmospheric self-purification capacity in neighboring regions on environmental pollution in one region, or the spatial spillover effect of atmospheric self-purification capacity in neighboring regions. The total effect means the sum of the direct effect and the indirect effect. Among them, SDM has a higher fitting degree. This paper mainly explains and analyzes the SDM results. Table 2 basically investigates the direct effect and indirect effect of atmospheric self-purification capacity (wlnAPI) on environmental pollution (lnAEP), or the impacts of atmospheric self-purification capacity in one region and its neighboring regions on environmental pollution. The total effect and direct effect both show that the coefficient of atmospheric self-purification capacity (wlnAPI) proves negative and passes the significance test. This demonstrates that atmospheric self-purification capacity helps to mitigate environmental pollution in one region and tallies with theoretical prediction. The reason is that atmospheric self-purification capacity signifies an inherent function of the environment to automatically eliminate pollutants and purify them. The pollutants that enter the atmosphere can be diffused and diluted to a wide space via physical, chemical, and biological actions under natural conditions, substantially reducing their concentration. Owing to the action of gravity and the washing of rain, heavier particulate matters fall on the ground. The pollutants can also decompose under the irradiation of light and the participation of other physics, which purifies the air and reduces atmospheric-pollution concentration. In terms of the impact of atmospheric self-purification capacity in neighboring regions, the coefficient of its indirect effect (wlnAPI) proves negative yet insignificant. To put it in other way, atmospheric self-purification capacity in neighboring regions has a spillover effect on local environmental pollution, with insignificant spillover effect. The reason lies in that atmospheric self-purification capacity is affected by many factors such as precipitation, ventilation, urban planning, pollution-source intensity, and vegetation coverage, which embody large spatial differences. With the transformation of social contradictions in China, central and local governments