## 4 Measurement Estimation and Analysis
### Robustness Test
As environmental pollution intensifies, China launches special plans for air pollution and raises stricter requirements and longer goals for environmental protection in major cities with environmental pollution, so as to promote the establishment of a working mechanism for joint prevention and control of air pollution in highly polluted regions. The Beijing-Tianjin-Hebei region has initially established joint prevention and control mechanism for air pollution, which forces relevant parties to optimize industrial structure in line with environmental regulation, gives full play to green-technology innovation in fog-haze governance, and ensures the emission of environmental pollutants within the scope of atmospheric self-purification capacity. Simultaneously, in the Beijing-Tianjin-Hebei region, with a large emission base of environmental pollutants, atmospheric self-purification capacity has a more significant effect on environmental pollution. Noticeably, in the Chengdu-Chongqing Economic Circle, the insignificant inhibitory effect of atmospheric self-purification capacity on environmental pollution probably results from the fact that pollutant emission mostly comes from motor vehicles. As an important platform for the development of the western regions in China, the Chengdu-Chongqing Economic Circle sees rapid economic development, and the number of motor vehicles keeps rising. In terms of the total effect, atmospheric self-purification capacity aggravates atmospheric environmental pollution in the Yangtze River Delta, the Pearl River Delta, and the northeastern region, yet the effect varies. In the Yangtze River, there is a significant positive correlation between atmospheric self-purification capacity and environmental pollution, with a coefficient of 10.035. In the Pearl River Delta and northeastern region, the coefficients reach 5.893 and 1.158, respectively, yet prove insignificant, indicating that atmospheric self-purification capacity does not serve as a key factor that affects the concentration values of environmental pollution in the Pearl River Delta and northeastern region. The Yangtze River Delta lies in southern China. With a developed economy and trade and high population density, cities are overloaded with population pressure, as well as excessive demand for resources and the environment. Besides, in industrial structure, the Yangtze River Delta relies heavily on industrialization, which becomes a main controlling factor for environmental pollution. With excess emission of pollutants, environmental bearing capacity oversteps the scope of atmospheric self-purification capacity, which aggravates environmental pollution in the region and its neighboring regions. China has issued _The Action Plan for the Prevention and Control of Air Pollution_ and _The Three-Year Action Plan to Win Blue Sky Protection Campaign_ as well as other documents, which take stricter measures on environmental assessment and pollution control in the Yangtze River Delta and provide policy support for improving environmental pollution. As Table 10 suggests, in the Pearl River Delta and northeastern region, environmental pollution is tied to apron strings of industrial structure. A higher proportion of secondary industry increases environmental pollution in the Pearl River Delta. The reason is that the rise of industrial output value kindles the increase in the emission of various pollutants and worsens environmental pollution. In the northeastern region, there is a significant negative correlation between economic development level and environmental pollution, or technological progress from economic development improves environmental level.