## 4 Measurement Estimation and Analysis
### Estimation Results and Analysis Statistics of Spatial Correlation Effect
The reason lies in that with the increase in population density in one region, the pressure on various resources and infrastructure in local and neighboring regions soars, and pollutant emission jumps, which pollutes and destroys the environment in neighboring regions. Yet, in the total effect, there is no significant correlation between population density and atmospheric environmental pollution. Population density affects environmental pollution in two ways, i.e., the scale effect and the agglomeration effect. In regions with high population density, citizens raise high requirements for clothing, food, housing, and transportation in daily life, which all cause lots of environmental pollution, and high residential density prevents the diffusion of pollutants. The increase in population density also poses pressure on the bearing capacity of land, expands the demand for land, causes the reduction in forest area, land degradation and other resource problems, and intensifies environmental pollution. Notably, albeit the increase in population has a negative impact on environmental pollution, population density, as an indirect indicator, cannot directly reflect the impact. The coefficients of the total effect and direct effect of built-up area (Inbua) show that there is a significant negative correlation between built-up area and environmental pollution. As the built-up area enlarges, urban scale expands. On the one hand, with greater pressure and capacity in environmental governance, they can increase investment in environmental governance, continually develop pollution-prevention technologies, and ameliorate the quality of atmospheric environment. On the other hand, with the expansion of urban scale, per capita income climbs, and citizens year for a better green life. They augment environmental awareness and appeal and prefer green products in their consumption structure. Various environmental-protection organizations continue to popularize environmental protection, launch green products and services, and improve local environmental quality. As the effect coefficient of per capita GDP (Iney) suggests, local per capita GDP has a significant negative correlation with environmental pollution, indicating that per capita GDP weakens local environmental pollution. The spatial spillover effect of per capita GDP proves negative, indicating that per capita GDP has a negative effect on environmental pollution in neighboring regions. The increase in local per capita GDP reduces the level of environmental pollution in neighboring regions. One possible reason is that the rise of local per capita GDP forms a siphonage, resulting in the inflow of industries and reducing the emission of environmental pollution in neighboring regions. Supposing the population size remains the same, the level of environmental pollution in neighboring regions can be remarkably reduced. The increase in the proportion of the secondary industry in GDP (Inin) has a significant positive effect on environmental pollution in local and neighboring regions. This is mainly because industry is the main source of the emission of environmental pollution, and various regions achieve the goal of rapid economic development by dint of industrial development. These exacerbate environmental pollution. By optimizing and adjusting of industrial structure, these industries and production factorsfeature high pollution and low efficiency and have a negative impact on the atmospheric environment of the transfer-in regions.