## 2 Theoretical Mechanism and Research Hypotheses
### The Effect of Atmospheric Self-Purification Capacity on the Governance of Environmental Pollution
Therefore, atmospheric self-purification capacity probably displays a spatial correlation effect among various regions. Coupled with spatial and geographical factors, environmental pollution may have an inter-regional effect. In terms of mechanism, the spatial correlation between atmospheric self-purification capacity in neighboring regions and local environmental pollution operates in three ways. First, the positive externality of the improvement of atmospheric self-purification capacity: ventilation and precipitation-based atmospheric self-purification capacity has a close tie with air-environment quality. When the atmospheric self-purification capacity index remains low, atmospheric diffusion capacity becomes weak, which hinders the removal and diffusion of pollutants and causes atmospheric environmental pollution [17]. The improvement of atmospheric self-purification capacity in neighboring regions will directly reduce environmental pollution from these regions by raising urban greening coverage and transferring polluting industries. In its neighboring regions, the drop in environmental pollution also reduces environmental pollution in one region. Second, the positive externality of air-pollution governance: cities with serious air pollution can hardly attract talent, high-tech, and other elements, which affects the accumulation level of urban capital and further undermines the quality and sustainability of economic growth. To better curb environmental pollution promotes the high-quality development of one region and enhances its attractiveness. Third, as \"hitchiking\" often occurs in environmental governance, the atmospheric self-purification capacity of one region can drive the governance of environmental pollution in neighboring regions (or a demonstration effect) in a limited space. To meet people's growing yearning for a better life, neighboring regions learn and implement advanced ideas of environmental-pollution governance, thus forming the spatial correlation effect of atmospheric self-purification capacity on environmental-pollution governance [30]. In brief, this paper proposes research Hypothesis 2. **Hypothesis 2 (H2). **_The improvement of atmospheric self-purification capacity in its neighboring regions helps to reduce environmental pollution in one region._  
The Threshold Effect of Atmospheric Self-Purification Capacity on the Governance of Environmental Pollution  
In consideration of the differences in regional economic development and social systems, the impact of atmospheric self-purification capacity on environmental pollution is also affected by multiple factors, environmental regulation in particular. With the improvement of policies and standards on environmental regulation, governments control the environmental-protection level of foreign investment and other factors more strictly and further strengthen the review of polluting industries. In this way, various regions enhance their atmospheric self-purification capacity and expand the pollution-reduction effect. Besides, various regions regulate or reduce the emission of environmental pollutants and control the emission within the scope of atmospheric environmental bearing capacity, to alleviate environmental pollution ([PERSON] and [PERSON], 2018) [31]. However, strict environmental regulation inhibits the reduction-emission effect of pollutants. The intensity of environmental regulation directly determines whether enterprises plan to include the cost of the governance of environmental pollution in production costs. Additionally, when governments tighten the grip on environmental regulation, corporate compliance cost increases, which restricts innovation in production technology to some degree [32]. Simultaneously, the rise of the expenses on environmental-pollution governance and the price of production factors cut down the upgrading of the enterprise's products.