打好污染防治攻坚战是全面建成小康社会的必然要求，推进以资源共享、责任共担为内核的区域协同治理则是妥善应对区域性、复合型大气污染问题，进而打赢污染防治攻坚战的重要手段。近年来，随着生态文明建设战略地位的不断提升，区域性、复合型大气污染的防治问题日益成为党和国家关心的重要议题。鉴于大气污染的空间外溢性，过去以行政区划为治理边界的属地管理模式已被证实无法适用于当下的大气污染防治，探索实施以区域协同为核心的大气污染治理模式已势在必行。在此背景下，京津冀及周边地区通过实施“2+26”城市大气污染联防联控政策实现了区域空气质量的整体改善，这无疑为国家治理区域性大气污染问题，乃至跨域环境污染难题都提供了一个全新的思路。有鉴于此，本研究聚焦区域大气污染协同治理问题，以“2+26”城市大气污染联防联控政策为研究对象，基于对政策效果的科学评估以及政策作用机制的深入剖析，深化对区域大气污染协同治理实践效果与作用机制的认识。

区域协同治理是协同治理理论在理论层面的延伸，大气污染联防联控政策则是协同治理理论的现实应用。因此，本研究以既有协同治理理论框架为基础构建区域大气污染协同治理结构——过程——效果分析框架，并据此进一步将研究问题细化为对“2+26”城市大气污染协同治理实践效果的评估以及对“2+26”城市大气污染协同治理静态结构性机制和动态过程性机制的探讨三个问题。为回答以上三个问题，本研究分别运用双重差分倾向得分匹配法对“2+26”城市大气污染协同治理的实践效果进行评估、运用社会网络分析对“2+26”城市大气污染协同治理的静态结构性机制进行刻画、运用扎根理论方法对“2+26”城市大气污染协同治理的动态过程性机制进行剖析。基于理论与实证分析，得出以下结论：

首先，“2+26”城市实施的大气污染联防联控政策显著改善了目标区域的大气污染状况；由于不同大气污染物的形成机制不尽相同，因此，不同的社会因素对不同类型的大气污染物往往会产生不同影响；联防联控政策对大气污染浓度的负向影响存在显著的动态效应，并且具有一定的时滞性，维持政策的长效运行至关重要；大气污染联防联控政策存在显著的时空异质性，气象条件在大气污染防治中主要发挥“助推”性的辅助作用。其次，“2+26”城市大气污染协同治理取得的显著成效得益于各级政府、不同部门之间协同网络关系的建立与稳固。在联防联控政策引导下，各级政府之间、不同部门之间的合作交流愈加频繁、相互间的联系趋于紧密，在区域大气污染协同治理领域已形成平等的、稳定的合作网络关系。最后，“2+26”城市大气污染协同治理优异成绩的取得也得益于对以目标为导向的引导性工具、以过程为导向的执行性工具、以结果为导向的保障性工具以及以能力为导向的赋能性工具等政策工具的综合运用，尽管对不同类型政策工具的应用程度存在差异，但四者彼此之间并非割裂、独立的存在，而是接续推进、彼此衔接的有机组合，共同构成“2+26”城市大气污染协同治理的基本政策工具箱。

“2+26”城市大气污染协同治理取得的显著成效为国家区域性、复合型大气污染问题治理提供了可资借鉴的重要经验，而其在体制机制建设、工具手段运用等方面存在的一些亟需改善的问题同样不容忽视。因此，面对日益严峻的大气污染防治形势，为推动“2+26”城市大气污染协同治理的深化完善，进一步提升国家治理区域性、复合型环境污染问题的水平与能力，结合前述理论与实证分析结果，本研究提出以下政策建议：第一，保障政策长效运行，实施多污染物协同控制；第二，深化多元主体合作，推动区域污染协同治理常态化；第三，优化协同治理手段，丰富污染治理政策工具箱；第四，归纳总结治理经验，助力破解跨域环境污染难题。

Fighting the tough battle of pollution prevention and control is an inevitable requirement for building a moderately prosperous society in all respects. Promoting regional collaborative governance with resource sharing and responsibility sharing as the core is an important means to properly deal with regional and complex air pollution problems, and then win the tough battle of pollution prevention and control. In recent years, with the continuous improvement of the strategic position of ecological civilization construction, the prevention and control of regional and complex air pollution has increasingly become an important topic of concern to the Party and the state. In view of the spatial spillover of air pollution, the territorial management model that used administrative divisions as governance boundaries in the past has been proven to be inapplicable to current air pollution prevention and control. It is imperative to explore and implement an air pollution control model with regional coordination as the core. In this context, Beijing-Tianjin-Hebei and surrounding areas have achieved overall improvement in regional air quality through the implementation of the “2+26” cities air pollution joint prevention and control policy. This undoubtedly provides a new way of thinking for the country to deal with regional air pollution problems and even cross-regional environmental pollution problems. In view of this, this study focuses on the regional collaborative governance of air pollution. Taking the “2+26” cities air pollution joint prevention and control policy as the research object. Based on the scientific evaluation of the policy effect and the in-depth analysis of the policy action mechanism, deepen the understanding of the practice effect and action mechanism of the coordinated governance of air pollution areas.

Regional collaborative governance is an extension of the collaborative governance theory at the theoretical level, and the joint prevention and control policy of air pollution is a practical application of the collaborative governance theory. Therefore, based on the existing theoretical framework of collaborative governance, this study constructs a structure-process-effect analysis framework for regional collaborative governance of air pollution, and further refines the research problem into the evaluation of the effect of the “2+26” cities air pollution regional collaborative governance, and the discussion of the static structural mechanism and the dynamic process mechanism of the “2+26” cities air pollution regional collaborative governance. In order to answer the above three questions, this study uses the differences-in-differences propensity score matching method to evaluate the policy effects of the “2+26” cities air pollution regional collaborative governance, uses social network analysis method to describe the static structural mechanism of “2+26” cities air pollution regional collaborative governance, and uses the grounded theory method to analyze the dynamic process mechanism of “2+26” cities air pollution regional collaborative governance. Based on theoretical and empirical analysis, the following conclusions are drawn:

Firstly, the joint air pollution prevention and control policy implemented by the “2+26” cities has significantly improved the air quality in the target area. Due to the different formation mechanisms of different air pollutants, different social factors often have different effects on different types of air pollutants. The negative impact of the joint prevention and control policy on the air pollution concentration has a significant dynamic effect, and it has a certain time lag, so it is very important to maintain the long-term operation of the policy. The air pollution joint prevention and control policy has significant spatial and temporal heterogeneity, and the meteorological conditions mainly play a “boosting” auxiliary role in the prevention and control of air pollution. Secondly, the success of the “2+26” cities air pollution regional collaborative governance benefits from the establishment and stability of the collaborative network relationship between governments at all levels and different departments. Under the guidance of the joint prevention and control policy, the cooperation and exchanges between governments at all levels and different departments have become more frequent, and the links between them have become closer. An equal and stable cooperative network relationship has been formed in the field of the regional collaborative governance of air pollution. Finally, the outstanding achievements of the “2+26” cities air pollution regional collaborative governance also benefit from the comprehensive use of policy tools such as the goal-oriented guiding tools, the process-oriented executive tools, the result-oriented guarantee tools, and the capability-oriented enabling tools. Although there are differences in the degree of application of different types of policy tools, the four are not separate and independent existences from each other, but an organic combination of continuous advancement and connection with each other. They constitute the basic policy toolbox for the regional collaborative governance of “2+26” cities air pollution.

The success of the “2+26” cities air pollution regional collaborative governance has provided important experience for the country’s regional and complex air pollution control. However, some problems that need to be improved in the construction of system and mechanism and in the application of tools and means cannot be ignored either. Therefore, in the face of the increasingly severe situation of air pollution prevention and control, in order to promote the deepening and improvement of the “2+26” cities air pollution regional collaborative governance, and further enhance the national level and ability to control regional and complex environmental pollution problems, combined the above theoretical and empirical analysis results, this study puts forward the following policy recommendations: first, ensure the long-term operation of the policy and implement the coordinated control of multiple pollutants; second, deepen the cooperation of multiple subjects and promote the normalization of regional pollution collaborative governance; third, optimize the means of collaborative governance and enrich the policy toolboxes of pollution control; Fourth, summarize the governance experience to help solve the problem of cross-domain environmental pollution.