Does the expansion of the joint prevention and control area improve the air quality?-Evidence from China's Jing-Jin-Ji region and surrounding areas.

The relationship between group size and alliance performance has long been concerned by the theoretical scholars, but their opinions vary from each other. China's air pollution prevention and control action in 2017-2018 expanded the core area of joint prevention and control from a minor alliance of "2 + 4" cities to a major alliance of "2 + 26" cities, which undoubtedly provides a good natural experiment for advancing theoretical research. In order to clarify whether the expansion of the cooperation scope of air pollution joint prevention and control will further improve the air quality, the paper uses the regression discontinuity design (RDD) to empirically analyze the original "2 + 4" cities and 22 newly added cities. The conclusions indicate that the expansion of the core area of air pollution joint prevention and control in Beijing-Tianjin-Hebei (Jing-Jin-Ji) region and surrounding areas is conducive to further improving regional air quality, and the improvement effect of the "2 + 4" cities in the original core area is greater than that of the newly added 22 cities. The improvement of air quality after the expansion of the alliance is attributed to the unified and coordinated supervision of trans-regional environmental protection agencies, which reduces the "free rider" behavior; the increase of alliance members is conducive to achieve better environmental governance and improve cooperation performance among alliance members. Therefore, it is proposed that when the joint prevention and control of air pollution area is mature, the city should expand the scope of the alliance appropriately, rationally control and strengthen the cooperation among cities within the regional alliance, and consolidate the achievements of inter-regional joint prevention and control air quality improvement.