Can regional integration control transboundary water pollution? A test from the Yangtze River economic belt

Based on the quasi-natural experiment of the establishment of the Yangtze River economic belt, this paper uses a difference-in-difference-in-differences model (DDD) to test the impact of regional integration on transboundary pollution. The results show that regional integration can significantly reduce transboundary water pollution. Regional integration reduces chemical oxygen demand (COD) by 0.357, ammonia nitrogen content (NH₃-N) by 0.155, and dissolved oxygen content (DO) by 0.088 in the downstream of the provincial boundary. And regional integration has a long-term inhibitory effect on transboundary pollution. The heterogeneity results show that the regional integration has a stronger inhibition effect on the transboundary pollution of the middle and lower reaches of the Yangtze River economic belt and the tributaries. At the government level, regional integration can reduce transboundary pollution by strengthening the government's environmental regulation and collaborative governance. At the industry level, it can inhibit transboundary pollution by promoting industrial structure optimization and green technology innovation. At the market level, regional integration can promote market competition to reduce transboundary pollution and expand market scale to intensify transboundary pollution. At the society level, regional integration can intensify transboundary pollution by promoting population agglomeration and effectively alleviate transboundary pollution by enhancing public environmental supervision.