The Effect of Urban Agglomeration Expansion on PM2.5 Concentrations: Evidence from a Quasi-natural Experiment

This study constructs a quasi-natural experiment based on the expansion of the Yangtze River Delta urban agglomeration (YRDUA) of China in 2010 to investigate the impact and inner mechanism of urban agglomeration expansion on fine particulate matter (PM2.5) concentrations through propensity scores in difference-in-differences models (PSM-DID) using panel data from 286 prefecture-level cities in China from 2003 to 2016. The results show that 1) urban agglomeration expansion contributes to an overall decrease in PM2.5 concentration, which is mainly achieved from the original cities. For the new cities, on the other hand, the expansion significantly increases the local PM2.5 concentration. 2) In the long term, the significant influence of urban agglomeration expansion on PM2.5 concentration lasts for three years and gradually decreases. A series of robustness tests confirm the applicability of the PSM-DID model. 3) Cities with weaker government regulation, a better educated population and higher per capita income present stronger PM2.5 reduction effects. 4) Urban agglomeration expansion affects the PM2.5 concentration mainly through industrial transfer and population migration, which cause a decrease in the PM2.5 concentration in the original cities and an increase in the PM2.5 concentration in the new cities. Corresponding policy suggestions are proposed based on the conclusions.