Are Small Cities More Environmentally Friendly? An Empirical Study from China

City sizes are rapidly expanding, and urban air pollution is a serious challenge in China. PM2.5 (fine particulate matter) is the primary pollutant of urban pollution. This study aimed to examine the correlations between PM2.5 and city size. In this paper, using the panel data of 278 cities in China from 2007 to 2016, we constructed a static and dynamic panel model based on the STIRPAT (Stochastic Impacts by Regression on Population, Affluence and Technology) analytical framework. We found that there was a significantly inverted N-shaped correlation between PM2.5 and city size. Two inflection points were found at 949,200 and 3,736,100. We found no evidence to support the EKC (Environmental Kuznets Curve) hypothesis, while the Pollution Haven Hypothesis gained support. The contradiction between PM2.5 and city size will exist for the long term. Policy recommendations were proposed based on our findings. Controlling the city size does not seem to be necessary for very large cities as they have passed the second inflection point. Cities with a growing population are under great pressure to prevent PM2.5 pollution and need to implement greater measures to reduce pollution.