Direct and spillover effects of urbanization on PM2.5 concentrations in China's top three urban agglomerations

China's rapid urbanization and severe PM2.5-dominated air pollution have received worldwide attention. It is important to reveal how urbanization affects the generation of PM2.5. A growing amount of research has examined the local urbanization impact on PM2.5 concentrations. However, few studies have quantified the influence of neighboring urbanization in terms of the spatial dependence of PM2.5 concentrations. Some studies considered these spatial effects but overlooked the natural controlling factors. In a comparative view among Beijing-Tianjin-Hebei, Yangtze River Delta, and Pearl River Delta urban agglomerations in China during 2000-2010, the direct and spillover effects of urbanization on PM2.5 concentrations were explored using spatial regression models that set urbanization and natural indicators as core and controlling variables, respectively. The results showed natural controlling factors were statistically significant; their inclusion in the regression model improved the reliability of estimation. It was also found that the spillover effect of urbanization substantially contributed to the spatial dependence of PM2.5 concentrations. Nevertheless, the direct and spillover effects of urbanization on PM2.5 concentrations differed among urban agglomerations and changed over time due to the complexity of urbanization processes. For the Beijing-Tianjin-Hebei urban agglomeration in 2010, the direct effect showed that the urban PM2.5 concentration increased by 0.138% averagely for every 1% increase of the urbanization level. Likewise, the spillover effect showed that urban PM2.5 concentration increased by 0.340% for every 1% increase in the average urbanization level of its neighboring cities. In addition, the conflict between PM2.5 abatement and urbanization level development in the three urban agglomerations was alleviated after 2007, reflected by the simultaneous improvement of urbanization level, urban air quality, and urbanization quality (the ratio of urbanization level to urban air pollution). This study provides policy makers with more detailed characterization about the impact of urbanization on PM2.5 concentrations, and it is suggested that not only the direct effect but also the spillover effect should be taken into account in guiding air pollution control of urban agglomerations.