The impacts of urbanization on fine particulate matter (PM_2.5) concentrations: Empirical evidence from 135 countries worldwide

Few attempts have been made to systematically investigate the impacts of urbanization on PM_2.5 concentrations in countries at different stages of economic development. In this study, a broad concept of urbanization that considers the transformations in the urban economy and the transport sector induced by urbanization is proposed to investigate the influence of urbanization on national PM_2.5 concentrations for underdeveloped, developing and developed countries during 1998–2014. The results indicate that urbanization has a significant relationship with PM_2.5 concentrations, but the magnitude of its influence varies among groups of countries with different development levels. First, the positive response of PM_2.5 concentrations to increased urbanization and transport-related emissions in underdeveloped countries are noticeably stronger than that in developing and developed countries. Second, for developing countries, urbanization, transport-related emissions and industrialization all have a significant positive effect on national PM_2.5 concentrations increase, although their impacts are unexpectedly smaller than those in the other groups of countries. Finally, increasing urbanization and the decrease in CO₂ emissions from manufacturing industry appear to reduce national average PM_2.5 concentrations in developed countries, while the decrease in transport-related CO₂ emission is likely to cause the rise in national average PM_2.5 concentrations.