The impacts of urbanization on fine particulate matter (PM2.5) concentrations: Empirical evidence from 135 countries worldwide.

Few attempts have been made to systematically investigate the impacts of urbanization on PM2.5 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 PM2.5 concentrations for underdeveloped, developing and developed countries during 1998-2014. The results indicate that urbanization has a significant relationship with PM2.5 concentrations, but the magnitude of its influence varies among groups of countries with different development levels. First, the positive response of PM2.5 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 PM2.5 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 PM2.5 concentrations in developed countries, while the decrease in transport-related CO2 emission is likely to cause the rise in national average PM2.5 concentrations.