How does population structure affect pollutant discharge in China? Evidence from an improved STIRPAT model

China is the most populous country in the world, and the pollution caused by the excessive population should not be underestimated. In recent years, China's population growth rate began to decline. Since 2003, the growth rate has dropped below 6‰, but the population base is still huge. This paper aims to study the influence of population structure on pollutant discharge. Using the improved STIRPAT model, we studied the panel data of 31 provinces from 2003 to 2017 to study the impact of population on pollutant discharge from the perspectives of gender, aging and urbanization. The results show that population affects pollutant discharge through three effects, among which gender effect and urbanization effect increase pollutant discharge, and gender effect has a greater impact on pollutant discharge than urbanization effect. But the aging effect helps to reduce pollutant discharge during the study period. The results also show that population size contributes to pollutant discharge in the east, west, and northeast. However, population size had no significant effect on pollutant discharge in the middle part. Therefore, it is necessary to consider the difference of population impact when making environmental policy effectively. Finally, some special issues are briefly discussed.