Evolution of south-north transport and urbanization effects on PM2.5 distribution with increased pollution levels in Beijing

Understanding the main factors affecting the spatial distribution of PM2.5 is necessary to propose effective measures to control urban air pollution. Regional PM2.5 transport and urbanization play important roles on the spatial distribution of PM2.5 concentrations. According to the hourly PM2.5 distribution data in Beijing between 2014 and 2017, we used gradient analysis to identify the positive, negative and nonsignificant effects of south-north transport and urbanization on PM2.5 distribution pattern. The south-north transport effect on the distribution of PM2.5 was evidenced by significant south–north PM2.5 gradient, and the south–north PM2.5 gradient increased as the pollution level increased except in summer. PM2.5 concentrations are influenced by strong emissions located in the south. The frequency of the positive urbanization effect decreased, and the frequency of the negative urbanization effect increased as the pollution level increased during all seasons. Further, the corresponding positive or nonsignificant urbanization effects on the distribution of PM2.5 concentration became negative. Our results demonstrate that urbanization contributes to the reduction of PM2.5 concentrations in urban areas in the case of lower PM2.5 pollution levels, and increased PM2.5 concentrations at higher PM2.5 pollution levels.