Spatiotemporal variations of PM2.5 concentration at the neighborhood level in five Chinese megacities

A thorough understanding of the spatiotemporal variations of PM2.5 concentrations is crucial for mitigating PM2.5 pollution with effective measures; however, few studies have investigated this issue based on the neighborhood scale. This study investigated the spatiotemporal variations of PM2.5 concentrations in 40 sites from five megacities by using hourly PM2.5 concentrations derived from air quality monitoring stations. Two relative indicators—the range and rate of change in PM2.5 concentration—were calculated to exclude the background levels of PM2.5 in different cities. Therefore, the differences between both the regions and between the sites within the same city were taken into consideration. Results showed that the within-city differences in PM2.5 concentration gradually increased from 2015 to 2017, and the differences were greater at a lower pollution level. Neighborhood-level PM2.5 concentration fluctuated mainly between 80% and 120% of each city's overall PM2.5 level. Combined with urban structure, the distribution of PM2.5 concentration during the four seasons presented three spatial patterns: the PM2.5 concentration was higher in the transition area, the PM2.5 concentration was higher in the core area than other areas, and the PM2.5 concentration had a relatively even distribution. Furthermore, the regional differences in the variations of PM2.5 concentrations depended on the PM2.5 concentrations differences among the cities; and the higher the PM2.5 pollution level, the greater was the observed regional difference. In addition, the range and rate of change in PM2.5 concentration had no significant differences among most sites at different pollution levels.