Spatiotemporal variations of PM_2.5 concentration at the neighborhood level in five Chinese megacities

A thorough understanding of the spatiotemporal variations of PM_2.5 concentrations is crucial for mitigating PM_2.5 pollution with effective measures; however, few studies have investigated this issue based on the neighborhood scale. This study investigated the spatiotemporal variations of PM_2.5 concentrations in 40 sites from five megacities by using hourly PM_2.5 concentrations derived from air quality monitoring stations. Two relative indicators—the range and rate of change in PM_2.5 concentration—were calculated to exclude the background levels of PM_2.5 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 PM_2.5 concentration gradually increased from 2015 to 2017, and the differences were greater at a lower pollution level. Neighborhood-level PM_2.5 concentration fluctuated mainly between 80% and 120% of each city's overall PM_2.5 level. Combined with urban structure, the distribution of PM_2.5 concentration during the four seasons presented three spatial patterns: the PM_2.5 concentration was higher in the transition area, the PM_2.5 concentration was higher in the core area than other areas, and the PM_2.5 concentration had a relatively even distribution. Furthermore, the regional differences in the variations of PM_2.5 concentrations depended on the PM_2.5 concentrations differences among the cities; and the higher the PM_2.5 pollution level, the greater was the observed regional difference. In addition, the range and rate of change in PM_2.5 concentration had no significant differences among most sites at different pollution levels.