Toward equitable air environment: Hourly spatiotemporal hotspot evolution and impact mechanisms of urban PM2.5 exposure risk

Abstract

As global urbanization accelerates, air pollution has become a critical environmental challenge affecting urban public health. Although previous studies under the population-exposure paradigm have revealed the relationship between human activity and air pollution, few have examined how the spatiotemporal variations and inequalities of PM_2.5 exposure risk (PER) relate to environmental characteristics. In particular, the influence of intraday population dynamics on PER has received limited attention. To address these gaps, we developed an environment-spatiotemporal-behavior-exposure analytical framework using big data and interpretable machine learning. Hourly-scale assessments of PER dynamics, identification of PER-evolving hotspots, spatial inequality analysis associated with multiple vulnerable groups, and systematic investigation of environmental factors impacting PER were performed. Using Shanghai as a case study, we found that spatial disparities in PER were primarily driven by resident population, whereas its temporal fluctuations were dominated by PM_2.5 concentration. Persistent PER hotspots were concentrated in high-density urban centers, sub-centers, and job-housing integrated zones within the western industrial cluster, with fluctuating PER hotspots surrounding them. Low-income and migrant groups were subject to higher PER, while spatial inequalities associated with the elderly and children covered broader spatial extents. Environmental factors exerted nonlinear effects on PER, including positive-negative conversion threshold effects, marginal saturation, non-monotonicity, and temporal sensitivity. Their interaction effects further revealed multi-phase fluctuation patterns. Based on these findings, targeted strategies emphasizing spatiotemporal collaborative governance were proposed. These strategies incorporated targeted interventions by time, location, and population group to mitigate PER and its inequality, thereby supporting healthy urban planning and air quality management.