PM_2.5 accumulation mechanism and health risks in basin-type cities: a case study of Linfen, China

Severe PM_2.5 pollution in basin-type cities arises from the combined effects of intensive emissions, complex terrain, and adverse meteorology. However, the accumulation mechanisms and associated health impacts in basin-type cities are still not fully characterized at high spatial resolution. This study integrates the WRF-CMAQ modeling system with the World Health Organization (WHO)-recommended Air Quality Plus (AirQ+) tool to investigate PM_2.5 accumulation and related health risks in Linfen, a coal-based basin city in northern China. High-resolution simulations for January and July 2020, validated against ground observations, are used to explore the roles of valley-controlled winds, planetary boundary layer dynamics, and dry-wet deposition processes. The results show that wintertime PM_2.5 levels are two to four times higher than those observed in summer, driven by shallow boundary layers (200-400 m), frequent temperature inversions, and weak winds that trap pollutants along the central valley corridor, leading to persistent high-concentration zones. Diurnal analyses at valley, hilly, and mountainous sites reveal terrain-dependent accumulation patterns, with pronounced morning peaks under winter inversions in valley areas and weaker, circulation-driven variability in mountainous regions. Deposition diagnostics indicate that dry deposition dominates in winter, while strong convective mixing and precipitation enhance wet deposition in summer, promoting more efficient removal. Population-weighted exposure fields coupled with AirQ+ show that winter-related fractions for all-cause and cardiovascular mortality substantially exceed national averages, with the most-exposed 20% of residents bearing a disproportionately high health burden. This study clarifies how emissions, complex terrain, and boundary-layer processes jointly control PM_2.5 accumulation in basin environments. It also provides an integrated framework for evaluating health risks and supporting equity-oriented air-quality management.