Atmospheric microplastic deposition in an urban agglomeration: Spatiotemporal dynamics, influencing factors and ecological risks.

Atmosphere is a crucial medium for the global cycling of microplastics (MPs), yet the environmental fate and ecological risks of atmospheric MPs in urban agglomerations remain largely unexplored. In this study, we conducted a year-round (four seasons), high-density (33 sites) passive sampling campaign across the Beijing-Tianjin-Hebei urban agglomeration to evaluate the spatiotemporal patterns, driving factors, and ecological risks of atmospheric MP deposition. The results showed that the average deposition flux was 61.89 ± 4.21 items·m-2·d-1, with significantly higher fluxes observed in Beijing and Tianjin than in Hebei, and in spring than in summer (ANOVA with Tukey's HSD, P < 0.05). The predominant characteristics of atmospheric MPs included black (53.41%) and white (27.28%) colors, fibrous morphology (90.06%), particle sizes below 1000 μm (66.72%), and polymer compositions dominated by polyethylene terephthalate (PET, 68.35%) and rayon (RY, 18.80%), with notable subregional and seasonal variations. Regional MP flux was significantly positively correlated with GDP, road density, and population density, while altitude exhibited negative correlation. At the subregional level, wind speed was the significant driver in Hebei. Seasonally, anthropogenic-MP flux correlations emerged only in autumn and winter. Distance-based redundancy analysis (db-RDA) further identified O3 and humidity as significant factors associated with variation in MP characteristics. Multi-index risk assessment based on deposition flux and characteristics revealed site-specific risk spanning "moderate" to "high" risk levels, with polymer type contributing the most. Collectively, this study elucidates the spatiotemporal patterns, hierarchical drivers, and ecological risks in the Beijing-Tianjin-Hebei urban agglomeration, providing a scientific reference for risk assessment and pollution control strategies.