Monsoon and Westerlies drive particle size sorting of microplastics in the riparian zone of the Tibetan Plateau interior

As the water tower of Asia, the source-sink dynamics of microplastics (MPs) on the Tibetan Plateau are regulated by complex atmospheric-hydrological interactions, yet habitat-specific driving mechanisms remain poorly understood. This study investigated 152 riparian sediment sites across Xizang and revealed significant size-dependent spatial distribution and distinct environmental fates. Microplastic abundance ranged from 0 to 3778 items/kg dry weight, with localized hotspots against a broad low-background pattern. Millimeter-sized fractions (1-5 mm) were mainly associated with monsoon precipitation and surface runoff (p < 0.05), forming high-abundance hotspots in the humid southeastern Tibetan Plateau. Conversely, micrometer-sized components (< 1 mm) exhibit showed a pronounced westward shift, persisting in the plateau interior via the Westerly Jet. Habitat-driven analysis demonstrates that river sediments act as transport corridors, controlled by hydrological and water-quality factors, whereas lake environments serve as ultimate sinks, accumulating high-density MPs through turbidity-driven ballast effects. Morphological and chemical characterization revealed a predominance of fibers (64.35%) and particles in the 1-3 mm size range (38.57%), with polyethylene terephthalate (PET) and polypropylene (PP) emerging as the primary polymer types (35.15% and 31.17%, respectively). Notably, despite the region's overall low pollution load (pollution load index: 1.36-2.47), the accumulation of high-hazard polymers like PA66 in headwater areas significantly elevates the Potential Ecological Risk Index, which reached 62.26 in the upper Yarlung Tsangpo River. This study elucidates the synergistic sorting mechanisms of MPs driven by atmospheric circulation and watershed hydrology, highlighting the need for targeted risk control in fragile aquatic ecosystems.