Spatial organisation of microplastics in road runoff across an urban network.

Road runoff is a major pathway by which microplastics are transported from urban surfaces to aquatic ecosystems, yet the spatial scales over which measurements represent road networks remain unresolved. This constrains source attribution, monitoring design and urban emission inventories. Using high-resolution µFTIR imaging, we quantified microplastic abundance, particle-size distribution and polymer composition in runoff across an interconnected urban road network following a rainfall mobilisation event, characterising twenty-one polymer types across four particle-size fractions. Concentrations spanned more than an order of magnitude (18-544 particles L⁻¹), varying sharply even between road only 114 m apart. Compositional dissimilarity showed no significant relationship with inter-site distance, instead revealing a mosaic of locally distinct polymer assemblages, with significantly greater heterogeneity in the urban core. Cellulose acetate, polyurethane, polymethyl-methacrylate and polypropylene dominated, reflecting diverse sources. In contrast, 25-50 µm particles consistently dominated, accounting for ∼84% of particles, indicating a common fine-particle transport signature despite pronounced spatial variability in polymer composition. These reveal for the first time, that fine-scale spatial heterogeneity, rather than geographic distance, is the defining characteristic of microplastic distributions in road runoff. Resolving this is therefore fundamental to establishing the representativeness of urban microplastic measurements, without which risk assessments remain inherently uncertain.