Quantification of microplastic by particle size down to 1.1 μm in surface road dust in an urban city, Japan

The impact of microplastics (MPs, plastic particles ≤5 mm) on ecosystems is of great concern. Road surfaces represent a significant source of MPs where plastic fragments are physically and chemically reduced to MPs. However, the literature lacks information on fragmentation tendencies below 11 μm. This study aimed to characterize the occurrence of MPs in road dust in different size fractions down to 1.1 μm. Road dust was collected at five sites near a major road in Kusatsu city, Japan, and partitioned by size into 13 fractions (1.1-850 μm). The coarser fractions accounted for a greater proportion of the dust. The percentage of organic matter, determined by loss on ignition, increased as the fractions became finer. Pyrolysis-gas chromatography-mass spectrometry was used to quantify 12 types of polymers in each fraction. The dust was found to contain nine types of MP, namely, polyethylene (PE), polypropylene (PP), polyvinylchloride (PVC), polystyrene (PS), styrene/butadiene rubber (SBR), acrylonitrile/butadiene/styrene resin (ABS), polycarbonate (PC), polymethylmethacrylate (PMMA), and polyamide 66 (PA66). The total MP concentration in road dust particles by particle size fraction (concentration_f) began to increase from the 125-250 μm fraction and remained elevated in finer fractions down to 1.1 μm, indicating that MPs in the road dust micronized to at least 1.1 μm. However, for individual polymer types, the tendency for concentration_f to increase or decrease with particle size fraction varied: the concentration_f of some polymers, such as PE and PVC, remained elevated in fractions down to 1.1 μm; the concentration_f of SBR, a rubber-MP, showed a stable or decreasing trend in fractions of 7.0-11 μm and finer. Particles of PE, PVC, and some other plastics might become increasingly finer, even down to 1.1 μm. Further research is needed to understand the comminution limits of these polymers under pertinent environmental conditions.