Unexpected deposition by rainfall of globally transportable microplastics (<25 μm) hovering over the megacity of Beijing

Microplastics of <25 μm have been identified as possessing globally transportable features, but the impact of precipitation on their transport remains unclear. Here, microplastics were detected in all ten rainfall events in the megacity of Beijing, with <25 μm microplastics present in eight rainfalls. Interestingly, microplastic abundance was tentatively linked to maximum rainfall intensity, ranging from 7590 to 136778 items·m‒3, with <25 μm microplastics making up 39.6(±27.5)% of the total count. The shapes of microplastics were fragments, fibers, and films. The composition of <25 μm microplastics significantly differed from the larger microplastics, although both mainly comprised polystyrene, polyethylene, and polypyrene. Distinct differences were noted for the microplastic communities among different rainfalls, suggesting atmospheric transport is a highly dynamic process. The deposited microplastics were unstable and highly fragmented according to the conditional fragmentation model. The microplastic deposition rate was calculated as 2‒463 μg·m‒2·d‒1 (146–8629 items·m‒2·d‒1), amounting to 76.31 tons annually in Beijing. Although <25 μm microplastics represented a negligible proportion (0.00‒1.24%) of the overall mass load of microplastics, their numerical abundance was high. Our results demonstrate precipitation is an effective mechanism for removing airborne microplastics, which may enter urban soils and waters and exacerbate microplastic burdens in the urban environment.