Wet Deposition of Globally Transportable Microplastics (<25 μm) Hovering over the Megacity of Beijing

Microplastics of size <25 μm possess globally transportable features, but the impact of precipitation on their transport remains unclear. Here, microplastics were detected in all 10 studied rainfalls in Beijing, with <25 μm microplastics present in 8 rainfalls. Interestingly, microplastic abundance (7590-136,778 items·m^-3) was tentatively linked to maximum rainfall intensity, with <25 μm microplastics making up 39.6 (±27.5)% of the total count. The composition of <25 μm microplastics differed from that of larger microplastics, although both mainly comprised polystyrene, polyethylene, and polypropylene. The microplastic communities differed among rainfalls, suggesting that atmospheric transport is a highly dynamic process. The first rainfall exhibited the highest microplastic abundance and community diversity after long-term exposure to dry atmospheric environment. The deposited microplastics were unstable and highly fragmented according to the conditional fragmentation model. The wet deposition rate of the microplastics was calculated as 2-463 μg·m^-2 (146-8629 items·m^-2) per rain, amounting to 25.44 tons per annum 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 that precipitation is an effective mechanism for removing airborne microplastics, which may enter urban soils and waters, exacerbate microplastic burdens in the environment, and cause potential risk for human health.