Airborne microplastic concentrations and deposition across the Weser River catchment

Microplastic (MP) appears to be omnipresent in the atmosphere, raising concerns about dispersion across environmental compartments, ecological consequences and human health risks by inhalation. To date, data on the sources of atmospheric MP and deposition to river catchment areas are still sparse. We, therefore, took aerosol and total atmospheric deposition samples in the catchment area of the large German river Weser to estimate microplastic deposition fluxes (DFs) at six specific sites and airborne MP concentrations. Sampling in rural, suburban, and urban environments and wastewater treatment plants (WWTPs) was performed, aiming at a variation in airborne MP pollution and elucidating potential MP source areas. Aerosol samples were taken twice in April and October while monthly total deposition samples were collected over a period from March to October. Microplastics were detected in all analysed aerosol samples by Raman spectroscopy down to 4 μm, and in all 32 total deposition samples by μFT-IR down to 11 μm. Average MP number concentrations of 91 ± 47 m^-3 were found in aerosol samples. The measured total MP number DFs ranged between 10 and 367 N m^-2 day^-1 (99 ± 85 mean ± SD) corresponding to total deposition of 0.05 ± 0.1 kg ha^-1 per year and to an estimated 232 metric tons of plastic being deposited in the Weser River catchment annually. MP number DFs were higher in urban than rural sites. An effect of WWTPs on the MP abundance in air was not observed. Polypropylene, polyethylene, polyethylene terephthalate, polyvinyl chloride, polystyrene, and silicone fragments were found as the predominant polymer types in total deposition samples, while polyethylene particles dominated in aerosol samples. The results suggest that proximity to sources, especially to cities, increase the numbers of MP found in the atmosphere. It further indicates that atmospheric MP considerably contributes to the contamination of both aquatic and terrestrial habitats.