All personnel taking samples was instructed to position themselves against the wind and sample undisturbed snow in front of them with bare hands to avoid contamination. If not stated otherwise, all laboratory ware was made of glass or stainless steel and thoroughly rinsed with Milli-Q water before use. All polymer-based items, which could not be replaced by alternatives (e.g., bottle caps and filter holders) were made of polytetrafluoroethylene, a polymer that cannot be detected within the current FTIR imaging settings (25). Airborne particles were filtered by dustboxes (DB1000, G4 before filtration, HEPA-H14 final filtration, Q = 950 m3 hour−1, Möcklinghoff Lufttechnik) in laboratories for particle sorting and FTIR analyses. All filtration steps were performed in a laminar flow cabinet (ScanLaf Fortuna, LaboGene). Cotton laboratory coats and clothes were worn to reduce contamination from synthetic textiles.

To account for possible contamination from plastic sample containers, two procedural blanks were created in the laboratory by filling the PVC and PE containers used with Milli-Q water and stored at −20°C for 4 days to simulate the freezing and thawing process. To assess possible contamination of the Bavarian snow samples from used tap water and the resin sealing in lids, a similar glass jar was filled with tap water and frozen by the citizen scientists. All blanks were processed in the same way as the snow samples. For the plastic containers, up to three FTIR imaging runs were performed because of low numbers found to obtain a reliable result, which represents the extent of MP contamination. No blanks were taken during field sampling. The amounts of MPs determined in the samples with available blanks were corrected by converting the quantity of MPs detected in the blank to the sample volumes (table S2). The number of particles per liter was calculated for each sample on the basis of the volume of melted snow. The polymer types, abundance, and sizes of MPs in procedural blanks are available in table S4. Blanks from the PE container harbored 256 N liter−1 and consisted primarily of PE. The PVC flask contained 308 N liter−1, most of which was polypropylene and PVC. The glass jar that had been prerinsed with tap water contained the highest number of MP (27,243 N liter−1) with a diverse mix of polymer types, mostly PE and rubber types 1 and 3. To address the uncertainty of all results regarding possible contamination, the propagation of uncertainty was calculated (45) as described in note S1.

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