Supplementary Materials for Plastic Rain in Protected Areas of the United States

Deposition samples were obtained through a collaboration with the National Atmospheric Deposition Program (NADP) at 11 National Park and Wilderness sites from the fall of 2017 through the summer of 2019 using Aerochem Metrics model 31 wet/dry collectors (ACMs). ACMs have a precipitation sensor that opens the wet bucket, and closes the dry bucket, while precipitation is occurring. If precipitation occurred in a given sampling week, wet samples were filtered through 0.45 μm polyethersulfone filters as part of the preparation of weekly wet deposition samples by NADP’s Central Analytical Laboratory (CAL) at the Wisconsin State Laboratory of Hygiene and sent to Utah State University (USU). Dry deposition samples were collected at monthly or bi-monthly intervals using custom-built dry sampling units (DSUs, 2“Brahney Buckets”) (25) manufactured at the University of Montana’s SensorSpace facility at the Flathead Lake Biological Station. During low dust deposition periods, occasionally, the dry deposition units were sampled bi-monthly so that they may accumulate more material. In total,we analyzed 236 wet samples and 103 dry samples. All field operators were provided monthly with sterile sampling equipment, personal protection, and instructions on how to minimize particle contamination. At USU, dry samples were removed from the DSU using a sterile ceramic razor blade and weighed into vacuum ionized pre-weighed vials. The total apparatus was then washed and filtered as above to obtain the full dry particulate mass. All equipment and glassware were triple-rinsed with Milli-Q water.