A team of researchers at Utah State University, who collected samples of microplastics in 11 national parks and wilderness areas in the western part of the US, are appalled at what they found.
The team examined weekly wet and monthly dry samples from these 11 sites. Using high-resolution atmospheric deposition data, the researchers, led by Utah State University Assistant Professor Janice Brahney, analyzed the samples to determine the composition of these plastics, identify their source and track their movement and fallout.
They calculated that over 1000 tons of microplastics are deposited onto protected lands in the western U.S. each year, equivalent to more than 123 million plastic water bottles.
"We were shocked at the estimated deposition rates and kept trying to figure out where our calculations went wrong," Brahney said. "We then confirmed through 32 different particle scans that roughly 4 percent of the atmospheric particles analyzed from these remote locations were synthetic polymers."
Microplastics have long been known to accumulate in wastewaters, rivers, and ultimately the worlds' oceans. But, as Brahney's team shows, they also accumulate in the atmosphere, ultimately ending up in remote and protected environments through a phenomenon known as atmospheric deposition.
Atmospheric deposition has often been described as the "cleansing" mechanism of the earth’s atmosphere is called. Airborne particles, gases, pollutants and other materials are carried through the atmosphere and deposited in soil and water through dry deposition; in wet deposition, these are mixed with suspended water in the atmosphere and washed out through rain, snow or fog.
"Several studies have attempted to quantify the global plastic cycle but were unaware of the atmospheric limb," Brahney said. "Our data show the plastic cycle is reminiscent of the global water cycle, having atmospheric, oceanic, and terrestrial lifetimes."
The study examined the source and life history of both wet (rain) and dry microplastic deposition. Cities and population centers were found to serve as the initial source of plastics associated with wet deposition, but secondary sources included the redistribution of microplastics re-entrained from soils or surface waters. In contrast, dry deposition of plastics showed indicators of long-range transport and was associated with large-scale atmospheric patterns. This suggests that microplastics are small enough to be entrained in the atmosphere for cross-continental transport.
The plastics deposited in both wet and dry samples mainly consisted of microfibers sourced from both clothing and industrial materials. Approximately 30 percent of the particles were brightly colored acrylic microbeads, likely derived from industrial paints and coatings.
As clear and white particles were not included — they did not meet the researchers' criteria for visual counting under magnification — the calculated deposition rates are likely to be an underestimate.
Other particles were fragments of larger pieces of plastic. The report notes that "this result, combined with the size distribution of identified plastics, and the relationship to global-scale climate patterns, suggest that plastic emission sources have extended well beyond our population centers and, through their longevity, spiral through the Earth system."
"This ubiquity of microplastics in the atmosphere and the subsequent deposition to remote terrestrial and aquatic environments raise widespread ecological and societal concerns," Brahney said. "Identifying the key mechanisms of plastic emission to the atmosphere is a first step in developing global-scale solutions."
The findings are reported in the June 12 issue of Science Magazine in the article, Plastic Rain in Protected Areas of the United States.
In a statement in response to the study, the American Chemistry Council said scientific research does not suggest that human health is harmed at current levels of exposure to microplastics, and it advocated a “risk-based” framework.
“ACC and its partners around the world are committed to helping people better understand what we know, based on science, about microplastics and their potential effects on human health and the environment,” the Washington-based group said.
“The current scientific literature does not suggest human health is affected from microplastics at current, very low levels of exposure, and in fact, a the World Health Organization recently concluded that there is no evidence to indicate a human health concern from drinking water,” ACC said.
It pointed to efforts to reduce plastic waste in the environment, such as the Alliance to End Plastic Waste and the ACC’s goal of having all plastic packaging by reused, recycled or recovered by 2040, and that all of it be recyclable or recoverable by 2030.
Brahney, Janice, Margaret Hallerud, Eric Heim, Maura Hahnenberger, Suja Sukumaran. "Plastic Rain in Protected Areas of the United States," Science, 12 June 2020 DOI: 10.1126/science.aaz5819