Plastic is ubiquitous in human environments—and it doesn’t stay put. Tiny plastic particles have been found in some of the most remote parts of the planet, showing up in ice cores drilled in the Arctic, in protected wilderness areas in the Western US, and on high mountains in the European Alps.
Researchers have long suspected that these minuscule particles were being transported long distances through the atmosphere before raining down or falling out of the atmosphere as dust. A modeling study published this week bolsters the evidence for this idea, modeling how microplastics from vehicle tires and brake pads hitch an atmospheric ride from urban streets to remote regions and the world’s oceans (Nat. Commun. 2020, DOI: 10.1038/s41467-020-17201-9).
In recent years, researchers tracing the movement of microplastics into the wild have focused on how the pollutants move through waterways, such as by washing from rivers into the oceans. But this pathway doesn’t explain how microplastics get to many remote regions, says Timothy Hoellein, an ecologist at Loyola University Chicago. Hoellein, who studies the effects of plastic on aquatic environments, was not involved in the atmospheric modeling research. His team has found microplastics in remote rivers within Yellowstone National Park in Montana, where “there’s nobody around,” he says. Atmospheric transport seemed like the only possible way microplastics could reach those rivers.
To model how microplastics relocate by air, atmospheric chemist Nikolaos Evangeliou and his team at the Norwegian Institute for Air Research focused on one source of microplastics: roadways. Traffic microplastics—produced when friction between the road and tires or between brake pads causes bits of plastic to break off or volatilize—are a relatively small source of these pollutants, Evangeliou says. Germany, for example, produced 12.6 million metric tons (t) of plastics in 2017, but only produces an estimated 100,000 t of tire-wear particles per year. Evangeliou and his colleagues chose to focus on traffic microplastics because they knew they could quantify them relatively well, he says.
They estimated traffic microplastic emissions around the world using greenhouse gas emissions data, which correlate with traffic intensity; information about the lifetime weight loss of returned tires; and other information. They then fed the estimate into atmospheric transport models for particulates. These models were developed to study the movement of fine aerosols that have implications for human health and the climate; with small adaptations, they can predict the movement of plastic microparticles, too.
Overall, the study suggests that atmospheric transport can indeed move plastic microparticles from densely populated areas to distant places, including the Arctic. The team estimates that each year, the atmosphere transports 140 metric kilotons (kt) of traffic-produced particles into the world’s oceans—equivalent to the amount delivered by rivers—and 86.1 kt to the world’s ice and snow cover. Evangeliou says the deposition of microplastics on the world’s already vulnerable snow and ice formations is particularly concerning. Since they are dark in color, these particles may absorb sunlight and accelerate melting, he says.
It’s been difficult to pinpoint the sources of the microplastics ecologists have been finding at their study sites, but this study is a step in that direction, Hoellein says. Janice Brahney, a biogeochemist at Utah State University, agrees. The work takes “first steps to figure out what are the major emission point sources into the atmosphere.” The next step is to validate modeling studies with field measurements, Brahney says.
Brahney’s working on it. Last month, her group published results of a survey of microplastic pollution in protected wilderness areas in the western US (Science 2020, DOI: 10.1126/science.aaz5819). She had set out to study patterns of dust deposition, but when she realized how much of the dust was actually microplastics, she pivoted: In her study, about 98% of soil samples from US protected areas contained microplastic pollutants. She used models from the US National Oceanic and Atmospheric Administration to trace where those particles originated in the region. “As an air mass moves through an urban center, it picks up plastic,” she says. She’s now working on a study that will link observational data and global atmospheric models.
While ecologists try to tease out the effects of microplastics on ecosystems, atmospheric chemists want to know more about their geophysical effects, which have been little studied. Some atmospheric aerosols serve as condensation sites that seed rain and snow formation, and some can affect regional temperatures by absorbing or reflecting sunlight. Researchers want to know whether microplastics behave similarly. Evangeliou says he’s now studying whether and how atmospheric microplastics affect the climate.
Both Brahney’s and Evangeliou’s studies raise more questions than scientists can answer yet, Brahney says. “It’s scary how little we know,” she says.