A new study examined how microplastics enter the atmosphere and found that roads are their most significant vector in the western U.S., highlighting a need for better waste management and less reliance on single-use plastics.
The study, published April 12 in PNAS, found that road dust generated 84% of atmospheric microplastics, while sea spray and agricultural soil dust accounted for 11% and 5%, respectively.
According to co-lead author Janice Brahney, an assistant professor of watershed sciences at Utah State University, the study was inspired by a desire to figure out how microplastics are moving through the environment, as well as how far.
"There's virtually nowhere you can go anymore where you wouldn't be encountering plastics in the atmosphere, and that's pretty alarming," Brahney said in an interview with The Academic Times. "So, trying to understand how it's getting there, I think, is pretty important."
To help answer this question, Brahney and her colleagues drew on 14 months of wet and dry deposition data, which accounts for rainwater along with anything else that might fall from the sky. They were able to identify a weekly rate for wet deposition, and a monthly rate for dry.
Both forms of deposition contain microplastics, which the National Oceanic and Atmospheric Administration defines as any plastic fragment under 5 millimeters long. The researchers could use this data to discern how much was falling from the sky at various remote locations throughout the western U.S.
But that information alone did not indicate the environmental source of the microplastics, so the team then turned to existing atmospheric transport models. These models have been developed to study how dust circulates through the atmosphere, allowing scientists to recreate atmospheric circulation at a given moment in time. Brahney and her colleagues applied their deposition data to these models to detect environmental sources of the microplastics.
The team found that roads are a major source of atmospheric microplastics because they chew up already worn plastic including litter such as bottles, and spit it back out into the atmosphere with paint, asphalt and other detritus as part of typical road dust. Roads "provide the mechanical energy to emit these plastics to the atmosphere," according to the study.
"Plastics emitted directly from population centers could be too large for long-range transport and get deposited nearby, where they can gradually degrade to microscopic sizes due to sunlight exposure, temperature changes, freezing and melting water and mechanical forces from vehicles," the study says, noting that transportation of plastic pellets could also be a factor.
Similarly, the constant churning of oceans helps emit plastic particles into the atmosphere. And agricultural activity may produce atmospheric microplastics through tilling and the use of plastic mulch or biosolids, a type of fertilizer derived from sewage. Biosolids absorb most microplastics found in wastewater, according to the study.
Brahney says that even the mere presence of microplastics in the atmosphere is cause for concern, as airborne plastic particles can travel anywhere on Earth, including places where waste wouldn't usually appear. This is especially true when the study also shows that these particles can remain in the atmosphere from anywhere between one hour to 6.5 days, with smaller fragments staying in the atmosphere for longer.
Prior research has also shown that microplastics can change soil properties and alter plant growth, and that small organisms such as plankton and worms can accidentally eat bits of microplastic. In December, a literature review of showed that humans, too, may suffer a range of ills due to microplastic ingestion, from higher levels of oxidative stress to an increased risk of cancer.
Even though this study was able to describe sources of atmospheric microplastics, researchers are still unsure about the effects of their presence. This is especially disconcerting for Brahney, because it shows how far behind microplastics research is compared to the scope of the problem. For example, Brahney and her colleagues also found that above some marine environments, microplastics were as concentrated in the atmosphere as other atmospheric aerosols combined.
"That was a little bit jarring, because we don't understand the implications of this, yet we're realizing so late that it's already so concentrated," Brahney said. "We're just really late to the game in recognizing this as a pollution source, and we don't yet understand the consequences of that."
While Brahney knows the U.S. is not going to give up its cars or roads anytime soon, there are still a number of other actions that both individuals and government could take to reduce plastic pollution, she says, such as improving the efficiency of plastic recycling, expanding oversight on manufacturers of single-use plastics and limiting use of such plastics.
According to Brahney, the U.S. ships much of its plastic waste to countries that lack the infrastructure to handle large amounts of it. From there, the plastic can easily reenter the environment and return to where it came from, via either the skies or the seas.
And it does return, as Brahney and her colleagues also note that most of the microplastic that arrives on land through the atmosphere likely originates in marine environments, "underscoring the cumulative role of legacy pollution in the atmospheric burden of plastic."
For future studies, Brahney will prioritize gathering more data from suspected emission sources such as roads, agricultural sites and sea spray. Filling these gaps will help improve the certainty of later research.
"After 70 years of producing plastic and not using it appropriately or managing it properly, it's now so ubiquitous in the world that it's become a new cycle we've created — it's everywhere and anywhere," Brahney said. "There's no ecosystem, no human, that's untouched by plastic waste, and it's an important threat to our environment, to human health and to ecosystem health."
The study, "Constraining the atmospheric limb of the plastic cycle," published April 12 in PNAS, was authored by Janice Brahney, Utah State University; Natalie Mahowald, Cornell University; Marje Prank, Cornell University and Finnish Meteorological Institute; Gavin Cornwell, Pacific Northwest National Laboratory; Zbigniew Klimont, International Institute for Applied Systems Analysis; Hitoshi Matsui, Nagoya University; and Kimberly Ann Prather, University of California San Diego.