Microplastic can be considered rubbish that never, ever disappears. New research on this seemingly invisible pollution shows how durable it can be if they go from land to sea to air and back again.
Every day, microplastics – often smaller than the head of a needle – infiltrate our oceans, our seafood and even our own bodies. Scientists now say the problem is more extreme than previously realized.
In a study conducted Monday in the Proceedings of the National Academy of Sciences, Researchers suggest that atmospheric microplastics – pieces of fiber and fragments in the air – may re-enter the atmosphere, even if they have settled on land or water.
Three striking findings are:
- 84 percent of the atmospheric microplastics in the western U.S. come from road dust.
- The largest concentration of atmospheric microplastics is estimated to be across the ocean.
- Although Antarctica produces no microplastic emissions, it is still polluted by microplastics.
What’s new – Co-author Natalie Mahowald is a professor at Cornell University specializing in Earth and Atmospheric Science. While previous research shows that microplastics can reach remote parts of the earth – far from their source, this study provides new and disturbing data, says Mahowald Reverse.
“Our paper suggests that microplastics do not only come from remote regions, but that they are also absorbed from the ocean, agricultural areas or roads into the atmosphere,” says Mahowald.
Furthermore, Mahowald and colleagues found that about 1 gigagram (Gg) of microplastic is located in the atmosphere in the western United States. For context, 1 gigagram equals 1,000,000 kilograms.
The study team found three sources that contribute to the spread of microplastics in the atmosphere above the Western US: roads, the ocean and dust from agricultural production.
- The largest contributor to microplastics was roads (84 percent)
- The second largest contributor was oceans (11 percent)
- The final and least contributing source was agricultural dust (5 percent)
On average, the study suggests that particles stay in the atmosphere during different periods and range from an hour to 6.5 days. It depends on the size and source of the microplastic.
How they did it – To measure microplastic abundance, the team collected information from data stations across the western United States for 14 months.
Armed with this data, the scientists modified atmospheric models to determine where the atmosphere of microplastics came from, spent their time in the atmosphere and where it accumulated above the earth’s surface.
They also investigated the size of microplastics and divided them into large, medium and small particles (although they used the medium-sized particles as their baseline for reference).
It only included microparticles that were larger than 4μm (micrometers), as it is more difficult to detect and measure smaller particles. Four micrometers is 0.004 millimeters.
By comparison, the head of a needle varies from half a millimeter to 1 millimeter, so microparticles can be very small.
Finally, the scientists applied their model to a global context to understand the greater impact of microplastics in the atmosphere.
Go into the details – While previous research has already found that microplastics can penetrate the Earth’s atmosphere, these data provide new insights into the variety of sources that contribute to the polluted distribution.
“Our combination of model and data suggests that the microplastics may not only come from road sources, but also from agricultural dust and oceans,” says Mahowald.
It is critical that roads do not specifically contribute to atmospheric microplastics. Instead, it is more about plastic particles being transported by winds or other means and deposited on roads, including roads that are far from human cities.
Tires traveling across these roads then break down plastic particles – which would otherwise be too large to climb into the atmosphere – into finer microplastics, making it possible to send them into the atmosphere.
However, we cannot discount the important role that oceans play in circulating microplastics. “Most continents were net importers of plastics from the marine environment, emphasizing the cumulative role of heritage pollution in the atmospheric burden of plastics,” the scientists write.
Why it matters – Eventually, the team found that microplastics could end up far from their original landfill. The research indicates that the greatest concentration of atmospheric plastic is above the ocean, while Antarctica has been found to have its own microplastic pollution – despite the fact that it was so far away that the microplastic originated.
“The idea of re-suspending microplastics means that all the uncontrolled plastics we accidentally dump into the sea or on land could end up in other remote regions,” says Mahowald.
Mahowald’s research also suggests that microplastic pollution is on the rise. And although recent research has found that microplastics are in the placenta of unborn children, we still do not really know how microplastics affect health.
“Given these preliminary findings, the accumulation and transport of microplastics in the natural environment could have negative and as yet unknown consequences for ecosystems and human health,” the study team wrote.
What’s next – The researchers write that their study ‘leads to more questions than it definitely answers’.
Some of these questions have to do with limitations in the data. The study is based on data from the Western USA, which means that there is a great deal of uncertainty about the application of these findings on a global scale. As the study authors put it: ‘plastic now spirals the world’.
The authors suggest, for example, that plastic is used more in road materials in Europe and Asia than in the US – which can change the amount of plastic that enters the atmosphere from the roads. The researchers also assume that Africa and Asia are the largest global source of plastics, and therefore further studies are needed on these continents.
Various agricultural practices around the world – such as greater use of plastic coverings in China’s agricultural fields – could lead to greater release of microplastics into the atmosphere in those regions. Meanwhile, coastal areas may experience a greater atmospheric concentration of microplastics from ocean spraying. Changing ocean currents can also affect the spread of microplastics.
Finally, the team argues that our ‘relative ignorance’ should encourage us to improve plastic waste management and technologies that can capture and remove plastic from the ocean.
Summary: Plastic pollution is one of the most pressing environmental and social issues of the 21st century. Recent work has highlighted the role of the atmosphere in transporting microplastics to remote locations. Allen et al., Nat. Geosci.12, 339 (2019) and J. Brahney, M. Hallerud, E. Heim, M. Hahnenberger, S. Sukumaran, Science368, 1257–1260 (2020)]. Here, in situ observations of microplastic deposition are combined with an atmospheric transport model and optimal estimation techniques are used to test hypotheses of the most probable sources of atmospheric plastic. The results suggest that atmospheric microplastics in the western United States come primarily from secondary sources of emissions, including roads (84%), the ocean (11%), and agricultural raw materials (5%). Using our best estimate of plastic resources and modeled transportation routes, most continents were net importers of plastic from the marine environment, emphasizing the cumulative role of heritage pollution in the atmospheric burden of plastic. This effort uses high-resolution spatial and temporal precipitation data, along with various hypothesis emission sources to limit atmospheric plastics. In line with global biogeochemical cycles, plastics are now spinning around the world with distinct atmospheric, oceanic, cryopheric and terrestrial residence times. Although progress has been made in the production of biodegradable polymers, our data suggest that non-biodegradable polymers exist through the Earth’s systems. Due to limited observations and understanding of the source processes, there remain major uncertainties in the transportation, deposition and source attribution of microplastics. We therefore give priority to future research directions to understand the plastic cycle.