The shape of microplastics determines how far they travel in the atmosphere

Just like the land and the ocean, the atmosphere is clouded by various pollutants. In recent years, a new form has been identified: micron-sized microplastic debris that can be carried by the jet stream across oceans and continents.

A Cornell collaboration developed a model to simulate the atmospheric transport of microplastic fibers and found that their shape plays a critical role in how far they travel. While previous studies suggested that these fibers were spherical, the research shows that flat fibers are more common and travel farther in the lower atmosphere.

Modeling has the potential to help scientists pinpoint the sources of widespread waste—which could inform policy efforts to reduce it.

The group’s paper, “Long-range atmospheric transport of microplastic fibers as affected by their shapes,” was published Sept. 25 in Nature Geoscience. The lead author is former postdoctoral fellow Shuolin Xiao.

Atmospheric microplastics result from a number of sources, from shredded tires and road dust to soda bottles that end up in the ocean. If the plastic breaks down or grinds, it can become small enough to be carried by the wind.

The project began when Qi Li, assistant professor in the Department of Civil and Environmental Engineering at Cornell Engineering and senior author of the report, contacted Natalie Mahowald, the Irving Porter Church Professor of Engineering. Li’s lab studies environmental fluid mechanics and hydrology, primarily as they relate to Earth’s lower atmosphere, and she was intrigued by Mahowald’s recent research, along with co-author Janice Braney of Utah State University, on the movement of microplastics in the air. Li also consulted with Donald Koch, a professor in the Smith School of Chemical and Biomolecular Engineering who has studied fundamental fluid dynamics about how fibers settle in turbulence.

“I realized, with my postdoc, that current global climate models assume that the shape of these fibers is a sphere,” Li said. “To date, we have no computationally feasible way to represent the settling velocity of these elongated fibers.”

By combining Koch’s theoretical insights with the extensive measurement data collected by Mahowald and Braney, Li and Xiao set out to create a more rigorous analysis system.

The result was a theory-based sedimentation rate model that could incorporate large-scale climate models, which revealed two key findings. Essentially, by treating flat fibers as spherical or cylindrical, previous studies have overestimated their deposition rates. Taking into account the flat shape of the fibers, they spend 450% more time in the atmosphere than previously estimated, and therefore travel greater distances.

The researchers also found that flat fibers made up the majority of the microplastic particles collected by Mahowald and Brahney.

In addition, the modeling suggests that the ocean may play a larger role in emitting microplastic aerosols directly into the atmosphere than previously known, according to Li.

“We can now more accurately pinpoint the sources of microplastic particles that will eventually be transported into the air,” she said. “If you know where they come from, then you can come up with a better management plan and policies or regulations to reduce plastic waste. This could also have implications for any heavy particles that are transported in the lower atmosphere, such as dust and pollen.

PhD student Yuanfeng Cui was a co-author.

The research was supported by the National Science Foundation, and computing resources were provided by the National Center for Atmospheric Research.

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