Shape can explain how some microplastics travel so far in the environment

Microplastics have turned up in some of the most remote places, such as the Antarctic and the summit of Mount Fuji. Yet models of atmospheric transport haven’t been able to fully explain how these plastic bits stray so far from the people that produced them. A new study finds that the shape of microplastics influences the distances they fly (Environ. Sci. Technol. 2023, DOI: 10.1021/acs.est.3c08209).

“You find these microplastics worldwide, even far away from likely sources,” says Andreas Stohl, a meteorologist at the University of Vienna and one of the study’s authors. Most of these microplastics seem to be fibers or relatively complex shapes, he says. “But most models assume that particles are spherical.”

Researchers have tools that work fairly well for predicting the transport of small particles with lengths of around 10 μm. But some microplastics can stretch up to 5 mm in one dimension. “The big knowledge gap is actually for these larger particles and especially when it comes to how they behave in the air,” says Mohsen Bagheri, another of the study’s authors and a physicist at the Max Planck Institute for Dynamics and Self-Organization. Many studies have investigated the motion of microplastics in liquids to work out the particles’ settling velocity, or the speed at which a material falls, in air, he says. But few have actually measured that motion in air.

To arrive at more realistic estimates, Stohl, Bagheri, and their colleagues watched how particles behave in air using an instrument called a Göttingen turret, which consists of an air-filled chamber and high-speed cameras. The team filmed microplastic spheres, straight fibers, and curved fibers of various sizes as they fell through the air. For pieces of the same volume, fibers had settling velocities up to 76% lower than that of the spheres, the team reports.

When the researchers plugged their numbers into an atmospheric transport model, they found that fibers spent around 4 times as long in the air as spheres did. “It’s really substantial,” Stohl says. In the simulations, spheres tended to disperse regionally, while fibers dispersed more or less globally, which may explain why past models have underestimated the distances microplastics can travel.

The study “fills an important gap” in the experimental knowledge of how fibers travel, says Qi Li, an atmospheric scientist at Cornell University who wasn’t involved in the study. And the team connected the results of their lab-scale experiments to transport in the atmosphere and its global implications, she adds.

Fibers’ low settling velocity means that not only can these microplastics cover vast horizontal distances, they can also be carried high into the atmosphere by updrafts, the authors of the study say. That height would increase the potential of microplastics to affect cloud formation by acting as sites for condensation or ice nucleation.

Microplastics in the atmosphere could also be degraded by light and release ozone-harming chlorine or bromine. As some plastics are composed of about 60% chlorine by mass, microplastics that make it to the stratosphere could potentially contribute to ozone destruction, Stohl says. But it’s not clear how much plastic makes it up there; researchers don’t have a good handle of how much microplastics pollution is emitted.

The study’s findings could also help explain the long-range transport of other large particles, including those produced naturally, such as volcanic ash, ice, and pollen, says Daria Tatsii, a meteorologist at the University of Vienna who was also an author of the study. And the results underscore the need to reduce plastic use because plastic waste inevitably ends up everywhere, she says. “It will get to you no matter where you are.”