Microplastics in Motion

Microplastics have conquered the earth. Tiny shards of colorful plastic are in the food that we eat. Toxic confetti floats upon the air we breathe. Plastic fragments drift in the oceans like colorful plankton. These plastic pieces were once everything from water bottles to car tires, grocery bags to bottle caps. But when those products disintegrate or wear down, they disperse small specks into the environment.

How, exactly, did microplastics manage to penetrate every single point on the planet? Recently, researchers at Cornell have been solving the mystery.

Five years ago, Qi Li, an assistant professor of civil and environmental engineering at Cornell, came across research announcing that microplastics had been found on a pristine, remote mountainside in southern France. “It caught my attention,” says Li. “I’d read that microplastics could be found in seafood, for example. But this was the first paper that said microplastics were transported through the atmosphere.”

And just like that, microplastics were in her wheelhouse. Li leads the Environmental Fluid Mechanics and Hydrology in the Built and Natural Environments lab, where she studies exchanges of momentum, energy and mass between earth’s surfaces — with a focus on urban environments — and the lower atmosphere. Constructing numerical models of these interactions, Li learns how cities interact with the atmosphere to affect weather. It is work that allows global models to predict the effects of climate change more accurately.

Big cities tend to have the worst levels of atmospheric microplastic pollution. In a recent study, Li, working with Yuanfeng Cui, a doctoral candidate at the School of Civil and Environmental Engineering, among other collaborators, used large eddy simulations – mathematical models for turbulence and fluid dynamics – to computationally analyze how buildings alter the way winds carry microplastics and other heavy particles through different layers of the urban environments.

What they found was that within the urban canopy – the space between the ground and rooftops – buildings create local wind patterns that affect particle movement and block vertical transport, causing microplastics to accumulate on the downwind side. As buildings get taller and more numerous, fewer particles escape the canopy. For particles that do reach above the buildings, their movement can be predicted using a model that balances turbulence, gravity and particle flow.

“The findings suggest that depending on the city morphology, the microplastics produced from urban traffic have different odds, or different chances, of escaping from the lower atmosphere,” Li said. “So in a city like New York it probably means that the pedestrians will be breathing in more of those microplastic particles, whereas in cities with a more open layout – even with the same kind of traffic productions – their contribution to potential non-local microplastic pollution could be higher.”

The microplastics on the isolated French mountainside, though, had travelled enormous distances. “It led me to think: Do we really know how these microplastic particles are transported?” said Li. “Compared to other particles that are carried around by the wind, what are their unique aspects?”

Li’s study of urban environments assumed, like most other models, that microplastics are a simple shape – spheres in the order of 40 microns in diameter. But plastic particles come in many different shapes, and this assumption, she imagined, could mean models were misrepresenting the time that plastics were spending in the air.

Li, together with former postdoc Shuolin Xiao, sought out the expertise of other collaborators at Cornell. With the support of the National Science Foundation they developed a new gravitational settling velocity model that takes into account the elongation, cross-sectional shapes, ambient turbulent environment and orientation of microplastic fibers.

In 2023, they published their findings in Nature Geoscience. With this new model, Li showed that previous models were indeed underestimating the time that some microplastics stay in the atmosphere. By one measure, replacing a spherical microplastic with one shaped like a flat fiber will increase atmospheric lifetime by as much as eight times.

It’s the difference between a baseball shooting through the air and a leaf slowly tumbling through the sky. And because the microplastics stay in the atmosphere for longer, the distances they can cover are much larger.

“This changes our understanding of how microplastic sources can be attributed,” Li said. “The majority of particles are fibers and can travel to these remote places such as national parks, far away from any city or other source of microplastics. If you know where they’re coming from, then you can come up with a better management plan and policies or regulations to reduce the plastic waste.”

Li’s research piqued the interest of scientists at Princeton University who specialize in air-sea mass exchange, including the generation of sea spray aerosols.

“I was talking to them about my speculation of sea sprays being the potential mechanism of microplastic transport into the atmosphere,” Li said, “but the exact mechanisms of how this might happen were not yet understood.”

In an ensuing research project that used high-speed cameras capturing up to 22,000 frames per second, the Princeton researchers detailed how bursting bubbles create tiny jet droplets that can propel microplastics as large as 280 microns in diameter into the air.

These mechanics were incorporated into a new global microplastic emission model and applied to different concentration maps. The models showed a wide range of possible ocean emissions, with the most recent data used by the researchers resulting in an estimate of the ocean emitting .1 mega-metric tons of microplastics into the atmosphere each year.

“It changes our understanding of the ocean’s role and its place in distributing microplastics around the world,” Li said. “Especially when you consider that 70% of the Earth is covered by oceans and they are the ultimate sinks of those microplastics.”

More work remains to be done to understand how, exactly, atmospheric microplastics impact climate. Currently, Li’s lab is studying how other aspherical particles travel through the atmosphere, including dust particles from natural and agricultural sources. “Hopefully,” she says, “an improved understanding will help us better model these small particles in large-scale climate models.”