Microplastics impact cloud formation, likely affecting weather and climate

The study, published in the journal Environmental Science and Technology: Air, demonstrated that microplastics act as ice nucleating particles, microscopic aerosols that facilitate the formation of ice crystals in clouds.

This means that microplastics could impact precipitation patterns, weather forecasting, climate modeling and even aviation safety by influencing how atmospheric ice crystals form clouds, explained Miriam Freedman, professor of chemistry at Penn State and senior author on the paper.

"Throughout the past two decades of research into microplastics, scientists have been finding that they're everywhere, so this is another piece of that puzzle," Freedman said. "It's now clear that we need to have a better understanding of how they're interacting with our climate system, because we've been able to show that the process of cloud formation can be triggered by microplastics."

In the controlled environment of the lab, the researchers studied the freezing activity of four different types of microplastics: low density polyethylene (LDPE), polypropylene (PP), polyvinyl chloride (PVC) and polyethylene terephthalate (PET). The team suspended the four types of plastics in small droplets of water and slowly cooled the droplets to observe how the microplastics affected ice formation.

They found that the average temperature at which the droplets froze was 5-10 degrees warmer than droplets without microplastics. Typically, an atmospheric water droplet without any defects freezes at about negative 38 degrees Celsius, explained Heidi Busse, a graduate student at Penn State and lead author on the paper. Any kind of defect in the water droplet, whether that's dust, bacteria or microplastics, can give ice something to form -- or nucleate -- around. That tiny structure is just enough to trigger the water droplet to freeze at warmer temperatures.

"In the case of our microplastics, 50% of the droplets were frozen by minus 22 degrees Celsius for most of the plastics studied," Busse said. "It turns out that if you introduce something insoluble, you introduce a defect into that droplet and it can nucleate ice at warmer temperatures."

What this discovery means for weather and climate is not entirely clear, explained Freedman, but it suggests that microplastics are likely already making an impact. She added that mixed-phase clouds, such as the puffy cumulus, blanket-like stratus and dark, ominous nimbus clouds, all contain a combination of liquid and frozen water. These clouds can be widespread throughout the atmosphere, including the classic "anvil" shaped clouds that can form during thunderstorms.

"When air patterns are such that a droplet gets lifted into the atmosphere and cools, that's when microplastics could be affecting weather patterns and forming ice in clouds," said Freedman, who is also affiliated with Penn State's Department of Meteorology and Atmospheric Science. "In a polluted environment with many more aerosol particles, like microplastics, you are distributing the available water among many more aerosol particles, forming smaller droplets around each of those particles. When you have more droplets, you get less rain, but because droplets only rain once they get large enough, you collect more total water in the cloud before the droplets are large enough to fall and, as a result, you get heavier rainfall when it comes."

In general, clouds cool the Earth by reflecting solar radiation, but certain clouds at certain altitudes can have a warming effect by helping to trap energy emitted from the Earth, Freedman explained. The amount of liquid water versus the amount of ice is important in determining to what extent clouds will have a warming or cooling effect. If microplastics are influencing mixed-phase cloud formation, Freedman said, they are likely affecting climate, too, but it's extremely difficult to model their overall effect.

"We know that the fact that microplastics can nucleate ice has far-reaching effects, we're just not quite sure yet what those are," Busse said. "We can think about this on many different levels, not just in terms of more powerful storms but also through changes in light scattering, which could have a much larger impact on our climate."

The researchers also found that environmental aging, the natural photochemical processes that aerosol particles experience over time, can significantly change how the particles interact with gases and vapors in the atmosphere. The team simulated environmental aging by exposing the microplastics to light, ozone and acids to see if it changed their ability to form ice.

They found that all the tested plastics could form ice, but aging generally reduced the ice-forming ability of LDPE, PP and PET. In contrast, aging increased the ice-forming ability of PVC due to slight changes on its surface caused by aging.

Next, the team is going to study a variety of additives that are commonly put into plastics, such as plasticizers, to get a better of sense of how commonly used plastics may affect the Earth's atmosphere.

"We know the full lifecycle of these plastic items we use every day could be changing the physical and optical properties of the Earth's clouds and, therefore, changing the climate in some way, but we still have a lot to learn about exactly what they are doing," Busse said.

Other authors on the paper are Devaka Ariyasena and Jessica Orris, graduate students at Penn State. The U.S. National Science Foundation supported this research, which was facilitated by the Materials Characterization Lab at Penn State.