The impact of microplastics on cloud formation and disruption of cloud formation mechanisms and climate has been revealed by a study conducted by the University of Pennsylvania, published Nov. 9 in the journal Acs ES&T Air. “Over the past two decades of research on microplastics, scientists have found that it's everywhere, so this is another piece of that puzzle,” explained Miriam Friedman, professor of chemistry at Penn State and senior author of the study. “Now it's clear. that we need to better understand how they interact with our climate system because we've been able to demonstrate that the process of cloud formation can be altered using microplastics.” According to the authors of the new study, microplastics act as ice nucleating particles, microscopic aerosols that promote the formation of ice crystals in clouds.

In a controlled laboratory environment, 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 plastic in small drops of water and slowly cooled the drops to observe how the microplastics affected ice formation. They found that the average temperature at which the droplets froze was 5 to 10 degrees Celsius higher than droplets without microplastics. Normally, a flawless droplet in the atmosphere would freeze at about -38°C, explained Heidi Busse, a graduate student at the University of Pennsylvania and lead author of the paper.

Any type of defect in a water droplet, whether it's dust, bacteria or microplastics, can cause something nuclear to form around the ice. This tiny structure is enough to cause the water droplet to freeze at higher temperatures. “In the case of our microplastic, 50 percent of the droplets of most of the plastics studied were frozen at minus 22 degrees Celsius,” Busse said. “It turns out that if you introduce something insoluble, you introduce a defect into that droplet, and it can form ice nuclei at higher temperatures.” It's not clear what significance this discovery has for weather and climate, Friedman summarizes, but it suggests that microplastics are probably already having an impact on climate change.

“When the structure of the air is such that a droplet rises into the atmosphere and cools, that's when microplastics can affect weather patterns and form ice in clouds,” she said. In general, clouds cool the Earth by reflecting solar radiation, but some clouds at certain altitudes can have a warming effect by helping to capture energy radiated by the Earth, Friedman explained. The amount of liquid water relative to the amount of ice is important in determining how much of a warming or cooling effect clouds will have.