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When a volcano erupts, it can spew ash high into the atmosphere—inserting aerosols right where clouds typically form. How exactly these aerosols impact cloud formation has long been a mystery to atmospheric scientists.

In a study in Science Advances, researchers from Lawrence Livermore National Laboratory (LLNL) analyzed 10 years of satellite data to determine that volcanic ash particles can trigger cloud formation by providing a surface for ice to coalesce.

"Our research helps close a significant knowledge gap about whether and how volcanic eruptions influence cloud formation," said LLNL scientist and author Lin Lin. "We show that volcanic ash particles can trigger ice cloud formation by acting as sites for ice nucleation."

Clouds reflect sunlight and trap heat, and because they cover about 70% of Earth's surface at any given time, they play a critical role in the planet's energy balance. For accurate atmospheric models, researchers must understand clouds and the aerosols that affect them. Volcanic eruptions offer a unique, real-world opportunity to observe how particles influence cloud properties.

The scientists examined radar and lidar data from two NASA missions, CloudSat and CALIPSO. By drawing from multiple datasets and instruments, they were able to piece together a coherent picture.

After ash-rich volcanic eruptions, the team saw clear and consistent changes in the satellite data. Clouds hosted fewer but larger ice crystals, and cirrus clouds—high, wispy clouds made mostly of ice—were more frequent. The same was not the case for ash-poor eruptions.

"At the beginning of the study, we did expect clouds affected by volcanic eruptions to look different from natural clouds, but not in the way we ultimately found," said Lin. "We anticipated that volcanic aerosols would lead to an increase in the number of ice crystals in clouds. But to our surprise, the data showed the opposite."

The group initially hypothesized that ice would form by spontaneously condensing from very cold water droplets, a process called homogeneous nucleation. Instead, they saw water congregate on the surface of the ash aerosols under the opposite mechanism, heterogeneous nucleation. After an ashy eruption, water latched onto the aerosols before it reached cold enough conditions to spontaneously freeze.

"The results completely overturned our original expectations," said Lin. "Letting go of our initial idea and developing a new explanation based on unexpected findings was both the hardest and most rewarding part of the process."

While they wait for the next major volcanic eruption—and therefore the next opportunity to test and validate their work—the team at LLNL has pivoted to exploring Arctic clouds and their role in global atmospheric models.