Protein Powder Makes Ice Crystals Flower - Eos

Clouds do more than just beautify a landscape—they also play a key role in Earth’s climate by both blocking and trapping radiation. To better understand the physics of clouds, researchers recently collected samples of glacial dust in Alaska. Dust within clouds facilitates the formation of ice crystals, which in turn can affect how rapidly clouds disperse. The team found that compared with dust from low-latitude desert environments, the dust they studied was a more active ice-nucleating material. Biological components like proteins present in the dust likely boosted its ice-nucleating properties, the researchers concluded.

“It’s in your ears, it’s in your clothes.” Non-Natural Amino Acid

Protein Powder Makes Ice Crystals Flower - Eos

In October 2019, Sarah Barr and Bethany Wyld embarked on a dust-chasing expedition in Alaska’s Copper River Valley near the city of Cordova. The fieldwork’s timing and location were intentional: In the late summer and autumn, the glacial silt that blankets the valley’s floodplains is regularly lofted into the atmosphere by powerful winds. And nature didn’t disappoint during the trip, said Barr, an atmospheric scientist at the University of Leeds in the United Kingdom. By the third day of the researchers’ 10-day excursion, intense winds had kicked up. “It was almost difficult to open the car doors,” said Barr. And dust was blowing everywhere, she said. “It’s in your ears, it’s in your clothes.”

Barr and Wyld, then an atmospheric scientist at the University of Leeds, used an apple-sized device known as a multistage cascade impactor to collect airborne dust in four size bins ranging from roughly 0.3 to 6.0 microns. Back in the laboratory, the team added each of those size-segregated samples to a different vial of ultrapure water, mixed each vial thoroughly, and then manually pipetted tiny droplets—each consisting of just 0.001 milliliter—onto a glass slide to mimic the water droplets typically found in clouds.

The researchers then started lowering the temperature and monitoring the rate at which the droplets froze. The first droplet froze at around −7°C, and by −20°C roughly a quarter of the droplets had turned to ice. That might seem like an obvious result, said Barr, but in fact, water that’s free of impurities will remain in a liquid state down to surprisingly low temperatures. “If you have perfectly pure water, it won’t freeze on its own until around −35°C to −38°C.”

The fact that the droplets froze at temperatures far warmer than expected was a testament to the dust they contained, the team concluded. Impurities like dust—and other particulate matter such as bits of pollen or volcanic ash—can function like ice-nucleating particles, said Barr. “You need a surface for the ice to form onto.”

When Barr and her collaborators compared their observations with literature data, however, they realized that their droplets were consistently freezing at warmer temperatures than other dust-laden droplets. Something in the chemistry—or biology—of the dust from the Copper River Valley must be boosting its ability to nucleate ice, the team concluded.

The researchers started by ruling out a chemical culprit. The team showed that the dust grains from Alaska had higher ice-nucleating activity at a given temperature than even dust grains that contained pure potassium feldspar, a mineral believed to be the most important one for ice nucleation. Something else must be going on, Barr and her colleagues surmised, and perhaps it had to do with biology.

The ice-nucleating activity of dust born in arid environments like deserts is dictated solely by its mineralogy, previous research has shown.

But dust that arises from more biologically active places tends to be correspondingly richer in biological material like proteins, and that “living” component of the dust can increase its ice-nucleating activity. That’s because some biological materials interact with water molecules by essentially shoving them into arrangements that begin to resemble an ice crystal, said Patrick Hayes, an atmospheric chemist at the University of Montreal who was not involved in the research. And sometimes that kick is all that’s needed to form ice, he said. “Once you have that template and things start to look like ice, things just kind of snap together.”

“Fungal proteins bind fairly strongly to mineral dust particles.”

It’s entirely conceivable, and even likely, that dust from the Copper River Valley is replete with biologically active material like proteins, Barr and her colleagues proposed. The region is rich in microorganisms, thanks to its forested landscape crosscut by rivers. Fungi in particular are known to produce proteins that can be sloughed off in water, where they mix with the glacial silt that produces the region’s dust, said Ben Murray, a team member and an atmospheric scientist at the University of Leeds. “Fungal proteins bind fairly strongly to mineral dust particles.”

To investigate whether proteins might be responsible for the ice-nucleating activity of their Alaskan dust, the researchers conducted a straightforward experiment: They boiled samples of their dust-laden water for 30 minutes, pipetted tiny droplets again, and measured the rate at which the droplets froze. If proteins were the culprit, there should be a decrease in ice-nucleating activity, said Murray, because high temperatures tend to destroy proteins’ delicate structures. “You denature the proteins,” he said.

That’s precisely what the researchers observed: On average, the droplets that had not been boiled froze more readily at a given temperature than the droplets that had been boiled. Those findings suggest that dust from Alaska’s Copper River Valley contains proteins, the team concluded. And that was unexpected, said Hayes. “The importance of the biological activity was new and quite surprising.”

The formation of ice crystals within a cloud can trigger precipitation and ultimately cloud dispersal. “If there’s enough ice nucleation going on, it’ll completely remove the cloud,” said Murray. “You might go from a state where you’ve got a big reflective cloud deck to a state where the cloud is gone.”

And right now, the majority of climate models are assuming that clouds stick around longer than they really do, said Murray. That negative feedback on climate change—because clouds block incoming solar radiation—might accordingly be too negative, he said. “We have to get the ice formation in these clouds right in order to be able to get these feedbacks right.”

—Katherine Kornei (@KatherineKornei), Contributing Writer

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