Protein Powder Makes Ice Crystals Flower

Clouds have an important impact on Earth’s climate, as they not only enhance the appearance of a landscape, but also help regulate radiation by blocking and trapping it. To gain a better understanding of how clouds function, scientists gathered samples of glacial dust in Alaska. This dust aids in the formation of ice crystals within clouds, which can influence how quickly they dissipate. The team discovered that the dust they examined was more effective at initiating ice formation compared to dust from desert areas near the equator. This is likely due to the presence of biological components, such as proteins, present in the dust.

Dust in the Wind

In October 2019, Sarah Barr and Bethany Wyld went on a research trip in Cordova, Alaska’s Copper River Valley, specifically to study the effects of strong winds on the glacial silt that covers the floodplains in late summer and autumn. According to Barr, a scientist at the University of Leeds in the UK, the timing and location of the fieldwork were carefully planned. The weather conditions during the 10-day expedition were as expected, with strong winds stirring up dust and making it difficult for the researchers to even open their car doors. Barr described the experience as having dust everywhere, from their ears to their clothes.

Barr and Wyld, who was an atmospheric scientist at the University of Leeds at the time, utilized a small device, similar to the size of an apple, called a multistage cascade impactor. This device was used to gather dust particles from the air in four different size categories, ranging from 0.3 to 6.0 microns. Once back in the lab, the team combined each of these size-separated samples with ultrapure water in separate vials. The vials were then thoroughly mixed and small droplets, each measuring just 0.001 milliliter, were manually pipetted onto a glass slide to imitate the water droplets commonly found in clouds.

Watching Droplets Freeze

The scientists began to decrease the temperature and observe how quickly the droplets froze. The initial droplet froze at approximately −7°C, and at −20°C, about a quarter of the droplets had frozen. This may appear to be a predictable outcome, but in reality, pure water can remain in liquid form at very low temperatures. According to Barr, “If the water is completely pure, it will not freeze until around −35°C to −38°C.”

The team determined that the droplets freezing at higher temperatures than predicted was proof of the presence of dust. According to Barr, impurities such as dust, pollen, or volcanic ash can act as ice-nucleating particles, providing a surface for the ice to form on.

Chemistry or Biology?

After comparing their findings with existing data, Barr and her team discovered that their droplets were freezing at higher temperatures than other dust-filled droplets. The team concluded that there must be something in the chemical or biological makeup of the Copper River Valley dust that enhances its ability to initiate ice formation.

The scientists initially eliminated a potential chemical cause. They demonstrated that the dust particles from Alaska had a greater ability to initiate ice formation at a specific temperature, even compared to those containing pure potassium feldspar, which is thought to be the most significant mineral for ice nucleation. Barr and her team reasoned that there must be another factor at play, and speculated that it could involve biology.

Past studies have demonstrated that the dust produced in dry regions, such as deserts, is influenced only by its mineral composition when it comes to its ability to initiate ice formation.

However, dust from areas with higher biological activity contains a higher amount of biological material, such as proteins, which can enhance its ability to form ice. According to Patrick Hayes, an atmospheric chemist at the University of Montreal who was not part of the study, this is because certain biological materials can push water molecules into arrangements that resemble ice crystals, ultimately leading to the formation of ice. Hayes also noted that sometimes this initial push is enough to trigger the formation of ice. In other words, once the template of ice is present, the rest falls into place.

It is possible, and highly probable, that the dust found in the Copper River Valley contains biologically active substances such as proteins, according to a proposal by Barr and her team. The valley’s abundance of microorganisms, due to its forested terrain and intersecting rivers, contributes to this. Murray, an atmospheric scientist from the University of Leeds and part of the team, explains that fungi, in particular, are known to release proteins that can mix with the glacial silt in the water and ultimately become part of the dust in the region. These fungal proteins are also known to strongly bind to mineral dust particles.

To determine if proteins play a role in the ice-nucleating properties of Alaskan dust, the scientists conducted a simple experiment. They heated samples of their dust-infused water for half an hour, extracted small droplets, and measured how quickly they froze. According to Murray, if proteins were the cause, there should be a decline in ice-nucleating activity due to the breakdown of their fragile structures at high temperatures. He explained, “The proteins become denatured.”

The team’s observations showed that, on average, droplets that were not boiled froze faster at a specific temperature compared to those that were boiled. These findings led the team to conclude that dust from Alaska’s Copper River Valley contains proteins. This was an unexpected discovery, according to Hayes, as the significance of biological activity was previously unknown.

When ice crystals form inside a cloud, it can lead to precipitation and eventually cause the cloud to break apart. According to Murray, if there is a sufficient amount of ice nucleation, the cloud will completely disappear. This can result in a change from a large, reflective cloud cover to no cloud at all.

According to Murray, most climate models currently operate under the assumption that clouds persist for a longer period than they actually do. This can result in an inaccurate negative feedback on climate change, as clouds serve to block solar radiation. Murray explains that accurately understanding how ice forms in clouds is crucial for properly accounting for these feedbacks.

—Katherine Kornei (@KatherineKornei), Contributing Writer

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The article titled “Protein Powder Causes Flowering Ice Crystals” by K. Kornei was published in the journal Eos on [DAY MONTH] 2023, with a citation of 104 and a DOI of https://doi.org/10.1029/2023EO230362.

Text © 2023. The authors. CC BY-NC-ND 3.0

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