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Scientists Study Ice Formation in the Atmosphere

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Proposing New Methods to Better Understand This Atmospheric Phenomenon

The formation of ice crystals in the atmosphere impacts cloud formation, weather, and precipitation amounts, and is essential for life on Earth. Therefore, understanding how ice crystals — which form by way of tiny particles in the sky thinner than the diameter of a human hair — are created in the atmosphere is crucial. Scientists are still learning about its formation and processes that help to unravel the puzzle. Two Stony Brook University scientists assessed recent research about atmospheric ice nucleation and highlighted promising methods in a review piece now illustrated on the current cover of Nature Reviews Physics.

Authors Daniel Knopf, professor of atmospheric sciences in the School of Marine and Atmospheric Sciences (SoMAS) at Stony Brook University, and former SoMAS research scientist, Peter Alpert, now at the Paul Scherer Institute in Switzerland, provided an overview of the investigations in their review piece titled “Atmospheric Ice Nucleation.”

They summarize the most recent advances in atmospheric ice formation research and center on leading questions that need to be addressed with answers that will enable scientists to improve predictions of atmospheric ice crystallization. Questions such as: Where does ice form in an aqueous miniscule aerosol particle or droplet? How does droplet size affect freezing? Does the nanometer-scaled ice germ display a cubic or hexagonal lattice structure? How can researchers describe ice formation to implement it in cloud and climate models?

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Snowflakes are a longer-term result of atmospheric ice nucleation that starts on a scale smaller than a human hair. Snowflakes illustrate the complexity of form and shape as a consequence of ice crystallization.  Credit: Brian Colle, SoMAS, Stony Brook University

“The process of atmospheric ice nucleation is tremendously complex, yet if we can fully explain and quantify the kinetic and thermodynamics of nucleation in relation to fundamental physics, we can improve prediction of ice formation when implemented in cloud and climate models,” explains Knopf, “something that would be of huge value for weather research and forecasting and climate prediction.”

The authors describe that recent advances in experimental and computational simulation studies shed light on the challenging intricacies of predicting ice formation and post forward-looking questions (as highlighted above) that will need to be addressed to improve cloud and climate models. Scientists ability to examine ice nucleation on the molecular scale has also advanced, opening up new avenues to research.

Reviewing these latest advances led Knopf and Alpert to propose a universal thermodynamic predictor of ice nucleation that would facilitate the description of atmospheric ice formation. This predictor is based on water activity — a measure of how much “free” water is available in the particles to initiate freezing. For example, in the presence of salts, water activity is less than for pure water, and as a consequence, the freezing efficiency decreases.

“Observations of actual ice nucleation are extremely challenging but are fascinating because while the initial crystal is miniscule, it then grows into mesmerizing, beautifully shaped snowflakes, which adds to the complexity,” says Knopf, who runs research at Stony Brook that examines many aspects of atmospheric ice formation in his Aerosol Research Laboratory.

Other advances that the authors highlight with this body of research internationally include research related to the role of interfacial free energy and pressure on ice nucleation rates, mobility regions of water that generate the ice nucleus, classical and non-classical pathways of nucleation, the types of ice polymorph that forms, the impact of solutes on freezing, and the role of nanopores as surface features promoting ice nucleation.

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