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Each year, billions upon billions of tons of CO₂ are pumped into the atmosphere. A significant proportion of this ends up in Earth's oceans, where it can react with water to form carbonic acid, which causes ocean acidification.

While much research has focused on how this process occurs deep inside the liquid, less attention has been paid to how this reaction proceeds at aqueous interfaces, where water meets another substance, for example, at the ocean surface.

In a study in PNAS, Cambridge and University College London researchers have found that COâ‚‚ can react within the very top layer of water, through a new "in and out" mechanism. In this process, instead of fully dissolving into the water, COâ‚‚ briefly dips into the surface layer, reacts, and then reemerges. This happens in a very thin layer, just a few molecules thick.

"It is like, instead of diving deep into the water to react, COâ‚‚ does a quick dip in the water, partially dissolving in the topmost layer of water where it can react to form carbonic acid. This acid species then returns to the surface and pops back out," said Samuel Brookes, first author of the paper and a Ph.D. student at Cambridge's Yusuf Hamied Department of Chemistry and Cavendish Laboratory.

The in and out mechanism challenges previous assumptions about where and how COâ‚‚ can turn into carbonic acid and shows that reactions can happen right at the water's surface, not just deep within it.

Understanding how COâ‚‚ reacts at the water's surface is crucial for improving climate models and predictions.

Scientists know from previous studies that large amounts of COâ‚‚ gather at the ocean's surface. Now, the Cambridge team suggests that this surface COâ‚‚ doesn't need to fully dissolve before reacting. Instead, it can react right where it sits, which means ocean acidification could happen faster than previously thought.

"Unfortunately, this suggests that current predictions about changes in ocean pH, which measures its acidity, may be underestimated," said Brookes. "This makes the need for accurate climate models even more urgent."

The scientists utilized machine learning to study this acidification process at the air-water interface. The models, trained on accurate quantum-level data, enabled them to simulate the reaction at the level of atoms and molecules. From these molecular level observations, the researchers could then make meaningful predictions such as reaction energies and pathways.

Researchers were surprised by the similarity between the reaction at the interface and in bulk solution. They expected that the reaction to form carbonic acid would be much harder at the water's surface than deeper in the water because of the fewer water molecules available, which can make chemical reactions more difficult.

"But because of this in and out mechanism, we found that the lack of surrounding water molecules at the surface didn't make the reaction harder as we had initially predicted, and the energy required to form carbonic acid was about the same at the surface as it was deeper in the water," said Brookes.

Having uncovered the underlying reaction mechanism, the team was also fascinated by just how much the reaction properties changed over such a short distance.

"By moving CO₂ just a fraction of a nanometer—going from being on top of the water to residing in the topmost layer—we practically cut the barrier to reaction in half," said Dr. Christoph Schran, head of the FAST group at the Cavendish Laboratory, which led the research.

"That we observed such drastic changes during the initial diffusion stage of reaction was remarkable. It made us wonder: Is there any other fundamental process that is amenable to such changes? What other reaction mechanisms can we uncover?"

The research team now plans to extend their modeling to include other species such as sodium, chloride, and carbonate ions, which are abundant in Earth's oceans. This will add a further layer of realism to their simulations, aiding in the more accurate prediction of pH trends and surface reactivity.