Scientists are discovering a way the Earth’s atmosphere is cleaning itself

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Human activities emit many types of pollutants into the air, and without a molecule called hydroxide (OH), many of these pollutants would continue to accumulate in the atmosphere.

How OH itself forms in the atmosphere has been viewed as a whole story, but new research published in the Proceedings of the National Academy of SciencesA research team including Sergei Nezhorodov, a professor of chemistry at the University of California, Irvine, reports that a strong electric field present at the surface between airborne water droplets and the surrounding air can create OH by a previously unknown mechanism.

It’s a discovery that reshapes how scientists understand how to purify the air of things like human-emitted pollutants and greenhouse gases, which OH can interact with and eliminate. “You need OH to oxidize hydrocarbons, otherwise they will build up in the atmosphere indefinitely,” said Nizhgorodov.

“OH is a key player in the story of atmospheric chemistry. It starts reactions that break down airborne pollutants and helps remove harmful chemicals like sulfur dioxide and nitric oxide, which are toxic gases, from the atmosphere,” said Christian George. An atmospheric chemist at the University of Lyon in France and lead author of the new study. “Thus, having a full understanding of its sources and sinks is key to understanding and mitigating air pollution.”

Before, researchers assumed that sunlight was the main driver of OH formation.

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“The conventional wisdom is that you have to make OH by photochemistry or redox chemistry. You have to have sunlight or metals acting as catalysts,” said Nizhgorodov. “What this paper is essentially saying is that you don’t need any of this. In pure water itself, OH can be formed spontaneously by the special conditions on the surface of the droplets.”

The team built on research by Stanford University scientists led by Richard Zari, who reported the spontaneous formation of hydrogen peroxide on the surfaces of water droplets. The new findings help explain unexpected findings from Zare’s group.

The team measured the concentrations of the hydroxide in different flasks — some with surface water and air and others with just water without any air — and tracked hydroxide production in the dark by embedding a “probe” molecule in the flasks that fluoresces when it reacts with OH.

What they saw was that the rates of OH production in the dark mirrored those of and even exceeded those of drivers such as sun exposure. “Enough OH will be generated to compete with other known OH sources,” Nizhgorodov said. “At night, when there is no photochemistry, hydrogen is still produced and produced at a higher rate than it would otherwise.”

The findings change the understanding of OH sources, Nizhgorodov said, something that will change how other researchers build computer models that attempt to predict how air pollution will occur.

“It could dramatically change air pollution models,” Nizhgorodov said. “OH is an important oxidizing agent within water droplets and the main assumption in the models is that the OH comes from the air, and is not produced in the droplet directly.”

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To determine whether the new OH-production mechanism plays a role, Nizkorodov believes the next step is to conduct carefully designed experiments in the real atmosphere in different parts of the world.

But first, the team expects the findings will impact the atmospheric research community.

“A lot of people will read this but they won’t believe it at first and they will try to reproduce it or they will try to run experiments to prove it wrong,” said Neskorodov. “There will be many follow-up lab trials on this for sure.”

He added that UCI is a key place for such science to continue, because other laboratories at UCI, such as that of Annemarie Carleton, professor of chemistry, focus their efforts on the role that water droplets play in the atmosphere.

more information:
Kangwei Li et al, Spontaneous dark formation of OH radicals at the water-atmosphere droplet interface, Proceedings of the National Academy of Sciences (2023). DOI: 10.1073/pnas.2220228120

Journal information:
Proceedings of the National Academy of Sciences

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