By | October 15, 2022
Scientists discover how Earth's atmosphere cleans itself

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Human activities release 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 was seen as a complete story, but in new research published in Proceedings of the National Academy of Sciencesa research team that includes Sergey Nizkorodov, a professor of chemistry at the University of California, Irvine, reports that a strong electric field that exists on the surface between airborne water droplets and the surrounding air can create OH through a previously unknown mechanism.

It’s a finding that stands to reshape how scientists understand how the air clears itself of things like pollutants emitted by humans and greenhouse gases, which OH can react with and eliminate. “You need OH to oxidize hydrocarbons, otherwise they would build up in the atmosphere indefinitely,” Nizkorodov said.

“OH is a key player in the story of atmospheric chemistry. It initiates the reactions that break down airborne pollutants and help remove harmful chemicals such as sulfur dioxide and nitric oxidewhich 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. “Therefore, having a complete understanding of its sources and sinks is key to understanding and mitigating air pollution. ”

Previously, scientists assumed that sunlight was the main driver of OH formation.

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

The team built on research from researchers at Stanford University led by Richard Zare who reported the spontaneous formation of hydrogen peroxide on the surfaces of water droplets. The new findings help to interpret the unexpected results from the Zare group.

The team measured OH concentrations in different vials—some containing an air-water interface and others containing only water without air—and tracked OH production in the dark by including a “probe” molecule in the vials that fluoresces when it reacts with OH.

What they saw is that OH production rates in the dark mirror and even exceed the rates of drivers exposed to sunlight. “Enough OH will be created to compete with other known sources of OH,” Nizkorodov said. “At night, when there is no photochemistry, OH is still produced and it is produced at a higher rate than would otherwise happen.”

The findings, Nizkorodov reported, change the understanding of the sources of OH, something that will change how other researchers build computer models that try to predict how air pollution occurs.

“It can change air pollution models quite significantly,” Nizkorodov said. “OH is an important oxidant inside water droplets and the main assumption in the models is that OH comes from the air, it is not produced directly in the drop.”

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

But first, the team expects the results to resonate with the atmospheric research community.

“Many people will read this but at first will not believe it and will either try to reproduce it or try to do experiments to prove it wrong,” Nizkorodov said. “There will certainly be many lab experiments following this up.”

He added that UCI is a great place for such science to continue to happen, as other labs at UCI, such as the Ann Marie Carlton Professor of Chemistry, are focusing their efforts on the role water droplets play in the atmosphere.

More information:
Kangwei Li et al, Spontaneous dark formation of OH radicals at the interface between aqueous atmospheric droplets, 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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