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Scientists uncover new way that Earth cleans its atmosphere

Air pollution, a major consequence of human activity, has long been a pressing environmental issue. The presence of hydroxide (OH) in the atmosphere plays a crucial role in the breakdown and elimination of pollutants. 

While the process of OH formation has been studied extensively, recent research published in the Proceedings of the National Academy of Sciences suggests that our understanding of OH generation might have been incomplete all along.

A research team, including Sergey Nizkorodov, a professor of chemistry at the University of California, Irvine, has discovered a previously unknown mechanism for the creation of OH. This groundbreaking finding could lead to a better understanding of how the air clears itself of pollutants and greenhouse gases, which are known to react with and be eliminated by OH molecules.

“You need OH to oxidize hydrocarbons, otherwise they would build up in the atmosphere indefinitely,” explained Nizkorodov. It has long been known that OH is an essential component in atmospheric chemistry, initiating reactions that break down airborne pollutants and removing toxic chemicals like sulfur dioxide and nitric oxide from the air. 

“Thus, having a full understanding of its sources and sinks is key to understanding and mitigating air pollution,” said study lead author Christian George, an atmospheric chemist at the University of Lyon.

Previously, researchers believed that sunlight played a major role in the formation of OH. “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 Nizkorodov. 

However, the new study reveals that OH can also be created spontaneously in the unique conditions on the surface of airborne water droplets, without the need for sunlight or metal catalysts.

The experts found that the strong electric field that exists at the interface between airborne water droplets and the surrounding air can generate OH through an entirely new mechanism. This discovery challenges long-held assumptions and opens up new avenues of research in the field of atmospheric chemistry.

The team built on research from Stanford University scientists led by Richard Zare, who had reported spontaneous formation of hydrogen peroxide on the surfaces of water droplets.

To study OH concentrations, the UCI scientists measured OH levels in different vials containing either an air-water surface or only water without air. They then tracked OH production in darkness by including a “probe” molecule that fluoresces when it reacts with OH.

The results showed that OH production rates in darkness were equivalent to or even exceeded those from sunlight exposure. “Enough of OH will be created to compete with other known OH sources,” said Nizkorodov. “At night, when there is no photochemistry, OH is still produced and it is produced at a higher rate than would otherwise happen.”

Nizkorodov explained that the findings could significantly change our understanding of the sources of OH and would, therefore, alter how other researchers build computer models that forecast air pollution. “OH is an important oxidant inside water droplets and the main assumption in the models is that OH comes from the air, it’s not produced in the droplet directly,” he said.

To determine whether this new OH production mechanism plays a role in air pollution, Nizkorodov suggested performing carefully designed experiments in the real atmosphere in different parts of the world. He added that UCI was a prime place for such research to continue happening because other labs at UCI, such as that of Ann Marie Carlton, professor of chemistry, focused their efforts on the role water droplets play in the atmosphere.

The project was funded by the European Research Council and involved researchers from France’s University Claude Bernard, China’s Guangdong University of Technology, and Israel’s Weizmann Institute.

Nizkorodov expects the new findings to create a ripple in the atmospheric research community. “A lot of people will read this but will not initially believe it and will either try to reproduce it or try to do experiments to prove it wrong,” he said. “There will be many lab experiments following up on this for sure.”

As scientists continue to investigate this newfound OH generation process, it may lead to improved strategies for mitigating air pollution, and a deeper understanding of how the Earth’s atmosphere naturally clears itself of harmful pollutants and greenhouse gases. This breakthrough discovery could have far-reaching implications not only for the scientific community but also for the future health of our planet. The study is published in the journal Proceedings of the National Academy of Sciences.

More details about OH

Hydroxyl radicals (OH) are highly reactive molecules made up of one hydrogen atom and one oxygen atom. OH is a critical component in the chemistry of the Earth’s atmosphere, and it plays a crucial role in the removal of pollutants such as methane and other greenhouse gases. OH is often referred to as the “detergent” of the atmosphere because it cleanses the air of pollutants.

OH is produced through a variety of mechanisms, including sunlight and chemical reactions between organic compounds and ozone. OH also has a short lifetime, typically on the order of seconds, meaning that it reacts quickly with other molecules in the atmosphere.

OH is not only important in the atmosphere, but it is also involved in a wide range of chemical processes in biological systems. For example, OH is a critical component in the body’s natural defense against pathogens, and it is also involved in the breakdown of drugs in the liver.

Despite its importance, OH is challenging to study because of its short lifetime and high reactivity. However, recent research at the University of California, Irvine, has shed new light on the formation of OH in water droplets, opening up new avenues for understanding its role in the atmosphere.


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