How Earth's atmosphere cleans itself

Human activities emit many kinds of pollutants into the air, and without a molecule called hydroxide (OH), many of these pollutants would keep aggregating in the atmosphere.

Now, an international research team has revealed that a strong electric field existing at the surface between airborne water droplets and the surrounding air can create OH by a previously unknown mechanism — a breakthrough that stands to reshape our understanding of how the air clears itself of things like pollutants and greenhouse gases. Their findings have been published in Proceedings of the National Academy of Sciences.

“OH is a key player in the story of atmospheric chemistry,” said lead author Christian George, from the University of Lyon. “It initiates the reactions that break down airborne pollutants and helps to remove noxious chemicals such as sulfur dioxide and nitric oxide, which are poisonous gases, from the atmosphere. Thus, having a full understanding of its sources and sinks is key to understanding and mitigating air pollution.”

Researchers had previously assumed that sunlight was the chief driver of OH formation. As explained by study co-author Sergey Nizkorodov, from the University of California, Irvine, “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.

“What this paper says in essence is you don’t need any of this. 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 Stanford University led by Richard Zare, which reported spontaneous formation of hydrogen peroxide on the surfaces of water droplets. The new findings help interpret the unexpected results from the Zare group.

The team measured OH concentrations in different vials — some containing an air–water surface and others containing only water without any air — and tracked OH production in darkness by including a ‘probe’ molecule in the vials that fluoresces when it reacts with OH. What they saw is that OH production rates in darkness mirror those and even exceed rates from drivers like sunlight exposure.

“Enough of OH will be created to compete with other known OH sources,” 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.”

Nizkorodov said the findings alter our understanding of the sources of OH, something that will change how other researchers build computer models that attempt to forecast how air pollution happens. He noted, “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.”

To determine whether this new OH production mechanism plays a role, Nizkorodov said the next step is to perform carefully designed experiments in the real atmosphere in different parts of the world. But first, he expects the results to make a splash 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.”

Image credit: iStock.com/sandsun