A fresh scientific discovery could sharpen our understanding of how to curb greenhouse gas pollution. The spotlight falls on the hydroxyl radical, OH, which behaves like a natural air detergent, helping remove harmful gases from the atmosphere.
Human activity injects a wide range of pollutants into the air, and without OH radicals — composed of a single hydrogen atom bound to oxygen — those pollutants would accumulate much more rapidly in the atmosphere. OH acts by cleaning the air, effectively breaking down other gases. It is especially important for controlling methane levels, a potent greenhouse gas that ranks just behind carbon dioxide in its impact on climate warming.
For years, scientists thought they understood how OH forms in the atmosphere. A recent study by a team of researchers, including Sergey Nizkorodov, a chemistry professor at the University of California, Irvine, reveals a new mechanism. They found that a strong electric field between water droplets and surrounding air can generate OH through a previously unknown process. The results were published in the Proceedings of the National Academy of Sciences.
This finding deepens our picture of how air is cleaned by OH, which reacts with and removes pollutants produced by human activities as well as greenhouse gases. “OH is required to oxidize hydrocarbons; otherwise they would linger in the atmosphere indefinitely,” Nizkorodov recalled.
Atmosphere renews itself, with OH playing a key role. It initiates reactions that break down pollutants and helps remove harmful substances from the air, including sulfur dioxide and nitric oxide, explained Christian George, an atmospheric chemist at the University of Lyon and the lead author of the study. “Having a full understanding of OH’s sources and sinks is essential for understanding and reducing air pollution,” he added.
Produced directly in water droplets
Earlier ideas suggested sunlight as the main driver of OH formation. Conventional wisdom held that OH arose from photochemistry or redox reactions requiring sunlight or metal catalysts. The new work shows that OH can arise spontaneously in pure water under the unique conditions at the water surface, without external triggers.
The researchers build on prior research from Stanford University scientists led by Richard Zare, which reported spontaneous generation of hydrogen peroxide at the surface of water droplets. The new findings help interpret some unexpected results observed by that group.
The team assessed OH concentrations in various bottle setups: some with air and a water–air surface, others with airless water, and they introduced a probe molecule that fluoresces when it reacts with OH. This allowed measurements of OH production in the absence of light.
Observations showed that OH production in the dark could outpace the rates seen in air exposed to sunlight. “At night, when photochemistry is absent, OH still forms, and often at a higher rate than during the day,” Nizkorodov noted.
These findings alter how scientists think about OH sources and will influence the way climate models simulate air pollution. “OH is an important oxidant in water droplets, and many models assume OH comes from the surrounding air rather than forming directly inside the droplet,” Nizkorodov explained.
To evaluate whether this new nighttime OH production mechanism matters in real-world conditions, the next step is to conduct carefully designed atmospheric experiments in diverse environments around the globe. Researchers anticipate that the results will require scrutiny by the broader atmospheric science community, with replication and verification forming a natural part of the process. “Many people will read this, but some may doubt it and try to reproduce the results,” Nizkorodov added.
Reference work: PNAS, 2023, 122(20): e2220228120, cited as forthcoming in the summary release. (PNAS, 2023, citation attributed.)
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