Could Our Atmosphere Eventually Purge Itself of Excess Pollutants?

The Greenhouse Gas Effect causes more solar radiation to be trapped by the atmosphere as the concentration of gases such as carbon dioxide, methane (CH4), and nitrous oxide increase. The discovery of a new mechanism for hydroxyl radical formation in the air could point to a way to accelerate automatic "scrubbing" of some of these GHG pollutants. Image: A Loose Necktie, CC

Though the findings are still in their early stages, this could be a path to cut how much solar energy the atmosphere is trapping close to the earth and slow the pace of global heating.

Reversing the trend is an entirely different problem because so many other climate-crisis-reinforcing feedback loops are already in place. But this still could represent a game-changer with more potential than most other approaches to drawing down carbon from the skies above us.

The discovery was made by a team of researchers at University of California Irvine (UCI), the National Center for Scientific Research at the University of Lyon, France, China’s Guangdong University, and the Weizmann Institute of Israel.

Sergey Nizkorodov, a professor of chemistry at UCI, and Christian George, a research chemist at Lyon, led the investigation.

The teams made their breakthrough while investigating a chemical compound made from oxygen and hydrogen known as hydroxyl radical (OH). It is made of just one atom of oxygen per every atom of hydrogen, unlike water which has the formulation H²O. But unlike water molecules, which are widely present in the atmosphere and, like carbon dioxide, also contribute to global heating by preventing reflected solar energy from escaping, OH molecules act as powerful oxidants.

Hydroxyl radicals are also unique in that their oxidation capacity works equally well whether operating in gaseous or aqueous (liquid) phases.

These molecules form naturally in the atmosphere. They are also a fundamental part of the self-cleaning mechanisms there, as Dr. Nizkorodov noted in comments about the new research.

“OH is a key player in the story of atmospheric chemistry,” he explained in a recent interview about their discoveries. “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.”

“You need OH to oxidize hydrocarbons, otherwise they would build up in the atmosphere indefinitely,” he continued.

Hydroxyl radicals are not in themselves a new discovery. Their presence and their importance in purging poisonous and heat-trapping greenhouse gases from the atmosphere has been known for some time. It has also been well-known that they are created in the atmosphere through a chemical process with sunlight providing the energy for it.

As such, though OH was understood to be important in cleansing the atmosphere, scientists did not see much which could be done to increase the presence of the compound in the atmosphere — and therefore accelerate the cleaning process — because just “adding more sunlight” to make more was not practical or desirable.

Fortunately for all, the research did not stop there. Work conducted by Dr. Richard Zare of Stanford University had recently discovered that a related oxidant, hydrogen peroxide (H2O2), can spontaneously form on the surface of water droplets. He had not been able to determine why but had been able to demonstrate the repeatability of the process.

Zare’s research pointed to an as-then-unknown process taking place. It also might be a clue to other atmospheric processes.

The University of California Irvine team took up the intellectual challenge of understanding what Zare had discovered. UCI’s unique experience in studying the role of water droplets in the atmosphere, led by leading scientists such as Ann Marie Carlton, a professor of chemistry there, gave them a strong starting point to build on.

The research team began their investigations by looking into whether how the rate of formation of OH was affected by the amount of sunlight present, and under other test conditions.

In classic experimental fashion, they prepared a series of vials with water molecules present, some with no air present in the vials and others with an air-water surface component, held in place by the high surface tension of water. Each vial had within it the ability to sensed the presence of OH production in the water, via a special sensor molecule embedded that fluoresces when OH is nearby.

They then began a series of tests, to determine what either causal factors might cause more or less OH to form. Of special interest of course was about how much sunlight might make a difference in producing far more OH.

What they found was a surprise. Under these unique controlled conditions, they were able to prove that the hydroxyl radical (OH) rates of production in darkness were equal to and often exceeded the rates of production in sunlight. What that meant was that, even though sunlight could help stimulate OH formation, there was something else present that was even more important to the process.

After further work, it turned out that the “something else” was the presence of a strong — and naturally occurring — electrical field formed between airborne water droplets and the surrounding air. That electrical field is related to the one formed in thunderstorms and the clouds with higher-than-normal concentrations of water vapor there, which in turn helps generate lightning.

The conclusion of the research is nothing less than “air-shattering”, to coin a phrase. As a minimum, it helps explain better some of the natural cleaning processes in the atmosphere when higher concentrations of the hydroxyl radicals are present. Taking it a step further, assuming a controllable and efficient means of deploying the technology was available, it could mean that guided electrical stimulation of air-and-water-vapor mixtures in the air could help eliminate higher percentages of toxic hydrocarbons from the atmosphere than happens by normal means.

At the very least, the researchers point out that this new mechanism for how OH molecules are formed needs to be incorporated in existing atmospheric models. It is likely that the role of OH molecules has been underestimated because there is more up there — and continuously forming spontaneously — than we knew.

As a minimum, this research does point to a new self-cleaning mechanism for the atmosphere that none of us previously knew existed.  It is also a mechanism that could be tapped to break down atmospheric hydrocarbons at a faster rate than already happens naturally.

Considering that this is believed to have little impact on the carbon dioxide component of greenhouse gas emissions, and that secondary self-reinforcing climate change processes such as the release of methane (NH4, another hydrocarbon) from melting permafrost are currently accelerating, it could be that investing heavily in artificial stimulation of hydroxyl radical formation may do little more than slow the pace of global heating, at best.

This research was funded in part by the European Research Council.

The paper describing the full results of the study, “Spontaneous dark formation of OH radicals at the interface of aqueous atmospheric droplets,” by Kangwei Li, et. al., was published in the April 3, 2023, issue of the Proceedings of the National Academy of Sciences of the United States of America.