Lightning Strikes a Blow for Cleaner Air

By Mark Miller

In 2012, a NASA jet flew through electrified, anvil-shaped storm clouds over the American Midwest as part of a meteorological study. Its instruments found the first evidence that lightning can create high levels of oxidants — molecules that play an important role in keeping our atmosphere clean.

Air Detergents

Oxidants clean the atmosphere by reacting with pollutants such as methane. When they do, they make the pollutants more readily soluble in water, which falls to the Earth as rain. This process helps clear the air of chemicals that contribute to global warming, reports Smithsonian Magazine. In fact, one of the oxidants observed, hydroxyl, has been named the “detergent of the atmosphere.”

Oxidant-Laden Lightning

The instruments on the jet flight measured the combined concentration of two types of oxidants generated by lightning and other electrified parts of clouds. One was the detergent-dubbed hydroxyl radical, or OH, and the other a related oxidant called the hydroperoxyl radical, or HO₂. The levels reached were thousands of parts per trillion. This is a substantial increase considering that the most previously observed was just a few parts per trillion.

"We are surprised by the extreme amounts of OH and HO₂ generated in thunderstorm anvils and cores. They are orders of magnitude larger than any previous atmospheric HO₂ or OH measurement," said Pennsylvania State University meteorologist and study author William H. Brune in the Smithsonian article.

Delayed Reaction

Since the study was largely conducted in 2012, why hasn’t it been published until 2021? According to Smithsonian Magazine, the data was put aside for years and not re-examined because the research team didn’t believe that the oxidant increase was caused by lightning. They attributed it to problems with their instruments. Fortunately, measurements were taken from both ground and sky, which allowed them to confirm the oxidant increase by comparing the two datasets.

"With the help of a great undergraduate intern, we were able to link the huge signals seen by our instrument flying through the thunderstorm clouds to the lightning measurements made from the ground," said Brune in a Penn State press release.

Global Impact

According to Smithsonian Magazine, the research team determined that anvil clouds may produce 2 to 16 percent of all hydroxides on Earth. Their calculations considered the average 1,800 lightning storms that occur globally at any time. But there is more to come.

"These results are highly uncertain, partly because we do not know how these measurements apply to the rest of the globe," said Brune in the release release. "We only flew over Colorado and Oklahoma. Most thunderstorms are in the tropics. The whole structure of high plains storms is different than those in the tropics. Clearly, we need more aircraft measurements to reduce this uncertainty."

If lightning is adding oxidants to the atmosphere at the levels their initial study indicates, it may be an important factor in ongoing climate research.