Lightning may be an important source of air-cleaning chemicals

Lightning could play an important role in flushing pollutants out of the atmosphere.

Observations from a storm-chasing airplane reveal that lightning can forge lots of air-cleansing chemicals called oxidants, researchers report online April 29 in Science. Oxidants help clear the air by reacting with contaminants like methane to form molecules that are more water soluble or stickier, allowing them to more easily rain out of Earth’s atmosphere or stick to its surface.

Researchers knew lightning produces nitric oxide, which can lead to the formation of oxidants such as hydroxyl radicals. But no one had seen lightning directly create lots of these oxidants.

In May and June 2012, a NASA jet measured two oxidants in storm clouds over Colorado, Oklahoma and Texas. One was the hydroxyl radical, OH. The other was a similar oxidant called the hydroperoxyl radical, HO₂. The combined concentration of OH and HO₂ molecules, generated by lightning and other electrified regions of air, reached up to thousands of parts per trillion in some parts of these clouds. The highest concentration of OH previously observed in the atmosphere was a few parts per trillion. The most HO₂ observed was about 150 parts per trillion.

“We didn’t expect to see any of this,” says William Brune, an atmospheric scientist at Penn State University. “We shelved the data … because it was just so extreme.” But lab experiments later showed that electricity really could generate such large quantities of OH and HO₂, helping confirm these oxidant signals were real.

About 1,800 lightning storms are thought to be raging around the world at any given moment, so Brune and colleagues came up with a ballpark estimate that lightning could account for 2 percent to 16 percent of atmospheric OH. A more precise estimate would require observing more thunderclouds. Understanding how lightning affects atmospheric chemistry could become more important as climate change sparks more lightning (SN: 4/6/21).