Lightning could play an important role in removing pollutants from the atmosphere.
Observations from a plane chasing storms reveal that lightning can forge many chemicals to purify the air called oxidants, researchers reported online on April 29 in Science. Oxidants help clean the air by reacting with pollutants such as methane to form more water-soluble or more sticky molecules, allowing them to rain more easily from the Earth’s atmosphere or stick to its surface.
Researchers knew that lightning produces nitric oxide, which can lead to the formation of oxidants such as hydroxyl radicals. But no one had seen that lightning directly created many of these oxidants.
In May and June 2012, a NASA jet measured two oxidants in storm clouds over Colorado, Oklahoma and Texas. One of them 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 the air, has 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 billion. 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 archived the data … because it was so extreme." But later laboratory experiments showed that electricity could actually generate such large amounts of OH and HO2, which helped confirm that these oxidizing signals were real.
About 1,800 lightning storms are thought to be spinning around the world at some point, so Brune and colleagues thought lightning could account for between 2 and 16 percent of atmospheric OH. A more accurate estimate would require observing more thunderstorm clouds. Understanding how lightning affects atmospheric chemistry may be more important as climate change causes more lightning to come out (SN: 4/6/21).