Lightning may play an important role in clearing the air of pollutants.
A storm-chasing airplane has shown that lightning can forge large amounts of oxidants. These chemicals cleanse the atmosphere by reacting with pollutants such as methane. Those reactions form molecules that dissolve in water or stick to surfaces. The molecules can then rain out of the air or stick to objects on the ground.
Researchers knew lightning could produce oxidants indirectly. The bolts generate nitric oxide. That chemical can react with other molecules in the air to make some oxidants. But no one had seen lightning directly create lots of oxidants.
A NASA jet got the first glimpse of this in 2012. The jet flew through storm clouds over Colorado, Oklahoma and Texas in both May and June. Instruments on board measured two oxidants in the clouds. One was hydroxyl radical, or OH. The other was a related oxidant. It’s called the hydroperoxyl (Hy-droh-pur-OX-ul) radical, or HO2. The airplane measured the combined concentration of both in the air.
Lightning and other electrified parts of the clouds sparked the creation of OH and HO2. Levels of these molecules rose to thousands of parts per trillion. That may not sound like much. But the most OH seen in the atmosphere before was only a few parts per trillion. The most HO2 ever seen in the air was about 150 parts per trillion. Researchers reported the observations online April 29 in Science.
“We didn’t expect to see any of this,” says William Brune. He’s an atmospheric scientist. He works at Pennsylvania State University in University Park. “It was just so extreme.” But lab tests helped confirm that what his team saw in clouds was real. Those experiments showed electricity really could generate lots of OH and HO2.
Brune and his team calculated how much of the atmospheric oxidants that lightning could produce around the world. They did this using their storm-cloud observations. The team also accounted for frequency of lightning storms. On average, some 1,800 such storms are raging around the globe at any point in time. That led to a ballpark estimate. Lightning could account for 2 to 16 percent of atmospheric OH. Observing more storms could lead to a more precise estimate.
Knowing how storms affect the atmosphere may become even more important as climate change sparks more lightning.