Lightning can generate a variety of air-cleansing chemicals called oxidants; researchers report online April 29 in Science. Oxidants contribute to air purification by reacting with contaminants such as methane to form more water-soluble or stickier molecules, allowing them to rain out of the Earth’s atmosphere or adhere to its surface more easily.

Nitric oxide is produced by lightning, resulting in the formation of oxidants such as hydroxyl radicals. However, no one had witnessed lightning directly generate a large number of these oxidants.

A NASA jet measured two oxidants in storm clouds over Colorado, Oklahoma, and Texas in May and June 2012. One of these radicals was the hydroxyl radical, OH. The other was a closely related oxidant known as the hydroperoxyl radical, HO2. In some parts of these clouds, the combined concentration of OH and HO2 molecules generated by lightning and other electrified regions of the air reached thousands of parts per trillion. Previously, the highest observed concentration of OH in the atmosphere was a few parts per trillion. The highest concentration of HO2 detected was approximately 150 parts per trillion.

“We were not expecting to see any of this,” says William Brune, a Penn State University atmospheric scientist. “We shelved the data… due to its extreme nature.” However, laboratory experiments later demonstrated that electricity can produce such large amounts of OH and HO2, confirming the validity of these oxidant signals.

With an estimated 1,800 lightning storms raging around the world at any given time, Brune and colleagues calculated that lightning could account for between 2% and 16% of atmospheric OH. A more precise estimate would necessitate the observation of additional thunderclouds. Understanding how lighting affects the atmosphere’s chemistry may become more critical as climate change results in increased lightning.

Journal Reference: W.H. Brune et al. Extreme oxidant amounts produced by lightning in storm clouds. Science. Published online April 29, 2021. DOI: 10.1126/science.abg0492.