Lightning Produces Molecules that Clean Greenhouse Gases from the Atmosphere

Nitrogen, oxygen and water vapor molecules are broken apart by lightning and associated weaker electrical discharges, generating the reactive gases NO, O3, HO2 and the atmosphere’s cleanser, OH. Credit: Jena Jenkins, Penn State

Lightning produces molecules that can “clean” the atmosphere, according to research published this month in Science (Brune et al., 2021) and authored by a number of NOAA research scientists. The research demonstrates not only that lightning produces hydroxyl (OH) and hydroperoxyl (HO₂), but also produces amounts much greater than expected due to weaker, subvisible electrical discharges. Researchers knew that lightning creates ions and new molecules including OH and HO₂, but the amounts observed in this experiment were up to 1000 times what researchers had previously observed in the troposphere.

Lightning, and even smaller electric discharges in the atmosphere, creates ozone (O3) and two oxidizing chemicals: hydroxyl (OH) and hydroperoxyl (HO₂). The OH and HO₂ molecules dominate atmospheric oxidation chemistry that removes the majority of trace gases including some greenhouse gases (GHG), particularly methane, which mitigates the GHG contribution to climate change.

This research in Science came from airborne measurements made during the Deep Convective Clouds and Chemistry (DC3) field campaign in 2012. A range of instrumentation collected over 100 atmospheric constituents both airborne and on the ground-based measurements by using a NASA DC-8 aircraft, a Gulfstream-V from NCAR, satellites, ground-based radar and lightning antenna stations.

This huge thunderstorm supercell was photographed from NASA’s DC-8 airborne science laboratory as it flew at an altitude of 40,000 feet southwest of Oklahoma City, Ok., during a DC3 mission flight May 19. The flight crew estimated the top of the thunderhead’s anvil extended above 45,000 feet altitude. (NASA / Frank Cutler)

NASA’s DC-8 Earth Science laboratory sports numerous probes to collect atmospheric samples. The aircraft, based at the Dryden Aircraft Operations Facility in Palmdale, Calif., participated in the DC3 campaign. Credit: NASA/Tom Tschida

The data in the research featured a flight spiraling up through an anvil cloud, example pictured above, which contain volatile and high energy storm systems. The DC3 campaign was designed to explore the impact of large thunderstorms on the concentration of ozone and other substances in the upper troposphere over the central and southern United States. Pilots flew L-shaped patterns at different altitudes around thunderstorm cells to analyze a variety of atmospheric components throughout different areas of the US: northeastern Colorado, west Texas to central Oklahoma and northern Alabama.

Atmospheric researcher Xinrong Ren of NOAA’s Air Resources Laboratory (ARL) was onboard the DC-8 to measure the Hydroxyl levels. While most of us prefer to avoid thunderstorms while traveling on an airplane, these particular flights were designed to fly directly around, through and near thunderstorms, while a lightning mapped array on the ground captured visible imagery. Xinrong took it in stride, but the turbulence was noticeable. On one occasion, they flew close to a storm and the NASA DC-8 got a lightning strike on its tail, but very minor damage meant the mission continued.

When the data was safely on the ground and assessed, the team wasn’t sure they trusted it; the levels of hydroxyl OH) and hydroperoxyl (HO₂) were much higher than had been observed, or even speculated about. “At the beginning we thought these huge amounts of chemicals couldn’t be right and might be due to the instrument noise,” said Xinrong.

“Fortunately the PI, Bill Brune, and his current students spent quite a bit of effort to continue investigating this phenomenon and were able to replicate it, ” according to Xinrong. Since the researchers had doubts about the data, it sat on a shelf for a while until a box model (or called zero-dimensional model) was used to simulate chemical processes in the lightning and to test our understanding of atmospheric chemistry using the observations.

At Penn State, Bill Brune and some of his students set up a lab experiment designed to see if the high levels of hydroxyl and hydroperoxyl could be due to sparks or subvisible discharges. In the box model, weaker electrical currents could produce enough energy to generate hydroxyl and hydroperoxyl. Video from the original flight confirmed several examples of smaller electrical discharges.

Researchers on this project intend to continue investigating the influence of lightning on atmospheric chemistry and air quality. Xinrong Ren plans to be involved in the measurement of nitrous acid, which is an important product in lightning and a major precursor of hydroxyl radical. The lab experiment can be used to simulate the lightning events to look at other important atmospheric molecules, such as reactive nitrogen species.