Scientists shocked to discover how much lightning may clean the atmosphere

Lightning may play a much bigger part than previously thought in a fundamental process that cleans our atmosphere of pollutants, according to the results of a new study.

New research published in in the journal Science suggests that electrical storms produce large amounts of molecules called oxidating radicals that break down gases like carbon monoxide and methane in the atmosphere. These atmospheric pollutants can contribute to global warming and damage the ozone layer. 

Carbon monoxide and methane make their way into the atmosphere from both natural and industrial sources. Methane is produced by plant decay, but is also released by oil and gas development and agriculture. Carbon monoxide and other polluting hydrocarbons can be produced by industry and wildfires.

But natural processes in the atmosphere, driven largely by sunlight, produce molecules called radicals, among the most important of which is hydroxyl. These molecules are very chemically active (meaning they are much more likely to react with other molecules) and can react with pollutants to form new compounds that are harmless or that can more easily attach to water and be rained out of the air. 

At first we didn't believe the signals that we got. They were so large, a thousand times larger than the largest we've ever seen.​​​​- Meterology professor William Brune

The new research led by William Brune, a distinguished professor of meteorology at Penn State University, found that lightning produces far more of these molecules than had been previously understood.  His work suggests that more than 10 per cent of the atmosphere's supply of these cleansing radicals could be produced by lightning storms.

Using planes to fly through storms

The work involved flying a NASA DC-8 research plane through deep convective thunderstorms to gather data. According to Brune, it was not as dangerous as it sounds.

"It was a lot of fun, actually. The pilots are just extraordinary. They know what they're doing. They know how to keep the plane safe. But it really is interesting because you get to see deep convection, which is very, very strong rising storms up close and personal," he told Quirks & Quarks host Bob McDonald.

"We were doing this study in 2012, flying over the central U.S. and trying to look at what was going into storms in terms of chemistry, what was coming out the top," Brune said. "And to our surprise, we saw very, very, very large amounts of OH [hydroxyl molecules]. At first we didn't believe the signals that we got. They were so large, a thousand times larger than the largest we've ever seen."

That data collected from the aircraft was compared to data gathered from ground-based radio receivers that monitor lightning flashes in the clouds. The two sets of data confirmed the production of high levels of hydroxyl radicals by lightning. 

"Lo and behold, the two overlapped in terms of where lightning was happening," said Brune. 

Hydroxyl radicals are produced when the energy of the lightning breaks down water vapour in the atmosphere. 

"You can think of it as water with a hydrogen atom removed and then wants that hydrogen back," Brune said. "So it becomes very reactive as it goes and tries to get the hydrogen back." 

That means the hydroxyl radical is very prone to reacting with molecules methane or carbon monoxide. 

Capturing lightning in a model

Brune admits that it is difficult to gauge the effectiveness of this process on a global scale. The results of this study are based on a limited number of flights over a small section of the United States. There is still a lot more information that needs to be gathered in order to create a global picture. But Brune believes that the lightning generated hydroxyl has a substantial impact world-wide. 

Previous models suggested that lightning wasn't a significant contributor to the cleansing process. 

"Our best estimate right now is anywhere from two per cent, which is pretty important, to more than 10 per cent, which is quite important for the total amount of cleansing of the atmosphere," he said.

Those estimates could change as the planet warms. Some climate change models suggest an increase in thunderstorm activity, which would mean the production of more hydroxyls and thus more atmospheric cleansing in the future. Other climate models suggest that there may not be as much lightning, but that it will be more intense, which would also alter the numbers.

In any case, future global pollution and climate change models will have to take this new understanding of atmospheric cleansing into account. Existing models may need updating. 

"If lightning-generated OH is 10 per cent effective, that would say that the models estimating the other part are 10 per cent too high," said Brune. "And so this is going to get people to think really hard about how do you actually put something that's created by a flash into a model? Tough, tough problem."

Written and produced by Mark Crawley