Lightning breaks apart nitrogen and oxygen molecules in the atmosphere and creates reactive chemicals that affect greenhouse gases. Now, a team of atmospheric chemists and lightning scientists have found that lightning bolts and, surprisingly, subvisible discharges that cannot be seen by cameras or the naked eye produce extreme amounts of the hydroxyl radical (•OH) and hydroperoxyl radical (•OOH).
The hydroxyl radical is important in the atmosphere because it initiates chemical reactions and breaks down molecules such as the greenhouse gas methane. •OH is the main driver of many compositional changes in the atmosphere.
‘Initially, we looked at these huge •OH and •OOH signals found in the clouds and asked, what is wrong with our instrument?’ said William H. Brune, distinguished professor of meteorology at Penn State. ‘We assumed there was noise in the instrument, so we removed the huge signals from the dataset and shelved them for later study.’
The data was from 2012 from an instrument on a plane flown above Colorado and Oklahoma, looking at the chemical changes that thunderstorms and lightning make to the atmosphere.
But a few years ago, Brune and co-workers looked at the data again and saw that the signals were really •OH and •OOH. They looked to see if these signals could be produced by sparks and subvisible discharges in the laboratory. Then they reanalysed the 2012 dataset and were able to link the huge signals seen by the instrument flying through the thunderstorm clouds to the lightning measurements made from the ground.
Brune notes that aeroplanes avoid flying through the rapidly rising cores of thunderstorms because it is dangerous, but can sample the anvil, the top portion of the cloud that spreads outwards in the direction of the wind. Visible lightning happens in the part of the anvil near the thunderstorm core.
‘Through history, people were only interested in lightning bolts because of what they could do on the ground’, said Brune. ‘Now there is increasing interest in the weaker electrical discharges in thunderstorms that lead to lightning bolts.’
Most lightning never strikes the ground, and the lightning that stays in the clouds is particularly important for affecting ozone, and important greenhouse gases, in the upper atmosphere. It was known that lightning can split water to form •OH and •OOH, but this process had never been observed before in thunderstorms.
What confused Brune’s team initially was that their instrument recorded high levels of •OH and •OOH in areas of the cloud where there was no lightning visible from the aircraft or the ground. Experiments in the lab showed that weak electrical currents, much less energetic than visible lightning, could produce these same components.
While the researchers found •OH and •OOH in areas with subvisible lightning, they found little evidence of ozone and no evidence of nitric oxide, which requires visible lightning to form. If subvisible lightning occurs routinely, then the •OH and •OOH these electrical events create need to be included in atmospheric models. Currently, they are not.
According to the researchers, ‘Lightning-generated •OH in all storms happening globally can be responsible for a highly uncertain but substantial 2–16% of global atmospheric •OH oxidation.’
Brune noted that the results are highly uncertain: it is not known how these measurements apply to the rest of the globe. Most thunderstorms occur in the tropics and have a different structure from thunderstorms over high plains.