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How lightning helps purify the atmosphere of certain pollutants

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Recent observations made inside US thunderstorms have shown that lightning changes atmospheric chemistry much more than previously estimated. Also, by participating in the natural production of free radicals, they increase the capacity of purification of the atmosphere in various pollutants.

Lightning constitutes the luminous part of lightning, and thunder the acoustic part. Often feared and feared, this phenomenon is nevertheless involved in the regulation of our environment. In other words, carrying beneficial effects. The few million lightning bolts that occur every day on Earth, for example, participate in the natural purification of the atmosphere of gases such as methane (CH4) or carbon monoxide (CO). A contribution that would be even more important than we thought according to recent work published in journals Geophysical Research JournalAtmospheres and Science.

When in 2012, a research aircraft flew through a cluster of thunderstorms to gain a better understanding of how they affected atmospheric chemistry, the data transmitted by the instrument stunned scientists. So much so that they were first considered aberrant and then put aside. However, subsequent evaluations and laboratory experiments showed that this was not the case. The measurements were indeed correct … But what did the instrument measure when it probed the upper part of the thunderstorms between Colorado and Oklahoma?

The undervalued role of the inconspicuous part of lightning

Data showed an abnormally high amount of hydrogen oxides, present as hydroxyl radicals (OH) and hydroperoxyl (HO2). If scientists knew lightning could create these free radicals * Going through the production of nitrogen oxides, the process had never been directly observed. More importantly, they discovered thata direct and massive production ofOH and D’HO2 also existed. Whether through the main discharge or, more surprisingly, by the little or no visible currents that surround it.

Simplified representation of the free radical formation process OH and HO2 as well as nitrogen oxides and ozone (intermediate). Credits: Jena Jenkins, State of Penn.

If even weak currents have a notable influence on atmospheric chemistry, it is ultimately not so surprising to have measured a particularly high level of hydrogen oxides. In addition to having been supported by laboratory experiments, the mechanism suggests that the lightning participates much more strongly in the purification of the atmosphere than one thought. Also, 2% to 16% of the global oxidation linked to the hydroxyl radical would be due to lightning.

“These results are very uncertain, in part because we do not know how these measures apply to the rest of the world”, says WH Brune, main author of the paper. “We only flew over Colorado and Oklahoma. Most thunderstorms occur in the tropics. The structure of thunderstorms in the great plains is different from those in the tropics. It is clear that we need more aerial measurements to reduce this uncertainty ”.

* It is recalled that a free radical is a molecule whose bonds are not saturated. Also, their chemistry is very active, explaining a very short lifespan in the atmosphere.



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