Lightning bolts break apart nitrogen and oxygen molecules in the atmosphere
and create 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 -- HO2.
The hydroxyl radical is important in the atmosphere because it initiates
chemical reactions and breaks down molecules like the greenhouse gas
methane. OH is the main driver of many compositional changes in the
atmosphere.
"Initially, we looked at these huge OH and HO2 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 an instrument on a plane flown above Colorado and Oklahoma
in 2012 looking at the chemical changes that thunderstorms and lightning
make to the atmosphere.
But a few years ago, Brune took the data off the shelf, saw that the signals
were really hydroxyl and hydroperoxyl, and then worked with a graduate
student and research associate to see if these signals could be produced by
sparks and subvisible discharges in the laboratory. Then they did a
reanalysis of the thunderstrom and lightning dataset.
"With the help of a great undergraduate intern," said Brune, "we were able
to link the huge signals seen by our instrument flying through the
thunderstorm clouds to the lightning measurements made from the ground."
The researchers report their results online today (April 29) in Science
First Release and the Journal of Geophysical Research -- Atmospheres.
Brune notes that airplanes 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 outward 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 gas, in the upper atmosphere. It was known that lightning can
split water to form hydroxyl and hydroperoxyl, but this process had never
been observed before in thunderstorms.
What confused Brune's team initially was that their instrument recorded high
levels of hydroxyl and hydroperoxyl 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 current, much less energetic than that of
visible lightning, could produce these same components.
While the researchers found hydroxyl and hydroperoxyl 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 hydroxyl and hydroperoxyl these
electrical events create need to be included in atmospheric models.
Currently, they are not.
According to the researchers, "Lightning-generated OH (hydroxyl) in all
storms happening globally can be responsible for a highly uncertain but
substantial 2% to 16% of global atmospheric OH oxidation."
"These results are highly uncertain, partly because we do not know how these
measurements apply to the rest of the globe," said Brune. "We only flew over
Colorado and Oklahoma. Most thunderstorms are in the tropics. The whole
structure of high plains storms is different than those in the tropics.
Clearly we need more aircraft measurements to reduce this uncertainty."
References:
W. H. Brune, P. J. McFarland, E. Bruning, S. Waugh, D. MacGorman, D. O.
Miller, J. M. Jenkins, X. Ren, J. Mao, J. Peischl. Extreme oxidant amounts
produced by lightning in storm clouds. Science, 2021; eabg0492 DOI:
10.1126/science.abg0492
Jena M. Jenkins, William H. Brune, David O. Miller. Electrical Discharges
Produce Prodigious Amounts of Hydroxyl and Hydroperoxyl Radicals. Journal of
Geophysical Research: Atmospheres, 2021; DOI:
10.1029/2021JD034557