Lightning and subvisible discharges produce molecules that clean the atmosphere
Penn State
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."
Other researchers at Penn State
include Patrick J. McFarland, undergraduate; David O. Miller, doctoral
recipient; and Jena M. Jenkins, doctoral candidate, all in meteorology and
atmospheric science.
Also working on the project were
Eric Bruning, associate professor of atmospheric science, Texas Tech University;
Sean Waugh, research meteorologist, and Donald MacGorman, senior research
scientist, both at NOAA National Severe Storm Laboratory; Xinrong Ren, physical
scientist, NOAA Air Resources Laboratory; Jingqiu Mao, assistant professor of
atmospheric chemistry, Univeristy of Alaska; and Jeff Peischl, senior
professional research assistant, Cooperative Institute for Research in
Environmental Sciences, University of Colorado, Boulder.
The National Science Foundation,
NASA, and the National Oceanic and Atmospheric Administration supported this
work.
