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March 19, 2003
Surprise! Lightning has Big Effect on Atmospheric Chemistry
Scientists were surprised to learn summer lightning over the U.S. significantly
increases regional ozone and other gases that affect air chemistry 3 to 8 miles
above Earth's surface.The amounts of ozone and nitrogen oxides created by lightning
surpass those generated by human activities in that level of the atmosphere.
Typically over the U.S., fossil fuel burning is the main cause of nitrogen
oxides (NOx), which lead to the formation of ozone near the Earth's surface.
However, above the Earth's surface in the free troposphere (3-8 miles high),
during the summer months, lightning activity increases NOx by as much as 90
percent and ozone by more than 30 percent.
Renyi Zhang of Texas A&M University, lead author of a paper that recently
appeared in the Proceedings of the National Academy of Sciences, suggests lightning
has distinct impacts on air chemistry over the U.S. Human activities dominate
the creation of these gases near the Earth's surface, but lightning plays a
bigger role in the free troposphere.
Depending on where ozone resides, it can protect or harm life on Earth. Most
ozone resides in the stratosphere (a layer of atmosphere between 8 and 25 miles
high), where it shields life on Earth from the sun's harmful ultraviolet radiation.
At the surface, ozone is a harmful pollutant that causes damage to lung tissue
and plants. In the troposphere (surface to 8 miles high) ozone also is a radiatively
active gas that affects climate.
About 77 million lightning bolts annually strike the U.S. Measurements before
and after lightning strikes have confirmed the generation of nitrogen oxides
in the atmosphere.
"Ironically, over the United States lightning accounts for only about 5 percent
of the total U.S. nitrogen oxide annual emissions and about 14 percent of the
total emissions in July," said Zhang. Although the largest source of NOx over
the U.S. is fossil fuel burning, lightning still plays a dominant role in influencing
the regional air chemistry.
The explanation is NOx from fossil fuel burning is released close to the Earth's
surface and is consumed rapidly by chemical reactions before being transported
upward. Lightning, however, directly releases NOx throughout the entire troposphere.
The lightening source over North America for NOx is sufficiently large, so
that it has implications on free troposphere NOx over other parts of the world,
most notably Europe, which is downwind of the U.S., given the prevailing westerly
flow in the Northern Hemisphere mid- latitudes.
NASA funded this research, because one mission of NASA's Earth Science Enterprise
is to assess and understand the primary causes of changes in Earth's system,
including man- made and natural causes.
The objective of Zhang's work is to assess the impact of how the U.S. human-induced
(mainly fossil fuel burning) and natural (lightning) sources contribute to
air pollution in the lower and upper troposphere. He collaborated with Dr.
Xuexi Tie of the National Center for Atmospheric Research (NCAR).
Zhang used lightning measurements from the ground-based National Lightning
Detection Network and the Optical Transient Detector (OTD) instrument to obtain
the number of lightning flashes over the U.S. The OTD, aboard the Microlab
satellite, is the world's first space-based sensor capable of detecting and
locating lightning events during day and night, with high detection efficiency.
This research was partially supported by NASA's New
Investigator Program in Earth Science and the Texas Air
Research Center. The National Science Foundation supports
NCAR.
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Contacts:
David E. Steitz
Headquarters, Washington
(Phone: 202/358-1730)
Rob Gutro
Goddard Space Flight Center, Greenbelt, Md.
(Phone: 301/286-4044)
Keith Randall
Texas A&M University
(Phone: 979/845-4644)
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The Reach of U.S. Summer Lightning's Effects
These two graphs show the extent and impact of summer lightning
over the United States extend throughout the Northern Hemisphere.
The graphs show lightning's impact on ozone (03)/smog (left)
and nitrogen oxide (NOx) (right) at 250 millibars (approx.
34,000 feet high) in July stretching across the Atlantic Ocean
toward Europe. CREDIT: Renyi Zhang, Texas A&M University
Lightning Has a Big Effect on Regional Pollution
Summertime lightning over the United States increases regional
pollution by significant amounts and also over a large portion
of the northern hemisphere, surpassing those by human activities.
CREDIT: Photograph by M. Garay
The Microlab Satellite and the OTD Lightning Detector
The Optical Transient Detector (OTD), aboard the Microlab
satellite, is the world's first space-based sensor capable
of detecting and locating lightning events in the daytime
as well as during the nighttime with high detection efficiency.
It was designed and built at Marshall Space Flight Center (MSFC).
Quantifying Lightning's Emissions
These computer model simulations show summer lightning over
the U.S. dominating emissions of NOx (bottom 3 graphs) and
O3 (top 3 graphs) levels in the free troposphere (between
5-10km).
The largest level of O3 (ozone) is depicted by the taller
blue bars during the months of July (7)-Oct.(10) in the top
middle graph. The largest level of NOx (nitrogen oxide) is
depicted by the taller green bars during all months in the
bottom middle graph.
Despite that fossil-fuel burning (surface emissions) represents
the largest source of NOx over the U.S.
The y-axis represents the percentage of change in concentrations
summed vertically over the altitudes and horizontally over
the entire contiguous U.S. Months are shown on the x-axis
with 1 denoting January through 12 denoting December. The
troposphere panels correspond to the altitudes between 0 to
10 km (0 to 6.2 miles). CREDIT: Renyi Zhang, Texas A&M
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