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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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March 19, 2003 |
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RELEASE: 03-110
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. Photo credit/M. Garay
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 tropopause (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.
For more information about the research and images, see:
http://www.gsfc.nasa.gov/topstory/2003/0312pollution.html
For more information about NASA and the Earth Science Enterprise
on the Internet, visit:
http://www.nasa.gov.
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