| 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. ###
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 |
