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Sci Tech
Ozone's origins traced to lightning
A greater amount of near-surface ozone over tropical Atlantic develops as a result of lightning.
DURING SUMMER ozone near the Earth's surface forms in most major U.S. cities when sunlight and heat mix with car exhaust and other pollution. But in other parts of the world, such as the tropical Atlantic, this low level ozone appears to originate naturally in ways that have left scientists puzzled. Atmospheric scientist David Edwards and his colleagues from the National Centre for Atmospheric Research (NCAR) and collaborators in Canada and Europe have studied this problem using satellite data from three NASA spacecraft, one from the European Space Agency (ESA), and a computer model from NCAR.
They were surprised to find that a greater amount of near-surface ozone over tropical Atlantic develops as a result of lightning instead of agricultural and fossil fuel burning.
Their findings appeared in the American Geophysical Union's Journal of Geophysical Research.
The formation of ozone involves several factors, such as lightning and pollution from agricultural and fossil fuel burning.
NASA satellites included Terra, the Tropical Rainfall Measuring Mission (TRMM), and Earth Probe/TOMS. ESA's ERS-2 satellite was also used to look at ozone, and NCAR's MOZART-2 (Model for OZone And Related chemical Tracers) computer model was used to simulate the chemical composition of the atmosphere.
Because the different satellite instruments could very well detect fires, lightning flashes, and the resulting pollution and ozone in the atmosphere, respectively, they provided a bird's-eye global view of what was going on, and the computer model helped to tie all the pieces together.
Fires create smoke and carbon monoxide, and lightning creates nitrogen oxides (NOx). All of these come together with other unstable compounds in a chemical soup, and sunlight helps trigger the reaction that helps form ozone.
The scientists found that in the early part of the year, the intense fires set by farmers for land-clearing and traditional cultivation in north-western Africa, just south of the Sahara Desert, resulted in very large amounts of pollution that they could track using satellite images as it spread over the Atlantic towards South America. This pollution greatly increased ozone at low altitudes near the fires.
However, when researchers looked at areas of elevated ozone levels measured by satellites and aircraft over the Atlantic south of the equator, they were more surprised to find that this ozone was caused mainly by lightning rather than the fires.
In other parts of the world, especially near cities, ozone near Earth's surface is often made from pollution as a result of industrial fossil-fuel burning and cars.
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