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Web Extra Friday, July 25

Lightning implicated in ozone over Africa

About five years ago, atmospheric scientists studying ozone concentrations over equatorial Africa and the southern hemisphere of the tropical Atlantic came across a puzzling situation. They expected to see high levels of ozone north of the equator, where subsistence farmers had set massive fires to clear the savannas of grass in preparation for planting as part of a traditional annual practice. Burning such great quantities of biomass releases large amounts of carbon monoxide and chemically reactive nitrogen oxides (NOx), which are important ingredients in forming ozone. But the researchers' instruments told a different story — ozone concentrations were higher south of the equator, where no fires were burning at the time.

With no apparent source for ozone formation in the southern hemisphere, "people in the atmospheric research community were scratching their heads, trying to work out where the ozone was coming from," says David Edwards of the National Center for Atmospheric Research in Boulder, Colo. "How do these pollutants move from the northern hemisphere to the southern hemisphere?" This was especially perplexing given that the pollutants would have traveled across the intertropical convergence zone, a well known atmospheric barrier to movement of air masses between hemispheres in the tropics.

Unfortunately, the technology needed to solve this so-called "ozone paradox" was still in development. Measurements of gases using weather balloons and instruments on aircraft could not provide a broad enough picture of the region, and satellite-based instruments that could make accurate concentration measurements in the lower atmosphere were only just coming online.

In the end, the coordinated efforts of four separate satellites, each responsible for a different measurement — carbon monoxide concentrations, nitrogen dioxide (NO2) concentrations, the number of fires burning and the number of lightning strikes in the region — helped to find a possible solution to the paradox. The combined data suggested that high lightning activity in the southern hemisphere was the culprit. Lightning provides the energy needed for the reaction of nitrogen and oxygen in the atmosphere to form nitrogen oxide compounds, leading to subsequent reactions to produce ozone.

Edwards is the project leader for one of these satellites, the Terra Measurement of Pollution in The Troposphere (Terra/MOPITT) satellite — a joint Canadian-United States venture that uses thermal and near-infrared radiation to measure carbon monoxide concentrations in the lower atmosphere. MOPITT can sample virtually the entire surface of Earth in only three days.

The MOPITT group teamed up with researchers from the second European Remote Sensing Satellite Global Ozone Monitoring Experiment, which measures nitrogen dioxide concentrations; the Tropical Rainfall Measuring Mission Visible and Infrared Scanner, which maps ground fires; and the TRMM Lightning Imaging Sensor, which counts lightning strikes by measuring cloud brightness. After feeding their data into a mathematical model, the researchers were able to predict where ozone was likely to form.

For the study reported in the April 17, 2003 issue of Journal of Geophysical Research, Edwards and his colleagues analyzed data from January 2001, a month when farmers in the northern hemisphere of Africa traditionally set their ground-clearing fires. Carbon monoxide was present in high concentrations in regions above the fires, but it did not flow south of the equator — blocked by the intertropical convergence zone, as expected. In the southern hemisphere, very few fires were present, but the number of lightning strikes was substantial. Correlating lightning strikes with nitrogen dioxide concentrations indicated clearly that lightning was the cause of the high levels of ozone in the southern hemisphere of Africa at this time of year.

"Satellite measurement of carbon monoxide brings new data to bear on the problem," says Anne Thompson of the Goddard Space Flight Center, who coined the term "ozone paradox" while analyzing atmospheric data she collected on the Aerosols99 oceanographic research voyage in January and February of 1999. "What Edwards has done is to carry the data analysis one or two steps further than was previously possible, which is a great accomplishment," she says.

The new technology that provided insight into the conundrum is as important as the solution itself. "This was the first time that information from multiple satellites was used to look at pollutants in the lower atmosphere," Edwards says. "In the 1980s, we thought it would be great if we could measure levels of carbon monoxide from space. Now, with MOPITT, we can study carbon monoxide in the lower atmosphere and see how it moves around." Not only is carbon monoxide is an important indicator of pollution from biomass burning, but it also indicates pollutants emitted by industrial processes in urban regions. Having satellites that can peer through many kilometers of upper atmosphere and measure concentrations of trace pollutants in the lower atmosphere will have many applications in the future, such as monitoring the effects of chemical spills, industrial accidents and environmental phenomena on the atmosphere. Currently, his team is examining data collected from the wildfires of Colorado last summer and is also monitoring the atmospheric transport of industrial pollutants from China across the Pacific to the western United States.

Tim Palucka
Geotimes contributing writer


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