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Winter 1998

Ozone above Indian Ocean linked to African lightning

by Bob Henson

In one of the first studies to trace lightning's chemical impact across thousands of miles, a team of atmospheric chemists has connected a region of elevated ozone levels in the eastern Indian Ocean with lightning produced in Africa. Louisa Emmons, a visiting scientist in NCAR's Atmospheric Chemistry Division (ACD), presented the results at the American Geophysical Union conference in December.

Emmons and colleagues examined a set of ozone data collected over four years between Japan and Antarctica for their paper, "Evidence of Transport Across the Indian Ocean of Ozone Produced from Biomass Burning and Lightning." Her coauthors were Didier Hauglustaine (France's Centre National de la Recherche Scientifique), Michael Newchurch (University of Alabama at Huntsville), Toshi Takao and Kouji Matsubara (Japan Meteorological Agency), and Guy Brasseur (ACD). The research was funded by NASA.

Lightning is known to produce nitrogen oxides (NOx) within thunderstorms. These chemicals may react with others in the presence of sunlight to produce ozone. Until now, most related studies have focused on measuring the production of NOx in the immediate vicinity of storms. However, the ozone produced has a long lifetime in the upper troposphere and thus could be carried over long distances. According to Emmons and colleagues, ozone from storms across southern Africa is being transported by the subtropical jet stream to Australia.

Ozone measurements between 3 and 10 kilometers (2-6 miles) in altitude over a large part of the eastern Indian Ocean were as high as 80 parts per billion, similar to a polluted day in a U.S. city and several times more than normal levels, says Emmons. To analyze the source of this ozone, she and colleagues used a new computer model of atmospheric chemistry called MOZART, developed at NCAR by Brasseur and Hauglustaine.

Results from MOZART indicate that the ozone did not descend from the stratosphere, the most obvious source. Another possible source was the burning of forests and grasses upwind in Africa. When biomass burning was removed from the model calculations, ozone levels remained high, but when African lightning was removed, the ozone levels dropped significantly. The MOZART results are consistent with the observations above.

"Although there are uncertanties in the model results," says Emmons, "they indicate that lightning has a far-reaching and significant impact on tropospheric chemistry."

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