BOULDER--In one of the first studies to trace lightning's chemicalimpact across thousands of miles, a team of atmospheric chemists hasconnected a region of elevated ozone levels in the eastern Indian Oceanwith lightning produced in Africa. The results will be presentedDecember 6 at the American Geophysical Union conference in San Franciscoby Louisa Emmons, a visiting scientist at the National Center forAtmospheric Research (NCAR). NCAR's primary sponsor is the NationalScience Foundation.
Emmons and colleagues examined a set of ozone data collected over fouryears between Japan and Antarctica for their paper, "Evidence ofTransport Across the Indian Ocean of Ozone Produced from Biomass Burningand Lightning" (AGU paper A12D-11). Her coauthors are DidierHauglustaine (France's Centre National de la Recherche Scientifique),Michael Newchurch (University of Alabama at Huntsville), Toshi Takao andKouji Matsubara (Japan Meteorological Agency), and Guy Brasseur (NCAR).The research was funded by the National Aeronutics and SpaceAdministration.
Lightning is known to produce nitrogen oxides (NOx) withinthunderstorms. These chemicals may react with others in the presence ofsunlight to produce ozone. Until now, most related studies have focusedon measuring the production of NOx in the immediate vicinity of storms.However, the ozone produced has a long lifetime in the upper troposphereand thus could be carried over long distances. According to Emmons andcolleagues, ozone from storms across southern Africa is beingtransported by the subtropical jet stream to Australia.
Ozone measurements between 2 and 6 miles in altitude (3-10 kilometers)over a large part of the eastern Indian Ocean were as high as 80 partsper billion, similar to a polluted day in a U.S. city and several timesmore than normal levels, says Emmons. To analyze the source of thisozone, she and colleagues used a new computer model of atmosphericchemistry called MOZART, developed at NCAR by Brasseur and Hauglustaine.
Results from MOZART indicate that the ozone did not descend from thestratosphere, the most obvious source. Another possible source was theburning of forests and grasses upwind in Africa. When biomass burningwas removed from the model calculations, ozone levels remained high, butwhen African lightning was removed, the ozone levels droppedsignificantly. The MOZART results are consistent with the observationsabove.
"Although there are uncertanties in the model results," says Emmons,"they indicate that lightning has a far-reaching and significant impacton tropospheric chemistry."
The University Corporation for Atmospheric Research, a consortium ofmore than 60 universities offering Ph.D.s in atmospheric and relatedsciences, manages NCAR.
Writer: Bob Henson
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