BOULDER -- 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. The results will be presented December 6 at the American Geophysical Union conference in San Francisco by Louisa Emmons, a visiting scientist at the National Center for Atmospheric Research (NCAR). NCAR's primary sponsor is the National Science Foundation.
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" (AGU paper A12D-11). Her coauthors are 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 (NCAR). The research was funded by the National Aeronutics and Space Administration.
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 2 and 6 miles in altitude (3-10 kilometers) 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."
The University Corporation for Atmospheric Research, a consortium of more than 60 universities offering Ph.D.s in atmospheric and related sciences, manages NCAR.