January 1999
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January 1999 |
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| Louisa Emmons. (Photo by Carlye Calvin.) |
Louisa and colleagues examined a set of ozone data collected over four years between Japan and Antarctica. Her coauthors on the AGU paper are Didier Hauglustaine (an ACD visitor from France's Centre National de la Recherche Scientifique), Michael Newchurch (visiting ACD from the University of Alabama at Huntsville), Toshi Takao and Kouji Matsubara (Japan Meteorological Agency), and ACD director Guy Brasseur.
Lightning is known to produce nitrogen oxides (NOx) within thunderstorms. These chemicals can react with others in the presence of sunlight to produce ozone. Until now, most studies have focused on measuring the production of NOx in the immediate vicinity of storms. However, the resulting ozone has a long lifetime in the upper troposphere and thus could be carried over long distances. According to Louisa 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 higher than normal levels, says Louisa. To analyze the source of this ozone, she and colleagues used MOZART, a new computer model of atmospheric chemistry developed at NCAR by Guy and Didier. (See 1998/1999 Highlights.)
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 Indian Ocean observations.
"Although there are uncertanties in the model results," says Louisa, "they indicate that lightning has a far-reaching and significant impact on tropospheric chemistry."
"It's widely recognized that sulfate aerosols are playing a major role in the climate system," says Jeff. "One important way that sulfur moves in the atmosphere is through transport by the earth's winds." But winds are not the whole story. For the past three years, Kiehl and colleagues Mary Barth (MMM/ACD), Phil Rasch (CGD), and Tim Schneider (CGD) have been developing an integrated model of climate and sulfur chemistry. The model includes the emission of natural and industrial sulfur into the earth's atmosphere.
To model how the sulfur gas changes into sulfate aerosol particles, the team included chemical processes and the chemical and physical effects of clouds, including clouds' ability to remove sulfates from the atmosphere. By fully integrating sulfur chemistry into the climate model, and by tagging the sulfates in the climate simulations by source region, the team could calculate the percentage of sulfates transported from one region (North America, Asia, Europe, or the rest of the world) to another. The researchers compared their model simulations of sulfur and sulfate aerosols with observations near the surface. According to Jeff, more comparisons with observations yet to be made far above the surface are needed to confirm the model findings. The study was supported by NASA and NSF.