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January 1999

AGU news: Ozone near Australia linked to African lightning

Louisa Emmons. (Photo by Carlye Calvin.)

In one of the first studies to trace lightning's chemical impact across thousands of miles, a study funded by NASA and organized by ACD has connected a region of elevated ozone levels in the eastern Indian Ocean with lightning produced in Africa. The results were presented by ACD's Louisa Emmons on 6 December at the American Geophysical Union (AGU) conference in San Francisco.

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."

Also at AGU: Tracking the sources of sulfates

From 50% to 60% of sulfate-aerosol pollution over the Pacific Northwest is coming from industrialized Asia, according to a new analysis conducted at NCAR. While the total column of air contains "imported" sulfate aerosols, near the surface most of the aerosols come from North American sources. In contrast, sulfates in Europe are coming primarily from European sources, both at the surface and higher in the atmosphere. Jeff Kiehl, head of CGD's Climate Modeling Section, presented the findings 7 December at the AGU meeting.

"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.

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Edited by Bob Henson, bhenson@ucar.edu

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