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About five years ago, atmospheric scientists studying ozone concentrations
over equatorial Africa and the southern hemisphere of the tropical Atlantic
came across a puzzling situation. They expected to see high levels of ozone
north of the equator, where subsistence farmers had set massive fires to clear
the savannas of grass in preparation for planting as part of a traditional annual
practice. Burning such great quantities of biomass releases large amounts of
carbon monoxide and chemically reactive nitrogen oxides (NOx), which are important
ingredients in forming ozone. But the researchers' instruments told a different
story — ozone concentrations were higher south of the equator, where
no fires were burning at the time.
With no apparent source for ozone formation in the southern hemisphere, "people
in the atmospheric research community were scratching their heads, trying to
work out where the ozone was coming from," says David Edwards of the National
Center for Atmospheric Research in Boulder, Colo. "How do these pollutants
move from the northern hemisphere to the southern hemisphere?" This was
especially perplexing given that the pollutants would have traveled across the
intertropical convergence zone, a well known atmospheric barrier to movement
of air masses between hemispheres in the tropics.
Unfortunately, the technology needed to solve this so-called "ozone paradox"
was still in development. Measurements of gases using weather balloons and instruments
on aircraft could not provide a broad enough picture of the region, and satellite-based
instruments that could make accurate concentration measurements in the lower
atmosphere were only just coming online.
In the end, the coordinated efforts of four separate satellites, each responsible
for a different measurement — carbon monoxide concentrations, nitrogen
dioxide (NO2) concentrations, the number of fires burning and the
number of lightning strikes in the region — helped to find a possible solution
to the paradox. The combined data suggested that high lightning activity in
the southern hemisphere was the culprit. Lightning provides the energy needed
for the reaction of nitrogen and oxygen in the atmosphere to form nitrogen oxide
compounds, leading to subsequent reactions to produce ozone.
Edwards is the project leader for one of these satellites, the Terra Measurement
of Pollution in The Troposphere (Terra/MOPITT) satellite — a joint Canadian-United
States venture that uses thermal and near-infrared radiation to measure carbon
monoxide concentrations in the lower atmosphere. MOPITT can sample virtually
the entire surface of Earth in only three days.
The MOPITT group teamed up with researchers from the second European Remote
Sensing Satellite Global Ozone Monitoring Experiment, which measures nitrogen
dioxide concentrations; the Tropical Rainfall Measuring Mission Visible and
Infrared Scanner, which maps ground fires; and the TRMM Lightning Imaging Sensor,
which counts lightning strikes by measuring cloud brightness. After feeding
their data into a mathematical model, the researchers were able to predict where
ozone was likely to form.
For the study reported in the April 17, 2003 issue of Journal of Geophysical
Research, Edwards and his colleagues analyzed data from January 2001, a
month when farmers in the northern hemisphere of Africa traditionally set their
ground-clearing fires. Carbon monoxide was present in high concentrations in
regions above the fires, but it did not flow south of the equator — blocked
by the intertropical convergence zone, as expected. In the southern hemisphere,
very few fires were present, but the number of lightning strikes was substantial.
Correlating lightning strikes with nitrogen dioxide concentrations indicated
clearly that lightning was the cause of the high levels of ozone in the southern
hemisphere of Africa at this time of year.
"Satellite measurement of carbon monoxide brings new data to bear on the
problem," says Anne Thompson of the Goddard Space Flight Center, who coined
the term "ozone paradox" while analyzing atmospheric data she collected
on the Aerosols99 oceanographic research voyage in January and February of 1999.
"What Edwards has done is to carry the data analysis one or two steps further
than was previously possible, which is a great accomplishment," she says.
The new technology that provided insight into the conundrum is as important
as the solution itself. "This was the first time that information from
multiple satellites was used to look at pollutants in the lower atmosphere,"
Edwards says. "In the 1980s, we thought it would be great if we could measure
levels of carbon monoxide from space. Now, with MOPITT, we can study carbon
monoxide in the lower atmosphere and see how it moves around." Not only
is carbon monoxide is an important indicator of pollution from biomass burning,
but it also indicates pollutants emitted by industrial processes in urban regions.
Having satellites that can peer through many kilometers of upper atmosphere
and measure concentrations of trace pollutants in the lower atmosphere will
have many applications in the future, such as monitoring the effects of chemical
spills, industrial accidents and environmental phenomena on the atmosphere.
Currently, his team is examining data collected from the wildfires of Colorado
last summer and is also monitoring the atmospheric transport of industrial pollutants
from China across the Pacific to the western United States.
Tim Palucka
Geotimes contributing writer
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