The NASA TRACE-A (Transport and Atmospheric Chemistry near the
Equator-Atlantic) field study was deployed in August 1992 to determine
the cause and source of high concentrations of ozone that accumulate
over the Atlantic ocean between southern Africa and South America
during the months of August through October. This pool of ozone was
initially discovered in the mid 1980's as a result of the re-analysis
of ozone measurements from two operational satellites using a newly
developed mathematical technique to extract the concentration of ozone
in the troposphere. The satellite data provided the first hints of
ozone spread over thousands of square kilometers over the Atlantic
Ocean at concentrations comparable to those found in many large cites
around the world during the summertime. The fact that the enhanced
levels of ozone over the Atlantic were observed to be the highest
during the southern hemisphere's springtime, a period of intense
burning of vegetation in both southern Africa and South America,
suggested a link between the biomass burning and the ozone pollution.
(See Figure 1) As additional satellite
data were analyzed an alternative source was suggested to be the
downward transport of ozone from the stratosphere linked to a sinking
motion of air prevalent over the region during the southern
hemispherical springtime.
The TRACE-A mission brought together a multi-year series of ground
based and balloon measurements, aircraft measurements over Brazil,
southern Africa, and the Atlantic ocean, and powerful computer models
of the tropical atmosphere. TRACE-A was a cooperative project between
NASA and the Brazilian Space Agency (INPE), involving over 200
scientists from US, Brazil and South Africa. The center piece of
TRACE-A was the NASA DC-8 flying laboratory based at the NASA Ames
Research Center. The DC-8 was instrumented with state-of-the-art
instruments for measurements of ozone and other gases that are
associated with the production of ozone in the atmosphere. Two INPE
aircraft from San Jose dos Campos, provided additional measurements
over Brazil. During TRACE-A, the DC-8 aircraft flew over 70,000 miles
from bases in Brasilia, Brazil, Johannesburg, South Africa, Windhoek,
Namibia, and Ascension Island, UK.
During TRACE-A, another comprehensive experiment, called SAFARI
(Southern African Fire-Atmosphere Research Initiative) conducted by a
team of European, African, and North American scientists, studied the
African fires to understand exactly what kind of vegetation burns and
how this material is transformed into the air pollution pall that
hangs over vast regions of Zimbabwe, Zambia, and Botswana during the
burning season.
The picture that emerges from TRACE-A together with SAFARI, is that
widespread biomass burning in both South America and southern Africa
is the dominant source of the initial pollutants that lead to the
formation of the high amounts of ozone over the South Atlantic Ocean.
Additionally, the air motion in this part of the world is favorable
for the accumulation of these pollutants over the Atlantic Ocean where
the intense sunlight can do its work in transforming them into ozone
and other pollution gases. The production of ozone occurred over
thousands of miles and was usually the greatest at altitudes between
15,000 to 50,000 feet where relatively high concentrations of nitrogen
oxides, which are needed for the production of ozone, were also
measured during TRACE-A.
Results from the TRACE-A mission clearly demonstrated that the cause
and source of the seasonally enhanced pool of ozone pollution over the
South Atlantic Ocean is due to burning of vegetation on a massive
scale in southern Africa and South America, combined with an enormous
stagnation region between the two continents. What remains unclear is
the source of the high contrations of nitrogen oxides also observed by
instruments aboard the DC-8 aircraft. Transport from surface sources
as well as production by lightning have been suggested as the two
majors sources for the high concentrations of nitrogen oxides.
Measurements of wide spread ozone plumes during the more recent
(e.g. September, 1996) NASA Pacific Exploratory Mission in the south
tropical Pacific Ocean (PEM-Tropics)
provide additional evidence of the impact that biomass burning is
having on the global atmosphere.