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.
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