The TRACE-A science team looked at the meteorology of the region and developed a hypothesis. Could biomass burning emissions carried by low-level flow from Africa and high-level flow from South America be fueling the production of high ozone in the south Atlantic? To test the hypothesis, two field studies (the IGAC/STARE SAFARI and TRACE-A experiments) measured the chemical composition of the atmosphere, and improved the sparse meteorological coverage in the S. Atlantic. The GTE/TRACE-A mission took NASA's DC-8 aircraft, equipped with a host of instruments and investigators to South America and southern Africa from Sept. 21 - Oct. 26 1992. Here's what we found:
Ed Browell from the (Chemisty and Dynamics Branch at LaRC) measured amazingly high tropospheric ozone in layers throughout the troposphere. The only time "clean" values were measured were when the DC-8 penetrated below the stratocumulous cloud. There ozone values were similar to values measured in the remote marine boundary layer measured during ocean cruises (such as the Polarstern cruise in 1988--see Nov 92 J. Atmos. Chem).
The aircraft found many fires that feed convection in Brazil. Note the surrounding air looks clean. This shows that convection often pumps fire emissions out of the boundary layer (the layer of air closest to the ground, often capped by a stable layer that inhibits upward air motion).
This air, polluted by fire emissions, is transported to the upper atmosphere by convection. Here transport in a convective cloud encountered by the aircraft on 27 Sep is simulated with the Penn State/NCAR three-dimensional mesoscale model, MM5, in collaboration with Wang and Tao of GSFC Code 912. Very high levels of pollutants were measured in the cloud outflow between 9 and 12 km. Convection can also greatly enhance ozone production when there is lightning, which produces NOx. We estimate that at least 35-40% of the NOx measured by the aircraft was from lightning in the cloud outflow.
In contrast, Africa fires feed a more stable atmosphere. Note the haze layer, which suggests that convection has not vented pollutants out of the boundary layer. The pale circles are burn scars, which are taken into account in the remote sensing estimates of biomass burning by Justice and Kendall of GSFC code 923.
Trajectory studies are used to follow the pathways taken by air parcels in the atmosphere. Upper level westerly flow from S. America typically carries ozone precursors to the S. Atlantic. In the tropics, low-level flow tends to be easterly, and there are persistent counter-clockwise circulations over the subtropical African continent and Atlantic. Lower-level air slowly recirculates over Africa, picking up more fire emissions, and slowly drifts west toward the Atlantic. Air tends to sink over the ocean in the region of the stratocumulus cloud. Though convection is less common over southern Africa during this time, some air reaches the upper troposphere. Near the equator this upper level air can travel west, but most air tends to leave the continent south of 30S traveling toward the east.
So what does this tell us about ozone production? Photochemical model results using ozone precursor concentrations measured by aircraft show a large amount of ozone is formed photochemically (up to 15 ppbv/day or 4DU/day from the surface to 4 km) near active burning on both continents. Even in the upper troposphere some distance from fires, ozone production remains positive (.5-3 ppbv/day or .5-1 DU/day) from the presence of diluted fire emissions and possibly lightning.
These photochemistry rates combine with circulation patterns--slowly carrying air west from Africa at low levels and more rapid post-convection flows in the upper troposphere--to create a column thickness of 10-30 DU above the background (a 30-100% increase) over the South Atlantic basic in October 1992. This is a unique interaction of meteorology and photochemistry. Without circulation over the "chemical cauldron" of the South Atlantic, emissions would disperse and be diluted. Without biomass burning emissions and lightning, ozone production would be small in this region.
What Does This Mean for Global Change? From TRACE-A, scientists have a greatly improved picture of tropical ozone to use in climate models to assess the impact of biomass burning on global warming. The study of the S. Atlantic ozone maxima is a paradigm for future EOS studies. Our ability to monitor ozone and key atmospheric constituents from space will continue. When problems are discovered, field experiments will probe the atmosphere in-situ to collect data. Each new piece of information improves our ability to understand and predict changes in the global climate.