Satellite-borne instruments have been used to measure ozone and monitor its global distribution. Ozone is predominately located above the troposphere, within the stratosphere. Residual methods have been used to apportion what fraction of the total column of ozone lies within the troposphere. In 1986, Jack Fishman, of the Chemistry and Dynamics Branch at LaRC, and his colleagues highlighted a persistent region of high ozone in the southern tropical Atlantic from TOMS data. In the tropics, where the total ozone column is thin (typically 220-300 DU), variablity in total ozone seen by TOMS is mostly due to tropospheric ozone.
In Fishman's groundbreaking work, the stratospheric portion of the total column was subtracted to look at the "tropospheric ozone residual". With his method, the highest tropospheric ozone on earth (during the months of August to November) was in the southern hemisphere tropics along the west coast of Africa. Also very interesting was the "tail" of high ozone extending from Africa east toward Australia.
This period corresponds to the period of maximum biomass burning during the dry seasons of South America and southern Africa. Every year the rural people of South America and Africa renew agricultural areas by burning, a practice once used more extensively in the northern hemisphere as well. The community began to look to biomass burning as a possible cause for this high ozone. But there are other possible causes. There could be a component of anthropogenic emissions coming from large cities such as Rio de Janeiro. There could be natural hydrocarbon emissions coming from vegetation on either continent. Stratospheric air could be descending into the troposphere in this region. With this uncertainty, an aircraft mission was scheduled through the GTE project office called TRACE-A (Transport and Atmospheric Chemistry near the Equator-Atlantic) for Sept.-Oct 1992. Meanwhile, satellite studies continued to learn about this region before the mission.
Chris Justice and Jackie Kendall in GFSC Code 923 have used AVHRR data (infrared data from polar orbiting satellites) to produce fire counts in southern Africa. Alberto Setzer in Brazil produced a similar product for South America.
The S. Atlantic is one of several regions in the tropics where stratocumulus clouds are always present. They correspond to very persistent anticyclones and cold water upwelling off the west coasts of continents. We suspected a possible problem with the TOMS data. TOMS cannot see below clouds, and assumed a climatological cloud height in this region that is too high. Below this climatological cloud, the TOMS algorithm adds a climatological ozone value. If the cloud is actually much lower than expected, too much tropospheric ozone is added. Our correction for actual cloud height didn't make the seasonal feature disappear, but did clean up the daily maps. This research was passed to the Ozone Processing Team (OPT), and version 7 TOMS will have a better cloud correction based on monthly mean ISCCP data (from the Global Climate Research project at GISS).
Meanwhile Bob Hudson at the UMD Dept. of Meteorology, and his student Jae-wan Kim, developed another tropical tropospheric ozone product from the high density TOMS data. Only accurate when averaged over about a 2 week period, due to difficulty of subtracting off a proper stratospheric column and TOMS uncertainty, their product confirms that the Atlantic is polluted and the Pacific is very clean.