Ozone over southern Africa during SAFARI-92 TRACE A

Characteristics of total O-3 in southern Africa and over the adjacent Atlantic during the IGAC/STARE/SAFARI-92/TRACE A (International Global Atmospheric Chemistry/South Tropical Atlantic Regional Experiment/Southern African Fire Atmospheric Research Initiative/Transport and Atmospheric Chemistry near the Equator-Atlantic) field experiments are described, Most of the analysis is based on data from the Nimbus 7/total ozone mapping spectrometer (TOMS) gridded O-3 data archive (version 6.0), which is used to examine O-3 in terms of seasonal and interannual variability. Total O-3 column variability is compared to the tropospheric O-3 column derived from balloon-borne ozonesondes at four fixed SAFARI-92/TRACE A sites (Ascension Island, Brazzaville, Okaukuejo, and Irene) from September 1 to October 23, 1992. All of these sites except Okaukuejo had regular ozonesonde launches from 1990 to 1992. Total O-3 and integrated tropospheric O-3 at the sounding sites showed the expected September-October maxima over southern Africa and the adjacent Atlantic Ocean. Statistical analysis of the TOMS record for 1979-1992 allows disaggregation of components contributing to total O-3 variability: Signals due to semiannual and annual cycles and the quasi-biennial oscillation are identified at the sounding sites. The tropospheric O-3 column estimated from integrated sondes (to similar to 16 km) at the four sites ranged from 24 to 62 Dobson units (DU) (mean, 45 DU) and averaged 15% of total O-3 at Irene (14 launches) and 19% of total O-3 at Ascension (20 launches). Tropospheric O-3 was higher at Ascension and Brazzaville than at the sites south of 15 degrees S because transport from biomass burning regions was more direct at these sites. This transport is seen in Hovmoller (time-longitude) plots of total O-3. A comparison of 1990-1992 integrated tropospheric O-3 amounts with the annual total ozone cycle shows that tropospheric ozone variations may account for all of the annual signal at Ascension (8 degrees S) and Brazzaville (4 degrees S) but only 30-40% of the seasonal total O-3 variation at Irene (26 degrees S). Hovmoller plots of daily TOMS O-3 over Southern Africa and the Atlantic show easterly flow of local O-3 maxima at 0 degrees-10 degrees S and westerly movement from 30 degrees-40 degrees S. At 0 degrees-10 degrees S the continent-ocean total O-3 gradient and Ascension and Brazzaville O-3 soundings are used to estimate a photochemical O-3 formation rate of 1-2 ppbv O-3/d over the Atlantic. This agrees with model calculations of moderately aged biomass burning emissions from SAFARI-92/TRACE A.