Ozone over southern Africa during SAFARI-92/TRACE A

Characteristics of total O3 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 O3 data archive (version 6.0), which is used to examine O3 in terms of seasonal and interannual variability. Total O3 column variability is compared to the tropospheric O3 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 O3 and integrated tropospheric O3 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 O3 variability: Signals due to semiannual and annual cycles and the quasi-biennial oscillation are identified at the sounding sites. The tropospheric O3 column estimated from integrated sondes (to ∼16 km) at the four sites ranged from 24 to 62 Dobson units (DU) (mean, 45 DU) and averaged 15% of total O3 at Irene (14 launches) and 19% of total O3 at Ascension (20 launches). Tropospheric O3 was higher at Ascension and Brazzaville than at the sites south of 15°S because transport from biomass burning regions was more direct at these sites. This transport is seen in Hovmöller (time-longitude) plots of total O3. A comparison of 1990-1992 integrated tropospheric O3 amounts with the annual total ozone cycle shows that tropospheric ozone variations may account for all of the annual signal at Ascension (8°S) and Brazzaville (4°S) but only 30-40% of the seasonal total O3 variation at Irene (26°S). Hovmöller plots of daily TOMS O3 over southern Africa and the Atlantic show easterly flow of local O3 maxima at 0°-10°S and westerly movement from 30°-40°S. At 0°-10°S the continent-ocean total O3 gradient and Ascension and Brazzaville O3 soundings are used to estimate a photochemical O3 formation rate of 1-2 ppbv O3/d over the Atlantic. This agrees with model calculations of moderately aged biomass burning emissions from SAFARI-92/TRACE A [Jacob et al., this issue; Thompson et al., 1996, this issue].