Ozone over southern Africa during SAFARI-92/TRACE A

Abstract

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 balloonborne 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].