WHERE DID TROPOSPHERIC OZONE OVER SOUTHERN AFRICA AND THE TROPICAL ATLANTIC COME FROM IN OCTOBER 1992 - INSIGHTS FROM TOMS, GTE TRACE-A, AND SAFARI 1992

The seasonal tropospheric ozone maximum in the tropical South Atlantic , first recognized from satellite observations [Fishman et al., 1986, 1991], Save rise to the IGAC/ STARE/SAFARI 1992/TRACE A campaigns (Int ernational Global Atmospheric Chemistry/South Tropical Atlantic Region al Experiment/Southern African Fire Atmospheric Research Initiative/Tr ansport and Atmospheric Chemistry Near the Equator-Atlantic) in Septem ber and October 1992. Along with a new TOMS-based method for deriving tropospheric column ozone, we used the TRACE A/SAFARI 1992 data set to put together a regional picture of the O-3 distribution during this p eriod, Sondes and aircraft profiling showed a troposphere with layers of high O-3 (greater than or equal to 90 ppbv) all the way to the trop opause. These features extend in a band from 0 degrees to 25 degrees S , over the SE Indian Ocean, Africa, the Atlantic, and eastern South Am erica. A combination of trajectory and photochemical modeling (the God dard (GSFC) isentropic trajectory and tropospheric point model, respec tively) shows a strong connection between regions of high ozone and co ncentrated biomass burning, the latter identified using satellite-deri ved fire counts [Justice et al., this issue]. Back trajectories from a high-O-3 tropical Atlantic region (column ozone at Ascension averaged 50 Dobson units (DU)) and forward trajectories from fire-rich and con vectively active areas show that the Atlantic and southern Africa are supplied with O-3 and O-3-forming trace gases by midlevel easterlies a nd/or recirculating air from Africa, with lesser contributions from So uth American burning and urban pollution. Limited sampling in the mixe d layer over Namibia shows possible biogenic sources of NO, High-level westerlies from Brazil (following deep convective transport of ozone precursors to the upper troposphere) dominate the upper tropospheric O -3 budget over Natal, Ascension, and Okaukuejo (Namibia), although mos t enhanced O-3 (75% or more) equatorward of 10 degrees S was from Afri ca. Deep convection may be responsible for the timing of the seasonal tropospheric O-3 maximum: Natal and Ascension show a 1- to 2-month lag relative to the period of maximum burning [cf. Baldy et al., this iss ue; Olson ct al., this issue]. Photochemical model calculations constr ained with TRACE A and SAFARI airborne observations of O-3 and O-3 pre cursors (NOx, CO, hydrocarbons) show robust ozone formation (up to 15 ppbv O-3/d or several DU/d) in a widespread, persistent, and well-mixe d layer to 4 km. Slower but still positive net O-3 formation took plac e throughout the tropical upper troposphere [cf. Pickering et al., thi s issue (a); Jacob et al., this issue]. Thus whether it is faster rate s of O-3 formation in source regions with higher turnover rates or slo wer O-3 production in long-lived stable layers ubiquitous in the TRACE A region, 10-30 DU tropospheric O-3 above a similar to 25-DU backgrou nd can be accounted for, In summary, the O-3 maximum studied in Octobe r 1992 was caused by a coincidence of abundant O-3 precursors from bio mass fires, a long residence time of stable air parcels over the easte rn Atlantic and southern Africa, and deep convective transport of biom ass burning products, with additional NO from lightning and occasional ly biogenic sources.