Study of ozone formation and transatlantic transport from biomass burning emissions over West Africa during the airborne Tropospheric Ozone Campaigns TROPOZ I and TROPOZ II

Using the results obtained over the Ivory Coast during the airborne Tropospheric Ozone campaign (TROPOZ) I (December 1987) and over West Africa and South America during the meridional airborne campaign TROPOZ II (January 1991), a study is made of biomass burning emissions in the northern African tropics, a region which is much less well documented than that of southern Africa. The analysis of data for O-3 and its precursors (CO, CH4, NO, and NOy) confirms the existence of important ozone formations over northern Africa during the Boreal winter with photochemical O-3 formation rates of 15-35 ppbv O-3 d(-1). The distributions obtained over the Ivory Coast show the presence of different layers associated with the Inter Tropical Convergence Zone (ITCZ), with an ozone- and precursor-laden Harmattan layer embedded between the southerly depleted monsoon layer below and the free troposphere easterly circulation above. In front of the ITCZ discontinuity, air parcels of different ages are identified in biomass burning plumes resulting from the transfer to altitude of compounds emitted by fires followed by ascending motions of this polluted air above the monsoon layer. Using a chemical box model, the chemical evolution of concentrations in these air parcels is simulated, confirming the explanations already proposed from the study of the dynamic context and history of the air parcels along their trajectories. The ozone measurements performed during TROPOZ II on both sides of the central Atlantic (Brazil, West Africa) support the hypothesis that because of the prevailing meteorology, smoke-laden air masses emanating from the burning regions in northern Africa can enter a meteorological regime which permits them to be transported out over the Atlantic where they can influence budgets over the south Atlantic Basin during the Austral summer.