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A meso-scale model was used to understand and describe the dynamical processesdriving high ozone concentrations observed during both dry and monsoon season in monthlyclimatologies profiles over Lagos (Nigeria, 6.6° N, 3.3° E), obtained withthe MOZAIC airborne measurements (ozone and carbon monoxide). This study focuseson ozone enhancements observed in the upper-part of the lowertroposphere, around 3000 m.Two individual cases have been selected in the MOZAIC datasetas being representative of the climatological ozone enhancements, to be simulatedand analyzed with on-line Lagrangian backtracking of air masses.  This study points out the role of baroclinic low-level circulations presentin the Inter Tropical Front (ITF) area. Two low-level thermal cells around azonal axis and below 2000 m,  in mirror symmetry to each other with respect to equator,form near 20° E and around 5° N and 5° Sduring the (northern hemisphere) dry and wet seasons respectively. They are causedby surface gradients – the warm dry surface being located polewardof the ITF and the cooler wet surface equatorward of the ITF.  A convergence line exists between the poleward low-level branch ofeach thermal cell and the equatorward low-level branch of the Hadley cell.Our main conclusion is to point out this line as a preferred locationfor fire products – among them ozone precursors – to be uplifted andinjected into the lower free troposphere.  The free tropospheric transport that occurs then depends on the hemisphereand season. In the NH dry season, the AEJ allows transport of ozone andprecursors westward to Lagos. In the NH monsoon (wet) season, fire productsare transported from the southern hemisphere to Lagos by the southeasterlytrade that surmounts the monsoon layer. Additionally ozone precursorsuplifted by wet convection in the ITCZ can also mix to the ones uplifted bythe baroclinic cell and be advected up to Lagos by the trade flow

Medium-range mid-tropospheric transport of ozone and precursors over Africa: two numerical case studies in dry and wet seasons