Tropospheric ozone over Equatorial Africa: regional aspects from the MOZAIC data

Abstract. We analyze MOZAIC ozone observations recorded over Equatorial Africa, from April 1997 to March 2003 to give the first ozone climatology of this region. The monthly mean vertical profiles have been systematically analyzed with monthly mean ECMWF data using a Lagrangian-model (LAGRANTO). We assess the roles played by the dynamical features of Equatorial Africa and the intense biomass burning sources within the region in defining the ozone distribution. The lower troposphere exhibits layers of enhanced ozone during the biomass burning season in each hemisphere (boreal winter in the northern tropics and boreal summer in the southern tropics). The monthly mean vertical profiles of ozone are clearly influenced by the local dynamical situation. Over the Gulf of Guinea during boreal winter, the ozone profile is characterized by systematically high ozone below 650 hPa. This is due to the high stability caused by the Harmattan winds in the lower troposphere and the blocking Saharan anticyclone in the middle troposphere that prevents any efficient vertical mixing. In contrast, Central African enhancements are not only found in the lower troposphere but throughout the troposphere. The boreal summer ozone maximum in the lower troposphere of Central Africa continues up to November in the middle troposphere due to the influx of air masses laden with biomass burning products from Brazil and Southern Africa. Despite its southern latitude, Central Africa during the boreal winter is also under the influence of the northern tropical fires. This phenomenon is known as the "ozone paradox". However, the tropospheric ozone columns calculated from the MOZAIC data give evidence that the Tropical Tropospheric Ozone Column (TTOC) maximum over Africa swings from West Africa in DJF to Central Africa in JJA. This contrasts with studies based on TOMS satellite data. A rough assessment of the regional ozone budget shows that the northern tropics fires in boreal winter might contribute up to 20% of the global photochemical ozone production. This study gives the first detailed picture of the ozone distribution over Equatorial Africa that should be used to validate both global models over this region and future satellite products.