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

Abstract

Abstract. We analyze ozone observations recorded over Equatorial Africa between April 1997 and March 2003 by the MOZAIC programme, providing the first ozone climatology deriving from continental in-situ data over this region. Threedimensional streamlines strongly suggests connections between the characteristics of the ozone monthly mean vertical profiles, the most persistent circulation patterns in the troposphere over Equatorial Africa (on a monthly basis) such as the Harmattan, the African Easterly Jet, the Trades and the regions of ozone precursors emissions by biomass burning. During the biomass burning season in each hemisphere, the lower troposphere exhibits layers of enhanced ozone (i.e. 70 ppbv over the coast of Gulf of Guinea in December-February and 85 ppbv over Congo in June-August). The characteristics of the ozone monthly mean vertical profiles are clearly connected to the regional flow regime determined by seasonal dynamic forcing. The mean ozone profile over the coast of Gulf of Guinea in the burning season is characterized by systematically high ozone below 650 hPa ; these are due to the transport by the Harmattan and the AEJ of the pollutants originating from upwind fires. The confinement of high ozone to the lower troposphere is due to the high stability of the Harmattan and the blocking Saharan anticyclone which prevents efficient vertical mixing. In contrast, ozone enhancements observed over Central Africa during the local dry season (June-August) are not only found in the lower troposphere but throughout the troposphere. Moreover, this study highlights a connection between the regions of the coast of Gulf of Guinea and regions of Congo to the south that appears on a semi annual basis. Vertical profiles in wet-season regions exhibit ozone enhancements in the lower troposphere due to biomass burning products transport from fires situated in the opposite dry-season hemisphere.

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

“…The AEJ is maintained in association with two diabatically forced meridional circulations, one associated with deep convection (equatorward edge), the other to dry convection (poleward edge), in the ITCZ and Saharan heat-low region respectively (Thorncroft and Blackburn, 1999;Parker et al, 2005). An overview of the major flows can be seen in Sauvage et al (2005), their Fig. 1, at three different tropospheric levels.…”

“…For this purpose we adopt the approach of numerical modeling at the meso-α scale in association with on-line Lagrangian backtracking of air masses (backtrajectories). This model allows to resolve dynamical processes such as turbulence and convection, on the contrary of Sauvage et al (2005) back-trajectories calculations. The numerical set-up and Lagrangian method are described in Sect.…”

“…These processes could not be investigated in the climatological study of Sauvage et al (2005). First, the backtrajectories calculations were not including diffusive and convective transport.…”

“…According to Sauvage et al (2005), biomass burning is responsible for high lower-tropospheric O 3 mixing ratios in monthly mean during dry (December to February, DJF) and monsoon (June to August, JJA) seasons, over West and Central equatorial Africa. The persistent O 3 enhancements were measured with the MOZAIC airborne programme downwind of emissions over a 1997-2004 period, and were related with a Lagrangian study using ECMWF monthly analysis to transport inside Harmattan and AEJ flows during the dry season and inside trades flow during the monsoon season.…”

“…Therefore the goal of the present study is to investigate and to improve the understanding of the dynamical meso-scale processes which allow to inject air masses loaded with biomass burning emissions, into the upper part of the lower troposphere. In order to perform this work, we focus on two cases chosen in the MOZAIC database, as being representative of the O 3 enhancements history underlined in Sauvage et al (2005), in terms of both dynamics and chemical structure of the atmosphere (altitude location of the ozone enhancement and origin of the enhancement). The first case is representative of the ozone enhancement at Lagos during the dry season, connected with African northern hemisphere biomass burning through Harmattan and AEJ flow.…”