Abstract
Abstract. The African Monsoon Multidisciplinary Analyses (AMMA) fourth airborne campaign was conducted in July–August 2006 to study the chemical composition of the middle and upper troposphere in West Africa with the major objective to better understand the processing of chemical emissions by the West African Monsoon (WAM) and its associated regional-scale and vertical transports. In particular, the french airborne experiment was organized around two goals. The first was to characterize the impact of Mesoscale Convective Systems (MCSs) on the ozone budget in the upper troposphere and the evolution of the chemical composition of these convective plumes as they move westward toward the Atlantic Ocean. The second objective was to discriminate the impact of remote sources of pollution over West Africa, including transport from the middle east, Europe, Asia and from southern hemispheric fires. Observations of O3, CO, NOx, H2O and hydroperoxide above West Africa along repeated meridional transects were coupled with transport analysis based on the FLEXPART lagrangian model. The cross analysis of trace gas concentrations and transport pathways revealed 5 types of air masses: convective uplift of industrial and urban emissions, convective uplift of biogenic emissions, slow advection from Cotonou polluted plumes near the coast, meridional transport of upper tropospheric air from the subtropical barrier region, and meridional transport of Southern Hemisphere (SH) biomass burning emissions. O3/CO correlation plots and the correlation plots of H2O2 with a OH proxy revealed not only a control of the trace gas variability by transport processes but also significant photochemical reactivity in the mid- and upper troposphere. The study of four MCSs outflow showed contrasted chemical composition and air mass origins depending on the MCSs lifetime and latitudinal position. Favorables conditions for ozone production were found for MCSs with increased MCS lifetime (>1.5 days), which allowed for more H2O2 formation, and with trajectories crossing the 10° N latitude, which increased CO transport to the upper troposphere. The upper tropospheric concentrations sampled in the MCS outflow regions showed mixed origins including local vertical convective transport, and uplifting of air from the low troposphere over the middle-east related to the summer Asian low pressure system or from the southern hemispheric fires.
Highlights
The West African Monsoon is characterized by the migration of zonally banded rainfall from the Guinea coast to the Sahel
The clusters are plotted every 24 h, the color corresponds to their altitude, and the size of each cluster is proportional to the number of particles
Considering type I air masses, this study shows that low H2O2 concentrations (
Summary
The West African Monsoon (hereafter WAM) is characterized by the migration of zonally banded rainfall from the Guinea coast to the Sahel. Based on the airborne MOZAIC program, Sauvage et al (2005) have showed the possible impact of southern hemispheric fires on the ozone observations north of the equator, near the guinean coast during the boreal summer. This hypothesis was further confirmed by Mari et al (2008) with an extensive study of fire tracers during the July–August 2006 period. We attempt to analyze the aircraft data in the light of the WAM main dynamical features to (1) provide the horizontal variability of trace gas species in the outflow region of MCSs and (2) document the regional scale distribution of trace gas species, mainly across the vegetation gradient over Benin and southwestern Niger and through the transition zone between the continent and the Gulf of Guinea
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