Chemical and aerosol characterisation of the troposphere over West Africa during the monsoon period as part of AMMA
1School of Environmental Sciences, University of East Anglia, Norwich, UK
2Laboratoire Interuniversitaire des Systèmes Atmosphériques (LISA), Universités Paris 12 et Paris 7, CNRS, Créteil, France
3Department fo Chemistry, Faculty of Sciences, Saint Joseph University, Beirut, Lebanon
4LATMOS, Université Paris VI, Université Versailles-St-Quentin, CNRS, Paris, France
5Laboratoire d'Aérologie, Université de Toulouse, CNRS, UMR, Toulouse, France
6Istituto di Scienze dell'Atmosfera e del Clima, Consiglio Nazionale delle Ricerche, Italy
7School of Earth, Atmospheric & Environmental Sciences, University of Manchester, Manchester, UK
8Centre National de Recherches Meteorologiques, Meteo-France, Toulouse, France
9Deutsches Zentrum für Luft- und Raumfahrt (DLR), Institut für Physik der Atmosphäre, Oberpfaffenhofen Wessling, Germany
10Laboratoire de Météorologie Physique, Université Blaise Pascal, Clermont-Ferrand, France
11Frontier Science Organization, Kanazawa University, Japan
12Department of Meteorlogy, University of Reading, Reading, UK
13Danish Meteorological Institute, Research and Development Division, Copenhagen, Denmark
14School of Earth and Environment, University of Leeds, Leeds, UK
15Institute of Environmental Physics, University of Bremen, Bremen, Germany
16Laboratoire de Météorologie Physique, Aubière, France
*now at: University of Toronto, Canada
Abstract. During June, July and August 2006 five aircraft took part in a campaign over West Africa to observe the aerosol content and chemical composition of the troposphere and lower stratosphere as part of the African Monsoon Multidisciplinary Analysis (AMMA) project. These are the first such measurements in this region during the monsoon period. In addition to providing an overview of the tropospheric composition, this paper provides a description of the measurement strategy (flights performed, instrumental payloads, wing-tip to wing-tip comparisons) and points to some of the important findings discussed in more detailed in other papers in this special issue.
The ozone data exhibits an "S" shaped vertical profile which appears to result from significant losses in the lower troposphere due to rapid deposition to forested areas and photochemical destruction in the moist monsoon air, and convective uplift of O3-poor air to the upper troposphere. This profile is disturbed, particularly in the south of the region, by the intrusions in the lower and middle troposphere of air from the Southern Hemisphere impacted by biomass burning. Comparisons with longer term data sets suggest the impact of these intrusions on West Africa in 2006 was greater than in other recent wet seasons. There is evidence for net photochemical production of ozone in these biomass burning plumes as well as in urban plumes, in particular that from Lagos, convective outflow in the upper troposphere and in boundary layer air affected by nitrogen oxide emissions from recently wetted soils. This latter effect, along with enhanced deposition to the forested areas, contributes to a latitudinal gradient of ozone in the lower troposphere. Biogenic volatile organic compounds are also important in defining the composition both for the boundary layer and upper tropospheric convective outflow.
Mineral dust was found to be the most abundant and ubiquitous aerosol type in the atmosphere over Western Africa. Data collected within AMMA indicate that injection of dust to altitudes favourable for long-range transport (i.e. in the upper Sahelian planetary boundary layer) can occur behind the leading edge of mesoscale convective system (MCS) cold-pools. Research within AMMA also provides the first estimates of secondary organic aerosols (SOA) across the West African Sahel and have shown that organic mass loadings vary between 0 and 2 μg m−3 with a median concentration of 1.07 μg m−3. The vertical distribution of nucleation mode particle concentrations reveals that significant and fairly strong particle formation events did occur for a considerable fraction of measurement time above 8 km (and only there). Very low aerosol concentrations were observed in general in the fresh outflow of active MCSs, likely as the result of efficient wet removal of aerosol particles due to heavy precipitation inside the convective cells of the MCSs. This wet removal initially affects all particle size ranges as clearly shown by all measurements in the vicinity of MCSs.