1Université Blaise Pascal, Clermont Université, Clermont-Ferrand, France
2Laboratoire de Météorologie Physique, CNRS, Clermont-Ferrand, France
3Centre National de Recherches Météorologiques, Météo-France, Toulouse, France
4Frontier Science Organization, Kanazawa University, Japan
5Laboratoire de Glaciologie et Géophysique de l'Environnement, CNRS, Grenoble, France
Abstract. While West Africa is recognized as being one of the global hot-spots of atmospheric aerosols, the presence of West African Monsoon is expected to create significant spatial and temporal variations in the regional aerosol properties through mixing particles from various sources (mineral dust, biomass burning, sulfates, sea salt). To improve our understanding of the complexity of the aerosol-cloud system in that region, the African Monsoon Multidisciplinary Analysis (AMMA) project has been launched, providing valuable data sets of in-situ and remote sensing measurements including satellites for extended modeling.
The French ATR-42 research aircraft was deployed in Niamey, Niger (13°30' N, 02°05' E) in summer 2006, during the three special observation periods (SOPs) of AMMA. These three SOPs covered both dry and wet periods before and after the onset of the Western African Monsoon.
State of the art physico-chemical aerosol measurements on the ATR-42 showed a notable seasonal transition in averaged number size distributions where (i) the Aitken mode is dominating over the accumulation mode during the dry season preceding the monsoon arrival and (ii) the accumulation mode increasingly gained importance after the onset of the West African monsoon and even dominated the Aitken mode after the monsoon had fully developed. An extended analysis of the vertical dependence of size spectra, comparing the three observation periods, revealed that the decreasing concentration of the Aitken mode particles, as we move from SOP1 (June) to SOP2a1 (July), and SOP2a2 (August), was less pronounced in the monsoon layer as compared to the overlying Saharan dust layer and free troposphere.
In order to facilitate to all partners within the AMMA community radiative transfer calculations, validation of satellite remote sensors, and detailed transport modeling, the parameters describing the mean log-normally fitted number size distributions as a function of altitude and special observation periods were summarized and subsequently related to simultaneously performed measurements of major aerosol particle chemical composition. Extended TEM-EDX analysis of the chemical composition of single aerosol particles revealed dominance of mineral dust (aluminosilicate) even in the submicron particle size range during the dry period, gradually replaced by prevailing biomass burning and sulfate particles, after the onset the monsoon period. The spatial and temporal evolution from SOP1 to SOP2a1 and SOP2a2 of the particle physical and chemical properties and associated aerosol hygroscopic properties are remarkably consistent.