While the Sahelian belt in West Africa stretches in the border between the global hot-spots of mineral dust and biomass burning aerosols, the presence of West African Monsoon is expected to create significant vertical and temporal variations in the regional aerosol properties through transport and mixing of particles from various sources (mineral dust, biomass burning, sulfates, sea salt). In order to improve our understanding of the evolution of the aerosol-cloud system over such region across the onset of the summer monsoon, 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 the African Monsoon Multidisciplinary Analysis (AMMA) project. 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. The parameters for the mean log-normal distributions observed in respective layers characterized by the different wind regimes (monsoon layer, SAL, free troposphere) are presented, together with the major particle compositions found in the accumulation mode particles. Thereby, results of this study should facilitate radiative transfer calculations, validation of satellite remote sensors, and detailed transport modeling by partners within and outside the AMMA community. Extended analysis of the chemical composition of single aerosol particles by a transmission electron microscope (TEM) coupled to an energy dispersive X-ray spectrometer (EDX) 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 of 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.