Temporal and vertical variations of aerosol physical and chemical properties over West Africa: AMMA aircraft campaign in summer 2006
While the Sahelian belt in West Africa stretches in the border between\nthe global hot-spots of mineral dust and biomass burning aerosols, the\npresence of West African Monsoon is expected to create significant\nvertical and temporal variations in the regional aerosol properties\nthrough transport and mixing of particles from various sources (mineral\ndust, biomass burning, sulfates, sea salt). In order to improve our\nunderstanding of the evolution of the aerosol-cloud system over such\nregion across the onset of the summer monsoon, the French ATR-42\nresearch aircraft was deployed in Niamey, Niger (13A degrees 30' N, 02A\ndegrees 05' E) in summer 2006, during the three special observation\nperiods (SOPs) of the African Monsoon Multidisciplinary Analysis (AMMA)\nproject. These three SOPs covered both dry and wet periods before and\nafter the onset of the Western African Monsoon.\nState of the art physico-chemical aerosol measurements on the ATR-42\nshowed a notable seasonal transition in averaged number size\ndistributions where (i) the Aitken mode is dominating over the\naccumulation mode during the dry season preceding the monsoon arrival\nand (ii) the accumulation mode increasingly gained importance after the\nonset of the West African monsoon and even dominated the Aitken mode\nafter the monsoon had fully developed. The parameters for the mean\nlog-normal distributions observed in respective layers characterized by\nthe different wind regimes (monsoon layer, SAL, free troposphere) are\npresented, together with the major particle compositions found in the\naccumulation mode particles. Thereby, results of this study should\nfacilitate radiative transfer calculations, validation of satellite\nremote sensors, and detailed transport modeling by partners within and\noutside the AMMA community.\nExtended analysis of the chemical composition of single aerosol\nparticles by a transmission electron microscope (TEM) coupled to an\nenergy dispersive X-ray spectrometer (EDX) revealed dominance of mineral\ndust (aluminosilicate) even in the submicron particle size range during\nthe dry period, gradually replaced by prevailing biomass burning and\nsulfate particles, after the onset of the monsoon period. The spatial\nand temporal evolution from SOP1 to SOP2a1 and SOP2a2 of the particle\nphysical and chemical properties and associated aerosol hygroscopic\nproperties are remarkably consistent.