Journal article

Temporal and vertical variations of aerosol physical and chemical properties over West Africa: AMMA aircraft campaign in summer 2006

Matsuki A, Quennehen B, Schwarzenboeck A, Crumeyrolle S, Venzac H, Laj P, Gomes L ...see all

Atmospheric Chemistry and Physics, vol. 10, issue 17 (2010) pp. 8437-8451

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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 (13A degrees 30' N, 02A
degrees 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.

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