1Laboratoire de Météorologie Physique, Université Blaise Pascal, Clermont-Ferrand, France
2Centre National de Recherches Météorologiques, Météo-France, Toulouse, France
3Frontier Science Organization, Kanazawa University, Japan
Abstract. In order to gain insights into the characteristics of the mineral dust fraction which actually serves as cloud condensation nuclei (CCN) including the related cloud processing, this study proceeded to directly collect CCN and compare their mixing states with that of the clear-sky aerosol particles. To pursue this goal, the French ATR-42 research aircraft equipped both with a counterflow virtual impactor (CVI) and community aerosol inlet was deployed in Niamey, Niger (13°30´ N, 02°30´ E) in August 2006 during one of the special observation periods (SOP) of the African Monsoon Multidisciplinary Analysis (AMMA) project.
Both cloud residual and clear-sky particles were collected separately and later analyzed individually using transmission electron microscope (TEM) and scanning electron microscope coupled with energy dispersive X-ray spectroscopy (SEM-EDX). The analysis revealed interesting characteristics on the coarse dust particles (Dp>1 μm), particularly those which likely had acted as CCN.
Traces of heterogeneously formed secondary sulfate, chloride and nitrate were found on many dust particles. These secondary species were particularly enhanced in clouds (i.e. cloud processing). The study illustrates that carbonates (Calcite, Dolomite) contained the secondary species in significantly larger frequency and amount than the silicates (Quartz, Feldspar, Mica, Clay), confirming that carbonates represent the most reactive fraction of the mineral dust. Surprisingly large fraction of the carbonate particles were already found in deliquesced form even in clear-sky conditions, most probably reflecting their extreme hygroscopicity following the reaction with HNO3 gas. There were also some indications that the large carbonate particles may be acting primarily as CCN under very low supersaturations, unless there is sufficient hygroscopic coatings on the silicates particles.