Abstract
Abstract. In order to gain insights into the characteristics of the mineral dust particles incorporated in the actual cloud droplets and the related cloud processing, 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°05' E) in August 2006 within the framework of the African Monsoon Multidisciplinary Analysis (AMMA) project. Cloud residual and clear-sky particles were collected separately and analyzed individually using a transmission electron microscope (TEM) and a scanning electron microscope coupled with an energy dispersive X-ray spectroscopy (SEM-EDX). The analysis revealed interesting characteristics on the coarse dust particles (Dp>1μm), particularly those which likely had acted as CCN. Traces of heterogeneously formed secondary sulfate, chloride and nitrate were found on many dust particles (though fraction of sulfate may be present in the form of gypsum as primary dust component). These secondary species were particularly enhanced in clouds (i.e. cloud processing). The study illustrates that calcium-rich particles assumed to be carbonates (Calcite, Dolomite) contained the secondary species in significantly larger frequency and amount than the silicates (Quartz, Feldspar, Mica, Clay), suggesting that they represent the most reactive fraction of the mineral dust. A surprisingly large fraction of the Ca-rich particles were already found in deliquesced form even in clear-sky conditions, most probably reflecting their extreme hygroscopicity, resulting from their reaction with HNO3 gas. Both silicate and Ca-rich particles were found dominant among the supermicron cloud residues, and they were supposed to be those previously activated as CCN. It is highly probable that the observed formation of soluble materials enhanced their cloud nucleating abilities.
Highlights
Mineral dust emitted into the atmosphere by the surfaces of the continental arid and semi-arid regions comprises an important fraction of the atmospheric aerosol
Presence of a silicate dust particle can be identified by the typical irregular shape and the dominant X-ray peaks corresponding to Al and Si (Fig. 2a)
More than 90% of such coarse particles exceeding 1μm in diameter composed of the following 3 major types, namely, silicate dust particle (Fig. 2a), sea salt (Fig. 2b), and spherical particles enriched in Ca (Fig. 2c)
Summary
Mineral dust emitted into the atmosphere by the surfaces of the continental arid and semi-arid regions comprises an important fraction of the atmospheric aerosol. The mineral composition of individual dust particles is quite diverse already at the initial entrainment into the atmosphere It is dependent on the soil type and the geological situation in the source regions, of which the soils derive. Many of the previous works were based mainly on the analysis of aerosols under cloud-free conditions They provided valuable information regarding the extent of dust modification in the atmosphere. Tropical Africa is one of the world’s largest sources of atmospheric dust and biomass-burning aerosols. Both particles play a major role in radiative forcing and in cloud microphysics, and are an important part of the WAM system which requires further study. An overview of the project can be found elsewhere (Redelsperger et al, 2006)
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