D. Scheuvens

Electron microscopy of particles collected at Praia, Cape Verde, during the Saharan Mineral Dust Experiment: particle chemistry, shape, mixing state and complex refractive index

A large field experiment of the Saharan Mineral Dust Experiment (SAMUM) was performed in Praia, Cape Verde, in January and February 2008. The aerosol at Praia is a superposition of mineral dust, sea-salt, sulphates and soot. Particles smaller than 500 nm are mainly mineral dust, mineral dust–sulphate mixtures, sulphates and soot–sulphate mixtures. Particles larger then 2.5μm consist of mineral dust, sea-salt and few mineral dust–sulphate mixtures. A transition range exists in between. The major internal mixtures are mineral dust–sulphate and soot–sulphate. Mineral dust–sea-salt mixtures occur occasionally, mineral dust–soot mixtures were not observed. The aspect ratio was 1.3–1.4 for dry particles smaller than 500 nm and 1.6–1.7 for larger ones. Parameterizations are given for dry and humid state. Although the real part of the refractive index showed low variation (1.55–1.58 at 532 nm), a multi-modal imaginary part was detected as function of particle size, reflecting the complex composition. Soot mainly influences the absorption for wavelengths longer than the haematite absorption edge, whereas for shorter wavelengths dust is dominating. The refractive index of the aerosol depends on the source region of the mineral dust and on the presence/absence of a marine component.

Ground-based off-line aerosol measurements at Praia, Cape Verde, during the Saharan Mineral Dust Experiment: microphysical properties and mineralogy

A large field experiment of the Saharan Mineral Dust Experiment (SAMUM) was performed in Praia, Cape Verde, in January and February 2008. This work reports on the aerosol mass concentrations, size distributions and mineralogical composition of the aerosol arriving at Praia. Three dust periods were recorded during the measurements, divided by transitional periods and embedded in maritime-influenced situations. The total suspended particle mass/PM10/PM2.5 were 250/180/74μg/m3 on average for the first dust period (17–21 January) and 250/230/83μg/m3 for the second (24–26 January). The third period (28 January to 2 February) was the most intensive with 410/340/130 μg/m3. Four modes were identified in the size distribution. The first mode (50–70 nm) and partly the second (700–1100 nm) can be regarded as of marine origin, but some dust contributes to the latter. The third mode (2–4 μm) is dominated by advected dust, while the intermittently occurring fourth mode (15–70 μm) may have a local contribution. The dust consisted of kaolinite (dust/maritime period: 35%wt./25%wt.),K-feldspar (20%wt./25%wt.), illite (14%wt./10%wt.), quartz (11%wt./8%wt.), smectites (6%wt./4%wt.), plagioclase (6%wt./1%wt.), gypsum (4%wt./7%wt.), halite (2%wt./17%wt.) and calcite (2%wt./3%wt.).

Particle chemical properties in the vertical column based on aircraft observations in the vicinity of Cape Verde Islands

During the second Saharan Mineral Dust Experiment (SAMUM-2) field campaign, particles with geometric diameters (d) between ∼0.1 and 25 μm were collected on board of the Deutsches Zentrum f¨ur Luft- und Raumfahrt (German Aerospace Center, DLR) Falcon aircraft. Size, chemical composition and mixing state of aerosols sampled (spatially and vertically resolved) along theWest African coastline and in the Cape Verde Islands region were determined by electron microscopy. A pronounced layer structure of biomass-burning aerosol and desert dust was present for all days during the sampling period from 23 January to 6 February. The aerosol composition of the small particles (d < 0.5 μm) was highly variable and in cases of biomass burning strongly dominated by soot with up to 90% relative number abundance. Internal mixtures of soot particles with mineral dust were not detected. Soot was only observed to mix with secondary sulphate. The coarse particles (d > 0.5 μm) were dominated by silicates. In the Cape Verde Islands region mineral dust is well mixed. The determination of source regions by elemental or mineralogical composition was generally not possible, except for air masses which were transported over the Gulf of Guinea. The real part of the refractive index showed little variation. In contrast, the imaginary part strongly depended on the abundance of soot (biomass-burning aerosol) and haematite (mineral dust).

Individual‐particle analysis of airborne dust samples collected over Morocco in 2006 during SAMUM 1

During the course of SAMUM 1 in May and June 2006, airborne samples were collected in southern Morocco at altitudes between 830 and 3340 m above ground. We analysed approximately 22 500 particles of 1–30 μm by automated individual-particle analysis with a scanning electron microscope (SEM) and an energy-dispersive X-ray (EDX) analyser. The major difference between samples is due to the presence and amount of chlorine- and sulphurbearing particles, pointing to a varying maritime influence and to different degrees of aging. In contrast, the desert dust component is very homogeneous independent of altitude, activated local source area, and dust storm intensity, implying very fast mixing of the entrained dust with a persistent regional background aerosol. Nevertheless, our study reinforces the usefulness of carbonates and palygorskite as ‘compositional fingerprints’ for mineral dust from source areas in northwestern Africa. The average median aspect ratio of most particle groups is in the range between 1.5 and 1.6. Higher values are found for S- and Cl-dominated particles (except sodium chloride), and internal mixtures of alumosilicates with carbonates or sulphates. Taken together with other studies, a tendency of increasing aspect ratios with longer transport distances can be observed for Saharan dust.

On Composition, Morphology, and Size Distribution of Airborne Mineral Dust

Mineral dust is a key player in the Earth system. The uplift, transport, and deposition of mineral dust have large impacts on different Earth’s compartments such as the deserts, oceans, rain forests, and, last but not least, the atmosphere. The mineral dust cycle influences the fertility of soils and the biological activity in oceans and may cause harm to flora and fauna including human beings. Moreover, it changes the climate system directly by absorption and scattering of radiation and indirectly by influencing the life cycle of clouds.