Saharan Dust Plume Research Articles

Dual‐wavelength Raman lidar observations of the extinction‐to‐backscatter ratio of Saharan dust

For the first time, height profiles of the extinction‐to‐backscatter ratio (lidar ratio) of desert dust particles were simultaneously measured at 355 and 532 nm. The observations were performed with an advanced Raman lidar during two long‐range Saharan dust outbreaks at Leipzig, Germany (51.3°N, 12.4°E), in August and October 2001. Measured desert‐dust lidar ratios are needed for a proper profiling of the climate‐relevant volume extinction coefficient of the dust particles with widely used standard backscatter lidars. Unexpectedly large lidar ratios, mainly between 50 and 80 sr, were found in the Saharan dust plumes. The lidar ratios at 355 nm were, on average, higher by 10%–30% than the ones at 532 nm, probably due to enhanced light absorption in the UV. The large lidar ratios can be explained by model calculations, available in the literature for 532 nm, which focus on the deviations between the scattering characteristics of spheres and spheroids.

Validation of the Saharan Dust Plume Conceptual Model Using Lidar, Meteosat, and ECMWF Data

Lidar observations collected during the Lidar In-space Technology Experiment experiment in conjunction with the Meteosat and European Centre for Medium-Range Weather Forecasts data have been used not only to validate the Saharan dust plume conceptual model constructed from the GARP (Global Atmospheric Research Programme) Atlantic Tropical Experiment data, but also to examine the vicissitudes of the Saharan aerosol including their optical depths across the west Africa and east Atlantic regions. Optical depths were evaluated from both the Meteosat and lidar data. Back trajectory calculations were also made along selected lidar orbits to verify the characteristic anticyclonic rotation of the dust plume over the eastern Atlantic as well as to trace the origin of a dust outbreak over West Africa. A detailed synoptic analysis including the satellite-derived optical depths, vertical lidar backscattering cross section profiles, and back trajectories of the 16–19 September 1994 Saharan dust outbreak over the easte...

The geochemistry of Cu, Cd, Zn, Ni and Pb in sediment cores from the continental slope of the Banc d’Arguin (Mauritania)

Trace and major elements were measured in sediment cores collected at the shelf and continental slope of the Banc d’Arguin (Mauritania). Most of the sediments have a calcium carbonate content >50% and as a consequence have a low aluminium (<1.5%) and iron (<1.0%) content. The concentrations of the latter two elements increase down slope. Based on the major elemental composition, the sediments seem to be well mixed and concentration gradients are only observed on a spatial distribution. Concentrations of Cu, Cd, Zn, Ni and Pb are low and approximately equal to “natural background” values, indicating negligible anthropogenic influences. An exception is Cd for which very high values were found in the northern most part of the research area, close to Nouadhibou. A possible explanation for this high Cd content can be the existence of phosphate rich sediments with a high affinity for Cd. Small increases in the available trace metal concentrations (exchangeable fraction, 0.1 N HCl treated) observed near the sediment surface could be attributed to early diagenetic processes, and not to pollution effects. The inputs from the Saharan dust plume with low trace metal contents, the transport of small particles with higher trace metal content down slope together with the escape of redissolved trace metals from the sediment, and the absence of a supply of metal rich material to the Banc d’Arguin, has created a metal poor environment. There is clear evidence of trace metal redistributions, associated with Mn recycling. Mean enrichment factors, calculated relative to the crustal abundance, show that Zn, Pb and Cd are equal to unity and Cu and Ni below unity. These observations suggest that the Cu and Ni contents in sediments of Banc d’Arguin are depleted relative to the accepted values in the earth crust. The very low trace metal concentrations detected in these sediments can be taken as “baseline” values for unpolluted sediments in the global ocean coastal zone.

Particulate trace element fluxes in the deep northeast Atlantic Ocean

Particulate fluxes of trace elements (Al, Cd, Co, Cu, Fe, Mn, Ni, P, Ti, V and Zn) in the northeast Atlantic Ocean (three positions at latitudes from 33°N to 54°N along ∼20°W) were measured using time-series sediment traps between March 1992 and September 1994. Significant variabilities of fluxes with season and depth (1000 m to maximum of 4000 m) were observed only for ‘biogenic elements’, such as Cd, Ni, Zn or P. On the other hand, we found a distinct large-scale increase of fluxes into the deep-sea traps to the south for Al, Co, Fe, Mn and V. We attribute this latitudinal gradient to the increasing influence of the Saharan dust plume. The biogenic flux decreased towards the south. This trend was clearly visible for Cd and P only. The fluxes of other ‘nutrient-like’ elements, such as Ni or Zn, exhibited a general decrease between 53°N and 33°N. We compared our sedimentation flux data with published data from the western North Atlantic basins. For this purpose we corrected the deep-sea fluxes of Cu, Mn, Ni and Zn for their lithogenic fractions on the basis of Al, with average crustal material and granitic rocks as references. The comparison indicates that these ‘excess’ fluxes are a factor of at least 2 higher in the western basins for the selected elements. Estimated fluxes are in good agreement with reported atmospheric deposition in the two areas. The noted imbalance between the non-lithogenic atmospheric input of Mn and the determined ‘excess flux’ in the deep northeast Atlantic indicates an additional input in the form of a lateral flux of dissolved Mn(II) species and scavenging onto sinking particles. With respect to the mechanism of sedimentation, a unique behaviour is noticed for the refractory elements Co, Fe, Mn, Ti and V, which were found to correlate with the vertical transport of Al (clay). The ‘excess’ fluxes of Cu, Ni and Zn are linearly related to C org, whereas the overall relation of Cd to P fluxes exhibits a molar Cd/P ratio of 2.0×10 -4, which is close to the ratio in the dissolved fractions in the northeast Atlantic.

Infrared radiative forcing of a Saharan dust plume in the North Atlantic

Infrared radiative forcing of a Saharan dust plume in the North Atlantic

Sources of atmospheric trace metals over the subtropical North Atlantic

Air filter samples were taken over the subtropical North Atlantic under the temporary influence of a Saharan dust plume. Analyses for trace metals showed that most of the determined elements (Al, Sc, Cr, Fe, Co, Zn, Cs, La, Ce, Sm, and Th) were predominantly of crustal origin. Additionally, for Mn and Se a significant marine contribution could be detected.

Distribution of aluminium in waters of the North East Atlantic 25°N to 35°N

Dissolved Al concentrations from the NE Atlantic, between 25°N and 35°N off the coast of NW Africa, range from 16 nM to 32 nM in a well-mixed surface layer extending to 150 m depth, increasing towards the maximum of the Saharan dust plume. Mediterranean outflow water is detected as a high Al anomaly corresponding to the salinity signal of the Mediterranean water. Other than in surface and Mediterranean waters Al concentrations are invariant with depth, but increase from 6 nM to 11 nM towards the continental margin. The magnitude of the dissolved Al signal in the surface layer is compatible with dissolution of Al from the highest of present estimates of dust input to this area. Comparison with data from other areas indicates that Al cannot be cycled through the oceans in parallel to silica as has been previously suggested.

The Large-Scale Movement of Saharan Air Outbreaks over the Northern Equatorial Atlantic

The intense and prolonged heating of air passing over the Sahara during the summer and early fall months forms a deep mixed layer which extends up to 15–20,000 ft during July, the warmest month. The dust-laden heated air emerges from West Africa as a series of large-scale anticyclonic eddies which move westward over the tropical Atlantic above the trade-wind moist layer, principally in the layer between 5000 and 15,000 ft (600–800 mb). Measurements made during BOMEX show that this Saharan air is characterized by high values of potential temperature, dust and radon-222 which confirm a desert origin. As the parcels of air within the layer proceed across the Atlantic the continuous fallout of particulate matter and the mixing at the base of the layer cause dust to be transferred to the lower levels where its concentration may become sufficiently great to produce dense haze at the surface over wide areas over the Atlantic and Caribbean in the latitude belt 10–25N. Nevertheless, measurements indicate that the dust concentration and associated haziness are greater at 10,000 ft than at the surface. The presence of Saharan air over the Caribbean can be recognized on conventional meteorological soundings as a virtually isentropic layer within which the potential temperature is about 40C; the mixing ratio within this layer generally remains fairly constant with height with typical mean values of 2–4 gm kg−1. The upper surface of the Saharan air layer, clearly visible from above as a sharply defined haze top, coincides with an inversion, above which the mixing ratio decreases rapidly with height. The lower portion of the isentropic Saharan air layer may be as much as 5–6C warmer than the normal tropical atmosphere; consequently, there is a strong suppressive inversion above the moist trade-wind layer. There is also a sharp horizontal temperature gradient between the Saharan dust plume and the normal tropical air mass; aircraft penetrations into Saharan air at heights of 700–800 mb show that the discontinuity between Saharan and non-Saharan air is front-like in character inasmuch as the potential temperature and mixing ratio may change by several degrees Celsius and several grams per kilogram, respectively, over a distance of just a few kilometers. Because of the steep (adiabatic) lapse rate in the Saharan air, the positive temperature anomaly diminishes rapidly with height and tends to vanish near 650 mb, above which the dusty air may be slightly cooler than the normal tropical environment. Associated with this large-scale temperature contrast is a wind maximum of up to 40–50 kt in the Saharan air layer, usually between 600 and 700 mb. The westward speed of the Saharan air mass is usually about 15 kt, requiring about 5–6 days to cross the Atlantic. The leading edge of the Saharan air is often found immediately to the rear (east) of a large-amplitude African disturbance which also migrates from Africa to the Caribbean during the summer months at about the same forward speed. Normally the strongest winds and highest dust concentrations in the Saharan air are found in the southeasterly winds behind the disturbance and are therefore associated with the so-called “surges in the trades” which are often observed in the tropical Atlantic. Saharan air pulses tend to leave the continent of Africa with a potential temperature of 43–44C, about 3–4C higher than that found in the Saharan layer over the Caribbean. This apparent cooling is due to net radiation losses within the Saharan air which amount to about 0.7C per day. At the same time the Saharan air sinks by 50–100 mb (a mean descending motion of 1–2 mm sec−1) between Africa and the Caribbean. A model is proposed which depicts the movement of Saharan air from Africa to the Caribbean and its interaction with African disturbances. Although it is not known what effect, if any, the dust plume has upon the growth or suppression of disturbances, it is clear that the warmth of the Saharan air has a strong suppressive influence on cumulus convection and that, as a result, the advancing dust pulse is often marked by rapid clearing behind the disturbances.