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West Africa and the adjacent oceanic regions are very important locations for studying dust properties and their influence on weather and climate. The SHADOW (study of SaHAran Dust Over West Africa) campaign is performing a multiscale and multilaboratory study of aerosol properties and dynamics using a set of in situ and remote sensing instruments at an observation site located at the IRD (Institute for Research and Development) in Mbour, Senegal (14° N, 17° W). In this paper, we present the results of lidar measurements performed during the first phase of SHADOW (study of SaHAran Dust Over West Africa) which occurred in March–April 2015. The multiwavelength Mie–Raman lidar acquired 3β + 2α + 1δ measurements during this period. This set of measurements has permitted particle-intensive properties, such as extinction and backscattering Ångström exponents (BAE) for 355/532 nm wavelengths' corresponding lidar ratios and depolarization ratio at 532 nm, to be determined. The mean values of dust lidar ratios during the observation period were about 53 sr at both 532 and 355 nm, which agrees with the values observed during the SAMUM-1 and SAMUM-2 campaigns held in Morocco and Cabo Verde in 2006 and 2008. The mean value of the particle depolarization ratio at 532 nm was 30 ± 4.5 %; however, during strong dust episodes this ratio increased to 35 ± 5 %, which is also in agreement with the results of the SAMUM campaigns. The backscattering Ångström exponent during the dust episodes decreased to ∼ −0.7, while the extinction Ångström exponent, though negative, was greater than −0.2. Low values of BAE can likely be explained by an increase in the imaginary part of the dust refractive index at 355 nm compared to 532 nm. The dust extinction and backscattering coefficients at multiple wavelengths were inverted to the particle microphysics using the regularization algorithm and the model of randomly oriented spheroids. The analysis performed has demonstrated that the spectral dependence of the imaginary part of the dust refractive index may significantly influence the inversion results and should be taken into account.

Total and polarized radiances provided by the Polarization and Directionality of Earth Reflectances (POLDER) satellite sensor are used to retrieve the microphysical and optical properties of the volcanic plume observed during the Eyjafjallajökull volcano eruption in 2010, over cloud-free and cloudy ocean scenes. We selected two plume conditions, fresh aerosols near the sources (three cases) and a downwind volcanic plume observed over the North Sea 30 h after its injection into the atmosphere (aged aerosols). In the near-source conditions, the aerosol properties depend on the distance to the plume. Within the plume, aerosols are mainly non-spherical and in the coarse mode with an effective radius equal to 1.50 (± 0.15) μm and an Ångström Exponent (AE) close to 0.0. Far from the plume, in addition to the coarse mode, there are smaller particles retrieved in the accumulation mode suggesting a mixture of sulfate aerosols and volcanic dust, resulting in an AE around 0.8. The properties of the aerosols also depend on whether the plume is fresh or aged. For the downwind (aged) plume, if non-spherical coarse particles as well as some fine mode particles are still retrieved, the AE is smaller, around ~ 0.4. In addition, the real refractive index (RR) values are larger for the downwind plume (1.42 < RR < 1.58) than for the near-source plume (1.38 < RR < 1.48). The mean Single Scattering Albedo (SSA) retrieved at 0.865 μm was estimated at 0.97 over some parts of the downwind and near-source plumes; despite the low accuracy of our retrievals, the derived SSA values suggest that the ash particles are rather absorbing. To consider the particle shape, a combination of spheroid models is used. Although the employed model enabled accurate modeling of the POLDER signal in case of non-spherical ash, our approach failed to model the signal over the optically thickest parts of the near-source plume. The most probable reason for this is speculated to be the presence of ice crystals within the plume. For the Aerosol Above Clouds (AAC) scenes, polarized measurements allowed the retrieval of the Optical Thickness (OT) and the AE of optically thin volcanic ash. We found that all the cloud parameters retrieved by passive sensors were biased due to the presence of the elevated volcanic plumes. Finally, thermal infrared measurements were used to identify the type of multi-layer scene (i.e. cirrus clouds or volcanic dust above liquid clouds) and the retrieval method also provided the OT of thin cirrus layers above the clouds near Iceland.

We present ground-based measurements of aerosol mass, optical properties and vertical extinction profiles acquired in M'Bour, Senegal (16.96° W; 14.39° N) from January 2006 to September 2008. This place of the word is all year long affected by the export of mineral dust as it moves westward to the north Atlantic ocean. The maximum in the dust activity is observed in summer (June–July), corresponding to a maximum in the aerosol optical thickness (above 0.5) and single scattering albedo (above 0.95). It also corresponds to a maximum in the top altitude of the transported aerosol layer (up to 6 km) and aerosol optical thickness scale height (up to 3.5 km) due to the presence of the Saharan Air Layer located between 2 and 6 km. The late summer shows an additional low level aerosol layer that increases in thickness in autumn. Severe dust storms are also systematically observed in spring (March) but with a lower vertical development and a stronger impact (factor 2 to 3) on the ground-level mass compared to summer. Sporadic events of biomass burning aerosols are observed in winter (January) and particularly in January 2006 when the biomass burning aerosol are advected between 1.5 and 3.5 km high. On average, the seasonal signal in the aerosol optical properties and vertical distribution is very similar from year to year over our 3 year monitoring.