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Total and polarized radiances provided by the Polarization andDirectionality of Earth Reflectances (POLDER) satellite sensor are used toretrieve the microphysical and optical properties of the volcanic plumeobserved during the Eyjafjallajökull volcano eruption in 2010, overcloud-free and cloudy ocean scenes. We selected two plume conditions, freshaerosols near the sources (three cases) and a downwind volcanic plumeobserved over the North Sea 30 h after its injection into the atmosphere(aged aerosols). In the near-source conditions, the aerosol propertiesdepend on the distance to the plume. Within the near-source plume, aerosolsare mainly non-spherical and in the coarse mode with an effective radiusequal to 1.75 (±0.25) μm and an Ångström exponent (AE)close to 0.0. Far from the plume, in addition to the coarse mode, there areparticles retrieved in the accumulation mode, suggesting a mixture ofsulfate aerosols and volcanic dust, resulting in an AE around 0.8. Theproperties of the aerosols also depend on whether the plume is fresh oraged. For the downwind (aged) plume, if non-spherical coarse particles aswell as some fine mode particles are retrieved, the AE is higher,around ~0.4. In addition, rather low values for the real partof the refractive index (RR) were retrieved for the fresh plume (1.38 < RR < 1.48). Single scattering albedo (SSA) values rangingbetween 0.92 and 0.98 were retrieved over some parts of the near-source plume;despite the low accuracy of our retrievals, the derived SSA values suggestthat the ash particles are rather absorbing. To consider the particle shape,a combination of spheroid models was used. Although the employed modelenabled accurate modeling of the POLDER signal in the case of non-spherical ash,our approach failed to model the signal over the optically thickest parts ofthe near-source plume. The most probable reason for this is the presenceof 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 cloudparameters retrieved by passive sensors were biased due to the presence ofthe elevated volcanic plumes. Finally, thermal infrared measurements wereused to identify the type of multilayer scene (cirrus clouds orvolcanic dust above liquid clouds) and the retrieval method also providedthe OT of thin cirrus layers above the clouds near Iceland

Retrieval of the Eyjafjallajökull volcanic aerosol optical and microphysical properties from POLDER/PARASOL measurements