Tropospheric aerosols in the Mediterranean: 1. Microphysical and optical properties

[1] Measurements of the aerosol properties were carried out at the island of Lampedusa, in the Mediterranean, in May 1999, as part of the Photochemical Activity and Ultraviolet Radiation modulating factors II campaign. Data from ground-based lidar and Sun photometer, and particle counters aboard an instrumented ultralight aircraft, are used in this study. Three different cases, when all the measurements were available in cloud-free conditions, were identified to derive the aerosol microphysical and optical properties. In one of these cases (18 May) the airmasses originated from Africa, and were loaded with a large amount of desert dust. In the other two cases (25 May and 27 May) the airmasses passed over Europe before reaching Lampedusa from North. The microphysical and optical properties of the aerosol strongly depend on the origin of the airmasses. The amount of particles in the 1–6 μm range of radii and the average aerosol surface area per unit volume are larger in the desert dust case than on 25 May and 27 May. The real part of the refractive index of the desert dust at 532 nm is between 1.52 and 1.58; its imaginary part is 5–7 × 10−3 and the single scattering albedo is about 0.7–0.75. The aerosol layer of 18 May closest to the surface, that probably contains a mixture of desert dust and marine aerosol, displays a smaller imaginary part (1.2 × 10−3) and a larger single scattering albedo (0.91). The aerosols originating from the North Atlantic and Europe have a real part of the refractive index between 1.35 and 1.49, and an imaginary part ranging from 8 × 10−4 to 1.8 × 10−2; the single scattering albedo at 532 nm (0.78–0.95) is larger than for desert dust values. The smallest value of the single scattering albedo (0.69) corresponds to an airmass originating from North, characterized by a large imaginary part of the refractive index. The asymmetry factor of the desert dust appears consistently larger for the desert dust (0.75–0.8) than for the other cases (0.61–0.72). The extinction-to-backscattering ratio, also derived from the measurements, is about 40 sr for the desert dust, and between 60 and 81 sr for the aerosol of northern origin. Simple estimates of the aerosol average direct shortwave radiative forcing at the top of the atmosphere indicate that all considered aerosol types induce a cooling. The radiative forcing per unit optical depth of the aerosol originating from North is about −37 Wm−2 over ocean and −(12–17) Wm−2 over land, while is −29 Wm−2 over ocean and −8 Wm−2 over land for desert dust. The largest forcing is however produced by the desert aerosols that generally display a considerably larger optical depth.

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