Ground-based measurements of aerosol optical depth and surface shortwave irradiance carried out at the Mediterranean island of Lampedusa during 2004-2007 are used to estimate the surface aerosol direct radiative forcing for desert dust (DD), urban/ industrial-biomass burning (UI-BB), and mixed aerosols (MA). The aerosol single scattering albedo, ω at 415.6 and 868.7 nm is derived at 60° solar zenith angle, θ from measurements of global and diffuse radiation using radiative transfer model calculations. The shortwave forcing efficiency (FEs) is derived, for θ between 20° and 75°, for the three identified classes of aerosol and for all the observed data (AD). The absolute value of FEs decreases for increasing θ for all the aerosol types. FEs varies between -185 and -81.7 W m-2 for DD, -168 and -84 W m-2 for UI-BB, -251 and -120.2 W m -2 for MA, and -208 and -106.5 W m-2 for AD. The daily average forcing efficiency (FEd) at the equinox is -67.2 W m -2 for DD, -59.0 W m-2 for UI-BB, and -93.2 W m -2 for MA. The forcing efficiency of DD, UI-BB, and MA at θ = 60° was calculated for three intervals of single scattering albedo (0.7 < ω < 0.8, 0.8 < ω < 0.9, 0.9 < ω < 1) at 415.6 and 868.7 nm. The absolute value Of FEs decreases with increasing ω at 868.7 nm for all aerosol types, while it decreases with increasing ω at 415.6 nm for UI-BB and MA and increases for DD. A 0.1 increment in the single scattering albedo at 868.7 nm produces a reduction in FEs by 25-30 W m-2, and a reduction by 10-15 W m-2 in FE d. Copyright 2009 by the American Geophysical Union.

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