Measurements of Mediterranean aerosol radiative forcing and influence of the single scattering albedo

Abstract

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 FEd.