Radiative properties of aerosols in Saharan dust outbreaks using ground-based and satellite data:: Applications to radiative forcing

We report on measurements of atmospheric transmission (AT(T)) and aerosol optical depth (AOD(T)) made at three wavelengths (368, 500, and 778 nm) with a spectroradiometer placed on Tenerife (28.5 degreesN, 16.31 degreesW), Canary Islands. Using the National Oceanic and Atmospheric Administration (NOAA) advanced very high resolution radiometer (AVHRR) channel 1, we also measured the aerosol optical depth (AOD(S)) and albedo over a region of the North Atlantic Ocean extending from 15 degrees -35 degreesN to 12 degrees -25 degreesW. We observe large changes in AT(T) and AOD(T), when dust outbreaks pass over this region. Using all these data, we derive the asymmetry factor (g), the single-scattering albedo (omega), and the local mean AOD(T), and we compute the direct radiative forcing DeltaF attributable to mineral dust. The local radiative forcing obtained is over the ocean DeltaF = -9.7 W/m(2) and for the land DeltaF = -4.5 W/m(2) with an error of +/- 25%. Extending these results to global-scale averages, we obtain values of DeltaF of -1.22 W/m(2) over the ocean and -0.57 W/m(2) over land. The forcings attributable to dust are comparable in magnitude to those reported in the literature for anthropogenic sulphate and for biomass burning aerosols.