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 o ptical depth (AOD(T)) made at three wavelengths (368, 500, and 778 nm) with a spectroradiometer placed on Tenerife (28.5 degreesN, 16.31 degreesW), Ca nary Islands. Using the National Oceanic and Atmospheric Administration (NO AA) advanced very high resolution radiometer (AVHRR) channel 1, we also mea sured the aerosol optical depth (AOD(S)) and albedo over a region of the No rth 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 outbreak s pass over this region. Using all these data, we derive the asymmetry fact or (g), the single-scattering albedo (omega), and the local mean AOD(T), an d we compute the direct radiative forcing DeltaF attributable to mineral du st. 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%. Extend ing 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 attribu table to dust are comparable in magnitude to those reported in the literatu re for anthropogenic sulphate and for biomass burning aerosols.