Global and diffuse shortwave irradiance during a strong desert dust episode at Granada (Spain)

This paper studies the influence of a strong Saharan dust event on diffuse and global (direct + diffuse) shortwave irradiance (0.305 to 2.800 μm) recorded at Granada (South Spain) during 6 September 2007. The contribution of coarse mineral dust particles to the aerosol load was evidenced from the large aerosol optical depth (AOD, between 0.8 and 1.5) and the small Angström exponent (between 0.1 and 0.25) values recorded by a sun-photometer. Global irradiance on 6 September decreased about 20% compared to low-aerosol load days, while diffuse irradiance increased 200–400% around solar noon. On that day, the diffuse component was nearly 60% of the global irradiance at 12:00 UTC and therefore the direct component was substantially reduced. The aerosol forcing efficiency (radiative forcing per unit of AOD) during this strong event was higher than most of the desert dust forcing efficiencies reported in the literature; an increase of one unit in the AOD at 440 nm led to a decrease in global solar irradiance at the surface of 123 W/m2. Additionally, low values of the single scattering albedo at 440 nm (mean value 0.90 ± 0.2) in relation to pure desert dust episodes were indicative of absorbing anthropogenic particles from the European and Mediterranean regions.