Impacts of Saharan dust and clouds on photosynthetically available radiation in the area off Northwest Africa

Radiation measurements in the area off Northwest Africa (research cruise, February 2008) and at Mindelo (Cape Verde Islands, May 2009) were used to investigate the impacts of Saharan dust and clouds on solar irradiance and on photosynthetically available radiation, to derive a relationship between dust aerosol optical depth (AOD) and photosynthetically available radiation and to determine the annual variations of photosynthetically available radiation. Three different kinds of atmospheric conditions were considered: cloudy skies, which decreased or increased the irradiance and dusty skies. The reduction by clouds was up to 67.2% at 400 nm and up to 84.4% at 700 nm. Enhancements of up to 21.9% at 400 nm and 34.0% at 700 nm were observed. The decrease by dust was up to 19.7% at 400 nm and up to 4.1% at 700 nm. Clouds decreased or increased the photosynthetically available radiation by up to 79.9% or up to 31.2%. The reduction by dust depended on the dust AOD and was between 3.6% and 12.3%. A linear relationship confirmed a decrease of photosynthetically available radiation of 1.2% by an increase of dust AOD of 0.1.