[1] A method dedicated to the investigation of direct radiative forcing of the main anthropogenic aerosol species (ammonium sulfate, black carbon, particulate organic matter) is presented. We computed the direct radiative aerosol forcing at the top of atmosphere (TOA), at the bottom of atmosphere (BOA), and into the atmospheric layer (ATM). The methodology is based on chemical, photometric, and satellite measurements. We first determined the optical properties of the main aerosol species and then computed their direct radiative impact at local scale. The method was applied to a periurban zone during the Expérience sur Site pour Contraindre les Modèles de Pollution et de Transport d'Emission experiment. Optical computations indicate that the single scattering albedo, for the total aerosol population in the external mixture, is equal to 0.83 ± 0.04 at 550 nm, indicative of a strong absorption of the solar radiation. At the same time the mean asymmetry parameter is equal to 0.59 ± 0.04, and the mean aerosol optical thickness is equal to 0.30 ± 0.02, at 550 nm. The anthropogenic urban aerosol layer reduces significantly the daily surface illumination (À24 W m À2 > DF BOA > À47.5 W m À2 ) by reflection to space (À6 W m À2 > DF TOA > À9 W m À2 ) and by absorption of the solar radiation into the atmosphere (17 W m À2 < DF ATM < 39 W m À2 ). The available resulting energy in the atmospheric column heats the lowermost part of the atmosphere from 1.1°K d À1 to 2.8°K d À1 . Our study shows that the black carbon particles have a large contribution to the BOA forcing (almost 50% of the total daily forcing), whereas the ammonium sulfate particles contribute only to about 10%. Conversely, the TOA daily forcing is mostly driven by the ammonium sulfate aerosol (around 50%).

We present results of direct aerosol radiative forcing over a French Mediterranean coastal zone based on one year of continuous observations of aerosol optical properties during 2005-2006. Monthly-mean aerosol optical depth at 440 nm ranged between 0.1 and 0.34, with high Angstrom coefficient (α N 1.2). The single scattering albedo (at 525 nm) estimated at the surface ranged between 0.7 and 0.8, indicating significant absorption. The presence of aerosols over the Mediterranean zone during summer decreases the shortwave radiation reaching the surface by as much as 26 ± 3.9 W m − 2 , and increases the top of the atmosphere reflected radiation by as much as 5.2 ± 1.0 W m − 2 . The shortwave atmospheric absorption translates to an atmospheric heating of 2.5 to 4.6 K day − 1 . Concerted efforts are needed for investigating the possible impact of the increase in heating rate on the maintenance of heat-waves frequently occurring over this coastal region during summer time.

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