ACPD Atmospheric Chemistry and Physics Discussions ACPD 1680-7375 Copernicus GmbH Göttingen, Germany 10.5194/acpd-8-12461-2008 Radiative budget in the presence of multi-layered aerosol structures in the framework of AMMA SOP-0 Raut J.-C. 1 Chazette P. 1 Laboratoire des Sciences du Climat et de l'Environnement, Laboratoire mixte CEA-CNRS-UVSQ, CEA Saclay, 91191 Gif-sur-Yvette, France 01 07 2008 8 4 12461 12528 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys-discuss.net/8/12461/2008/acpd-8-12461-2008.html The full text article is available as a PDF file from http://www.atmos-chem-phys-discuss.net/8/12461/2008/acpd-8-12461-2008.pdf

This paper presents radiative transfer calculations performed over Niamey in the UV-Visible range over the period 26th January – 1st February during the African Multidisciplinary Monsoon Analysis (AMMA) international program. Climatic effects of aerosols along the vertical column have required an accurate determination of their optical properties, which are presented in for a variety of instrumented platforms: Ultralight aircraft, Facility for Airborne Atmospheric Measurements (FAAM) research aircraft, AERONET station. Measurements highlighted the presence of a multi-layered structure of mineral dust located below and biomass-burning particles in the more elevated layers. Radiative forcing was affected by both the scattering and absorption effects governed by the aerosol complex refractive index (ACRI). The best agreement between our results and AERONET optical thicknesses, ground-based extinction measurements and NO<sub>2</sub> photolysis rate coefficient was found using the synergy between all the instrumented platforms. The corresponding averaged ACRI were 1.53 (&plusmn;0.04)–0.047i (&plusmn;0.006) and 1.52 (&plusmn;0.04)–0.008i (&plusmn;0.001) for biomass-burning and mineral dust aerosols, respectively. Biomass-burning aerosols were characterized by single-scattering albedo ranging from 0.78 to 0.82 and asymmetry parameter ranging from 0.71 to 0.73. For dust aerosols, single-scattering albedo (asymmetry parameter) ranged from 0.9 to 0.92 (0.73 to 0.75). The solar energy depletion at the surface is shown to be ~ &minus;21.2 (&plusmn;1.7) W/m<sup>2</sup> as a daily average. At the TOA, the radiative forcing appeared slightly negative but very close to zero (~ &minus;1.4 W/m<sup>2</sup>). The corresponding atmospheric radiative forcing was found to be ~19.8 (&plusmn;2.3) W/m<sup>2</sup>. Mineral dust located below a more absorbing layer act as an increase in surface reflectivity of ~3–4%. The radiative forcing is also shown to be highly sensitivity the optical features of the different aerosol layers (ACRI, optical thickness and aerosol vertical distribution).

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