Radiative budget in the presence of multi-layered aerosol structures in the framework of AMMA SOP-0 J.-C. Raut and P. Chazette Laboratoire des Sciences du Climat et de l'Environnement, Laboratoire mixte CEA-CNRS-UVSQ, CEA Saclay, 91191 Gif-sur-Yvette, France
Abstract. This paper presents radiative transfer calculations performed over Niamey in
the UV-Visible range over the period 26th January–1st February 2006
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 here 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 NO2 photolysis
rate coefficient was found using the synergy between all the instrumented
platforms. The corresponding averaged ACRI at 355 nm were 1.53 (±0.04)
−0.047i (±0.006) and 1.52 (±0.04) −0.008i (±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 ~−21.2
(±1.7) W/m2 as a daily average. At the TOA, the radiative
forcing appeared slightly negative but very close to zero (~−1.4 W/m2).
The corresponding atmospheric radiative forcing was found to be
~19.8 (±2.3) W/m2. 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 sensitive to
the optical features of the different aerosol layers (ACRI, optical
thickness and aerosol vertical distribution).
Citation: Raut, J.-C. and Chazette, P.: Radiative budget in the presence of multi-layered aerosol structures in the framework of AMMA SOP-0, Atmos. Chem. Phys., 8, 6839-6864, doi:10.5194/acp-8-6839-2008, 2008.