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Abstract
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).
Highlights
In contrast to the radiative forcing attributed to greenhouse gases, which may be estimated to a reasonably high degree of accuracy, the uncertainties related to aerosol radiative forcings remain very large
The best simulation of the photolysis rate of NO2 molecule has been obtained using the Approach 1 (A1) approach with ultra-light aircraft (ULA)-derived optical properties since the modelled J(NO2) photolysis rates superimpose on the measured one within 1%, whereas the Approach 3 (A3) approach gives within 5% discrepancy and the Approach 2 (A2) approach gives within 12% discrepancy
The mean AOD from A1 and A2 approaches and the mean daily TOA and BOA radiative forcings for the 26th January, 28th January and 1st February are located in the centre of the almost Gaussian curve, indicating that our radiative calculations, which are in the range −3 to 0 W/m2 for FTOA and −23 to −19 W/m2 for FBOA are representative of the region for typical conditions (AOD values are close to 0.5–0.6)
Summary
In contrast to the radiative forcing attributed to greenhouse gases, which may be estimated to a reasonably high degree of accuracy, the uncertainties related to aerosol radiative forcings remain very large. The Sahelian region is a major global source of biomass-burning aerosol during the dry season (Bond et al, 2004) with maximum emissions occurring from December to March (Swap et al, 2003) Their black carbon content depending on fire intensity and vegetation density clearly affects the radiation balance. Accurate calculations of the radiative effects of dust and biomass-burning aerosols through the atmospheric column require high quality measurements of their microphysical and optical properties. These properties are derived in this study from a variety of instrumented platforms, including ground-based, airborne and integratedcolumn remote sensing. The sensitivity of the radiative effects to optical features of the different layers will be presented
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