1. IntroductionWest Africa is the biggest dust and biomass burning smoke aerosol source of the world [1,2]. Massive dust plumes regularly propagate from the continent to the ocean and as far as Florida particularly during the summer in the so-called Saharan air Layer [3,4]. These mineral particles exert a strong influence on the radiative balance and the climate [5,6]. The years of rainfall deficits over West Africa are marked by anomalies in the dynamics of the monsoon system and mineral dust has kept increasing during drought years [7,8]. But the cause of this continuous dust increase is still unknown because it may be linked to the increase in wind intensity during dry years and/or a decrease in vegetation [9,10]. Several studies using short time and spatially limited observed data have been conducted to characterize these particles and their impacts on the regional climate. Mineral dust particles were investigated during the SHADE campaign over the West Africa, while their radiative effects were measured in the solar spectrum by ground-based measurements [11,12]). The particles found in [11] during field measurements were only dust with relatively low absorption properties. During the winter time, mixed particles composed of dust and biomasses burning are found by [13]. The properties of this mixture are very poorly documented [14,15].
The modeling approach has been used by some authors [16-18] to study aerosols spatio-temporal distribution and their impacts on the climate.
Despite these studies, the spatio-temporal distribution of aerosols and their radiative impacts on the climate is still an investigation topic.
A long time-series (2000-2010) of in-situ measurements (AERONET data) is used in this study to characterize and evaluate the aerosols properties and their impacts on climate over the Sahel considering 4 stations: Dakar (Senegal), Agoufou (Mali), Ouagadougou (Burkina-Faso) and Banizoumbou (Niger). These stations (particularly Dakar) are ideally placed to study the spatiotemporal distribution of coarse (mineral dust) and fine (biomass burning) particles over the Sahel. We present first the data and the methodology used. A seasonal characterization of aerosols properties at each station is performed considering the same period (2000-2010) in the second part. Finally, the shortwave radiative impact of aerosols is assessed.
2. Data and Methodology2.1. Presentation of AERONET Network and StationsThe AErosol RObotic NETwork (AERONET) program is a federation of ground-based remote sensing aerosol networks established by the NASA and PHOTONS (University of Lille 1, CNES, and CNRS-INSU) and is greatly expanded by collaborators from national agencies, institutes, universities, individual scientists, and partners [19]. More than 200 instruments (photometers) were installed around the world. These instruments provide the spectral optical thickness and other parameters such as size distribution, the index of refraction and the Angstrom coefficient. These parameters are obtained from the results of inversion algorithm [20]. The Sahelian stations used in this study are displayed in Figure 1: Banizoumbou (Niger), Agoufou (Mali), Ouagadougou (Burkina Faso) and Dakar (Senegal).
The second dataset used is the Multi-Angle Imaging Spectro Radiometer (MISR) sensors aeronet optical depth [21] in the aim to characterize the regional scale coverage and maxima of aerosols.
2.2. MethodologyData used in this work are AERONET aerosol optical depth and radiative properties products (level 2) available on http://aeronet.gsfc.nasa.gov/file_help.html. The AOD is available at 532 nm which corresponds to the maximum of solar radiation. Other parameters such as the angstrom coefficient and single scattering albedo are derived from the spectral AOD. The aerosol optical properties retrieval algorithm is extensively described in [20] and [22]. The radiative properties of aerosol particles are calculated with the radiative transfer model of the Global Atmospheric Model—GAME [23,24].
3. Results and Discussions3.1. Spatio-Temporal Distribution of the MISR Aerosol Optical DepthFigure 2 shows the seasonal cycle of the AOD over West Africa from the sensor MISR averaged between 2000 and 2010. If we assume that aerosols maxima are closest to the source regions as in [25], sources of aerosols are mainly located over the Northern part of the Sahel—and over the Sahara; especially during the summer (JJA) and spring (MAM) periods. During JJA, dust sources are located at the border of Mauritania/Mali, southern Algeria and at the border of Niger-Tchad (near to the Bodele depression). The latter region has been identified as a major dust source by several studies [26,27]. During the spring, only the Sahel maxima are visible. These two sources are permanent features because they are active all year long. During MAM, a maximum is found off the West Africa which results from the low-levels transport by the Harmattan Winds. The maximum of biomass burning occurs in winter (DJF) and the maxima are mainly located over the Gulf of Guinea.
After diagnosing the aerosols maxima, we used the AERONET data to characterize their optical properties and radiative impacts over the Sahel.
Figure 3 shows the seasonal cycle of the AERONET aerosol loading (AOD) averaged between 2000 and 2010 for four AERONET stations at 532 nm. The concentration of aerosols is stronger in spring and summer with the peaks located in March and in June. The AOD is higher in March in Banizoumbou and Ouagadougou with respective values of 0.83 and 0.88; and during June for Dakar (AOD = 0.66) and Agoufou (AOD = 0.89). Table 1 summarizes the seasonal mean of AOD (550 nm) for considered AERONET stations during DJF, MAM, JJA and SON averaged between 2000 and 2010. Weaker values of AOD are recorded for all stations during DJF and SON compared to the spring (MAM) and summer (JJA) seasons. The maximum in JJA may be explain by the fact that during that period, strong African Easterly Waves (AEWs) and gravity density associated with me-
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E) climate using a long time series (2000 to 2010) from AERONET data. We first used the MISR aerosol optical depth (AOD) averaged between 2000 and 2010 to identify dust maxima regions. They are mainly located over Eastern Mauritania-Northern Mali and Eastern Niger-Western Tchad (near the Bodele depression). Moreover the aerosol loading is stronger during the spring (MAM) and particularly during the summer (JJA). The analysis of AERONET data at 4 Sahelian stations (Dakar, Agoufou, Banizoumbou and Ouagadougou) shows that Sahel is under dust influence all year long. The dust season is mainly located between March and October, with two peaks of AOD recorded in March and June; while the maximum of biomass burning activity is recorded in winter (December to February). The radiative forcing of aerosols on the Sahelian climate results mainly in a cooling as well as at the surface than at the top of the atmosphere (TOA) but the forcing is stronger at the surface and particularly during the summer (JJA) and spring (MAM) periods. Although there is not a sharp difference between DJF and SON periods from a station to another one when considering the aerosol loading (AOD), the radiative forcing is stronger at the surface and weaker at the TOA during DJF for most stations. The analysis of the force effectiveness (ratio between AOD and the radiative forcing) suggests that the BOA radiative forcing is mainly influenced by the nature of aerosols.