Climatological Analysis of Aerosols Optical Properties by Airborne Sensors and in Situ Measurements in West Africa: Case of the Sahelian Zone

Abstract

This paper deals with the climatology of aerosols in West Africa based on satellite and in situ measurements between 2001 and 2016 and covers four sites in the Sahelian zone. There are indeed Banizoumbou (13.541˚N, 02.665˚E), Cinzana (13.278˚N, 05.934˚W), Dakar (14.394˚N, 16.959˚W) and Ouagadougou (12.20˚N, 1.40˚W) located respectively in Niger, Mali, Senegal and Burkina Faso. Thus, an intercomparison between the satellite observations and the in situ measurements shows a good correlation between MODIS and AERONET with a correlation coefficient R = 0.86 at Cinzana, R = 0.85 at Banizounbou, R = 0.84 at Ouagadougou and a low correlation coefficient R = 0.66 calculated on the Dakar site. Like MODIS, SeaWiFS shows a very good correspondence with measurements of the ground photometer especially for Banizoumbou (R = 0.89), Cinzana (R = 0.88) and Dakar (R = 0.75) followed by a low correlation coefficient calculated on the Ouagadougou site (R = 0.64). The performance of these airborne sensors is also corroborated by the calculation of root mean square error (RMSE) and the mean absolute error (MAE). Following this validation, a climatological analysis based on aerosol optical depth (AOD) shows the seasonality of aerosols in West Africa strongly influenced by the climate dynamics illustrated by the MERRA model reanalysis. This seasonal spatial distribution of aerosols justifies the temporal variability of the particles observed at the different sites in the Sahel.

Seasonal Behavior of Aerosol Vertical Concentration in Dakar and Role Played by the Sea-Breeze

“…The heavy aerosol layer is located around 1000 m asl during nighttime (Figure 3(j), Figure 3(k)). This strong attenuated backscatter signal indicates a tick aerosol layer, in agreement with Senghor et al [37], Adams et al [30], and Bado et al [42]. Léon et al [43] have observed from lidar measurements a thick aerosol layer located be-tween 1000 and 2000 m at the Western coastal of Dakar in September 2001.…”

Contribution of Satellite Observations in the Optical and Microphysical Characterization of Aerosols in Burkina Faso, West Africa

“…An analysis of this figure allows us to locate aerosols in Burkina Faso in March with a maximum in the eastern part and in June marked by a massive presence of aerosols on the northern side. All this illustrates the effect of aerosol emissions in the Sahara probably due to the proximity of this area containing the main sources of dust [12]. These are located in the northern part of the country, more precisely in the northwestern part of Mali, and on the eastern side towards Niger, including Bodélé in Chad, which is considered the largest source of dust emissions in the world [9].…”

“…In addition, aerosol products supplied by SeaWiFS are mainly applicable and valid for oceanic regions [29] [30] and are available on NASA's website (https://giovanni.gsfc.nasa.gov/giovanni/). All these details about the SeaWiFS sensor are clearly provided by Nébon et al [12].…”

“…MISR was launched in 1999 on the TERRA satellite in polar orbit at an altitude of 705 km [12]. It has a temporal resolution of 16 days and nominal spatial resolutions of 250 m, 275 m and 1 km with radiances measured at 1.1 km resolution for the inversion of aerosol properties qualified as level 2 with a pixel size of 17.6 km × 17.6 km.…”

“…However, these annual emissions can vary from 130 Mt to 1600 Mt [9]. Thus, West Africa is marked by a permanent presence of dust throughout the year with a seasonal cycle characterized by peaks that are observed between October and March [1] [10] [11] [12]. During the summer (June-July-August), which is the winter period, the strong dust pulses are due to local emissions caused by convective systems responsible for the rains in the Sahel and the wet deposition of particles during this period [5].…”