Variability of aerosol vertical distribution in the Sahel

Abstract

We present and discuss ground-based and satellite observations of aerosol optical properties over Sahelian Africa for the years 2006, 2007 and 2008. <br><br> This study combines data acquired by three ground-based micro lidar systems located in Banizoumbou (Niger), Cinzana (Mali) and M'Bour (Senegal) in the framework of the African Monsoon Multidisciplinary Analysis (AMMA), by the AEROsol RObotic NETwork (AERONET) sun-photometers and by the space-based Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) onboard Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO). <br><br> The 2006 seasonal pattern of the aerosols vertical distribution is presented. It shows the presence of aerosol mainly confined in the lower levels of the atmosphere during the dry season, with the aerosol layer increasing in vertical extension and absolute values in spring, reaching the largest values in summer in correspondence with a progressive clearing up of the atmosphere at the lowermost levels. <br><br> Aerosol produced by biomass burning are observed mainly during the dry season, when north-easterly air masses pass over large biomass burning areas before recirculating over the measurement sites. This kind of aerosol is present mainly in layers between 2 and 6 km of altitude, although episodically it may show also below 2 km, as observed in Banizoumbou (Niger) in 2006. Biomass burning aerosol is also observed during spring when air masses originating from North and East Africa pass over sparse biomass burning sources, and during summer when biomass burning aerosol is transported from the southern part of the continent by the monsoon flow. <br><br> The summer season on the whole is characterized by a large presence of desert dust along the entire Sahelian region, widespread in altitude with Aerosol Optical Depths above 0.2. <br><br> The interannual variability in the three year monitoring period is not very significant. An analysis of the aerosol transport pathways, aiming at detecting the main source regions, revealed that air originated from the Saharan desert is present all year long and it is observed in the lower levels of the atmosphere at the beginning and at the end of the year. In the central part of the year it extends upward and the lower levels are less affected by air masses from Saharan desert when the monsoon flow carries inland air from the Guinea Gulf and Southern Hemisphere. Biomass burning is mainly confined in the higher layers and observable in winter and in autumn

Lidar ratio of Saharan dust over Cape Verde Islands: Assessment and error calculation

“…Simultaneously, the Ångström exponent dropped to k < 0.2. This is a typical signature of dust and suggests that during this period dust was the dominating if not the only aerosol component [ Toledano et al , 2009; Cavalieri et al , 2010]. However, from the photometer data it is not possible to infer the vertical extent of the dust layer.…”

Mineral dust over west and central Sahel: Seasonal patterns of dry and wet deposition fluxes from a pluriannual sampling (2006–2012)

“…Both in Cinzana and Banizoumbou slightly lower but still high DO fluxes (~0.30 g m −2 d −1 ) are also recorded at the end of January. From January to April, the dust transported from the Sahara to the Sahel is confined in very shallow surface layers [i.e., Léon et al ., ; Cavalieri et al ., ], producing high surface dust concentrations but moderate AODs. This situation does not favor the long‐range transport of dust but may enhance the efficiency of dry deposition.…”

Statistical relationship between surface PM10 concentration and aerosol optical depth over the Sahel as a function of weather type, using neural network methodology

“…Severe dust storms are systematically observed in spring (March) but with a lower vertical development (maximum extinction below 1.5 km) and a stronger impact on the ground‐level mass compared to summer [ Léon et al ., ]. Similar trends were evidenced over the African continent using 3 year measurements of the Cloud‐Aerosol Lidar with Orthogonal Polarization instrument on board the CALIPSO satellite [ Cavalieri et al ., ]. From 2006 to 2008, analysis of lidar profiles overpassing the stations of Banizoumbou (Niger) and Cinzana (Mali) showed the succession of low dust layers (below 3 km) from November to March and elevated (>3 km) dust layers from May to October.…”