Variability of aerosol vertical distribution in the Sahel

Abstract

In this work, we have studied the seasonal and inter-annual variability of the aerosol vertical distribution over Sahelian Africa for the years 2006, 2007 and 2008, characterizing the different kind of aerosols present in the atmosphere in terms of their optical properties observed by ground-based and satellite instruments, and their sources searched for by using trajectory analysis. 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 the CALIPSO satellite (Cloud-Aerosol Lidar and Infrared Pathfinder Observations). <br><br> During winter, the lower levels air masses arriving in the Sahelian region come mainly from North, North-West and from the Atlantic area, while in the upper troposphere air flow generally originates from West Africa, crossing a region characterized by the presence of large biomass burning sources. The sites of Cinzana, Banizoumbou and M'Bour, along a transect of aerosol transport from East to West, are in fact under the influence of tropical biomass burning aerosol emission during the dry season, as revealed by the seasonal pattern of the aerosol optical properties, and by back-trajectory studies. <br><br> Aerosol produced by biomass burning are observed mainly during the dry season and are confined in the upper layers of the atmosphere. This is particularly evident for 2006, which was characterized by a large presence of biomass burning aerosols in all the three sites. <br><br> 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> During summer months, the entire Sahelian region is under the influence of Saharan dust aerosols: the air masses in low levels arrive from West Africa crossing the Sahara desert or from the Southern Hemisphere crossing the Guinea Gulf while in the upper layers air masses still originate from North, North-East. The maximum of the desert dust activity is observed in this period which is characterized by large AOD (above 0.2) and backscattering values. It also corresponds to a maximum in the extension of the aerosol vertical distribution (up to 6 km of altitude). In correspondence, a progressive cleaning up of the lowermost layers of the atmosphere is occurring, especially evident in the Banizoumbou and Cinzana sites. <br><br> Summer is in fact characterized by extensive and fast convective phenomena. <br><br> Lidar profiles show at times large dust events loading the atmosphere with aerosol from the ground up to 6 km of altitude. These events are characterized by l...

Airborne observations of mineral dust over western Africa in the summer Monsoon season: spatial and vertical variability of physico-chemical and optical properties

“…The lowermost extent of the dust plumes above the boundary layer varies between 1.5 and 2 km, whereas the uppermost extent was found to be remarkably constant at 5 km. The vertical extent, but also the structural and temporal variability of the dust vertical profiles is in agreement with summertime observations by various authors Carlson and Prospero, 1972;Karyampudi et al, 1999;Formenti et al, 2003;Reid et al, 2002;McConnell et al, 2008;Tesche et al, 2009;Léon et al, 2009;Ben-Ami et al, 2009;Cavalieri et al, 2010). In particular, our data show the day-to-day variability of the dust vertical profile, in good agreement, albeit with lower statistical representation, with the long-time series of lidar data presented by Léon et al (2009) andCavalieri et al (2010).…”

“…Layers of very low concentrations decreasing monotonically with height were also observed (profiles during flights V024, V027 and V035). Such diversity is consistent with the climatology of aerosol profiles in the area proposed by Cavalieri et al (2010) based on ground-based and spaceborne lidar observations. Over land, the aerosol layers were always spectrally neutral, indicating than mineral dust dominated the aerosol load.…”

Seasonal cycle of desert aerosols in western Africa: analysis of the coastal transition with passive and active sensors

“…The mix of layers of desert aerosol and other types of aerosols (i.e., biomass burning) is very rare (Chou et al, 2008;Heese and Wiegner, 2008) in our region of interest. During the dry season, mineral aerosols are observed in the atmospheric surface layer ranging 0.5 to 1 km while the aerosols emitted through biomass burning are carried to higher levels, up to 5 km altitude (Cavalieri et al, 2010). Nevertheless, classification errors are possible for low values of the mineral dust occurrence frequency (MDOF) and at frontal zones between layers of different substances (Adams et al, 2012).…”

Atmospheric dust modeling from meso to global scales with the online NMMB/BSC-Dust model – Part 2: Experimental campaigns in Northern Africa

“…The data has to be treated with particular caution due to its weak sensitivity to boundary layer aerosol owing to high desert surface reflectivity (Martin, 2008;Badarinath et al, 2010), or cloud contamination (Hsu et al, 1999). OMI also tends to indicate heavy biomass burning in the Sahel (Basart et al, 2009;Cavalieri et al, 2010) and it may overestimate the winter AOD relative to MISR and MODIS (Ahn et al, 2008;Zhao et al, 2010).…”