1. The INDAAF dust monitoring in Sahel : an opportunity to constrain
the dust mass budget in regional simulations
Marticorena1, A. Féron1, C. Gaimoz1, F. Maisonneuve1, J. L. Rajot 2,1, G. Siour1,
M. Coulibaly3, A. Diallo4, S. Der Ba5, S.G. Dorego5, I. Koné3, A. Maman6, , T. NDiaye4 , M. Sene5 and A. Zakou6.
1 LISA, UMR CNRS 7583, UPEC,UPD; IPSL, Créteil, France ; 2 iEES Paris, UMR IRD 242, Bondy, France;
3IER Cinzana, Mali; 4IRD M’Bour, Senegal; 5CNRA Bambey, Senegal, Niger, 6IRD Niamey, Niger.
2. Instrumentation
1. The INDAAF Sahelian stations
PM10 concentration
Since 2006, a set of stations (M'Bour and Bambey, Senegal; Cinzana, Mali; and Banizoumbou, Niger) dedicated to the monitoring of mineral dust are operating in the Sahel. Initiated in the frame of the AMMA (African Monsoon Multidisciplinary Analysis) program, this monitoring activity is now part of the French national long-term observation network INDAAF (International Network to study Deposition and Atmospheric composition in Africa).
The main objective is to provide quantitative information to constrain the dust mass budget at the regional scale. Simple and robust instrumentation allows to monitor simultaneously the particles surface concentration, the deposition fluxes and the vertically integrated atmospheric dust load. More than 10 years of data are now available that document the variability of mineral dust in the Sahel.
The surface concentration of particulate matter is measured with a TEOM equipped with a PM10 inlet with a 5 min time step.
Deposition
Total deposition is sampled weekly with an “inverted Frisbee“ type passive collector and wet deposition is collected for each rain event with an automatic MTX collector. Dust deposit is collected with water, dried and weighted. The deposition fluxes is computed as a function of the surface of the collector and the duration of sampling.
Figure 2a: Views of the instrumentation at the station of Bambey (Senegal)
Aerosol Optical Depth (AOD)
Figure 2b: View of the TEOM (inside the building) at the station of Banizoumbou (Niger).
The aerosol optical depth is proportional to the vertically integrated dust load. It is measured with sunphotometer from the AERONET international network with a 15 min time step in clear sky conditions.
mm.yr-1
Longitude (°)
Latitude (°)
Meteorological parameters
Basic meteorological parameters are recorded to document the local climatic conditions and to sustain the interpretation of the dust measurements. Wind speed and direction, air temperature and relative humidity and precipitation are recorded with a 5min time step.
Figure 2b: View of a sunphotometer from the AERONET network at Cinzana (Mali)
Figure 1: Location of the stations and mean ( annual precipitation rates (from Lebel and Ali, 2009)
3. TIME SERIES
Total deposition fluxes also exhibit a marked seasonal cycle (Marticorena et al., 2017). In Banizoumbou and Cinzana, the maximum is recorded in the wet season (Fig. 4). It is due to wet deposition of the dust emitted locally by convective systems and washed out by the precipitation associated with these systems. In M’Bour and Bambey, the maximum is recorded in the spring like the concentration maximum and is due to dry deposition. The deposition fluxes of the last years in Banizoumbou and Cinzana are among the lowest of the 10 years (Fig. 5).
Figure 5 : Annual total deposition fluxes from 2006 to 2016.
Figure 3 : Monthly mean PM10 concentrations measured at the four stations from 2006 to The blue boxes highlight the rainy seasons
The PM10 concentrations exhibit a typical seasonal cycle (Fig. 3), with a maximum between February and March associated with Saharan dust transport and high concentrations at the beginning of the monsoon season due to local dust emission by convective systems (Marticorena et al., 2010, Kaly et al., 2015). They appear slightly lower during the last years, both in the dry and wet seasons, consistently with the decrease of the local wind speed (Bergametti et al., 2017).
Figure 4 : Monthly mean total deposition fluxes measured at the four stations from 2006 to The blue boxes highlight the rainy seasons
4. CONCLUSION
The measurements of the Sahelian stations of the INDAAF network are available since They allow to investigate the long-term variability of the mineral dust load in this region of emission, transport and deposition. The variability of the parameters that are continuously monitored are driven by different processes : regional transport and local emissions controls the variability of the surface concentrations, dust transport at different altitude modulates the variability of the aerosol optical depth, local emissions and precipitation drive the variability of the deposition fluxes in Niger and Mali while Saharan dust transport is responsible for the dry deposition that dominates the total deposition in Senegal.
This data sets provides a unique opportunity to test the capability of regional dust models to properly reproduce simultaneously these three parameters and thus to reliably represent all the involved processes.
It will be used as a validation data set in the frame of the European DUSTCLIM project (ERA4CS) to evaluate long-term regional dust simulations and to estimate the benefit brought by the assimilation of satellite data.
Figure 3 : Deposition fluxes (in g m-3.yr-1) measured by passive deposition collectors inside and close to the North of Africa : Senegal, Mali and Niger (This work); Niger (Drees et al., 1993); Lybia (O'Hara et al., 2006); Ghana (Breuning-Madsen et al., 2004; *Dry season only), Ivory Coast (Stoorvogel et al., 1997; *Dry season only); Mediterranean basin (Spain : Avila et al. (1997); Balearic Islands : Fiol et al., (2005); Corsica : Bergametti et al., (1989); Löye-Pilot et al., (1986); Turkey : Kubilai et al., (2000); Kriti : Mattson and Nehlén, (1994); Barbados (Prospero et al., 2010).
Figure 6 : Monthly mean total Aerosol Optical Depth (level 2; 675 nm) measured at the four stations from 2006 to The blue boxes highlight the rainy seasons
The Aerosol Optical Depths measured at the 3 stations (Fig. 6) do not exhibit an east to west gradient as marked as the PM10 concentrations and deposition fluxes. They exhibit a seasonal cycle similar to the one of the surface concentrations, but shifted toward the rainy season. This is clearly highlighted by the time series of the ratio between the PM10 concentrations and the AOD (Fig. 7). In average, an AOD of 1 corresponds to a surface concentration higher than 500 µg m-3 in the winter (Dec-Jan) and lower than 100 µg m-3 in the summer. The seasonal cycle of this ratio is due to changes in the depth and the altitude of the dust layers (Yahi et al., 2013).
References
Bergametti et al., 2017, Dust uplift potential in the Central Sahel: an analysis based on 10 years of meteorological measurements at high temporal resolution, J. Geophys. Res. Atmos., 122, /2017JD
Kaly, et al., 2015, Variability of mineral dust concentrations over West Africa monitored by the Sahelian Dust Transect, Atmos. Res., Atmos. Res., ,
Marticorena et al., 2010, Temporal variability of mineral dust concentrations over West Africa: analyses of a pluriannual monitoring from the AMMA Sahelian Dust Transect, Atmos. Chem. Phys., 10,
- Marticorena et al., 2017, Mineral dust over West and Central Sahel: seasonal patterns of dry and wet deposition fluxes from a pluriannual sampling ( ), J. Geophys. Res.., 122, doi: /2016JD
Yahi et al., 2013, Statistical relationship between surface PM10 concentration and aerosol optical depth over the Sahel as a function of weather type using neural network methodology, J. Geophys. Res., 118,
Figure 7 : Monthly mean ratio concentration /AOD (at 675 nm) measured at the four stations from 2006 to The blue boxes highlight the rainy seasons 1