The AMMA mulid network for aerosol characterization in West Africa

Three ground based portable low power consumption microlidars (MULID) have been built and deployed at three remote sites in Banizoumbou (Niger), Cinzana (Mali) and M'Bour (Senegal) in the framework of the African Monsoon Multidisciplinary Analyses (AMMA) project for the characterization of aerosols optical properties. A description of the instrument and a discussion of the data inversion method, including a careful analysis of measurement uncertainties (systematic and statistical errors) are presented. Some case studies of typical lidar profiles observed over the Banizoumbou site during 2006 are shown and discussed with respect to the AERONET 7-day back-trajectories and the biomass burning emissions from the Combustion Emission database for the AMMA campaign

Assessing the micropeds-size distribution of desert soils erodible by wind

International audienceThe microped size distribution of erodible soils is one of the main factors controlling the emissions of mineral aerosol. To date, the microped size distributions of desert soils in natural field conditions are not well known and thus the existing dust source models cannot be applied to natural sites. Based on dry sieving and elemental analysis of 26 soil samples collected in arid and semiarid regions, it is shown that the microped size distributions of these desert soil samples involve a combination of four major mass size populations. These four size populations correspond to salty soils, fine quartz sand, coarse dune sand and aluminosilicate silt. We show that the microped size distributions of the various desert soils, according to their mineralogical characteristics, can be calculated by a mixture involving two or three of these four populations. The main field of application for this scheme is the characterization of the erodible soil size distributions for modelling the atmospheric dust cycl

Contribution of the different aerosol species to the aerosol mass load and optical depth over the northeastern tropical Atlantic

Simultaneous ground-based measurements of chemical composition, size distribution, and column optical thickness at 670 nm of atmospheric aerosols have been performed at Sal Island (Cape Verde) during the winter season when African dust is transported in the lower troposphere. Mineral dust and, occasionally, sea salt dominate the aerosol mass load, whereas the excess sulfates plus the carbonaceous aerosol (particulate organic matter and black carbon) contributions to the mass load remain lower than 5% on average. We compute the total aerosol optical depth (AOD) by combining optical properties derived from measured size distributions and vertical concentration profile of each aerosol type estimated from surface elemental concentrations and meteorological observations. Results are very consistent with direct Sun photometer measurements. This allows us to derive the chemical apportionment of AOD in this region: mineral dust from Africa controls the total AOD and generally dominates AOD even in the absence of dust outbreak; on average, sea salt, excess sulfate, and carbonaceous aerosols all together only contribute to an averaged background AOD of 0.04 at 670 nm

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

This work aims at assessing the capability of passive remote-sensed measurements such as aerosol optical depth (AOD) to monitor the surface dust concentration during the dry season in the Sahel region (West Africa). We processed continuous measurements of AODs and surface concentrations for the period (2006-2010) in Banizoumbou (Niger) and Cinzana (Mali). In order to account for the influence of meteorological condition on the relationship between PM10 surface concentration and AOD, we decomposed the mesoscale meteorological fields surrounding the stations into five weather types having similar 3-dimensional atmospheric characteristics. This classification was obtained by a clustering method based on nonlinear artificial neural networks, the so-called self-organizing map. The weather types were identified by processing tridimensional fields of meridional and zonal winds and air temperature obtained from European Centre for Medium-Range Weather Forecasts (ECMWF) model output centered on each measurement station. Five similar weather types have been identified at the two stations. Three of them are associated with the Harmattan flux; the other two correspond to northward inflow of the monsoon flow at the beginning or the end of the dry season. An improved relationship has been found between the surface PM10 concentrations and the AOD by using a dedicated statistical relationship for each weather type. The performances of the statistical inversion computed on the test data sets show satisfactory skills for most of the classes, much better than a linear regression. This should permit the inversion of the mineral dust concentration from AODs derived from satellite observations over the Sahel

Saharan dust impacts on air quality: What are the potential health risks in West Africa?

Despite the proximity of the Sahara, very few studies about dust impact on air quality and human health have been conducted in West Africa. The lack of data is a major constraint on our understanding of the impacts on human health in this area. We analyzed PM10 concentrations and horizontal visibility recorded in four West African stations. The pollution levels often exceed the standards defined by many countries/regulatory authorities and have been associated with serious health risks outside Africa. Over the Sahelian stations, 45.6% of the days between November 2006 and March 2007 were likely to impact human health and the studied Sudanian population was exposed to potential health effects every 5 days

AMMA dust experiment : an overview of measurements performed during the dry season special observation period (SOP0) at the Banizoumbou (Niger) supersite - art. no. D00C14

International audienceIn the frame of the African Monsoon Multidisciplinary Analysis-Special Observation Period 0 (AMMA SOP0) field campaign, a sophisticated instrumental setup specially designed for documenting the characteristics of aerosols in the surface boundary layer has been operated for 1 month (13 January to 13 February 2006) at the Banizoumbou supersite in Niger. The analysis of the long-term routine sunphotometer measurements shows that the intensive measurements were performed during a transition phase between a period when biomass burning aerosols are present in the atmospheric column and another when mineral dust is clearly predominant. On the basis of ground-level observations, three types of events can be defined. (1) Type B events are those corresponding to advection of Biomass burning plumes from southern Sahel. They represent only 3% of the measurement period. (2) Type D events are those during which mineral dust concentrations are larger than 200 mu g m(-3). They represent 35% of the measuring time, and coincide with advection of mineral dust from distant sources to the measurement site. The composition of the dust differs significantly from one episode to the other, depending on the trajectory of air mass that transported it. (3) Type L events are those that can be explained by occurrence of local erosion and that are of short duration. During them, the aerosol size distribution is dominated by a coarser mode (9 mu m) than during the events of the D type dominated by a finer mode (4.5 mu m). Elemental analysis also shows that mineral dust is relatively rich in Fe and poor in Ca, which is typical of the Sahelian aerosols