Contact LISA : B. Marticorena

PI : J-L. Redelsperger (CNRM) , Thierry Lebel (LTHE)

AMMA AMMA (African Monsoon Multidisciplinary Analysis) is an international research initiated on a French initiative, which aims to study the African monsoon and its variability at a daily to a seasonal scale as well as its consequences on dynamics, hydrology and the composition of the atmosphere. A component is also dedicated to the study of the impacts of variability of the African monsoon in terms of water resources, food security and health situation. (http://www.amma-international.org/spip.php?rubrique2).

Structured around an international steering committee, the program was financed by the CEE (AMMA-EU integrated project; 2005-2009) and the French research organizations through an Action Program of Interagency (API AMMA-France). It is currently supported by the national research programs of the INSU (ECCO; LEFE; ORE; ..) the French ministry of foreign affairs (CORUS) and the French National Research Agency (ANR).

LISA is strongly invested in the "Chemistry and Aerosols" component of this program.

Mineral Dust :

A major question in the Sahel is to quantitatively assess the local dust emissions by wind erosion and their modulation by meteorological factors (wind speed and precipitation) and by the dynamics of the vegetation and the evolution of anthropogenic activities.

In West Africa, mineral dust emission is mainly related to strong winds associated with the passage of meso-scale convective systems(MCS) at the beginning of the rainy season over bare cultivated surfaces. Despiyte their short duration, these events are very intense and lead to the formation of real dust "walls" in the atmosphere (photo 1). However, the passage of these convective systems is most often accompanied of precipitations leading to the wash-out of locally produced aerosols. One of the questions is to know whether these events actually are an important source of transportable aerosols at great distances, i.e. whether dust emissions due to MCS's are higher than the deposition they induce. In addition, precipitations allow the growth of seasonal vegetation, natural or not, that gradually inhibit wind erosion. The influence of natural or cultivated vegetated areas on mineral dust emissions is critical to estimate Sahelian dust emissions and their evolution since the last drought period (70-80 years) and in the context of global warming and population growth.

During the dry season, the Sahel zone is not emitting dust but is subject to high levels of mineral dust related to dust transport from sources located in the Sahara. The evaluation of the direct radiative effect of desert aerosols over West Africa imposes to determine not only the optical properties of Sahel aerosols but also those of Saharan aerosols. Indeed, the physicochemical, especially optical, properties of aerosols in the Sahel, notably differs from those of Saharan aerosols. In particular, Sahel aerosols being rich in iron oxides than their Saharan counterparts, they are likely to absorb more effectively infrared telluric radiation, and therefore induce a warming effect of air masses in which they are transported.

Finally, aerosols across the Sahel-Sahara region are transported to great distances on the tropical North Atlantic, where their deposit is likely to contribute significantly to the intake of nutrients to the oceanic biosphere. The differences in terms of iron, phosphorus and silica content, in aerosol and in terms of solubility are expected according to the origin of these mineral aerosols. Moreover, the presence of organic compounds brought by the monsoon flow also appears as a determinant of the solubility of these aerosols in wet season.

To address these issues, a strategy combining field measurements and numerical modeling was developed, with the deployment of a super-site ground Banizoumbou (Nigeria), devices of dedicated airborne measurement (AVIRAD), long-term (Sahelian Dust Transect) measurements, and the implementation of specific modeling tools (CHIMERE-Dust; RAMS; vegetation STEP model) fed, tested or validated at the regional level using remote sensing products.

Example of dust emissions by a convective system Banizoumbou (Niger)

Monitoring of wind erosion on two plots, a bare (PA) and the other cultivated in traditional way (PB), with "sand traps" (photo top left and yellow dots on the right photo) and a "saltiphone" (lower left photo) detects the impact of moving sand grains

Instrumental device deployed on the Banizoumbou super-site to measure the physicochemical properties of aerosols (Nigeria): a container was buried which emerges the particles (PIP) isokinetic collector connected to the different instruments installed in the container

Collection of precipitations after the passage of a convective system on the Banizoumbou super-site (Nigeria): the collection of the sample is made in a sequential manner during the rainfall event; the collected particles by filtration of rain samples show a net decrease of workload in aerosols along the event.

 

Chemistry :

Tropical areas play a key role on the evolution of the chemical composition of the troposphere, since the major part of the oxidizing capacity (determined mainly by the levels of ozone and radical OH) of the troposphere is concentrated, with a significant impact on the lifetime of greenhouse gases like methane. These regions are subject to anthropogenic, natural and gas precursors sources (NOx, VOC) of the photo-oxidants. The development of convective systems in monsoon period allows the rapid transport of these precursors, from the surface to the upper troposphere.

The real impact of these species on the oxidative capacity of the middle and upper atmosphere is still poorly assessed in these areas, by ignorance of the chemical processes and transport involved. A detailed characterization of atmospheric interest species, accompanied by dynamic measures is therefore necessary to understand the effect of convection on the balance sheet of the photo-oxidants.

Also, a set of physicochemical data was collected during the period of intensive observations of the AMMA campaign in the summer of 2006, and in particular, airborne observations (O3, NOx, NOy, VOCs and H2O2) have been implemented by LISA on French research aircrafts (ATR-42 and Falcon). In addition, 0D deterministic modeling approaches and statistics applied to situations in pre and post-convective samples have been used to determine the original primary and/or secondary compounds and quantify the contribution of convection (compared to the long-range transport) on the levels of concentrations observed.

View of the ATR-42 which measures chemicals and aerosols implemented by LISA