TT2b: EOP/LOP Aerosols Monitoring and Radiation

TT-leaders: Béatrice Marticorena, Francesco Cairo

Detailed information: TT2b document: EOP/LOP Aerosols Monitoring and Radiation (latest update: 23.01.2006, pdf, 0.7 MB)



Scientific justification and objectives

West Africa is the world's largest source of biomass burning aerosols and mineral dust. Satellite sensors consistently indicate that these aerosol plumes are the most widespread, persistent and dense found on Earth. The effect of dust and carbonaceous aerosols on climate change is one of the largest uncertainties in the Earth radiative budget. They directly impact visible and infrared radiation and the microphysics of clouds, and have the potential to be exported over great distances by prevailing winds and atmospheric waves. Mineral dust exported from western Africa over the subtropical Atlantic Ocean also represents a significant nutrient input (mainly ferrous species) for remote marine ecosystems. The seasonal cycle of dust and smoke is directly linked to meteorological processes in the monsoon.

For example, a clear correlation was established between drought occurrences and increase in desert dust load in the atmosphere not only in the Sahel but also far downwind from African sources in the transport zone of Caribbean islands. These two simultaneous increases (dust haze in the Sahel, concentration of long range transported dust) have been interpreted as being due to an increase of the local dust emissions in the Sahel during the drought periods. This increase is attributed to the appearance of additional Sahelian sources in regions where lowest rainfall has led to an observed decrease of the vegetation cover rate. The contribution of the Sahelian belt to the mineral dust emission from North Africa has been further questioned based on numerical simulations of the mineral dust cycle performed with a global transport model. A correct simulation of dust concentrations over the Northern Tropical Atlantic Ocean and of the seasonal pattern of the Saharan plume requires the inclusion of Sahelian sources with a contribution of 30-50 % of the global dust emissions. These Sahelian emissions were attributed to regions affected by climatic changes and/or anthropogenic disturbance. From these results, some authors concluded that the Sahel was the major source of mineral dust in North Africa.

Similarly, the atmospheric content of biomass burning aerosol is expected to be connected to El Niño events. Indeed during years of regional drought, such as those in southern Africa associated with El Niño events, the area burned decreases by about half. It is believed that this is caused principally by a decrease in fuel availability. Since gas and aerosol emissions from biomass directly depend on the burned areas, the relationships between the atmospheric content of carbonaceous aerosols atmospheric content and climatic indicators of the intensity of El Niño events are investigated.

Because the aerosol emissions and transport are controlled by climatic parameters (directly or indirectly through vegetation variations), a long term monitoring of the aerosol atmospheric content at the regional scale is required to elucidate these complex relationships.

In recent decades, remote sensing has offered the opportunity to document on a large spatial and temporal scale the aerosol load over Western Africa. However the retrieval of the aerosol content over land surfaces is much more complex than over ocean so that, to date, the available information is only semi-quantitative (Aerosol Index TOMS; Infrared Different Dust Index). New sensors, such as MODIS, are starting being used to fill this important gap. In this respect, ground based monitoring is absolutely essential to

1. document properly the annual and interannual variability of the atmospheric aerosols content over the African continent for improved understanding of the controlling processes and
2. quantify their optical properties in order to better constrain their radiative impact.

Such monitoring was initiated in western Africa in the 90's through two networks in particular: PHOTONS/AERONET and IDAF.

PHOTONS is a part of the international sunphotometer Aerosol Robotic Network (AERONET), which was developed to provide aerosol information from ground-based measurements all over the world. It provides globally distributed near-real time observations of the aerosol spectral optical thickness and sky radiance as well as derived parameters such as particle size distributions, single-scattering albedo and complex refractive index. The AERONET data have been widely used to establish climatology of aerosol properties for the validation of satellite aerosol retrievals and regional-to-global simulations of the aerosol atmospheric content. To date, more than 10 years of worldwide distributed data from the AERONET network of ground-based radiometers are available. They are suited to reliably and continuously derive the detailed aerosol optical properties in key locations. From 1994, 14 stations have been installed in western Africa, with different period and duration of observations. Ten stations are operational today. PHOTONS/AERONET is a French National Observatory for Environmental Research dedicated to aerosol observation supported by the French national agencies for Space Studies (CNES) and for Earth and Universe studies (INSU).

The IDAF (IGAC/DEBITS/Africa) network is the African component of the international program DEBITS (International Global Atmospheric Chemistry / Deposition of Important Biogeochemically Trace Species) devoted to the monitoring of the gas and aerosol concentrations in the atmosphere and in wet and dry atmospheric deposition. It was created in 1994 and is, like PHOTONS, a French National Observatory for Environmental Research (ORE). The 5 IDAF ORES stations are located in the 3 main African ecosystems: dry savannah, wet savannah and equatorial forest. The main objectives of the IDAF network are to analyse the evolution trends of the chemical composition of aerosol, precipitations and gas as a function of the main seasonal emission sources. The data are used to identify the physical and chemical processes controlling the wet and dry deposition processes and, finally, to improve the parameterisation of these processes in atmospheric models.

In addition to these pre-existing networks, a "Sahelian Dust Transect" will be equipped in order to document specifically the mineral dust content and its transport toward the North Atlantic Ocean. One of the characteristics of the mineral dust originating from the Sahara and Sahelian regions is that it can be transported at different altitudes. Typically, during summer, mineral dust from the Sahara is transported across the North Atlantic Ocean above the MBL within the Saharan Air Layer (SAL). This is supported by various lidar measurements offshore western Africa as well as in split-window satellite retrievals of GOES data, and it becomes evident when looking at the combination of surface concentration and the column-integrated aerosol optical depth of mineral dust measured over the Cap Verde Islands. Recently, intensive field campaigns including aircraft measurements (CLAIRE-LBA, PRIDE, AEROSE) showed that long range transport of Saharan dust can also take place within the surface layer. The altitude of the dust transport layer, and in particular the position of the dust layers relative to clouds can significantly change the dust radiative impact. Different altitude transport patterns will also induce different deposition patterns and thus impact significantly the regional dust budget. To take into account this variability, three stations have been selected along the main dust transport pathway, for which the surface concentration, the vertically integrated dust amount, the vertical distribution and the wet and dry deposition will be monitored during the EOP period.