With an exploding population, massive urbanization and uncontrolled deforestation, West Africa is faced with major change, which could see anthropogenic1 pollution increase threefold between 2000 and 2030. What impact will this have on public health and ecosystems, as well as on the weather and climate?

This is the issue addressed by the European DACCIWA program, which brings together 16 partners, including CNRS, Universite Toulouse III - Paul Sabatier, Universite Pierre et Marie Curie, Universite Paris Diderot and Universite Blaise Pascal - Clermont-Ferrand II.

It will eventually make it possible to improve weather and climate forecasting in this little-studied region. With a major airborne and ground-based measurement campaign carried out in early summer, the program has delivered its first results, showing in particular that a large proportion of the pollution comes from open landfills.

The DACCIWA program, funded by the European Union and coordinated by the Karlsruhe Institute of Technology in Germany, focuses on the connections between weather, climate and air pollution in West Africa, from Ivory Coast to Nigeria. For the first time, it has enabled scientists to carry out a complete study of the impacts of natural and anthropogenic emissions on the atmosphere of the region and on human health, thanks to a comprehensive field campaign in June and July 2016.

The air over the Gulf of Guinea is a unique, complex mixture of various gases and aerosols2, of both natural and human origin, including monsoon winds laden with sea salt, Saharan winds carrying dust, forest and domestic fires, open landfills in towns, large numbers of oil tankers off the coasts, oil rigs and an aging car population.

At the same time, the multiple layers of clouds that develop in this environment have a major effect on the weather and climate. However, atmospheric composition and its effect on cloud formation and dispersal have never been studied in this region. As a result, weather and climate models are still incomplete with regard to aerosol-cloud interactions in a chemical environment as complex as that of the Gulf of Guinea.

Among the resources implemented in the DACCIWA program, three research aircraft have been used to monitor how air pollution from ports and large coastal cities (Abidjan, Accra, Lome, Cotonou, Lagos) affects inland areas. Major resources have also been deployed on the ground: for two months, three heavily instrumented sites, in Ghana, Benin and Nigeria, continuously measured clouds and many of the physical phenomena that play a part in their formation and dispersal. In addition, weather balloons were released several times a day all across the region, while ad hoc measurements of urban air pollution as well as health surveys were carried out in Abidjan (Ivory Coast) and Cotonou (Benin).

Initial results show, surprisingly, that a large part of the pollution is organic in origin, caused by continuous low-temperature burning in open landfills. The particles produced reduce the amount of solar radiation reaching the ground, modifying daytime changes in temperatures, wind and cloud, as well as atmospheric dynamics.

From now until 2018, the researchers will analyze the data, supplemented with satellite observations, with the aim of proposing an explanatory mechanism for the interaction between atmospheric chemistry, aerosols, formation and dispersal of low clouds, radiation budget and precipitation. The goal is to improve weather, climate and air quality models, which will support environmental policies for sustainable development of the region, and provide useful information about the health consequences of air pollution.