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Discussion papers
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 11 Apr 2019

Submitted as: research article | 11 Apr 2019

Review status
This discussion paper is a preprint. A revision of the manuscript is under review for the journal Atmospheric Chemistry and Physics (ACP).

Remote biomass burning dominates southern West African air pollution during the monsoon

Sophie L. Haslett1,a, Jonathan W. Taylor1, Mathew Evans2,3, Eleanor Morris2, Bernhard Vogel4, Alima Dajuma4,5, Joel Brito6, Anneke M. Batenburg7, Stephan Borrmann7, Johannes Schneider7, Christiane Schulz7, Cyrielle Denjean8, Thierry Bourrianne8, Peter Knippertz4, Régis Dupuy9, Alfons Schwarzenböck9, Daniel Sauer10, Cyrille Flamant11, James Dorsey1,12, Ian Crawford1, and Hugh Coe1 Sophie L. Haslett et al.
  • 1School of Earth and Environmental Sciences, University of Manchester, Manchester, UK
  • 2Wolfson Atmospheric Chemistry Laboratories, Department of Chemistry, University of York, York, UK
  • 3National Centre for Atmospheric Science, University of York, York, UK
  • 4Institute of Meteorology and Climate Research, Karlsruhe Institute of Technology, Karlsruhe, Germany
  • 5L'Université Félix Houphoët-Boigny, Abidjan 01, Côte D'Ivoire
  • 6Laboratory for Meteorological Physics (LaMP), University Blaise Pascal, Aubière, France
  • 7Particle Chemistry Department, Johannes Gutenberg University Mainz/Max Planck Institute for Chemistry, 55128 Mainz, Germany
  • 8CNRM, Université de Toulouse, Météo-France, CNRS, Toulouse, France
  • 9Laboratoire de Météorologie Physique, Université Clermont Auvergne, Aubière, France
  • 10Institut für Physik der Atmosphäre, Deutsches Zentrum für Luft-und Raumfahrt, Oberpfaffenhofen Wessling, Germany
  • 11LATMOS/IPSL, Sorbonne Université, UVSQ, CNRS, Paris, France
  • 12National Centre for Atmospheric Science, University of Manchester, Manchester, UK
  • anow at: Department of Environmental Science and Analytical Chemistry, Stockholm University, Stockholm 11418, Sweden

Abstract. Vast quantities of agricultural land in southern and central Africa are burnt between June and September each year, which releases large concentrations of aerosols into the atmosphere. The resulting smoke plumes are carried west over the Atlantic Ocean at altitudes between 2 and 4 km. As only limited observational data in West Africa have existed until now, whether this pollution has an impact at lower altitudes has remained unclear. The Dynamics-Aerosol-Chemistry-Cloud Interactions in West Africa (DACCIWA) aircraft campaign took place in southern West Africa during June and July 2016, with the aim of observing gas and aerosol properties in the region in order to assess anthropogenic and other influences on the atmosphere.

Results presented here show that a significant mass of aged accumulation mode aerosol was present in the southern West African boundary layer, over both the ocean and the continent. A median dry aerosol concentration of 6.2 µg m−3 (standard temperature and pressure (STP)) was observed over the Atlantic Ocean upwind of the major cities, with an interquartile range from 5.3 to 8.0 µg m−3. This concentration increased to a median of 11.1 µg m−3 (8.6 to 15.7 µg m−3) in the immediate outflow from cities. In the continental air mass away from the cities, the median aerosol loading was 7.5 µg m−3, with an interquartile range of 4.2 µg m−3. The accumulation mode aerosol population over land displayed similar chemical properties to the upstream population, which implies that upstream aerosol is a significant source of aerosol pollution over the continent. The upstream aerosol is found to have most likely originated from central and southern African biomass burning. This demonstrates that biomass burning plumes are being advected northwards, after being entrained into the monsoon layer over the eastern tropical Atlantic Ocean. It is shown observationally for the first time that they contribute up to 80 % to the regional aerosol loading in the boundary layer of southern West Africa during the monsoon season.

As a result, the large and growing emissions from the coastal cities are overlaid on an already substantial aerosol background. On a regional scale this renders cloud properties and precipitation less sensitive to future increases in anthropogenic emissions. Such high background loadings will lead to greater pollution exposure for the large and growing population in southern West Africa. These results emphasise the importance of including aerosol from across country borders in the development of air pollution policies and interventions in regions such as West Africa.

Sophie L. Haslett et al.
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Sophie L. Haslett et al.
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Short summary
Three aircraft datasets from the DACCIWA campaign in summer 2016 are used here to show there is a background mass of pollution present in the lower atmosphere in southern West Africa. We suggest that this likely comes from biomass burning in central and southern Africa, which has been carried into the region over the Atlantic Ocean. This would have a negative health impact on populations living near the coast and may alter the impact of growing city emissions on cloud formation and the monsoon.
Three aircraft datasets from the DACCIWA campaign in summer 2016 are used here to show there is...