Remote biomass burning dominates southern West African air pollution during the monsoon
Authors: Haslett Sophie L. , Taylor Jonathan W. , Evans Mathew , Morris Eleanor , Vogel Bernhard , Dajuma Alima , Brito Joel , Batenburg Anneke M. , Borrmann Stephan , Schneider Johannes , Schulz Christiane , Denjean Cyrielle , Bourrianne Thierry , Knippertz Peter , Dupuy Régis , Schwarzenböck Alfons , Sauer Daniel , Flamant Cyrille , Dorsey James , Crawford Ian , Coe Hugh 
Published: December 2019

Abstract. Vast stretches of agricultural land in southern and
central Africa are burnt between June and September each year, which
releases large quantities of aerosol 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 monsoon 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 (5.9 to 10.5 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 monsoon layer
over southern West Africa. Results from the COSMO-ART (Consortium for Small-scale Modeling – Aerosol and Reactive Trace gases) and GEOS-Chem
models support this conclusion, showing that observed aerosol concentrations
over the northern Atlantic Ocean can only be reproduced when the
contribution of transported biomass burning aerosol is taken into account. As a result, the large and growing emissions from the coastal cities are
overlaid on an already substantial aerosol background. Simulations using
COSMO-ART show that cloud droplet number concentrations can increase by up
to 27 % as a result of transported biomass burning aerosol. On a regional scale this renders cloud properties and precipitation less sensitive to
future increases in anthropogenic emissions. In addition, 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.