Overview of aerosol optical properties over southern West Africa from DACCIWA aircraft measurements
Authors: Denjean Cyrielle , Bourrianne Thierry , Burnet Frederic , Mallet Marc , Maury Nicolas , Colomb Aurélie , Dominutti Pamela , Brito Joel , Dupuy Régis , Sellegri Karine , Schwarzenboeck Alfons , Flamant Cyrille , Knippertz Peter 
Published: April 2020

Abstract. Southern West Africa (SWA) is an African pollution hotspot but a
relatively poorly sampled region of the world. We present an overview of
in situ aerosol optical measurements collected over SWA in June and July 2016 as
part as of the DACCIWA (Dynamics-Aerosol-Chemistry-Clouds Interactions in
West Africa) airborne campaign. The aircraft sampled a wide range of air
masses, including anthropogenic pollution plumes emitted from the coastal
cities, long-range transported biomass burning plumes from central and
southern Africa and dust plumes from the Sahara and Sahel region, as well as
mixtures of these plumes. The specific objective of this work is to
characterize the regional variability of the vertical distribution of
aerosol particles and their spectral optical properties (single scattering
albedo: SSA, asymmetry parameter, extinction mass efficiency, scattering
Ångström exponent and absorption Ångström exponent: AAE). The first
findings indicate that aerosol optical properties in the planetary boundary
layer were dominated by a widespread and persistent biomass burning loading
from the Southern Hemisphere. Despite a strong increase in aerosol number
concentration in air masses downwind of urban conglomerations, spectral
SSA were comparable to the background and showed signatures of the absorption
characteristics of biomass burning aerosols. In the free troposphere,
moderately to strongly absorbing aerosol layers, dominated by either dust or
biomass burning particles, occurred occasionally. In aerosol layers
dominated by mineral dust particles, SSA varied from 0.81 to 0.92 at 550 nm
depending on the variable proportion of anthropogenic pollution particles
externally mixed with the dust. For the layers dominated by biomass burning
particles, aerosol particles were significantly more light absorbing than
those previously measured in other areas (e.g. Amazonia, North America), with
SSA ranging from 0.71 to 0.77 at 550 nm. The variability of SSA was mainly
controlled by variations in aerosol composition rather than in aerosol size
distribution. Correspondingly, values of AAE ranged from 0.9 to 1.1, suggesting
that lens-coated black carbon particles were the dominant absorber in the
visible range for these biomass burning aerosols. Comparison with the literature
shows a consistent picture of increasing absorption enhancement of biomass
burning aerosol from emission to remote location and underscores that the
evolution of SSA occurred a long time after emission. The results presented here build a fundamental basis of knowledge about the
aerosol optical properties observed over SWA during the monsoon season and
can be used in climate modelling studies and satellite retrievals. In
particular and regarding the very high absorbing properties of biomass
burning aerosols over SWA, our findings suggest that considering the effect
of internal mixing on absorption properties of black carbon particles in
climate models should help better assess the direct and semi-direct
radiative effects of biomass burning particles.