Abstract
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.
Highlights
Atmospheric aerosols play a crucial role in the climate system by altering the radiation budget through scattering and absorption of solar radiation and by modifying cloud properties and lifetime
For cases related to dust, urban pollution and background condition, we clearly observe the monsoon layer up to 1.5 km a.m.s.l., which is characterized by weak to moderate wind speeds (2 to 10 m s−1, the latter corresponding to dust cases) and a flow from the south-west (220–250◦)
The peculiar dynamics of the region lead to a chemically complex situation, which enabled sampling of various air masses, including long-range transport of biomass burning from central Africa and dust from Sahelian and Saharan sources, local anthropogenic plumes from the major coastal cities, and mixtures of these different plumes
Summary
Atmospheric aerosols play a crucial role in the climate system by altering the radiation budget through scattering and absorption of solar radiation and by modifying cloud properties and lifetime. Considerable uncertainties remain about the contribution of both natural and anthropogenic aerosol to the overall radiative effect (Boucher et al, 2013). Large uncertainties are related to the complex and variable properties of aerosol particles that depend on the aerosol source and nature as well as on spatial and temporal variations. During transport in the atmosphere, aerosol particles may undergo physical and chemical aging processes altering the composition and size distribution and the optical properties and radiative effects. The capability of reproducing this variability in climate models represents a real challenge (Myhre et al, 2013; Stier et al, 2013; Mann et al, 2014). Intensive experimental observations in both aerosol source and remote areas are of paramount importance for constraining and evaluating climate models. Most back trajectories originated from the marine atmosphere and coastal areas south of the sampling area
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