Light absorption properties of aerosols over Southern West Africa

Abstract

<p><strong>Abstract.</strong> Southern West Africa (SWA) is an African pollution hotspot but a relatively poorly sampled region of the world. We present an overview of <i>in-situ</i> aerosol optical measurements collected over SWA in June and July 2016 as part as 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: <i>SSA</i>, asymmetry parameter, extinction mass efficiency, scattering Ångström exponent and absorption Ångström exponent: <i>AAE</i>). 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 of aerosol number concentration in air masses downwind of urban conglomerations, spectral <i>SSA</i> 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, <i>SSA</i> varied from 0.81 to 0.92 at 550 nm depending on the variable proportion of anthropogenic pollution particles externally mixed with the dust. Biomass burning aerosol particles were significantly more light absorbing than those previously measured in other areas (e.g. Amazonia, North America) with <i>SSA</i> ranging from 0.71 to 0.77 at 550 nm. The variability of <i>SSA</i> was mainly controlled by variations in aerosol composition rather than in aerosol size distribution. Correspondingly, values of <i>AAE</i> 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 literature shows a consistent picture of increasing absorption enhancement of biomass burning aerosol from emission to remote location and underscores that the evolution of <i>SSA</i> occurred a long time after emission.</p> The results presented here build a fundamental basis of knowledge about the <p>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 assessing the direct and semi-direct radiative effects of biomass burning particles.</p>

Absorption closure in highly aged biomass burning smoke

“…consistent with this picture of aged haze having a reduced BrC contribution compared to freshly emitted aerosol. Other recent studies have also shown AAE close to 1 in African biomass burning smoke that had been aged for several days (Zuidema et al, 2018;Denjean et al, 2019). These optical properties are also in a similar range to measurements by Saturno et al (2018), who measured smoke from southern Africa once it had reached Amazonia after 10 days transport.…”

Influx of African biomass burning aerosol during the Amazonian dry season through layered transatlantic transport of black carbon-rich smoke

“…pro- 445 posed that the CL derived from the ultraclean upper tropospheric air.In the vicinity of the western African coast, similar tropospheric stratification has been observed in a recent aircraft campaign. Aerosol and trace gas profile measurements over the Gulf of Guinea in July 2016, measured under the influence of aged BB plumes originating in central Africa, revealed two distinct aerosol layers, where the upper one, centered at 3.8 km altitude, was enriched in rBC (~0.3 µg m -3 ), CO 450 (~340 ppb), and OA (~65 µg m -3 )(Denjean et al, 2019; Flamant et al, 2018). Moreover,Weinzierl et al (2011) reported fvol = 26 % (13 -52 %, p3 -p97) for BB plumes near the western African coast in 2008 during SAMUM-2, which are larger than the fvol within the UPL in this study.…”

Influx of African biomass burning aerosol during the Amazonian dry season through layered transatlantic transport of black carbon-rich smoke

“…In the vicinity of the western African coast, similar tropospheric stratification was observed in a recent aircraft campaign. Aerosol and trace gas profile measurements over the Gulf of Guinea in July 2016, measured under the influence of aged BB plumes originating in central Africa, revealed two distinct aerosol layers, where the upper one, centered at 3.8 km altitude, was enriched in rBC (∼ 0.3 µg m −3 ), CO (∼ 340 ppb), and organics (∼ 65 µg m −3 ) (Denjean et al, 2019;Flamant et al, 2018). Moreover, Weinzierl et al (2011) reported f vol = 26 % (13 %-52 %, p 3 -p 97 ) for BB plumes near the western African coast in 2008 during SAMUM-2, which are larger than the f vol within the UPL in this study.…”