Overview of aerosol optical properties over southern West Africa from DACCIWA aircraft measurements

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.

Source–Receptor Relationships and Cluster Analysis of CO2, CH4, and CO Concentrations in West Africa: The Case of Lamto in Côte d’Ivoire

“…CO 2 has a different seasonal cycle lagging by one months after CH 4 , and CO. The background CO value in June and July (≈117 ppb) is comparable to that obtained by Denjean et al [49] (≈180 ppb) during the DACCIWA (Dynamics-Aerosol-Chemistry-Clouds Interactions in West Africa) campaign in June and July 2016 in polluted coastal cities (e.g., Abidjan, Accra and Lomé).…”

“…Clustering is a multivariate statistical technique designed to explore a structure within a dataset with unknown prior properties [52]. The technique aims at affecting data to significant classes by The evaluation of the CO 2 , CH 4 , and CO background signals is important to infer the concentration increases due to regional or local emissions, e.g., [45,49,50]. We have defined the background (in blue in Figure 5) as the concentrations measured when CO concentrations [18,34,51] are below their fifth percentile within a 7-day moving-window.…”

Unexpected Biomass Burning Aerosol Absorption Enhancement Explained by Black Carbon Mixing State

“…As part of the DACCIWA project (Knippertz et al, 2015), the ATR‐42 airborne platform of SAFIRE performed 20 flights over the Gulf of Guinea and inland from 29 June to 16 July 2016 (Flamant, Knippertz, et al, 2018). Aerosol and trace gas measurements revealed two distinct BBA layers over SWA (Denjean et al, 2020; Flamant, Knippertz, et al, 2018; Haslett et al, 2019), namely, (i) a lower layer located in the marine boundary layer (BL‐BBA) and (ii) an upper, about 2 km‐thick layer centered at ~3 km above sea level (FT‐BBA). In the DACCIWA region, the widespread low level clouds commonly lead to a decoupled BL which top is located at about 500 m above sea level deep away from the coast and about 800 m at the coast (Flamant, Deroubaix, et al, 2018).…”

Direct and semi-direct radiative forcing of biomass-burning aerosols over the southeast Atlantic (SEA) and its sensitivity to absorbing properties: a regional climate modeling study

“…6) the monthly-mean SSA (for all aerosols over the whole atmospheric column and at 550 nm) obtained by RegCM (2003RegCM ( -2015 and ALADIN (2000ALADIN ( -2015 with the recent MACv2 (year 2005) climatology over the box_S (5-15 • S, 15-25 • E). We recall that monthly sun-sky photometry statistics (from AERONET; Dubovik and King, 2000) were used as part of the MACv2 climatology (Kinne, 2019). The comparison indicates that the ALADIN SMK simulation is able to capture the seasonal cycle of SSA, especially between April and October.…”