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Abstract
Abstract. This study underlines the important role of the transported black carbon (BC) mass concentration in the West African monsoon (WAM) area. BC was measured with a micro-aethalometer integrated in the payload bay of the unmanned research aircraft ALADINA (Application of Light-weight Aircraft for Detecting IN situ Aerosol). As part of the DACCIWA (Dynamics–Aerosol–Chemistry–Cloud Interactions in West Africa) project, 53 measurement flights were carried out at Savè, Benin, on 2–16 July 2016. A high variability of BC (1.79 to 2.42±0.31 µg m−3) was calculated along 155 vertical profiles that were performed below cloud base in the atmospheric boundary layer (ABL). In contrast to initial expectations of primary emissions, the vertical distribution of BC was mainly influenced by the stratification of the ABL during the WAM season. The article focuses on an event (14 and 15 July 2016) which showed distinct layers of BC in the lowermost 900 m above ground level (a.g.l.). Low concentrations of NOx and CO were sampled at the Savè supersite near the aircraft measurements and suggested a marginal impact of local sources during the case study. The lack of primary BC emissions was verified by a comparison of the measured BC with the model COSMO-ART (Consortium for Small-scale Modelling–Aerosols and Reactive Trace gases) that was applied for the field campaign period. The modelled vertical profiles of BC led to the assumption that the measured BC was already altered, as the size was mainly dominated by the accumulation mode. Further, calculated vertical transects of wind speed and BC presume that the observed BC layer was transported from the south with maritime inflow but was mixed vertically after the onset of a nocturnal low-level jet at the measurement site. This report contributes to the scope of DACCIWA by linking airborne BC data with ground observations and a model, and it illustrates the importance of a more profound understanding of the interaction between BC and the ABL in the WAM region.
Highlights
Black carbon (BC) is one of the major contributors affecting the Earth’s climate system
An overview is presented with respect to the small-scale vertical variability of BC that was measured with ALADINA during the 2-week period (2–16 July 2016) of DACCIWA
Weather conditions were typical for the West African monsoon (WAM), mainly influenced by the occurrence of a low-level jet and low-level clouds, which are shown with ground-based monitoring by a wind profiler and ceilometer
Summary
Black carbon (BC) is one of the major contributors affecting the Earth’s climate system. Emitted BC caused by incomplete combustion is insoluble in water and strongly absorbs solar radiation in the visible spectrum (Bond et al, 2013). Once BC is emitted into the atmosphere, ongoing physical and chemical reactions, like secondary aerosol formation, influence the composition and mixing state of aerosol particles. Aged BC can act as cloud condensation nuclei after hygroscopic growth in the atmosphere (Zhang et al, 2008), which contributes to the indirect aerosol ef-. The understanding of the BC life cycle is still poor, as the monitoring of the type and source of combusted products, ageing processes in the atmosphere, and the current knowledge of wet deposition differ in terms of the measurement methods used, and simulations show high discrepancy The understanding of the BC life cycle is still poor, as the monitoring of the type and source of combusted products, ageing processes in the atmosphere, and the current knowledge of wet deposition differ in terms of the measurement methods used, and simulations show high discrepancy (e.g. Li et al, 2017)
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