Abstract
In the framework of the Saharan Mineral Dust Experiment (SAMUM) in 2008, the mixing of the urban pollution plume of Dakar (Senegal) with mineral dust was studied in detail using the German research aircraft Falcon which was equipped with a nadir-looking high spectral resolution lidar (HSRL) and extensive aerosol in situ instrumentation. The mineral dust layer as well as the urban pollution plume were probed remotely by the HSRL and in situ. Back trajectory analyses were used to attribute aerosol samples to source regions.We found that the emission from the region of Dakar increased the aerosol optical depth (532 nm) from approximately 0.30 over sea and over land east of Dakar to 0.35 in the city outflow. In the urban area, local black carbon (BC) emissions, or soot respectively, contributed more than 75% to aerosol absorption at 530 nm. In the dust layer, the single-scattering albedo at 530 nm was 0.96 − 0.99, whereas we found a value of 0.908 ± 0.018 for the aerosol dominated by urban pollution. After 6 h of transport over the North Atlantic, the externally mixed mode of secondary aerosol particles had almost completely vanished, whereas the BC agglomerates (soot) were still externally mixed with mineral dust particles.
Highlights
Mineral dust (MD) and light-absorbing black carbon (BC), or soot, respectively, are the two dominant light-absorbing components of the tropospheric aerosol
Radiative properties of fresh MD and of dust mixed with anthropogenic pollution such as BC are in the focus of current research for two reasons: first, deserts are by far the strongest sources for airborne particulate matter on a global scale representing about half of the annual particle emission at the global scale (Forster et al, 2007), and second, driven by growing
The section closes with the assessment of the impact of aerosol mixing on the chemical composition and on the index of refraction
Summary
Mineral dust (MD) and light-absorbing black carbon (BC), or soot, respectively, are the two dominant light-absorbing components of the tropospheric aerosol. The mixing of dust with anthropogenic pollution from industrialized areas was investigated in several studies for the outflow regimes of Asian (Parungo et al, 1994; Clarke et al, 2004; Quinn et al, 2004) or African (Kandler et al, 2007; Chou et al, 2008) arid areas and deserts. In these studies, samples were collected hours to days away from the source region, giving sufficient time for efficient mixing of the aerosol. Key objectives tackled by the study were the modification of the dust aerosol optical depth (AOD) and the change in dust aerosol size distributions, mixing state and chemical composition caused by the anthropogenic emissions
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