Browsing by Author "Kangueehi, Kaukurauee Ismael"

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    Aerosol trace metal concentration and dissolution characteristics from known dust emitters in southern Africa
    (Stellenbosch : Stellenbosch University, 2017-12) Kangueehi, Kaukurauee Ismael; Fietz, Susanne; Frank, Eckardt; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
    ENGLISH ABSTRACT: Dust can be a source of micronutrients to surrounding areas such as oceans and terrestrial regions. The deposition of dust can provide trace elements to the open oceans, which can increase primary production and ultimately remove carbon dioxide from the atmosphere, therefore reducing global warming. Previous remote sensing studies have shown that southern African is a prominent dust emitting region and can potentially provide micronutrients to oceanic regions which might be depleted in some bioactive trace elements. Hysplit modeling software was used to estimate the long distance transport of dust emissions observed in southern Africa between January 2005 and December 2008. The observations revealed that most of the dust emissions occur during spring and winter seasons, with very little emissions in autumn. Most of the dust emissions tend to travel off the Namibian coastline towards the north-west Africa regions and are mainly influenced by strong south easterly trade winds. A strong air mass migrates towards the Indian Ocean and as far as the Australian continent due to the effects of the westerlies. Fewer air masses travel towards the nutrient-limited regions of the Atlantic Southern Ocean and central eastern Indian Ocean. The locations further north of the southern Africa preferentially travel towards the north west Atlantic Ocean, because the westerlies are not strong enough to transport air-masses towards the southern oceanic regions. This study also revealed that the prominent dust emitting sites in southern Africa are two ephemeral rivers, Kuiseb and Omaruru River as well as two ephemeral pans, the Etosha Pan in Namibia and Makgadikgadi Pan in Botswana. Emissions from these sources tend to travel towards north west Atlantic Ocean and south east Indian Ocean, with the exception of the Etosha Pan, which has emissions that travel towards the northern regions. These emitters were investigated for particle size distribution, mineralogical characteristics and trace elemental concentrations. The role of ephemeral rivers in southern Africa as potential sources of micronutrients to marine environments has not been previously investigated extensively. Most previous studies focussed on the ephemeral pans. Particle sizes can be an indicator of how far the sediments can potentially travel and of the trace elemental solubility. Etosha Pan has the finest grain sizes, while the Makgadikgadi had the coarsest grain size. Omaruru and Kuiseb River showed medium grain size variation. Our dissolution experiments showed, however, that the dissolution of the sediments is mostly influenced by the mineralogy rather than the particle sizes. The two pans appeared to be enriched in calcite, silica oxide and quartz, while the two rivers were more enriched in kaolinite, quartz, illite and muscovite. High trace element solubility in the Etosha Pan is most probably attributed to the high calcite content, which is highly soluble. A continuous flow through method proved to be effective and inexpensive. This study is one of the few in southern Africa which aimed at modelling the air mass pathways from dust emissions that have been observed instead of just creating simulations. Our findings highlight the importance of additional studies to prove the dissolution and quality of dust in dry regions as potential contributors to marine primary production.
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    Southern African dust characteristics and potential impacts on the surrounding oceans
    (Stellenbosch : Stellenbosch University, 2021-12) Kangueehi, Kaukurauee Ismael; Fietz, Susanne; Eckardt, Frank; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
    ENGLISH ABSTRACT: This study supports understanding the potential impact of dust aerosols from southern Africa have on the proximal ocean ecosystems. Dust can release essential nutrients and thus fertilize the ocean, which affects the food-web and the carbon dioxide concentrations in ocean waters, i.e. climate. Dust that fertilizes the phytoplankton communities in open oceans stimulates the drawdown of carbon dioxide from the atmosphere through the process of photosynthesis. Key here is that the nutrients must be released from the aerosol particles, as they can only be uptaken by biological organisms in soluble form. However, mineral dust emitted from arid environments differ from dust emitted through industrial activities; as a result the solubility and, by extent, potential impact on the ocean may also differ. In this study, I investigated mineral characteristics and solubility of dust from three distinct regions, one with a strong human footprint (Saldanha Bay), one with mostly natural mineral dust (Namib Desert) and one off shore over the Southern Ocean. The first study targeted Saldanha Bay, a town that hosts the largest port in South Africa, with exports of up to 60 million tons of iron and manganese ore annually and is home of a steel plant and a smelter. Satellite images and photos from the area have shown extensive dispersion of dust from the area. Solubility leaching experiments revealed that dust collected in this town is highly soluble (bioaccessible) for trace metals such as Fe (up to 28%), Cu (up to 33%), Pb (up to 45%) and Zn (up to 38%). Phytoplankton communities in open oceans are sometimes depleted in these trace metals, and thus, such high solubility of dust from Saldanha Bay can prove to be an important nutrient supplier to surrounding oceans. In addition, air mass trajectories revealed that this readily available dust most likely affects the southeast Atlantic and Indian Ocean. The major implication was that harbour towns can be essential sources of trace metals to proximal oceans. Secondly, I present results from the largest non-playa environment in the Namib Desert. Some of the prominent dust emitters located in the Namib Desert provide, feed and source of hundreds of tons of mineral dust to proximal surface ocean waters. The solubility of the mineral dust, in contrast to the mixed (anthropogenic and natural sourced) dust mentioned above, is much lower (e.g. up to 2 % for Fe, up to 7.8 % for Cu, up to 16% for Zn, up to 9% for Mn, 8% for Pb). These solubilities are in similar range to other semi-arid to arid major global dust sources such as the Sahara Desert in the Northern Hemisphere. Differences in mineralogy and particle size distribution at the sampling stations are the main drivers of the solubility. For example, the solubility of trace metals such as Fe, Zn and Pb is higher at the inland station than at the coastal station. The aggregated particles along the coastal stations are less soluble probably because of persistent foggy conditions and less sunlight. The inland stations had less foggy conditions and more fine-grained particles which are rich in FeO oxides. Air mass trajectory modelling indicated that this mineral dust, with its low solubility, typically travels towards the southeast Atlantic Ocean, but can also reach the nutrient poor areas in the Southern Ocean in some cases. This study highlighted the role of non-playa environments as important dust sources and that mineralogy coupled with particle size are closely related to trace metal solubility. In addition, to assessing the solubility and potential impact of dust with natural and anthropogenic imprint, we investigated the solubility of aerosols collected over the Southern Ocean south of South Africa on board on the research vessel SA Agulhas II. The solubility of the trace metals over the Southern Ocean ranges from 0.5 to 41% Fe, 9 to 48% Al, 0.1 to 88 % Mn, 25 to 72 % Zn and 3.3 to 39 % Co. This study found large variability in trace metal composition and fractional solubility in dust aerosols collected from three distinct environments in southern Africa and south of southern Africa. Our study highlighted the importance of southern African dust of both anthropogenic and natural sources as a nutrient supplier to surrounding coastal and open oceans.