Aerosol trace metal concentration and dissolution characteristics from known dust emitters in southern Africa

Kangueehi, Kaukurauee Ismael (2017-12)

Thesis (MSc)--Stellenbosch University, 2017.

Thesis

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.

AFRIKAANSE OPSOMMING: Stof kan 'n bron van mikrovoedingstowwe wees vir omliggende gebiede soos oseane en landstreke. Die afsetting van stof kan spoorelemente aan die oop oseane verskaf, wat primêre produksie kan verhoog en uiteindelik koolstofdioksied uit die atmosfeer verwyder, wat sodoende globale verwarming verminder. Vorige afstand waarnemings studies het getoon dat suidelike Afrika 'n prominente stof vrystellende streek is en moontlik mikronutriënte kan verskaf aan oseaanstreke wat in sommige bioaktiewe spoorelemente uitgeput is. Hysplitmodelleringsagteware is gebruik om die langafstand vervoer te beraam van stofvrystellings wat in suidelike Afrika waargeneem is tussen Januarie 2005 en Desember 2008. Hierdie waarnemings het getoon dat die meeste stofvrystellings gedurende die lente- en winterseisoene plaasvind, met baie min vrystelling tydens herfs. Die meeste van die stofvrystellings is geneig om van die Namibiese kus na die noordwestelike Afrika-gebiede te reis en word hoofsaaklik deur sterk Suid-Oosterse handelswinde beïnvloed. ‘n Sterk lugmassa migreer na die Indiese Oseaan en so vêr as die Australiese vasteland as gevolg van die uitwerking van die westewinde. Minder lugmassas reis na die voedingsarm gebied van die Suidelike Oseaan en Sentraal-Oos- Indiese Oseaan. Die stof van areas verder noord van die suidelike Afrika streek reis verkieslik na die noordwestelike Atlantiese Oseaan, want die westewinde is nie sterk genoeg om lugmassas na die suidelike oseaanstreke te vervoer nie. Hierdie studie het ook aan die lig gebring dat die prominente stof vrystellers in suider-Afrika twee kortstondige riviere, die Kuiseb en Omaruru-rivier sowel as twee kortstondige panne, die Etosha Pan in Namibië en Makgadikgadi Pan in Botswana is. Stof vrystelling van hierdie bronne is geneig om te reis na die Noordwes-Atlantiese Oseaan en Suid-Oos Indiese Oseaan, met die uitsondering van die Etosha Pan, wat vrystelling het wat na die noordelike streke reis. Hierdie stof vrystellers is ondersoek vir deeltjiegrootteverdeling, mineralogiese eienskappe en spoorelementkonsentrasies. Die rol van kortstondige riviere in Suider-Afrika as potensiële bronne van mikrovoedingstowwe in mariene omgewings is nie voorheen breedvoerig ondersoek nie. Die meeste vorige studies het gefokus op die kortstondige panne. Partikelgroottes kan 'n aanduiding wees van hoe vêr die sedimente potensieel kan beweeg en elementêre oplosbaarheid van die verskillende stof vrystellingsbronne kan opspoor. Etosha Pan het die fynste korrel groottes, terwyl die Makgadikgadi die grofste korrel grootte gehad het. Omaruru- en Kuisebrivier het 'n middlematige korrelgrootte variasie getoon. Die twee panne was verryk in kalsiet, silikaoksied en kwarts, terwyl die twee riviere meer kaoliet, kwarts, illiet en muskoviet ryk was. Hoë oplosbaarheid in die Etosha Pan word waarskynlik toegeskryf aan die hoë kalsiet inhoud, wat hoogs oplosbaar is. Ons resultate het getoon dat die dissolusie van die sedimente meestal deur die mineralogie eerder as die deeltjiegroottes beïnvloed word. 'n Deurlopende vloei metode was effektief en goedkoop. Hierdie studie is een van die min in suider Afrika wat daarop gemik was om die lugmassas te modelleer van stofvrystellings wat in ag geneem is, in plaas van om net simulasies te skep. Ons bevindings beklemtoon die belangrikheid van bykomende studies om die dissolusie en kwaliteit van stof in droë streke te bewys as moontlike bydraers tot primêre produksie in mariene stelsels.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/102728
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