Southern African aerosol trace metal concentration and dissolution kinetics characteristics. Aerosol trace metal concentration and dissolution from known dust sources in Southern Africa

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 modelling 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. 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. 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. - Abstract as displayed in the - Abstract booklet. The presentation on the day may differ from the - Abstract.