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

[摘要] 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 andultimately remove carbon dioxide from the atmosphere, therefore reducing global warming. Previous remotesensing studies have shown that southern African is a prominent dust emitting region and can potentiallyprovide micronutrients to oceanic regions which might be depleted in some bioactive trace elements. Hysplitmodeling software was used to estimate the long distance transport of dust emissions observed in southernAfrica between January 2005 and December 2008. The observations revealed that most of the dust emissionsoccur during spring and winter seasons, with very little emissions in autumn. Most of the dust emissions tendto travel off the Namibian coastline towards the north-west Africa regions and are mainly influenced by strongsouth easterly trade winds. A strong air mass migrates towards the Indian Ocean and as far as the Australiancontinent due to the effects of the westerlies. Fewer air masses travel towards the nutrient-limited regions ofthe Atlantic Southern Ocean and central eastern Indian Ocean. The locations further north of the southernAfrica preferentially travel towards the north west Atlantic Ocean, because the westerlies are not strong enoughto transport air-masses towards the southern oceanic regions. This study also revealed that the prominent dustemitting sites in southern Africa are two ephemeral rivers, Kuiseb and Omaruru River as well as two ephemeralpans, the Etosha Pan in Namibia and Makgadikgadi Pan in Botswana. Emissions from these sources tend totravel 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 elementalconcentrations. The role of ephemeral rivers in southern Africa as potential sources of micronutrients to marineenvironments has not been previously investigated extensively. Most previous studies focussed on theephemeral pans. Particle sizes can be an indicator of how far the sediments can potentially travel and of thetrace elemental solubility. Etosha Pan has the finest grain sizes, while the Makgadikgadi had the coarsest grainsize. 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 particlesizes. The two pans appeared to be enriched in calcite, silica oxide and quartz, while the two rivers were moreenriched in kaolinite, quartz, illite and muscovite. High trace element solubility in the Etosha Pan is mostprobably attributed to the high calcite content, which is highly soluble. A continuous flow through methodproved to be effective and inexpensive. This study is one of the few in southern Africa which aimed atmodelling the air mass pathways from dust emissions that have been observed instead of just creatingsimulations. Our findings highlight the importance of additional studies to prove the dissolution and quality ofdust in dry regions as potential contributors to marine primary production.

[发布机构] Stellenbosch University