Browsing by Author "Vickery, Kathryn J"

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    The nature of pan sediments: A case study on dust supply from the Makgadikgadi pans, Botswana
    (2014) Vickery, Kathryn J; Eckardt, Frank
    Understanding dust sources is essential, as aerosols from arid basins have widespread effects on atmospheric, biological and terrestrial processes on regional to global scales. This thesis presents the results of chemical and morphological analysis for sediments from Sua Pan, Botswana, which forms part of the Makgadikgadi Basin, southern Africa's most productive dust source. The analysis was performed in order to evaluate pan sediment characteristics and erosivity controls including the analysis of weather and climatological conditions. We analysed 41 grab samples consisting of crusts, fluff, and soil from 12 locations within a 144 km² grid (centred on 25.959°E and -20.5754°S) as well as 6 airborne samples collected using frisbees, BSNE traps and PI-SWERL exhaust, between July and October 2011. Dry sieving and laser diffraction for all 41 samples revealed a dominance of uni-modal, sand sized sediments, with the particle size distribution controlled by evaporite minerals, inferred from sample digestion. XRD and ICP analysis on 8 and 22 samples respectively, identified evaporite minerals including halite, thenardite, mirabilite and trona, in addition to carbonate minerals, which represent the young transient mineralogy of the pan. Older metal oxides were also present, originating from parent substrates. The addition of QEMSCAN analyses performed on 5 airborne and 1 surface sample, identified feldspar and pyroxene, along with quartz in its amorphous state, which was further confirmed through SEM analysis. QEMSCAN evaluated 255 minerals, many of which were below the detection limits of the 20 minerals identified through XRD analysis. QEMSCAN further attributed mineralogy and morphological detail to 27 000 individual grains per sample, positively linking airborne samples with their surface source. Seven years of twice daily MODIS images were analysed to identify the emission signature of the Makgadikgadi complex, revealing a number of synoptic states associated with emissions, peaking in July and driven largely by ridging anticyclones. The emission signature of the pans revealed that they respond to diurnal, seasonal and inter-annual controls, moisture dynamics and crust conditions with anthropogenic stresses also being evident. This study has interpreted the driving mechanisms and limitations associated with both the micro and macro scale of dust entrainment, advancing the current understanding of the Makgadikgadi as a dust source through interpreting these controls at the local scale.
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    Southern African dust sources as identified by multiple space borne sensors
    (2010) Vickery, Kathryn J; Meadows, Michael E
    Mineral aerosols emitted from arid and semi-arid regions effect global radiation, contribute to regional nutrient dynamics and impact local soil and water quality. Satellite imagery has been central to the identification and determining the distribution of source areas and the trajectories of dust around the globe. This study focuses on the dryland regions of Botswana, Namibia and South Africa. It uses the capabilities of the ultraviolet channels provided by the older Total Ozone Mapping Spectrometer (TOMS), the Ozone Monitoring Instrument (OMI) (a TOMS follow up), the visible bands of Moderate Resolution Imaging Spectroradiometer (MODIS), and the Spinning Enhanced Visible and Infrared Imager (SEVIRI) onboard Meteosat Second Generation (MSG). This study compares various dust detection products but also focuses on the application of thermal infrared bands from MSG through the usage of the new "Pink Dust" visua lisation technique using channels 7 (8.7 ~m), 9 (lO.8 ~m), and 10 (12.0 ~m). This multisensor approach resulted in a regional maps highlighting the distribution of source points and establishing some of the prevalent transport pathways and likely deposition zones. Southern African dust sources include a few large and many small pans, subtle inland depressions and ephemeral river systems, which are subject to a range of climatic conditions as part of the Kalahari and Namib region. This work in particular examines if source points are productive due to favourable climatic conditions. The debate around transport limit verses supply limit can only be solved at the local scale which requires observation at higher spatial and temporal resolution as provided by the latest dust detection products. MSG and MODIS in particular have shown distinct source point clusters in Etosha and the Makgadikgadi Pans which based on the courser resolution of older TOMS, have so far been treated as homogeneous sources. Data analyses reveal 327 individual dust plumes over the 2005-2008 study period, some of which are more than 300 km in length. These are integrated into existing climate and weather records provided by National Centers for Environmental Prediction (NCEP) data. The results identified a set dust drivers such as the Continental High Pressure, Bergwinds, Tropical Temperate and West Coast Troughs, and Westerly and Easterly Wave lows. This enhances our ability to predict such events, in particular, if transport acts as the limiting driver. Some of these find ings also have the potential to enhance our knowledge of the aerosol generation process elsewhere. The quality of findings are still limited by problems associated with dust plume substrates and clearly require significant surface validation relating to hydrological and climatic controls at the micro-scale. It is furthermore evident that no current instrument fully meets the requirements of the mineral aerosol research community.