School of Earth Sciences - Theses

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    Evaluation of aerosol representation by ACCESS-CM2 with increased aerosol-chemistry complexity in the Southern Ocean
    Wadlow, Imogen ( 2020)
    This thesis identifies the inherent biases of aerosol parameters within the next-generation Global Climate Model (GCM); the Australian Community Climate and Earth System Simulator Coupled Model (ACCESS-CM2). GCMs poorly represent clouds and aerosols over the Southern Ocean, resulting in systematic shortwave (SW) radiation biases with widespread global energy budget impacts. This research determines whether a more complex, physically-representative aerosol-chemistry scheme may reduce the Southern Ocean radiation bias, and the inherent aerosol biases established within ACCESS-CM2. Southern Ocean aerosols are dominated by sea-salt and biogenic products. This study ran a control ACCESS-CM2 simulation and three perturbation simulations, which altered either the representation of Primary Marine Organic aerosols, sea-salt, or implemented a fully interactive chemistry scheme respectively. A suite of ground-based and satellite observations was collected and compared against each simulation to establish model bias respective to key aerosol metrics including Aerosol Optical Depth (AOD), Cloud Con- densation Nuclei (CCN) and SW radiation. Simulation biases were explained through model-observation comparisons of aerosol chemistry, size and number parameters. Overall, ACCESS-CM2 exhibited a substantial positive bias in Aerosol Optical Depth (AOD) and SW radiation, and underestimated Cloud Condensation Nuclei concentration. This research suggests that increasing the complexity of aerosol schemes was able to provide a closer model agreement with observed aerosol metrics of AOD and CCN. Fully interactive chemistry provided the best reduction in both AOD and CCN bias. However, modified aerosol schemes have negligible effects upon the inherent SW radiation bias in ACCESS-CM2, suggesting further research into cloud schemes is necessary.
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    Aerosol contributions to speleothem geochemistry
    Dredge, Jonathan ( 2014)
    There is developing interest in cave aerosols due to the increasing awareness of their impacts on the cave environment and speleothems. This study presents the first multidisciplinary investigation into cave aerosols and their potential contribution to speleothem geochemistry. Aerosols are shown to be sourced from a variety of external emission processes, and transported into cave networks. Both natural (marine sea-spray, terrestrial dust) and anthropogenic (e.g. vehicle emissions) aerosol emissions are detected throughout caves. Internal cave aerosol production by human disruption has also been shown to be of importance in caves open to the public. Aerosols produced from floor sediment suspension and release from clothing causes short term high amplitude aerosol suspension events. Cave aerosol transport, distribution and deposition are highly variable depending on cave situation. Cave morphology, ventilation, and environmental conditions will influence how aerosols are distributed through cave networks. Aerosol deposition monitoring in Obir Cave, Austria has shown the significance of cave chamber size in aerosol transport, with large open chambers presenting higher levels of deposition. Modern monitoring of suspended aerosol concentrations, CO2 and temperature in Gough’s Cave, Cheddar Gorge have presented a strong relationship with cave ventilation processes. Temporal variations of aerosol levels have demonstrated the ability of aerosol monitoring to record seasonal ventilation shifts, beyond anthropogenic influences. Aerosol minima (based on 24 hours) provide a representation of natural aerosol baseline conditions without diurnal anthropogenic influences. Aerosols have shown a quicker recovery to natural background levels when compared to CO2 and T, making aerosols a sensitive and effective monitoring tool. When used in combination with more established monitoring methods, suspended aerosol monitoring is a beneficial addition to cave environmental studies. Theoretical modelling and calculations based on modern aerosol monitoring have established that aerosol contributions are highly variable. In some instances, modern aerosol supply is sufficient to account for speleothem geochemistry concentrations entirely. Aerosol contributions are of greatest significance under slow growth or hiatus scenarios and high aerosol deposition scenarios. Geochemical and stratigraphical analysis of a flowstone core from Gibraltar has highlighted the importance of hiatus events for future aerosol studies. Hiatus events provide a unique opportunity to investigate the type and amount of aerosol deposition and accumulation. Marine aerosol contributions have been quantified in the Gibraltar flowstone core and account for 18.5% of speleothem Sr. Sr isotopic analysis has confirmed the significance of marine aerosol contributions. Flowstone analysis has also demonstrated the ability of speleothems to record shifts in the supply of highly radiogenic terrestrial dust. Bio-aerosol deposits and bacterial colonisation have been identified as a potential source of trace element bioaccumulation and flowstone coloration in Yarrangobilly Caves, Australia. Bio-aerosols have shown to be deposited throughout cave networks. Inorganic aerosol deposition may provide a nutrient supply to cave surfaces allowing for, and sustaining microbial colonisation.