Browsing by Subject "Ocean and Atmospheric Science"

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    Open Access
    Chemical composition and pH of marine aerosols in the summer-time marine boundary layer of the Atlantic sector of the Southern Ocean
    (2023) Xokashe, Sive; Altieri, Katye
    Aerosol acidity is a fundamental parameter of aqueous chemistry that impacts the lifetime of pollutants, biogeochemical cycles, human health, and climate. However, despite this importance, the effects of aerosol pH on these processes are difficult to constrain, in part because there are no direct methods to measure aerosol pH. The lack of observations in the remote clean atmosphere makes quantification of aerosol pH even more challenging. This dissertation aims to predict the spatial and temporal variation of aerosol pH and its drivers in the marine boundary layer of the summertime Southern Ocean. Results are presented from Ambient Ion Monitor (online method) and high volume sampler (offline method) based chemical analysis of aerosol species sampled during a cruise from Cape Town (34.11°S, 18.03°E) to Antarctica (70°S, 2.11°W). In addition, a thermodynamic model, ISORROPIA-II, is used to predict the pH of aerosol. The thermodynamically predicted aerosol pH from both the online and offline methods are generally acidic (i.e., <7). The modelled pH of particles with a diameter less than 1 µm (submicron) from the offline method ranged from -0.67 to 4.37 with an average value of 0.27, while the pH of particles with a diameter greater than 1 µm (supermicron) ranged from 1.67 to 4.68 with an average value of 2.98. For the online method, which only sampled particulate matter with a diameter less than 2.5 µm in size (PM2.5), the pH ranged from -0.68 to 4.46 with an average value of 2.07. The variation in predicted submicron pH was best explained by the sodium to sulfate (Na+ /SO4 2- ) ratio, while no relationship was observed with temperature or relative humidity. For both supermicron and the PM2.5 aerosols measured online, the pH was best explained by the molar ratio of the inorganic aerosol species. In addition, the supermicron pH varied as a function of relative humidity while for the PM2.5 did not. Using a combination of aerosol concentration, windspeed, and sea ice concentration data, along with air mass back trajectories, it was found that generally, aerosol samples that originated from the open ocean were relatively more acidic compared to those that had air mass back trajectories originating from sea ice covered surface ocean. This was attributed to the partitioning of hydrogen chloride (HCl). One of the major limitations in this study was a lack of aerosol gas concentrations to constrain the model (e.g., ammonia (NH3), nitric acid (HNO3), HCl). Therefore, to improve the predicted aerosol pH, measurements of aerosol gas species is highly recommended. Additionally, it was found that when an offline method is used, an online method proves useful for validation of possible sampling artefacts that may be occurring in the offline method.
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    Towards Rogue Wave Characterization in False Bay, South Africa
    (2019) Salonen, Nicholas M; Vichi, Marcello; Rautenbach, Christo
    Rogue waves are extreme oceanic waves which exceed twice the significant wave height of the wave record. In False Bay, South Africa, a shallow sea mount called rocky bank was suggested by Shipley (1964) to be the cause for increased wave energy along the eastern periphery of False Bay, where it is not uncommon for coastal users, particularly rock fishermen, to be washed off of the rocky coastline. The shallow bathymetry associated with rocky bank refracts the incoming open ocean swell and is thought to focus the wave energy toward sections of this coastline. This investigation aims to understand which offshore wave conditions could be conducive to rogue wave development along the eastern periphery of False Bay, South Africa. To do so, the phase-averaging spectral wave numerical model SWAN was used to investigate various wave conditions which are known to enter False Bay. The focusing effect which rocky bank supposedly has on the incoming wave energy was investigated by deepening and shallowing rocky bank's depth. This study showed that rocky bank does cause a varying degree of wave focusing which appears to be most dependent on the wave period, with higher wave periods leading to greater extents of refraction. According to this study, the average conditions which impact the south-western coastline of South Africa are conducive to wave focusing by rocky bank and despite the rarity of rogue wave events, coastal users are still affected by them. Calm seas have the ability to mislead users that make use of these dangerous rocky coasts, when the event of a rogue wave in small seas can still be sufficient to wash a person into the sea. A study by de Vos & Rautenbach (2019) suggested that users are at a higher risk, for a variety of coastal activities, during (seemingly) good conditions due to the increased human exposure. Since this study only simulated monochromatic wave conditions, one would expect the results to be significantly different if multiple swell systems were simulated concurrently, as this introduces the possibility of the dispersive focusing mechanism for swells that travel at different speeds and thus increasing the likelihood of a rogue wave. The knowledge acquired from this study is a great step towards understanding the wave climate of False Bay. This information can be used to implement possible warning systems which would hopefully be respected by the community and ultimately lessen the lives lost to extreme waves along False Bay's 'Death coast'.
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    Wet and dry troughs over Southern Africa during early summer
    (1992) Barclay, Jennifer Jayne; Jury, Mark R; Washington, Richard
    The synoptic scale structure of troughs transiting southern Africa in October and November is examined. Cases are chosen on the basis of an upper trough being present over southern Africa, and a minimum horizontal and vertical temperature change. Wet and dry troughs are differentiated by the extent and amount of interior rainfall produced. Once selected, a spatial and temporal framework was used on surface, upper-level synoptic maps and radiosonde sections. Individual and composite time-height and spatial sections are analysed for anomalies of temperature, geopotentials, kinematic, vorticity and divergence fields, dewpoint, dewpoint depression, mixing ratio, dry and total static energy and, equivalent potential temperature. European Centre for Medium Range Forecast (ECMWF) maps of vertical motion are analysed and ECMWF data were exclusively used in the wet and dry case study. The essential features of wet troughs include a large amplitude upper westerly wave with a diffluent and northward displaced sub-tropical jet stream, slow movement, westward tilted trough in the vertical and a negative - positive dipole where a high is located south of the low pressure system. In comparison dry troughs are characterised by a small amplitude upper wave, rapid movement, no tilted trough in the vertical, and a stationary high-pressure system over the western interior. Radiosonde moisture variables, circulation anomalies and ECMWF fields of moisture flux give evidence in the wet cases of inflow from the north-east, in conjunction with a ridging anticyclone south of the continent. In dry cases the trajectory of flow is north-westerly and the supply of moisture is limited. In the ECMWF composite maps of vertical motion, lift is weak over the interior in dry cases consistent with a gentle slope in the divergence profile. For wet cases upward motions are intense and widespread over the interior consistent with a steep slope in the divergence profile, and compensated by descending motions over the adjacent oceans along 30° S band. Precipitation in wet events is a combination of dynamical forcing, prefrontal moisture and unstable lower troposphere. In dry events, precursor moist inflow is limited, weak instability and, a gentle slope in the divergence/convergence fields are not conducive to sustain lift.