https://www.um.edu.mt/library/oar/handle/123456789/11480 2026-04-27T07:37:32Z 2026-04-27T07:37:32Z https://www.um.edu.mt/library/oar/handle/123456789/101803 2023-03-24T06:52:48Z 2016-01-01T00:00:00Z

Title: Dielectric properties of biological tissue in medical applications Abstract: Accurate knowledge of the dielectric properties of organ tissues is useful for many medical applications. Numerous studies have been published characterising various tissues but large variability in literature data still exists. Also, there exist conflicting ideas as to whether in-vivo dielectric properties differ from those measured from ex-vivo samples. Additionally, there exists a lacuna in knowledge of dielectric properties characterising biological tissues above 20 GHz. In this work, these lingering questions were addressed by initially conducting a thorough validation of the open-ended coaxial probe measurement system together with a complete assessment of the uncertainties in the measurements. Following that, Ex-vivo measurements on muscle, liver and kidney were conducted and wideband dielectric models were derived from 500 MHz to 40 GHz. Also, the differences between in-vivo dielectric measurements and those measured ex-vivo are investigated by conducting in-situ measurements for up to six minutes after animal death. This study showed that for the frequency range under study (500 MHz - 40 GHz) the differences are statistically significant but still lie within the uncertainty values. Therefore, it can be concluded that it is possible to substitute in-vivo measurements with ex-vivo, given that the organ is kept well hydrated. Additionally, the dielectric measurement techniques for solid/semisolid materials are reviewed. In particular, the conventional reflection and transmission waveguide measurement techniques are presented together with various inversion algorithms. This resulted in a comprehensive library of waveguide methods which can be used for the characterisation of various solid materials. Following accurate implementation of these methods using experimental and numerical studies on standard materials, dielectric measurements on cortical bone were conducted. This proved to be very challenging, mainly because of the extensive sample preparation required to fit the material in the waveguide aperture. Therefore, an alternative method using open ended waveguide is proposed for further validation. The implementation of this method is outlined, describing the forward analytical solution required to solve the inverse problem. Description: PH.D.PHYSICS

2016-01-01T00:00:00Z https://www.um.edu.mt/library/oar/handle/123456789/101522 2022-09-07T09:58:35Z 2016-01-01T00:00:00Z

Title: Modelling the formation and radiative effects of secondary organic aerosols in a climate model Abstract: Aerosols are known to cause changes in temperature and precipitation as a result of their effect on radiation and cloud droplets, and hence have a strong influence on the climate. Organic aerosols that form via the chemical oxidation of a gaseous precursor are called Secondary Organic Aerosols (SO As). The organic nature of these SOAs results in very different optical properties than other aerosols. Most climate models do not take into account SOAs or their radiative properties, however, recent studies have started including small suites of SOAs in order to reduce model biases. The aim of this research is to include the SOAs and their radiative properties to the Regional Climate Model, RegCM4 model (which does not currently model SOAs) in order to reduce the model biases and produce more reliable climate simulations. To achieve this, a recent gas phase module, the CB6r2, was coupled with RegCM4 to produce a more holistic suite of VOCs and chemical mechanisms, most notably ethyne, benzene, and pinene and their corresponding oxidation products. These gas phase products were coupled to Secondary Organic Aerosol Model (SORGAM) to produce the SOAs in RegCM4. The resulting aerosols were aggregated into six categories to reduce the size of the output and group the optical properties. The CB6r2 and SORGAM modules were successfully coupled with RegCM4, and the model now produces SOAs that interact with the radiation scheme. However, the current configuration of emission and chemical boundary conditions used for the CB6r2 do not produce a reliable chemical output. As a result of this, the SOA products are not yet reliable. Nevertheless, RegCM4 simulations using CB6r2, and the coupled CB6r2-SORGAM were run and analysed to identify methods that would improve the SOA simulations with RegCM4. The analysis also revealed a reduction in temperature and shortwave radiation over Europe when running RegCM4 with SORGAM, thereby showing the potential of SOA system in RegCM4. Description: PH.D

2016-01-01T00:00:00Z https://www.um.edu.mt/library/oar/handle/123456789/100936 2022-08-31T06:53:18Z 2016-01-01T00:00:00Z

Title: Regional dispersion of pollutants with special reference to Etna emissions as measured at the Giordan Lighthouse GAW station Abstract: Trace gas measurements of ozone, sulphur dioxide and nitrogen oxides together with meteorological parameters have been collected between 2011 and 2014 at Giordan Lighthouse Global Atmosphere Watch station in Gozo, Malta. Additionally, measurements of carbon monoxide, carbon dioxide, methane and water vapour were collected between 2012 and 2014. This research concentrates on the monitoring of these trace gases with a view to identify the origin of pollution sources and studying their dispersion. Ozone data gathered during this study was compared to that collected between 1997 and 2008 to analyse long term trends. A statistically significant decreasing trend was in fact found for ozone from 2003 to 2014 which was preceded by a statistically significant increase in the period between 1997-2003. Seasonal variations were evident in all the trace gas measurements and their variations with wind direction were also found to be statistically significant. The latter variations highlighted the effect of natural emissions from Etna, as well as local anthropogenic emissions from the main island of Malta as well as ship emissions in the Malta-Sicily channel. Etna' s volcanic eruptions were found to be one of the main natural sources of pollution affecting the Maltese Islands. Research was carried out to gather information about Etna's eruptions which involved the Maltese Islands, starting with historical eruptions dating back to the 14th century to more recent ones. The dispersion of Etna's ash plume was modelled using PUFF, which provided tephra deposit load and ash concentrations. Three different eruptive scenarios that characterize Etna's recent activity were considered; the first scenario representing the 2001 eruption (Sc1), the second scenario representing the July 1998 eruption (Sc2) whilst the third scenario represents the recent activity from 2011 onwards (Sc3). It was found that the time taken for the volcanic ash plume to reach the Maltese Islands when the wind direction is toward the Southwest ranges from 4 to 8 hours. The effect of wind speed and direction was also studied and it emerged that the probability that an Etna volcanic plume reaches Malta during an eruption is around 13% per annum. The now calibrated model produces daily forecasts of deposit load and cumulative area of volcanic ash dispersal. This allows for provision of adequate alerts to civil aviation authorities and Malta airport which is of direct use to local communities and aviation. Insights into the high levels of sulphur dioxide measured at Giordan Lighthouse were also explored by evaluating the potential effect of Etna's volcanic SO2 plume. Investigation was carried out by examining relationships between SO2 concentrations gathered at Giordan Lighthouse and those measured at Etna by the ultraviolet scanning spectrometer network FLAME (FLux Automatic Measurements) of the Istituto Nazionale di Geofisica e Vulcanologia, Osservatorio Etneo (INGV-OE) between 2011 and 2013. Seventy case studies with anomalous S02 peaks observed in Gozo were inspected by evaluating the strength of association between Giordan Lighthouse and INGV-OE records using statistical analysis. Statistically significant correlation was found in 40 of these cases. Results show for the first time the impact of Etna's volcanic S02 plume on the island of Malta and its potential effects on the local environment. On the other hand, shipping in the Malta-Sicily channel was found to be a great source of anthropogenic emissions affecting the Maltese Islands. This channel is a very busy route, and thus an Automatic Identification System (AIS) receiver was installed to be able to gather information on the geographic distribution of maritime traffic and its activities. A database was developed to store this information which was used to come up with two methods of measuring ship's traffic near the Maltese Islands between 2012 and 2014. One procedure involves taking hourly counts of the number and type of vessels and averaging them on a daily basis. The other method entails the comparison of vessels present in a narrow strip between Malta and Sicily on a daily basis, to come up with a yearly average number of vessels crossing the channel. The first method resulted in an average of 256 vessels present at any point in time in the area surrounding the Maltese Islands, with the highest values observed during summer with an average peak in July of 356 vessels. The second method showed that an average of 40,000 vessels cross the Malta-Sicily channel annually. These figures, although high, only quantify a portion of the entire situation, since they are based on data gathered by one AIS antenna, and thus coverage was limited. The majority of vessels were found to be cargo vessels in both methods. Additionally, through a collaborative agreement, the AIS data for the year 2012 was inputted into the Ship Traffic Emission Model (STEAM), developed by the Finnish Meteorological Institute to give results of oxides of sulphur and nitrogen as well as carbon dioxide emissions. This was the first time that the STEAM model was used in the Mediterranean and that such a study related to shipping activities near the Maltese Islands was carried out. Description: PH.D.PHYSICS

2016-01-01T00:00:00Z https://www.um.edu.mt/library/oar/handle/123456789/93898 2022-04-18T09:00:50Z 2016-01-01T00:00:00Z

Title: Controlling stochastic currency risk exposure optimally Abstract: Investors operating in countries adopting different currencies face an additional risk in the form of currency exchange rates. This thesis aims at deriving the optimal hedging strategy for such investors through the use of futures and forwards. Having described the processes underlying the economic framework which affect the investor, the theory of stochastic optimal control will be used to formulate and solve this the problem mathematically. As an attempt to solve the problem analytically, the dynamic programming approach will first be employed. However, since the resulting Hamilton-Jacobi-Bellman equation involves a highly non-linear second order partial differential equation, such a solution is hard to obtain in closed form and so we resort to numerical techniques. To this end we shall employ the Markov chain approximation method, in which a sequence of optimal stochastic control problems for Markov chains will be solved via the dynamic programming approach. The latter will lead to a sequence of functional equations, which have to be solved for the approximating value function. An approximate solution to these functional equations will then be obtained numerically via the Implicit method which, provided the approximating Markov chains are locally consistent, converges to the original controlled stochastic integral equation. Furthermore under this local consistency, the solutions to the functional equations are also known to converge to the value function of the original stochastic optimal control problem. Description: B.SC.(HONS)STATS.&OP.RESEARCH

2016-01-01T00:00:00Z