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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.

Thermally generated convection and precipitation over volcanoes : numerical modelling of flow over Montserrat

Poulidis, Alexandros-Panagiotis January 2015 (has links)
Atmospheric flow over orography is a classic research area, while the atmospheric response to surface heating has become a focus more recently in the context of solar heating and forest fires. Here, for the first time, these forcing mechanisms are superposed to examine atmospheric flow over a mountain with a heated summit, i.e. an active volcano. Intense rainfall over active volcanoes is known to trigger dangerous volcanic hazards, from remobilising loose surface material into lahars or mudflows, to initiating explosive activity such as pyroclastic flows. The effect of a heated volcanic surface on the atmospheric circulation is investigated here – including examining the triggering of precipitation over the volcano. Recent activity at the Soufri`ere Hills Volcano (SHV), Montserrat, Eastern Caribbean, is a well-documented example of such rainfall–volcano interactions. Hence, Montserrat is used as a template for the experiments, although the experimental setup is general so the results will have applicability for other tropical island volcanoes. The Weather Research and Forecasting (WRF) atmospheric model has been used for the study, run in an idealised configuration with horizontal grid sizes down to 100 m. Initially, the effect of the heated surface is studied through idealised simulations over a Gaussian mountain with an imposed surface temperature anomaly on the volcano summit. Subsequently, a digital elevation model (DEM) of Montserrat is used to study the effects over this specific island. The atmospheric structure in most simulations is that of a typical tropical setting – easterly TradeWinds, capped by a temperature inversion. In these cases, localised convection triggered by the heat source can overcome convective inhibition and force deep convection, if there is sufficient convective available potential energy. A significant increase in precipitation over the volcano covering a 4 km2 area is consistently simulated for surface temperature anomalies above 40°C, an area-average value that is exceeded at the SHV. For a range of realistic atmospheric conditions, covering up to 18% of days in a relevant climatological study in the Caribbean, the precipitation increase is well above the observed threshold (5-10 mm hr⁻¹) required to trigger explosive volcanic activity. Hence, the thermal forcing of the atmosphere due an active, but non-erupting, volcano appears to be an important factor in rainfall-volcano interactions and should be taken into account in hazard assessment.

Iodine speciation in marine aerosol

Yodle, Chan January 2015 (has links)
Iodine chemistry in the marine aerosol plays important roles in the marine boundary layer such as ozone destruction and new aerosol particle formation. Iodine has a complex chemistry in the gas and aerosol phases and to date, what controls iodine speciation, the interactions and roles of individual iodine species are not well understood. This research aims to identify key controls on iodine speciation in marine aerosol. Effects of filter types on iodine and extraction methods were tested to provide optimum conditions for extraction of iodine species. Coupling of ion chromatography and inductively coupled plasma – mass spectrometry (IC-ICP-MS) for the measurement of iodine speciation was developed to provide a reliable analytical method. These optimised methods were used to determine iodine speciation in samples collected during cruises in the Atlantic Ocean (AMT21) and the Pacific Ocean (TransBrom and SHIVA). Major ions were also determined in these samples by ion chromatography (IC) with the results providing insight into the chemical characteristics aerosol samples. A high variability of the total soluble iodine (TSI) was observed between AMT21 (12–82, median 30 pmol m-3) and TransBrom (1.6–27, median 6.9 pmol m-3) and SHIVA (5.9–15, median 8.4 pmol m-3). The proportions of iodide (I-), iodate (IO3-) and soluble organic iodine (SOI) on the three cruises also showed a high variability: AMT21: I- 5.2–39%, median 14%; IO3- 36–99%, median 66%; and SOI 13–47%, median 28%, TransBrom: I- 8.8–64%, median 46%; IO3- 1.8–65%, median 6.2%; and SOI 5.9–50%, median 39%, SHIVA: I- 22–79%, median 42%; IO3- 17–66%, median 39%; and SOI non-determinable value–41%, median 14%. Three main types of aerosol with distinctive iodine speciation were identified: polluted aerosol, clean seasalt and mineral dust. pH seems to play an important role in regulating iodine speciation. The formation of HOI and the reduction of iodate are driven by acidic conditions in polluted aerosol. In clean less acidic seasalt aerosol, a high iodine enrichment was observed (565–1675, median 725), especially in the fine mode aerosol. For mineral dust, uptake of HIO3 on calcium carbonate surfaces seems to dominate.

Approximate vs. purely numerical approaches for full waveform modelling of global earth structure

Parisi, Laura January 2015 (has links)
This thesis focuses on the global seismic forward modelling of body and surface waveforms in realistic 3-D Earth models using approximate and purely numerical methods. Firstly, we investigate two techniques: (i) the Born approximation, which should be valid for media with weak heterogeneity; and, (ii) the full ray theory approach, which should be valid for smooth media. We find that the Born approximation has a very limited domain of validity. It only models accurately surface waveforms with wave periods longer than T~80 s–90 s in existing earth models, and for T > 120 s–130 s when models with stronger heterogeneity are considered. On the other hand, the full ray theory is valid for almost all the earth models considered, failing only for unrealistically rough models. Hence, there is scope to build future improved global tomographic models using this technique, which is computationally very efficient. We then use a purely numerical technique, the spectral element method, to assess the quality of a recently built global radially anisotropic mantle model (SGLOBErani). We find that it explains independent seismic data slightly better than a previous widely used model (S40RTS). Moreover, our tests find small data misfit differences between isotropic and anisotropic versions of the models considered, which highlight the difficulties in constraining 3-D radially anisotropic structure. Finally, we carry out forward modelling experiments of short-period (T > 5 s) body waves travelling through the Earth’s lowermost mantle and investigate the effects of isotropic (1-D and 3-D), anisotropic and attenuation structure on wave propagation. We find that phase interference can change the shape and apparent arrival-time of wave pulses. This can give rise to apparent SH-SV wave splitting, even when isotropic earth models are used. This suggests that caution should be taken when interpreting SH-SV splitting of deep mantle body waves exclusively in terms of anisotropy in the lowermost mantle.

Evaluating the natural attenuation of synthetic organic chemicals in a chalk aquifer

Hampson, Karen January 2013 (has links)
Hydrocarbon contaminants regularly leach into groundwater because of human activities and accidental chemical spills. Here, the contaminants pose a threat to the environment and valuable drinking water resources. Many subsurface environments are able to purify the groundwater of such contaminants, thus lowering the cost of remediation. This has sparked an interest in the use of naturally occurring processes to degrade contaminants in the subsurface and has opened up an area of research focusing on „natural attenuation‟. For the successful application of natural attenuation and in order to obtain regulatory approval a reliable assessment of the mechanisms responsible for the removal of contaminants in an aquifer is essential. This thesis presents evidence to support the natural attenuation of isoproturon (IPU) and sulphanilamide (SULPH) in a chalk aquifer and assesses in-situ biodegradation by: (a) analysing historical monitoring data to identify and support biodegradation processes at the contaminated field site; (b) the development of in-situ microcosm approaches, (c) determining the catabolic activity present across the site using ex-situ microcosm studies that replicate in-situ aquifer conditions and, (d) identifying and quantifying biodegradation in the aquifer (of SULPH) using compound specific isotope analysis. Based on the historical data and ex-situ microcosms IPU biodegradation was found to be occurring at the site; albeit at very low levels (2 %). It is suggested that the high concentrations of other contaminants at the site (up to 650 mg L-1), compared to the low IPU concentrations (9 μg L-1) may have impeded IPU biodegradation. However, even these low levels of IPU biodegradation may be important in aquifers exhibiting long residence times. The historical data and ex-situ microcosms further indicated the biodegradation of around 50% of the SULPH present at the site. The development of novel compound specific isotope analysis indicates 56 % of the SULPH has undergone biodegradation at the site. These three approaches; historical data, ex-situ microcosms and compound specific isotope analysis, provide strong evidence to support the occurrence of natural attenuation at the site. Of particular originality, is the identification of natural attenuation of SULPH in a chalk aquifer and the quantification of its biodegradation using compound specific isotope analysis.

Variability of the global ocean carbon sink (1998 through 2011)

Landschutzer, Peter January 2014 (has links)
In this thesis a newly developed 2–step neural network approach is used to reconstruct basin–wide monthly maps of the sea surface partial pressure of CO₂ (pCO₂) at a resolution of 1°X1° for both the Atlantic Ocean from 1998 through 2007 and the global ocean from 1998 through 2011. From those, air–sea CO₂ flux maps are computed using a standard gas exchange parameterization and high–resolution wind speeds. Observations form the basis of the studies conducted in this thesis. The neural network estimates benefit from a continuous improvement of the observations, i.e., the Surface Ocean CO₂ Atlas (SOCAT) database. Additionally, bottle samples were collected along the UK–Caribbean line to investigate the variability of the sea surface pCO₂ and its drivers. The neural network derived pCO₂ estimates fit the observed pCO₂ data with a root mean square error (RMSE) of about 10 μatm in the Atlantic Ocean from 1998 through 2007 and about 12 μatm in the global ocean from 1998 through 2011, with almost no bias in both studies. A check against independent pCO₂ data reveals a larger RMSE, in particular in regions with strong pCO₂ variability and gradients. Temporal mean contemporary flux estimates for the Atlantic Ocean (-0.45±0.15 Pg C ·yr⁻¹) and the global ocean (-1.54±0.65 Pg C ·yr⁻¹) agree well with recent studies. Trends and variabilities within the considered time periods are strongly influenced by climate modes. The global results from 1998 through 2011 reveal the strongest variability of the air-sea CO₂ fluxes in the Equatorial Pacific (±0.12 Pg C · yr⁻¹,±1σ), mainly driven by the El Niño Southern Oscillation (ENSO) climate mode. Trends towards a strengthening of the Southern Ocean carbon sink (-0.36±0.07 Pg C ·yr⁻¹ · decade⁻¹) from 1998 through 2011 are potentially linked to the recent weakening of the Southern Annular Mode (SAM) index.

Communicating climate change in Internet discussion fora : processes and implications

Hsu, Po-Han January 2014 (has links)
Communicating climate change issues in the Internet era requires new strategies that incorporate online communication. The rapid growth of new media and widespread use of the internet has marked everyday lifestyles in modern society. Information on a wide range of social issues, including climate change, is disseminated and debated through online discussions in internet fora. In this research, communication on internet fora and other potential forms of online social interaction are explored, to identify ways to enhance climate change communication on the Internet. The thesis raises three research questions to explore the communication context of internet fora discussion, namely: what are characteristics of the communication process on internet fora? Who is involved in the communication process? What influences do these online communication activities have on users’ everyday activities? The research applies a mixed-methods approach of analysing the usage of Internet fora and the contents of fora communication activities to explore these questions. This includes qualitative reviews of topic-thread discussions to reveal users’ roles in discussions, as well as surveys of fora users. It is argued that with increasing levels of interaction among communicators (people who post or reply to articles in order to express or respond ideas) on internet fora, these communicators are mobilised to join the online discussion process, competing for opinion leadership. The online discussions further contribute to the formation of opinions on climate change, as climate change and related issues are discussed The thesis thereby aims to contribute to the development of effective approaches for opinion formation and climate change communication online, and to encourage individuals to discuss changing behaviour patterns and public engagement of greenhouse gas reduction actions.

The role of place attachment in proactive and reactive adaptation to flood risk

Quinn, Tara January 2014 (has links)
This research examines the role of relationship with place in reactive and proactive adaptation to flood risk in England. There is currently little research into how floods affect relationship with place and how this relationship may determine adaptive behaviour. As the social limits to adaptation are increasingly recognised, theories about relationship with place offer a new perspective as to how flood risk is interpreted and acted on. This research examines how place attachment and place meaning interacts with flood risk at the household and community scale, and the role of place attachment in involvement in adaptation planning decisions. Fieldwork took place in two locations, Cumbria, where adaptation measures were in response to flooding that occurred in 2009 and Barnes, an area that is at risk of flooding and is part of the Thames Estuary 2100 plan that addresses flood risk in the coming century. Place attachment, place meaning, social capital and trust in institutions were examined using face-to-face surveys (n=380). In order to gain a deeper understanding of the relationship between place and flood experience in-depth interviews were carried out with fourteen older adults in Cumbria. The study finds that place attachment predicts choices and behaviours in pro-active adaptation to flood risk. When adaptation is reactive the strength of the relationship between adaptation behaviour and place attachment weakens. Following a flood, place related meanings change, this research finds that social aspects of place become more important to the individual and shape how place is experienced and can act to increase adaptive capacity. These findings demonstrate the dynamic nature of relationship with place and suggest that place attachment plays a different role in involvement in adaptation decisions depending on whether behaviour is in anticipation of, or in reaction to, environmental risk.

Responses of boreal vegetation to recent climate change

Barichivich, J. January 2014 (has links)
The high northern latitudes have warmed faster than anywhere else in the globe during the past few decades. Boreal ecosystems are responding to this rapid climatic change in complex ways and some times contrary to expectations, with large implications for the global climate system. This thesis investigates how boreal vegetation has responded to recent climate change, particularly to the lengthening of the growing season and changes in drought severity with warming. The links between the timing of the growing season and the seasonal cycle of atmospheric CO2 are evaluated in detail to infer large-scale ecosystem responses to changing seasonality and extended period of plant growth. The influence of warming on summer drought severity is estimated at a regional scale for the first time using improved data. The results show that ecosystem responses to warming and lengthening of the growing season in autumn are opposite to those in spring. Earlier springs are associated with earlier onset of photosynthetic uptake of atmospheric CO2 by northern vegetation, whereas a delayed autumn, rather than being associated with prolonged photosynthetic uptake, is associated with earlier ecosystem carbon release to the atmosphere. Moreover, the photosynthetic growing season has closely tracked the pace of warming and extension of the potential growing season in spring, but not in autumn. Rapid warming since the late 1980s has increased evapotranspiration demand and consequently summer and autumn drought severity, offsetting the effect of increasing cold-season precipitation. This is consistent with ongoing amplification of the hydrological cycle and with model projections of summer drying at northern latitudes in response to anthropogenic warming. However, changes in snow dynamics (accumulation and melting) appear to be more important than increased evaporative demand in controlling changes in summer soil moisture availability and vegetation photosynthesis across extensive regions of the boreal zone, where vegetation growth is often assumed to be dominantly temperature-limited. Snow-mediated moisture controls of vegetation growth are particularly significant in northwestern North America. In this region, a non-linear growth response of white spruce growth to recent warming at high elevations was observed. Taken together, these results indicate that net observed responses of northern ecosystems to warming involve significant seasonal contrasts, can be non-linear and are mediated by moisture availability in about a third of the boreal zone.

Effects of climate and land management changes on conservation of Mediterranean cork oak woodlands and their bird communities

Correia, Ricardo January 2014 (has links)
Cork oak woodlands are a keystone habitat for Mediterranean biodiversity but are currently undergoing a global decline and widespread management changes driven by social, economic and climatic factors. I examine the effects of both climate and land management changes on cork oak woodlands and their bird communities across the western Mediterranean Basin. Future climatic scenarios indicate that up to 60% of current cork oak woodlands may become unsuitable by 2080, especially in southern areas where they will be restricted to microclimatic refugia sites. Increasing aridity, particularly in drier microclimates, will potentiate a decline of cork oak tree condition resulting in impacts across the food web that ultimately have a negative effect on breeding bird abundance and diversity, particularly for tree-dependent forest species. Farmland and shrubland birds respond mainly to habitat features modified by land management and their abundances increase in areas with open and heterogeneous ground cover. Current abandonment, intensification and overexploitation trends are likely to have negative effects on their populations but climate change can also play an important role if it provides an additional stimulus for land management changes. The unique bird assemblages of North African cork oak woodlands seem particularly vulnerable to both factors and should therefore be considered a conservation priority. Nonetheless, climate change may benefit species of Mediterranean origin and seems to have enabled the establishment of resident populations of traditionally migratory bird species in the Iberian Peninsula. This may provide new conservation opportunities for declining migratory species as long as adequate winter habitat areas are available. Adaptation strategies should aim to incorporate biodiversity-friendly management practices, promote cork oak afforestation in areas that will become climatically suitable and evaluate alternatives to provide similar economic and environmental services in the regions where cork oak woodlands are likely to disappear.

Carbon structures and Mg-based materials for gas sorption

Tian, Mi January 2014 (has links)
Hydrogen is an alternative energy carrier for both mobile and stationary applications, which can effectively alleviate greenhouse gas emissions and reduce dependence on fossil fuels. The other promising approach in reducing greenhouse gas emissions is carbon capture. Mg-based materials have been considered as a promising hydrogen storage system due to their high hydrogen capacity (up to 7.6 wt.%), high abundance, low cost and lightweight. Different carbon structures have also drawn considerable interests for hydrogen storage and carbon capture. In this research, the nanostructured carbon was produced in a cold plasma reactor designed in-house as additives for improving hydrogen storage properties of Mg-based materials and CO2 storage of MgO. The effects of the plasma reactor’s flow rate, temperature and power were evaluated for the formation of the carbon structures. TEM shows that the carbon consists of spherical particles of 40.8±8.7 nm in diameter and graphene sheets. Further thermal treatment of the plasma carbon was carried out to enhance the surface area. The treatment conditions were optimized through response surface methodology (RSM). The effects of the treatment temperature, time and pressure on BET surface area and yield were studied. The predicted BET surface area and yield by RSM were found to agree with the experimental values. The optimum treatment conditions for the plasma carbon (PC) were found to be: temperature = 950°C and time = 120 min, pressure = 100 kPaCO2 gas flow. The optimized PC was mixed as an additive with 20h-milled MgH2/TiC for improvement of hydrogen storage properties. RSM optimized the mixing time and the content of PC in the (MgH2/TiC + PC) composite. The results demonstrated that both mixing time and the content of plasma carbon (PC) significantly affected the hydrogen storage properties. The effects of the PC, activated carbon (AC) and carbon nanotubes (CNTs) on hydrogen storage properties of MgH2/TiC were studied. PC, AC and CNTs showed positive effects on reducing hydrogen desorption temperature and improving the adsorption kinetics of the 20h-milled MgH2/TiC. PC shows the best effect due to its unique structure. The mechanism of the effects of the three carbon structures on hydrogen storage was discussed. ABSTRACT II The optimized PC was also mixed with MgO, both by ball milling and chemical coprecipitation methods to form porous carbon supported MgO for CO2 storage and separation. The results indicated that the chemically synthesized MgO+PC calcined at 800 °C (referred to as MgO/PC-800) showed the most promising CO2 storage capacity up to 6.16 mmol/g at 25 °C and 1500 kPa CO2 pressure. The introduction of PC improves the CO2 adsorption capacity of the chemical synthesized MgO due to improved surface area. The dual-site Langmuir (DSL) model was employed to predict adsorption equilibria of CO2/H2 gas mixtures, which well simulated the behaviors of pure CO2 adsorption and H2 adsorption, and can be used to predict the binary CO2/H2 gas mixture separation.

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