• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 6
  • 4
  • 1
  • Tagged with
  • 150
  • 12
  • 11
  • 8
  • 7
  • 6
  • 6
  • 5
  • 5
  • 4
  • 4
  • 4
  • 4
  • 4
  • 3
  • 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.
1

Tropospheric formaldehyde retrievals with GOME-2

Hewson, Will January 2014 (has links)
Biogenic Volatile Organic Compounds (BVOCs) emitted by terrestrial ecosystems impact air quality and climate. The most important BVOC for global tropospheric composition is isoprene, whose annual global emissions (400-600 Tg C/year) account for ~50% of the total global BVOC budget. Tropical ecosystems are generally thought to be responsible for 70-90% of the global isoprene budget. Satellite observations of formaldehyde (CH[subscript 2]O), a high-yield, short lifetime product of isoprene oxidation, provide top-down constraints on surface isoprene emissions. Errors in retrieved satellite slant column densities (SCD - trace gas concentration along the instrument's line of sight) are typically in the region of 40% for scenes with little cloud and aerosol contamination. Error sources arise from instability in the differential optical absorption spectroscopy (DOAS) retrieval procedure, largely due to low signal to noise ratios frequently encountered with space-borne UV spectrometers, coupled to CH[subscript 2]O's faint absorption signal. The essential conversion of SCDs to vertical column densities (VCDs - vertical trace gas concentration extending from the Earth's surface), needed for application of retrieved CH[subscript 2]O values to geochemical modelling schemes, is a further significant contributor to product error (30-60%), being strongly influenced by aerosol, cloud and albedo inhomogeneities at sub-pixel level. A new global 5 year CH[subscript 2]O product (2007-2011) generated with global ozone monitoring experiment 2 (GOME-2) satellite radiance data is presented. SCDs for this are generated with optimised spectroscopic fit settings derived from an extensive sensitivity study of input retrieval parameters for the DOAS portion of the retrieval. SCDs are then converted to vertical column densities VCDs using air mass factors calculated with a significantly upgraded retrieval scheme, accounting for atmospheric radiative transfer due to viewing geometry, surface albedo and atmospheric scattering. New GOME-2 CH[subscript 2]O VCDs are then evaluated against GEOS-Chem modelled CH[subscript 2]O VCDs, and compared to a suite of associated environmental parameters.
2

Investigation of the chemistry of the demethylation of pectin by halide ion

McRoberts, W. C. January 2012 (has links)
No description available.
3

Remote sensing of photosynthetic-light-use efficiency

Nichol, Caroline Jane January 2000 (has links)
Narrow waveband spectral reflectance, chlorophyll fluorescence and gas exchange measurements were made sequentially on individual leaves of <i>Vicia faba</i> at three levels of 500, 1000 and 1500 mmol m<sup>-2</sup> s<sup>-1</sup>. PRI was calculated from the reflectance data and correlated with LUE calculated from gas exchange and photosystem II( PS II) efficiency measured from chlorophyll fluorescence. PRI was linearly related to photosystematic LUE and PS II efficiency over the range of light levels and followed the diurnal light response of photosynthetic light-use-efficiency. The relationship between PRI and LUE was then investigated over boreal forests in Canada. Reflectance measurements of four stands were made from a helicopter-mounted spectroradiometer during the 1994 growing season. Eddy covariance towers were measuring CO<sub>2</sub> fluxes during this time and LUE was calculated from these data. A strong linear relationship was found between PRI and LUE for the four sites, and when expressed on a functional type basis of conifer and deciduous, the relationships were stronger still. The relationship between PRI and LUE at the canopy scale was further investigated in boreal forest in Siberia during the "green-up" period from winter into spring in 2000. During this time the photosystems were under stress as a result of extremes of temperature (from -20°C to +35°C) coupled with a high radiation load. Reflectance measurements of four stands were made from a helicopter-mounted spectroradiometer and PRI was calculated from these data. Eddy covariance towers were operating at each site and offered a means to calculate LUE. A significant linear relationship was apparent between PRI and LUE although the relationship was weaker than that found between PRI and LUE in the Canadian boreal forest. Collating the data for all the boreal forests highlighted a difference between the slopes of the PRI:LUE relationship. Reflectance measurements were also made from the eddy covariance tower of the Scots pine canopy and needles were sampled simultaneously for xanthophyll pigment determination. Strong linear relationships were observed between PRI, the epoxidation state of the xanthophyll cycle and LUE over the seasonal change and the diurnal cycle.
4

Mass spectrometric characterisation of priority pollutants

Mudge, Angela January 1999 (has links)
The majority of the work described in this thesis has involved the development and application of two-step mass spectrometry for the <i>in situ</i> analysis of priority pollutants. With this approach - laser desorption laser photoionisation mass spectrometry, abbreviated here as L2MS - a pulsed (infrared, visible or UV) laser is used to desorb neutral molecules which are then positioned using a second pulsed (UV) laser. The resulting product photoions are then analysed using time-of-flight mass spectrometer. A detailed comparison of L2MS versus other mass spectrometric techniques for the analysis of environmental samples has been carried out. The underlying physical principles behind L2MS are described, together with the instrumentation used and the experimental procedures and protocols that were developed. The majority of the work presented has focused on the polycyclic aromatic hydrocarbons (PAHs) and related compounds directly from their host matrices, photoionisation at a wavelength of 193 nm or 266 nm. The matrices investigated included soils, sediments, extracts and aerosol particulates. Although, these samples were part of a complex system the selectivity inherent in L2MS enabled simple mass spectra to be obtained, showing each PAH as a molecular ion. A number of experiments have been carried out using certified reference materials to investigate the reproducibility of the spectra generated and the feasibility of using L2MS for quantitative as well as semi-quantitative studies on "real-world" samples. In parallel with this work, the same reference materials have been interrogated using a variety of other analytical techniques, such as gas chromatography mass spectrometry (GC MS), field desorption mass spectrometry (FD MS), laser desorption/ionisation mass spectrometry (LD1 MS), and high performance liquid chromatography (HPLC). These techniques, together with L2MS, were applied to analyse both the intact samples ("<i>in situ</i>" studies) and Soxhlet extracts. Using GCMS and HPLC enabled the identification of most of the PAHs present in the extracts. Whereas, FD was found to produce a complex spectra which was difficult to interpret. Finally LDI MS could prove to be useful for screening, as sample preparation and production of a spectra is simple. However, under the ionisation wavelength employed, 337 nm, during this work spectra were difficult to produce.
5

The vertical distribution, associations and mobility of Pb and other elements in an ombrotrophic peat bog

Freeman, Alex January 2001 (has links)
From the results of this study, elevated Pb concentrations of up to 350<i> </i>mg/kg(dry weight) were found mainly in the top 0-30 cm of the solid phase peat. This was consistent with the deposition of anthropogenic Pb particularly since the onset of the Industrial Revolution. Peak concentrations of 328 and 165<i> </i>mg/kg were found at 4-6 cm and 20-22 cm respectively. Porewater Pb concentratiom were typically very low with a maximum value of ~0.04 mg/l and values <0.01 mg/l below 30 cm. The two peaks in porewater Pb were coincident with those in the solid phase peat and the porewater/solid phase ratio of Pb concentrations did not vary over significantly over these top sections. From this information alone, it appeared that there was a simple equilibrium distribution of Pb between the solid and aqueous phase and that very little of the total Pb was present in the porewater. The peak at 20-22 cm in both the solid phase and the porewater did, however, coincide with the maximum in porewater organic carbon concentration. Determination of the proportion of Pb in humic extracts showed, however, that very little of the total Pb was associated with humic materials at this depth. In addition, from the distributions and humic-associations of other elements, processes including active plant uptake of nutrients (Mn, Na, K, Ca), reductive dissolution (Fe) and mineral dissolution (Al, Ti) did not affect the vertical distribution of Pb within the peat. Finally, there was good agreement between the <sup>206</sup>Pb/<sup>207</sup>Pb isotope ratios obtained for the solid phase peat, the porewater and each of the humic extracts, particularly in the uppermost sections of the core where the isotope signature changes from 1.136 to 1.177 in the solid phase, from 1.143 to 1.179 in the porewater and, for example, 1.143 to 1.180 in the humic acid extract. The similarity of the isotopic profiles provides further evidence to support the post-depositional vertical immobility of Pb in ombrotrophic peat bogs. Overall this work has provided a dated concentration profile for Pb which can be used as a historical record of environmental Pb contamination. It has also extended the understanding of biogeochemical processes occurring in peat bogs although these do not influence the vertical concentration profile of Pb. In particular, the use of stable Pb isotopes established that even humic complexation after deposition was unlikely to affect the vertical distribution of Pb.
6

The use of radio-nuclides (unsupported 210Pb, 7Be and 137Cs) in air, rain and undisturbed soil as environmental tools

Choubedar, Feraidoun January 2000 (has links)
Soil inventories of atmospherically derived <sup>210</sup>Pb (radioactive half lives 22.3 years) were used to determine deposition patterns of atmospherically derived aerosols averaged over decades at low altitude sites (51-6° N, 0.6° W) and quantify the enhancement in aerosol deposition by aerodynamically rough canopies (forest) relative to short vegetation (grass). <sup>210</sup>Pb and <sup>7</sup>Be (radioactive half life 53.4 days) in surface air and rain (55.6° N, 3.9° W) were measured to determine the concentration of these radionuclide isotopes in the environment. Non-destructive gamma-spectrometry was used for all 3 parts using high purity germanium (HPGe) detectors. From the soil samples <sup>137</sup>Cs (radioactive half life 30.2 years) are being measured and reported. The average inventories to the canopies compared to the short vegetation for <sup>210</sup>Pb and <sup>137</sup>Cs for Geescroft forest were 25.7% and 60.8% higher than the open field inventories, respectively; the corresponding values for Broadbalk forest were 30.9% and 68.9% for <sup>210</sup>Pb and <sup>137</sup>Cs, respectively. Although these two forests are not far from each other and have the same altitude, the excess inventories for Broadbalk forest are larger than Geescroft forest and this could be due to the fact that this forest is acting as an edge, because of its width. The averaged flux for Rothamsted forest soils for <sup>210</sup>Pb was determined as 211 ± 6 Bq m<sup>-2</sup> y<sup>-1</sup> per metre of rainfall, and for the soil and vegetation in the open area 167 ± 6 Bq m<sup>-2</sup> y<sup>-1</sup> per metre of rainfall. The annual average concentrations in surface air were 0.19 ± 0.06 and 2.71 ± 0.66 mBq m<sup>-3</sup> for <sup>210</sup>Pb and <sup>7</sup>Be, respectively. The annual fluxes of <sup>210</sup>Pb and <sup>7</sup>Be in rain were determined as 68 ± 10 and 787 ± 29 Bq m<sup>-2</sup> y<sup>-1</sup> per metre of rainfall, respectively. There was a seasonal variation for <sup>7</sup>Be concentrations in air and rain giving spring high. The highest concentrations for <sup>210</sup>Pb were obtained when the air mass origin was from continent. From the air and rain data for <sup>210</sup>Pb and <sup>7</sup>Be, similar washout scavenging ratios (W<sub>s</sub>) were obtained, indicating they are scavenged from the atmosphere with a similar rate. The dry deposition of <sup>210</sup>Pb and <sup>7</sup>Be carrier aerosols were estimated as 41 ± 13% and 35 ± 9% of the total deposition for <sup>210</sup>Pb and <sup>7</sup>Be, respectively.
7

Organic-mineral interactions across the benthic boundary layer in the northeast Atlantic Ocean

Good, Emily Irene January 2002 (has links)
Bulk elemental and biochemical yields in samples collected across the benthic boundary layer (BBL) in the Northeast Atlantic indicated the BBL to be a key site of organic matter (OM) alteration and played a pivotal role in determining the organic carbon: surface area ratio (OC: SA) ratio of material deposited in the sediments in this region. Evidence for the seasonal nature of OM inputs to the BBL in the Northeast Atlantic was found in two of the three investigated sites. The seasonal deposition of material associated with the spring bloom led to the presence of a substantial layer of phytodetritus material at one site. Although this material was not rich in OC, it contained elevated yields of labile biochemicals that suggested the potential importance of such material to deep-sea benthic nutrition. The majority of OM within the sediments in the deep Northeast Atlantic appeared to be sorbed to mineral grains displaying SA-normalised OC loadings below those typically observed on continental margins (i.e. <0.5<i> </i>mg QC m<sup>-2</sup>) indicating that OM remineralisation processes are extremely efficient in this region. An investigation into the possible influences on OC preservation and burial in the Northeast Atlantic and in other diverse continental margin sediments, suggested that only the length of time that sedimentary OM was exposed to oxic porewaters displayed a consistent relationship with sediment OC: SA ratios and OC burial efficiencies. Sediment oxygen exposure time, therefore, appear to be a primary control on OM burial.
8

The use of an atmospheric chemistry-transport model (FRAME) over the UK and the development of its numerical and physical schemes

Vieno, Massimo January 2005 (has links)
The relatively long runtime (days) of the FRAME model (Fine Resolution Atmospheric Multi-pollutant Exchange) was a limitation for using the model as a policymaker’s tool. Introducing a new and faster numerical scheme (Finite Volume Method) reduced the runtime by a factor of ~36. The FRAME model is now capable of performing 100 runs in 3 days. Introducing high-stack point sources in the emissions inventory led the FRAME model to overestimate sulphur concentrations in areas near strong point sources. The missing process was the plume rise of high-stack emissions which improved surface sulphur concentrations in those areas. The low-level emissions injection height has also been improved introducing a specific sector emission height (i.e. NOx emissions from cars at 1 m and ammonia emissions from housed livestock at 3 m) giving a better performance in predicting ammonia and oxidised nitrogen surface concentrations. The FRAME model used a wind dataset derived from Jones (1981). This dataset uses the geostrophic wind rose and has an unusually high frequency of winds from a northerly direction. A different approach was chosen and a new wind dataset has been derived from radiosonde measurements from various stations across the FRAME domain. The new wind dataset enhanced the export of pollutants with an associated reduced deposition within the FRAME domain. Validation of the FRAME model was made carrying out a comparison between observations from various measurement networks, for surface concentrations and wet deposition and model prediction. A detailed analysis of model versus observations was made focusing on how the model is representative of an entire grid square (5 x 5 km<sup>2</sup>) whereas observation sites are more representative of the land-use type in which they are located.
9

Laboratory and modelling studies of phenols relevant to the atmosphere

Harrison, Mark January 2003 (has links)
In order to investigate the kinetics and selectivity of the liquid phase nitration of phenol, reliable procedures for the production of the nitrating agents N<sub>2</sub>O<sub>5</sub> and CINO<sub>2</sub> were established. Production of N<sub>2</sub>O<sub>5</sub> was achieved <i>via</i> on-line mixing of NO<sub>2</sub> (1%) with O<sub>3</sub>(~5%) in the 2:1 ratio in a darkened reaction vessel. CINO<sub>2</sub> was produced by conversion of N<sub>2</sub>O<sub>5</sub> using NaCI solution (4 M). The concentration of nitrating agents was determined by bubbling the gas stream through water to yield nitrate which was analysed using Ion Chromatography and also by off-line reduction to nitrite followed by analysis using the sulphanilamide/UV method. A method for quantitative solid phase extraction with phenol in the aqueous phase. Nitration experiments were conducted under both acidic and basic conditions over a temperature range relevant to environmental processes. Results indicate the formation of 4-nitrosophenol (additionally characterised using GC-MS) as well as 2- and 4-nitrophenol (additionally characterised using GC-MS) as well as 2- and 4-nitrophenol. The product ratio of the products species has been shown to be dependent upon both the nitrating agent on pH. Finally, a chemical kinetic model has been developed to quantify the relative nitration pathways of aromatics in the troposphere. A box model, coded using FACSIMILE software, has been used to partitioning of benzene and phenol into the liquid phase and to assess the relative importance of the gas and liquid phases. The model includes the phase partitioning of 21 species and focuses on the conversion of benzene to phenol and finally nitrophenol in both the gas and liquid phase. Results indicate that the liquid phase contributes significantly to the production of nitrophenols in the troposphere. The system was shown to be sensitive to the assumed liquid water content over the range 3x10<sup>-9</sup> to 3x10<sup>-6</sup> as well as the droplet diameter and temperature. The accuracy of the liquid phase rate coefficients for the phenol + OH and phenol + NO<sub>3</sub> reactions were also shown to be very significant although altering the liquid phase benzene rate coefficient had little impact on the system as a whole. The model was also extending to include the partitioning of the nitrophenol products. This allowed an estimation to be made regarding the fate of the product species. At the benchmark <i>Lc</i> value of 3x10<sup>-7</sup>, used to describe tropospheric cloud conditions, some 58% of the nitrophenols are produced by liquid phase processes whereas less than 2% of the nitrophenols remains in the liquid. This suggests that a great deal of the nitrophenol that may be observed in the gas phase is actually produced through liquid phase pathways.
10

Development of an atmospheric transport model simulating concentration and deposition of reduced nitrogen over the British Isles

Fournier, Nicolas January 2002 (has links)
Firstly, improvement in the model run-time was sought. By developing a parallel implementation using High Performance Fortran, a speed-up by a factor of 69 is achieved on a Cray <i>T3E</i> (128 processors). In addition, an alternative diffusion discretisation, the implicit Finite Volume method, is implemented, producing a speed-up of 34. Combined, these two approaches yield a speed-up of 2346. The results of this new version compare well with those of the previously validated sequential code. Secondly, attention is focused on the wet deposition process for reduced nitrogen, a common and recognised weakness of existing UK models. Here, in contrast with previous approaches, the directional orographic enhancement of precipitation and wet deposition from the seeder-feeder effect is investigated and then explicitly taken into account. Furthermore, a more realistic description of the wet deposition parameterisation is developed. Comparison with measurement data shows that these two considerations improve significantly the reduced nitrogen wet deposition results. These and other developments of the models result in a robust and significantly improved tool for atmospheric reduced nitrogen predictions. Moreover, they provide a foundation for further use of the model in a variety of applications. The model is applied here to consider the transport and deposition of oxidised nitrogen and sulphur, to evaluate regional budgets of reduced nitrogen for the British Isles and to test scenarios for 2010.

Page generated in 0.0413 seconds