695 research outputs found

    Study of Photochemical Formation of Hydroxyl Radical in Natural Waters

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    This dissertation mainly focuses on the sources of the hydroxyl radical (•OH) from photochemical reactions in natural waters, in particular from reactions involving dissolved organic matter (DOM). Firstly, an accurate method for estimating •OH formation rate during long-term irradiation was developed. It was observed that previous methods for measuring •OH formation rates in the natural waters, which were based upon sequentially determined cumulative concentrations of probe photoproducts, significantly underestimated actual •OH formation rates. It was found that the underestimation was mainly due to the degradation of the probe photoproducts and that only ‘instantaneous’ formation rates were appropriate for accurately estimating •OH photochemical formation rates. ‘Instantaneous’ •OH formation rates were obtained by adding probes to a sub-sample at each time point during the long-term irradiation and irradiating the sub-sample for a short time. By employing this approach, •OH formation rates were measured during a photoflocculation study in natural waters. In addition to •OH formation, hydrogen peroxide concentration, dissolved organic carbon, total dissolved nitrogen, water optical properties, and iron speciation were measured. The results showed that in iron- and DOM-rich water samples •OH appears to be mainly produced from the Fenton reaction initially, but subsequently from other sources, in particular DOM photoreactions. In order to elucidate possible photoreaction sites and mechanisms of •OH photoformation from DOM, phenolic compounds were used as model DOM chromophores. •OH quantum yields (Фs) at 280~340 nm were measured by •OH trapping reaction with benzene. It was found that many phenolic acids are capable of producing •OH, especially those with para hydroxyl and carboxyl groups (especially 2,4-dihydroxybenzoate and 4-hydrodroxybenzoate), which have markedly high •OH Фs. By conducting methane trapping and competition kinetics experiments, it was confirmed that free •OH was produced from these compounds. The results suggest that hydroxybenzoic acid moieties within DOM play an important role in the photoproduction of •OH. Finally, it was hypothesized that a quinoid enol tautomer present as a water cluster was responsible for •OH production from phenolic compounds with para hydroxyl and carboxyl groups. Phenolic acids with para hydroxyl and carbonyl groups are common components of lignin, which is a major source of DOM in freshwaters. In order to examine the lignin phenolic composition of natural samples, a simplified method using alkaline copper oxide oxidation coupled with solid-phase extraction and high performance liquid chromatography (HPLC) was developed. In this study, an interlaboratory comparison of the simplified HPLC approach with the conventional, but much more complex and expensive, high pressure reaction vessel GC-MS method was also conducted. The agreement between the two different methods was generally very good. A major benefit of this simplified HPLC method is that it allows any investigator with standard HPLC equipment to analyze lignin components, whereas previously only a small number of specialized labs were able to perform these analyses

    Spectroscopic Characterization of Dissolved Organic Matter: Insights into Composition, Photochemical Transformation and Carbon Cycling

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    This dissertation explores processes affecting the composition of dissolved organic matter (DOM) and how DOM composition changes in sunlit surface waters and in the dark interior ocean. Simulated solar irradiations were used to investigate the impact of photochemistry on terrestrial waters and deep ocean DOM. The photochemically mediated processes observed in Dismal Swamp samples included (i) light induced flocculation of up to 12% of the organic matter and 84% of the dissolved iron originally present; (ii) 74-88% mineralization of dissolved organic carbon (DOC) and 95-99% bleaching of chromophoric DOM (CDOM) during 110 days of irradiation; and (iii) nearly complete loss of the biochemical markers for terrestrial DOM: lignin phenols, CDOM absorption and fluorescence, and aromaticity determined by nuclear magnetic resonance (NMR) spectroscopy. Extensively photo-degraded terrestrial DOM exhibited spectroscopic signatures similar to DOM isolated from ocean water (except that it lacked protein-like fluorescence and appeared to contain excess carboxyl carbon), and photo-degraded deep ocean DOM exhibited optical properties similar to surface ocean DOM. The heretofore-unexamined DOM removal process of light induced flocculation was further investigated using solid-state 13C NMR and infrared spectroscopy. Photochemical decarboxylation and production of alkyl functionality drives the initial phase of photochemical flocculation, while adsorption to iron flocculates is important during later phases of the process. Carboxyl amides appeared to resist mineralization, but were susceptible to photochemical flocculation. A fraction of the photodegraded DOM is more susceptible to mineral adsorption, which may be an important pathway for DOM export from surface waters to the sediments and subsequent preservation. Advanced solid-state 13C NMR characterization of DOM isolated by reverse osmosis — electrodialysis (RO/ED) from marine environments with varying biogeochemistries revealed new insights into the biodegradation of carbohydrates as well as preservation of carboxyl groups and condensed aromatic structures in the ocean\u27s interior. Quaternary anomeric carbons were identified as a potentially important structural component of the poorly characterized pool of bio-refractory carbohydrates. The present biogeochemical paradigm for ocean DOC cycling, the three-pool model, is re-examined along with the three-pool photoreactivity classification system. A new conceptual model is proposed, which incorporates both biological and photochemical reactivity of dissolved organic matter

    Virginia Institute of Marine Science Forty-Seventh Annual Report (1988)

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    For the period ending June 30, 1988.https://scholarworks.wm.edu/vimsannualrpt/1024/thumbnail.jp

    The 1989 NASA-ASEE Summer Faculty Fellowship Program in Aeronautics and Research

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    The 1989 NASA-ASEE Summer Faculty Fellowship Program at the Goddard Space Flight Center was conducted during 5 Jun. 1989 to 11 Aug. 1989. The research projects were previously assigned. Work summaries are presented for the following topics: optical properties data base; particle acceleration; satellite imagery; telemetry workstation; spectroscopy; image processing; stellar spectra; optical radar; robotics; atmospheric composition; semiconductors computer networks; remote sensing; software engineering; solar flares; and glaciers

    Oxygen Isotopes as a Tracer of DOM Processes in River-Estuary Systems

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    In the biogeochemical study of dissolved organic matter (DOM) in natural waters, stable isotopes are used to provide insight into both the sources of DOM and the processes affecting its alteration. Through the research presented here, oxygen isotopes are incorporated into the study of DOM through the adaptation of a pyrolysis elemental analysis isotope ratio mass spectrometer method, and sample preparation using two-stage ultrafiltration. The application of oxygen isotopes to the study of DOM is demonstrated in two studies. First, natural abundance of δ18O in DOM is explored in the Delaware estuary. Using a two end-member mixing model, DOM oxygen isotopes are compared with carbon isotopes, DOC concentrations and elemental ratios to determine if changes are a result of two water bodies mixing. Isotope values are decoupled from concentration values and indicate that from an isotopic perspective two end member mixing is completed by mid bay. A conceptual model is presented with comparison to the observed data. However, the extension of this model to DOM δ18O values show that additional processes affect the DOM oxygen along the estuary transect. Oxygen isotopes are also used in an enrichment study to assess the sources of DOM oxygen in the photochemical oxidation of organic matter. In a series of laboratory irradiations using York River and Dismal Swamp water amended with either 18O-enriched dissolved oxygen or water, the amount of 18O incorporated into the high molecular weight dissolved organic matter was determined. For both sites, a fraction of the oxygen in DOM was photochemically incorporated from dissolved oxygen, although a larger fraction was incorporated from water. The differences in incorporation between sites are attributable to DOM compositional differences. An oxygen budget is proposed for the observed Dismal Swamp photochemical DOM oxidation. DOM oxygen isotopes provide an additional dimension in the investigation of DOM sources and alteration processes. As DOM δ18O values appear to be affected by processes that do not alter the δ13C values and are not correlated with the water δ18O value, it provides an additional parameter for characterizing high molecular weight DOM

    Advances in Understanding the Molecular Composition of Dissolved Organic Matter and Its Reactivity in the Environment

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    Dissolved organic matter (DOM) is the ultimate product of Earth\u27s systems dynamics. DOM chemical signature is strongly shaped by the interaction among Earth\u27s spheres, such as the atmosphere, the geosphere, the biosphere, and the hydrosphere, but also life and human activity. DOM source, composition, photochemical alteration and availability affect freshwater ecosystems, their carbon and nitrogen fluxes and, thus, the global carbon and nitrogen cycles. The aim of this thesis was to gain an understanding of the molecular composition of DOM and its photochemical and biological reactivity in an environment impacted by anthropogenic disturbance. The York and James River systems within the Cheasapeake Bay watershed provided an excellent study site for such studies. The experiments included monitoring the alteration of DOM and its subcomponents dissolved organic nitrogen (DON) and carbon (DOC) from both natural and anthropogenic sources during photochemical and biological processes. My combined analytical and statistical approach identified the molecular signature of the photolabile and the photoproduced DOM and thebiolabile and the bioproduced DOM ding these processes. My approach depicted differences in DON assimilation by the York River biota depending on the DOM source, where anthropogenic DON showed more bioavailability than naturally derived DON. Furthermore, anthropogenically-derived DON showed an intense bioavailability in the freshwater end member of the James River, VA. These findings suggest that anthropogenic DON is highly reactive in the natural environment and that simple assays examining net consumption or production of bulk DON pools are inadequate for assessing its bioavailability. The studied photochemical alterations of natural and anthropogenic DOM induced production of newly dissolved organic nitrogen (DON) even from natural sources that are relatively N-poor. My experimental results demonstrated that photochemistry transforms DON from complex structural entities to ammonia, aliphatic molecules, and low carbon number molecules that might enhance microbial metabolism, and eventually increases CO2 emissions and reduces DOM concentrations in stream ecosystems

    Annual Research Report, 2009-2010

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    Annual report of collaborative research projects of Old Dominion University faculty and students in partnership with business, industry and governmenthttps://digitalcommons.odu.edu/or_researchreports/1001/thumbnail.jp

    Environmental Dynamics of Dissolved Organic Matter and Dissolved Black Carbon in Fluvial Systems: Effects of Biogeochemistry and Land Use

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    Black carbon (BC) is an organic residue formed primarily from biomass burning (e.g., wildfires) and fossil fuel combustion. Until recently, it was understood that BC was highly recalcitrant and stabilized in soils over millennial scales. However, a fraction of the material can be solubilized and transported in fluvial systems as dissolved BC (DBC), which represents on average 10% of the global export of dissolved organic carbon (DOC) from rivers to coastal systems. The composition of DBC controls its reactivity, and it has been linked with a variety of in-stream processes that induce both carbon sequestration and evasion of COâ‚‚ from aquatic systems, which suggest DBC may have a significant contribution within the global carbon cycle. The primary objectives for the thesis were to elucidate environmental factors that control the fate and transport of DBC in fluvial systems. Ultra-high resolution mass spectrometry was used to characterize DBC on a molecular scale whereas benzenepolycarboxylic acids were used to quantify and characterize BC in both dissolved and particulate phases (PBC). Sinks for polycondensed DBC were linked to a series of in-stream biogeochemical processes (e.g., photodegradation, metal interactions); whereas photooxidation of particulate charcoal led to production of DBC, suggesting photodissolution as a previously unrecognized source of DBC to fluvial systems. Coupling of DBC with PBC, however, was hydrologically constrained with sources varying over temporal scales and land use regimes. For DBC in particular, an enrichment of heteroatomic functionality was observed as a function of anthropogenic land use. Furthermore, land use coupled with stream order (a proxy for in-stream processing as defined by the River Continuum Concept) could explain significant spatial variability in organic matter (e.g., DOC) composition within an anthropogenically impacted system. With an increase in wildfire frequency projected with on-going climate change trends, parallel projections for increases in BC production are also expected. Furthermore, conversion of natural landscapes for urban and agricultural practices is also expected to continue in the coming decades. Thus, it is imperative to reach a comprehensive understanding of processes regulating the transport of DBC in fluvial systems with efforts to constrain future BC budgets and climate change models

    Feasibility Study for an Aquatic Ecosystem Earth Observing System Version 1.2.

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    International audienceMany Earth observing sensors have been designed, built and launched with primary objectives of either terrestrial or ocean remote sensing applications. Often the data from these sensors are also used for freshwater, estuarine and coastal water quality observations, bathymetry and benthic mapping. However, such land and ocean specific sensors are not designed for these complex aquatic environments and consequently are not likely to perform as well as a dedicated sensor would. As a CEOS action, CSIRO and DLR have taken the lead on a feasibility assessment to determine the benefits and technological difficulties of designing an Earth observing satellite mission focused on the biogeochemistry of inland, estuarine, deltaic and near coastal waters as well as mapping macrophytes, macro-algae, sea grasses and coral reefs. These environments need higher spatial resolution than current and planned ocean colour sensors offer and need higher spectral resolution than current and planned land Earth observing sensors offer (with the exception of several R&D type imaging spectrometry satellite missions). The results indicate that a dedicated sensor of (non-oceanic) aquatic ecosystems could be a multispectral sensor with ~26 bands in the 380-780 nm wavelength range for retrieving the aquatic ecosystem variables as well as another 15 spectral bands between 360-380 nm and 780-1400 nm for removing atmospheric and air-water interface effects. These requirements are very close to defining an imaging spectrometer with spectral bands between 360 and 1000 nm (suitable for Si based detectors), possibly augmented by a SWIR imaging spectrometer. In that case the spectral bands would ideally have 5 nm spacing and Full Width Half Maximum (FWHM), although it may be necessary to go to 8 nm wide spectral bands (between 380 to 780nm where the fine spectral features occur -mainly due to photosynthetic or accessory pigments) to obtain enough signal to noise. The spatial resolution of such a global mapping mission would be between ~17 and ~33 m enabling imaging of the vast majority of water bodies (lakes, reservoirs, lagoons, estuaries etc.) larger than 0.2 ha and ~25% of river reaches globally (at ~17 m resolution) whilst maintaining sufficient radiometric resolution
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