Chemistry & Biochemistry Theses and Dissertations
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Item EXAMINING QUINONE CONTRIBUTION TO THE OPTICAL PROPERTIES OF CHROMOPHORIC DISSOLVED ORGANIC MATTER(2024) Ashmore, Rachel; Blough, Neil; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Chromophoric Dissolved Organic Matter (CDOM) in natural waters is largely responsiblefor absorption of light and photochemistry in the water, impacting environmental reactions and aquatic life. The composition of CDOM is greatly varied based on source, photochemical reactions, and natural cycles. The impact of quinone moieties on this structure and photochemical and redox reactions involving CDOM remains the subject of controversy. To investigate the impact of quinone structure on optical properties, model quinone compounds were thoroughly characterized by their optical properties and reactions with sodium dithionite and sodium sulfite. A series of methyl-substituted p-benzoquinones, a methoxy p-benzoquinone, and a range of napthoquinones and anthraquinones were investigated. These model compounds were characterized according to their quinone and hydroquinone molar absorptivities and fluorescence quantum yields. Sodium dithionite reduction of quinones and the impact of structure on the products of this reaction was investigated by reducing the quinones with both sodium dithionite and sodium sulfite and comparing the optical properties of the products to those of the quinone and hydroquinone. The spectra of dithionite reduced p-benzoquinones and napthoquinones suggested the presence of products other than the hydroquinone. Sulfite is produced in solution as a result of dithionite reduction of quinones. Model quinones were therefore also reduced with sodium sulfite to investigate the impact of this side reaction on the dithionite reduction products. High performance liquid chromatography (HPLC) was used to further investigate and quantify the products of dithionite reduction of quinones and the importance of sulfite interference. Although some of the model quinones react with sulfite to form a proposed sulfonated hydroquinone product, based on the observed extent of this reaction in dithionite reductions, the structures of quinones likely to be found in CDOM, and their relatively small contribution to CDOM optical properties, the sulfite reaction was determined to not significantly impact the study of quinone moieties in CDOM. Dithionite selectively reduces quinones, while borohydride reduces ketones, aldehydes, and quinones. Therefore, in CDOM samples, dithionite can be used to isolate the effects of quinone moieties on the optical properties. Dithionite reduction was used to analyse CDOM standards and natural water extracts from the North Pacific Ocean and the Chesapeake Bay to investigate quinone contribution to their optical properties. These results are compared to borohydride reduction results from Cartisano and McDonnell to compare the contribution of quinones to that of ketones and aldehydes. (1, 2) Dithionite reduction showed small impacts on absorbance and fluorescence, whereas significant changes in both were observed for borohydride reduction. Therefore, the optical changes observed under borohydride reduction are attributed to primarily ketones and aldehydes rather than quinones. Model quinones showed significant changes in fluorescence intensity due to dithionite reduction, which are largely not observed for CDOM standards and natural water extracts, further supporting the conclusion that their role in CDOM optical properties is small.Item A DETAILED OPTICAL ANALYSIS OF CHROMOPHORIC DISSOLVED ORGANIC MATTER AND C18 EXTRACTED ORGANIC MATTER IN THE CHESAPEAKE BAY(2023) McDonnell, Shannon Marie; Blough, Neil V; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Chromophoric dissolved organic matter (CDOM) is a large portion of the open ocean dissolved matter pool which contributes largely to ocean color. The composition and distribution of CDOM is essentially controlled by in-situ biological production, terrestrial inputs, photochemical degradation, and microbial consumption. Estuarine environments contain particularly diverse CDOM composition due to their large variety of inputs and shoreline land usage in addition to the mixing of freshwater and salt water. Developing a further understanding of CDOM variation and composition will help develop and improve satellite remote sensing algorithms, help us understand CDOM’s role in the global carbon, nitrogen, and oxygen cycles, and may help to prioritize in-situ sampling for water quality monitoring in areas of concern. The use of inherent optical properties combined with pH titration and chemical reduction with sodium borohydride (NaBH4), helps to probe the molecular composition of CDOM and its spatial variability. Detailed studies of CDOM from the Chesapeake Bay are limited with many studies only investigating the main channel of the Bay and neglecting the various tributaries. Also, there is a lack of studies which specifically probe the molecular composition of the CDOM samples. To address this, an in-depth analysis of the optical properties of CDOM and C18 extracted organic matter (C18-OM) from the Chesapeake Bay, focusing on various inputs, was performed. Chemical reduction with NaBH4 and pH titration were employed to probe the presence of specific functional groups and their contribution to overall optical properties, and how they vary between locations. Spectral slope (S300-700), E2:E3 absorption ratio, fluorescence intensity, and apparent quantum yield of fluorescence (AQY) were used to analyze 170 samples from various tributaries in the Chesapeake Bay. Overall, this study suggested 1) there may be multiple inputs of CDOM within the Chesapeake Bay 2) the Top of the Bay and central channel of the Bay are impacted by the heavy terrestrial input from the Susquehanna River 3) A lack of correlation between phytoplankton fluorescence and CDOM absorption suggest phytoplankton are not an immediate source of CDOM within the Chesapeake Bay and 4) removal of protein and phytoplankton fluorescence after sample filtration indicates these species must exist in aggregates >0.2 µm. Optical analysis combined with pH titration and NaBH4 reduction investigated the variation between 9 C18-OM extracts from various regions in the Chesapeake Bay and a humic material standard Suwannee River Fulvic Acid (SRFA). Additionally, this study investigated the validity of the Charge-Transfer (CT) model using the optical properties of model compounds. This study suggested 1) certain absorbing and emitting species are lost during C18 extraction but extracts are still representative of their CDOM 2) nearly identical optical responses to pH titration and NaBH4 reduction suggest similar chromophore content throughout the Chesapeake and 3) CT interactions leading to long wavelength absorption are more prevalent in Suwannee River Fulvic Acid (SRFA) than they are in the Chesapeake. To compare the molecular and optical properties of the Chesapeake Bay to other locales, these extracts were compared to extracts from the Delaware Bay (DEL), Equatorial Atlantic Ocean (EAO) and North Pacific Ocean (NPO) in addition to reference materials Suwannee River Fulvic Acid (SRFA), Pony Lake Fulvic Acid (PLFA), and Elliott Soil Humic Acid (ESHA). This study showed 1) composition of deprotonatable and reducible chromophores within the Chesapeake and Delaware Bays is nearly identical but different from the oceans 2) despite being estuaries and containing a mixture of fresh and ocean water, CDOM within both Bays looks terrestrially dominated 3) deep ocean extracts from the Atlantic and Pacific exhibit similar optical response to pH titration, NaBH4 reduction, and NaBH4 reduction combined with pH titration suggesting the similarity of deep ocean waters from both ocean basins.Item INVESTIGATION OF THE PRODUCTION AND DECAY PATHWAYS OF SUPEROXIDE BY CHROMOPHORIC DISSOLVED ORGANIC MATTER(2022) Le Roux, Danielle Marie; Blough, Neil; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Chromophoric dissolved organic matter (CDOM) in natural waters absorbs sunlight which leads to the production of a suite of reactive intermediates and reactive oxygen species (ROS) such as superoxide (O2⦁-) and hydrogen peroxide (H2O2). A significant amount of research over the years has investigated the sources and sinks of these two ROS. The currently accepted sequence of reactions for their production involves photochemically produced one-electron reductants (OER) within CDOM reacting with dissolved oxygen to form O2⦁-, which undergoes self-dismutation to produce H2O2. A previously used method to detect radical species with CDOM has been modified herein to be conducted simply using a fluorometer. Production rates of OER and H2O2 were measured for a variety of samples and correlations between the rates and optical/structural properties of the samples indicate that lower molecular weight species produce more OER and H2O2. Based on the stoichiometry of the mechanism above, the ratio of the production rate of OER to that of H2O2 should be two. However, ratios from five to sixteen were obtained, which suggests that O2⦁- undergoes oxidative reactions that compete with dismutation. The possibility of a light-dependent pathway for O2⦁- decay has been proposed but had yet to be explicitly demonstrated. Herein this sink is directly shown through O2⦁- spiking experiments. Rapid consumption of the O2⦁- spike occurs if injected into a sample during irradiation, as compared to a spike introduced into the sample in the dark, suggesting the presence of a light-dependent sink. Extensive data analysis and kinetic modeling of the O2⦁- decay data has allowed for approximations as to the extent of the sink and its decay rate constant. O2⦁- and H2O2 are environmentally important species, and a significant amount of work has been done on modeling their concentrations in natural waters. Based on the work here, O2⦁- is produced at higher concentrations than previously believed, which has implications on the modeling of O2⦁- and H2O2 in natural waters. Additionally, the light-dependent oxidative sink of O2⦁- could be with moieties within CDOM, providing further insight to the photochemical transformation of DOM during transit from terrestrial sources to marine waters.Item CHROMOPHORIC DISSOLVED ORGANIC MATTER (CDOM) IN THE OPEN OCEAN: OPTICAL AND CHEMICAL PROPERTIES AND THEIR RELATION TO CDOM STRUCTURE AND SOURCES.(2019) Cartisano, Carmen Marie; Blough, Neil V; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The carbon contained as dissolved organic matter (DOM) in the Earth’s oceans is an important factor in the global carbon cycle, but studying and tracking DOM in the aquatic environment can be challenging. However, the light-absorbing and emitting subcomponents of DOM, called chromophoric dissolved organic matter (CDOM) and fluorescent dissolved organic matter (FDOM) can be directly probed using absorption and fluorescence spectroscopy, respectively. Detailed studies on CDOM from the open oceans are limited with many of the existing studies having very limited data sets (only select wavelengths or indices). To address this, the optical properties of CDOM from a variety of geographic locations (North Pacific Ocean: NPO, Equatorial Atlantic Ocean: EAO, Middle Atlantic Bight: MAB, Delaware River and Delaware Bay) were compared, and chemical tests performed (sodium borohydride (NaBH4) reductions and pH titrations). The responses to the chemical tests along with similarities and differences in the optical properties were examined to compare the structures present in terrestrial, coastal and open ocean samples. A long-pathlength capillary waveguide spectrometer was used to characterize open ocean CDOM samples, with the need for a calibration and validated protocol addressed prior to use. The optical properties of the NPO samples did not vary significantly at depths from ~300-4500 meters with only the surface samples showing significant differences. Solid phase extraction of the natural waters did remove unique absorbing and emitting bands in the UV region that could be marine in origin, while enriching the “humic-like” fraction. The open ocean samples showed similarities to the coastal and riverine samples including: 1) monotonically decreasing and unstructured absorbance with increasing wavelength; 2) loss of absorption upon NaBH4 reduction at all wavelength, with the largest percent loss in the visible; 3) enhanced absorption with increasing pH with spectral changes that occurred over the same pH ranges as the pKas of carboxylic acids and phenols; 4) attenuation of absorption enhancement with increasing pH following reduction at most wavelengths. These similarities not only suggest that there are structural similarities throughout all samples, but also indicate that there may be a terrestrial source of CDOM in the open ocean.Item QUANTITATIVE ANALYSIS OF THE ATMOSPHERIC OXIDATION OF ISOPRENE USING MODELS AND MEASUREMENTS: IMPACTS ON SURFACE OZONE(2019) Marvin, Margaret Rosemary; Wolfe, Glenn; Salawitch, Ross; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The oxidation of isoprene – one of the most abundant volatile organic compounds (VOCs) in our atmosphere – significantly impacts the formation of surface ozone, which is detrimental to public health. Computer models simulate the complex relationships between ozone and VOCs like isoprene and are used to guide policy decisions directed at improving ozone. However, uncertainties in the emissions and chemistry of isoprene limit the accuracy of modeled ozone. This body of work comprises a quantitative analysis of atmospheric isoprene oxidation that strives to identify and improve such uncertainties through the combination of models with measurements. Measurements used in this work mainly comprise in situ observations from the Southeast Nexus (SENEX) aircraft campaign, which sampled atmospheric composition across the isoprene-rich summertime Southeast US. I have prepared two models – the Framework for 0-D Atmospheric Modeling (F0AM) and the Comprehensive Air Quality Model with Extensions (CAMx) – to drive simulations of atmospheric isoprene oxidation, which are evaluated against observations from SENEX. Using F0AM, a photochemical box model, I demonstrate that several commonly-used mechanisms significantly underestimate measured mixing ratios of formaldehyde, a high-yield product of isoprene oxidation, by 0.5–1 ppb across a wide range of NOx conditions. The consistent underestimation of formaldehyde suggests a deficit of VOC oxidation among all considered mechanisms. Although the cause for this deficit remains elusive, I provide recommendations for improving the simulated production of formaldehyde upon isoprene oxidation in the Carbon Bond version 6 revision 2 (CB6r2) mechanism, commonly used for air quality modeling. Using CAMx, a three-dimensional chemical transport model, I produce a standard air quality modeling scenario that simulates atmospheric composition across the continental US for the summer of 2013. Evaluation of this scenario reveals that the emissions of isoprene from the Biogenic Emissions Inventory System (BEIS) are underestimated in the Southeast US by at least 40%. Finally, implementation of improvements in the emissions and chemistry of isoprene within the CAMx modeling framework increases the net photochemical production of surface ozone by up to 0.5 ppb hr−1 and shifts surface ozone production regimes more NOx-limited, relative to the standard platform for regional air quality modeling.Item Investigating the Mechanism of Phenol Photooxidation by Humic Substances(2014) Sikorski, Kelli Ann; Blough, Neil V; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)It is well established that organic pollutants such as phenols are degraded in the presence of chromophoric dissolved organic matter (CDOM) and sunlight in natural waters. Early work attributed the photochemical loss of phenols to the involvement of photoproduced reactive oxygen species (ROS) such as singlet oxygen (1O2), hydroxyl radical (*OH) or peroxy radicals (RO2*). However, evidence for the involvement of triplet excited states of aromatic ketones/aldehydes within CDOM has accumulated in the literature. To probe the mechanism of the photosensitized loss of phenols by humic substances (HS), the dependence of the initial rate of 2,4,6-trimethylphenol (TMP) loss (RTMP) on dioxygen concentration and irradiation wavelength was examined both for a variety of untreated as well as borohydride-reduced HS and C18 extracts from the Delaware Bay and Mid-Atlantic Bight. The effect of [O2] and borohydride-reduction of SRFA was also examined for a series of substituted phenols of varying one-electron reduction potentials. We find that RTMP was inversely proportional to dioxygen concentration at [O2] > 50 μM, a dependence consistent with reaction with triplet excited states, but not with 1O2 or RO2. Modeling the dependence of RTMP on [O2] provided rate constants for TMP reaction, O2 quenching and lifetimes compatible with a triplet intermediate. Borohydride reduction significantly reduced TMP loss, supporting the role of aromatic ketone triplets in this process. However, for most samples, the incomplete loss of sensitization following borohydride reduction, as well as the inverse dependence of RTMP on [O2] for these reduced samples, suggests that there remains another class of oxidizing triplet sensitizer, perhaps quinones. However, the results of the wavelength dependence reveal that the sensitization is driven primarily by shorter wavelength UV-B and UV-A absorbing moieties, consistent with the involvement of aromatic ketones and aldehydes but appearing to exclude the longer wavelength (visible) absorbing quinones as sensitizers. An inverse dependence of Φ on one-electron reduction potential was observed where DMOP ≈ TMP > 4-MOP > 4-MP > phenol. Similar dependencies were observed for TMP and 4-MOP in the dependence of Rprobe on [O2] whereas DMOP did not exhibit a substantially lower Rprobe at high [O2] as would be expected for a triplet sensitization mechanism. Moreover, that a significant amount of sensitization is observed following borohydride reduction of SRFA for DMOP under high [O2], as well as the very low sensitization observed at low [O2] indicates that a separate pathway, unrelated to triplets, may be important for the mechanism of DMOP photooxidation by chromophoric dissolved organic matter.Item Effect of Borohydride Reduction and pH on the Optical Properties of Humic Substances(2014) Schendorf, Tara Marie; Blough, Neil V; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Despite decades of research, the structural basis for the optical properties of chromophoric dissolved organic matter (CDOM) and humic substances (HS) are still not clear. Through several analytical techniques, it is known that CDOM contains carbonyls (aromatic ketones, aldehydes, and quinones), carboxylic acids, and phenols. The charge-transfer model proposed to explain the optical properties of these materials assigns the short-wavelength absorption (<350 nm) and fluorescence emission to electron donors (phenols) and acceptors (carbonyls), while the long-wavelength absorption is attributed to charge-transfer interactions among these species. Because carbonyls are reducible species a method was developed to eliminate them and to investigate its effects on the optical properties of HS in relation to their structure. In addition, the effect of pH on the optical absorption spectra for both untreated and borohydride reduced HS was examined and related to the deprotonation of carboxylic acids and phenolic moieties.Item MODELING THE PH DEPENDENT OPTICAL PROPERTIES OF AQUATIC, TERRESTRIAL AND MICROBIAL HUMIC SUBSTANCES(2013) Heighton, Lynne Page; Mignerey, Alice C; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Humic substances (HS) and chromophoric dissolved organic matter (CDOM) are ubiquitous, widely impacting environmental processes, yet despite decades of study the link between structure and the unique optical properties evident in HS/CDOM remains elusive. Model compounds derived from a solely microbial source, as well as terrestrial sources from both aquatic environments and soil systems, exhibit many of the same optical properties despite their disparate methods of generation and sources. All show a pH dependent absorbance, exhibit increasing absorbance as wavelength decreases and a loss of absorbance upon borohydride reduction. The link between colored humic substances is their ability to form electronic interactions that extend long wavelength absorbance. The underlying processes by which charge transfer bands or electronic interactions in HS/CDOM are generated are investigated by optical and potentiometic titrations of untreated and borohydride reduced material. Borohydride reduction targets carbonyl functional groups such as aromatic ketones and quinones. The reduction of these groups affects the optical properties by reducing long wavelength absorbance and causing a blue shift in the fluorescence emission spectra. A direct comparison of divergent sources of fulvic and humic acids including an aquatic fulvic acid, Suwannee River Fulvic Acid (SRFA) and a microbial source of fulvic acid, Pony Lake Fulvic Acid (PLFA), the soil derived humic acids, Elliott (EHA) and Leonardite Humic Acids (LHA), an aquatic humic acid Suwannee River Humic Acid (SRHA) as well as Lignin Alkali Carboxylate (LAC) highlights differences between sources of humic material as exemplified by borohydride induced optical changes such as absorbance intensity in the UV and visible range, difference (DA), fractional difference, spectral slope (S), fluorescence excitation-emission matrix spectra (EEMS), and differential emission spectra (DF) as well as quantum yield. Traditional Raman spectroscopy, although capable of providing relevant chemical functional group information, cannot be applied to untreated CDOM because of high fluorescence background. Surface enhanced Raman scattering (SERS) provides the capability of overcoming the florescence background, thus providing useful Raman spectral data. SERS spectra of model compounds and CDOM were collected using roughened silver electrodes. Functional groups were identified from selected borohydride reduced CDOM SERS spectra.Item Advanced Receptor Models for Exploiting Highly Time Resolved Data Acquired in the EPA Supersite Project(2012) Ke, Haohao; Ondov, John; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Receptor models have been widely used in air quality studies to identify pollution sources and estimate their contributions. A common problem for most current receptor models is insufficient consideration of realistic constraints such as can be obtained from emission inventories, chemical composition profiles of the sources, and the physics of plume dispersion. In addition, poor resolving of collinear sources was often found. With the high quality time-, composition-, and size-resolved measurements during the EPA Supersite project, efforts towards resolving nearby industrial sources were made by combinative use of Positive Matrix Factorization (PMF) and the Pseudo-Deterministic Receptor Model (PDRM). The PMF modeling of Baltimore data in September 2001 revealed coal-fired and oil-fired power plants (CFPP and OFPP, respectively) with significant cross contamination, as indicated by the high Se/Ni ratio in the OFPP profile. Nevertheless, the PMF results provided a good estimate of background and the PMF-constrained emission rates well seeded the trajectory-driven PDRM modeling. Using NOx as the tracer gas for χ/Q tuning, ultimately resolved emissions from individual stacks exhibited acceptable tracer ratios and the emission rates of metals generally agreed with the TRI estimates. This approach was later applied to two metal pollution episodes in St. Louis during in November 2001 and March 2002 and met a similar success. As NOx measurements were unavailable at those metal-production facilities, highly-specific tracer metals (i.e., Cd, Zn, and Cu) for the corresponding units were used to tune χ/Qs and their contributions were well resolved with the PMF-seeded PDRM. Opportunistically a PM2.5 excursion during a windless morning in November 2002 allowed the extraction of an in-situ profile of vehicular emissions in Baltimore. The profiles obtained by direct peak observation, windless model linear regression (WMA), PMF, and UNMIX were comparable and the WMA profile showed the best predictions for non-traffic tracers. Besides, an approach to evaluate vehicular emission factors was developed by receptor measurements under windless conditions. Using SVOC tracers, seasonal variations of traffic and other sources including coal burning, heating, biomass burning, and vegetation were investigated by PMF and in particular the November traffic profile was consistent with the WMA profile obtained earlier.