Browsing by Subject "Photodegradation"
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Item Photochemical Production of Reactive Intermediates in Inland Surface Waters(2017-09) McCabe, AndrewReactive intermediates form when dissolved natural organic matter (DOM) absorbs sunlight in surface waters. These reactive intermediates include triplet excited states of dissolved organic matter (T*), reactive oxygen species, carbonate radical, and halide radicals. They are associated with a variety of physicochemical processes, including carbon and metal cycling, pathogen inactivation, and reactions with trace organic contaminants. T* is particularly important in these processes because it can react either through electron or energy transfer mechanisms and it is responsible for the formation of secondary reactive intermediates, such as singlet oxygen and radicals. The quantity and composition of DOM are key variables that control the rate and efficiency of T* formation, defined as the ratio of the rate of T* formation to the total rate of light absorption. As DOM is transported through aquatic environments, its composition is altered by natural and anthropogenically-influenced biogeochemical processes. Here, DOM composition is related to the reactivity of T* in stormwater and in temperate wetlands, two important aquatic systems involved in the production and transport of DOM. The rate and efficiency of T* formation were measured with two chemical probes, 2,4,6-trimethylphenol and trans,trans-2,4-hexadienoic acid, that quantify rates of T* electron transfer and energy transfer, respectively. DOM composition was characterized using absorption spectrophotometry, fluorescence spectroscopy, and Fourier transform ion cyclotron mass spectrometry. Within our sample set, the observed range in the efficiency of T* formation is <1%–14%, and shows a distinct dependence on watershed vegetative land cover and open water extent. The rate of T* formation increases with the concentration of dissolved organic carbon (DOC) while the efficiency of T* formation is independent of DOC. The data reported here suggests that DOM derived from vascular plants has a dual role, controlling both the rate of light absorption and the efficiency of T* formation.Item The role of photodegradation in plant litter decomposition in grassland ecosystems(2009-08) Brandt, Leslie AlysonDecomposition of plant litter is the primary process by which carbon and nutrients are returned from plants to the soil and atmosphere. Although plant litter decomposition is primarily driven by plant litter chemistry, temperature, and precipitation these factors have failed to fully explain decomposition patterns in arid and semiarid grassland ecosystems. In my dissertation, I tested the hypothesis that solar radiation, particularly in the UV range (280-400 nm) contributes to the decomposition process in these systems via the process of photodegradation. In a three-year field study in the semiarid shortgrass steppe in Colorado, I examined whether photodegradation by UV radiation played a role in plant litter decomposition and whether the role of photodegradation in the decomposition process was affected by plant litter chemistry and precipitation. In a series of laboratory experiments, I examined the pathways by which mass is lost via photodegradation. In a two-year cross-site field experiment, I examined whether photodegradation may explain the difference in litter decomposition patterns among mesic, semiarid, and arid grassland ecosystems. The combined results of this research show that photodegradation is an important process in plant litter decomposition in mesic grassland ecosystems as well as arid and semiarid grassland ecosystems, accounting for up to 50% of litter mass loss. Results also show that litter mass loss via photodegradation is the result of photochemical production of carbon dioxide, which can be up to 4 g C m-2 y-1 in arid ecosystems. This research has important implications for future basic research in biogeochemical modeling, photochemistry of natural compounds, and plant litter decomposition in arid ecosystems.