Theses and Dissertations from UMD
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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM
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Item Developing an optimization approach for estimating incident solar radiation at earth surface from multiple satellite data(2019) Zhang, Yi; Liang, Shunlin; Geography; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Surface incident shortwave radiation (ISR) is a crucial parameter in the land surface radiation budget. Many reanalysis, observation-based, and satellite-derived global radiation products have been developed but often have insufficient accuracy and spatial resolution for many applications. In this dissertation, I propose an optimization-based method (OB-Algorithm) based on a radiative transfer model for estimating surface ISR from Moderate Resolution Imaging Spectroradiometer (MODIS) Top of Atmosphere (TOA) observations by optimizing the surface and atmospheric variables with a cost function. This approach consisted of two steps: retrieving surface bidirectional reflectance distribution function parameters, aerosol optical depth (AOD), and cloud optical depth (COD); and subsequently calculating surface ISR. I also adapted the algorithm to VIIRS data and performed global validation with 34 Baseline Surface Radiation Network (BSRN) sites for both instantaneous and daily mean ISR. Researches on estimating daily and diurnal ISR was also made on Advanced Himawari Imager (AHI) and Advanced Baseline Imager (ABI). Geostationary satellites capture diurnal ISR variation better than polar-orbiting ones, especially in cloudy cases. Validation against measurements at seven Surface Radiation Budget Network (SURFRAD) sites resulted in an R2 of 0.91, a bias of -6.47 W/m2, and a root mean square error (RMSE) of 84.17 W/m2 for the instantaneous results. Validation at eight high-latitude snow-covered Greenland Climate Network (GC-Net) sites resulted in an R2 of 0.86, a bias of -21.40 W/m2, and an RMSE of 84.77 W/m2. These validation results show that the proposed method is much more accurate than the previous studies (usually with RMSEs of 80-150W/m2). The VIIRS ISR results at seven SURFRAD showed RMSEs of 83.76 W/m2 and 27.78 W/m2 for instantaneous and daily ISR, respectively at SURFRAD sites. Results at 34 BSRN sites showed RMSEs of 106.68 W/m2 and 32.76 W/m2 for instantaneous and daily ISR, respectively at BSRN sites. Validation of instantaneous and daily AHI ISR at eight OzFlux sites shows an R2 of 0.93, a bias of 0.52 W/m2 and an RMSE of 106.52 W/m2 for instantaneous results and an R2 of 0.95, a bias of -0.12 W/m2 and an RMSE of 22.49 W/m2 for daily mean ISR. Validation of instantaneous and daily ABI ISR at seven SURFRAD sites shows an R2 of 0.93, a bias of 8.71 W/m2 and an RMSE of 102.30 W/m2 for instantaneous results and an R2 of 0.95, a bias of -2.38W/m2 and an RMSE of 27.17 W/m2 for daily mean ISR. Ten years of optimization-based ISR products (OB-Product) in the Amazon region from MODIS data were produced and compared with existing products. The inter-comparison with CERES-SYN and GLASS ISR products show similar spatial and temporal patterns among the three datasets. Validation of my product at BSRN sites in this area provides an improved accuracy compared to GLASS product and a significant finer resolution compared to CERES-SYN product.Item Investigating the mechanism of the hydrogen peroxide photoproduction from chromophoric dissolved organic matter(2014) Zhang, Yi; Blough, Neil V; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The photochemical pathways for H2O2 production from chromophore dissolved organic matter (CDOM) were investigated extensively, employing in part a selective sodium borohydride reduction method to examine the structural basis of the H2O2 formation. Estimates of the lifetime of possible H2O2 precursor(s) have been acquired by examining the dependence of H2O2 production rates on dioxygen concentration. The results indicate that H2O2 arises from intramolecular electron transfer from an excited singlet donor to a ground-state acceptor. Possible donors include substituted phenols, while possible acceptors included quinones, which unlike ketones/aldehydes are not irreversibly reduced by borohydride. The relationship between the rate of H2O2 formation and the rate of reducing intermediate(s) formation upon irradiation of CDOM was thoroughly investigated employing radical molecular probe. The results obtained from the dependence of dioxygen concentrations and wavelengths indicate that approximate 90% of produced one-electron reducing intermediates is converted to superoxide. The stoichiometric ratio between the H2O2 and total reducing radicals suggests that 67% of the photochemically produced superoxide decays through oxidant sinks other than dismutation to H2O2. The effect of adding external phenol electron donors on the H2O2 production rate was also examined. Substantially enhanced rates of H2O2 production are observed, which is substantially inhibited by borohydride reduction (40-50%), similar to the loss of TMP previously reported. In addition, H2O2 production rate increases as the dioxygen concentration decreases, consistent with reaction with a triplet state intermediate. The results all indicate that an additional pathway of H2O2 formation is introduced in the presence of sufficient high concentration electron donors. Reaction between the phenol electron donors and the excited triplet state of aromatic ketones/aldehydes yields a ketyl radical that subsequently react with O2 to form O2- and then H2O2. The enhancement generally follows the reduction potential of the added phenols, as DMOP> MOP> PHE, with the exception of TMP. In the case of TMP, secondary radical reactions could be the cause of this difference.Item MEASUREMENT OF INVERSE DIFFUSION FLAME QUENCHING LIMITS(2013) Zhang, Yi; Sunderland, Peter B; Fire Protection Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Quenching limits of inverse diffusion flames were measured for different conditions. The flames were laminar and axisymmetric and were obtained by injecting various oxidizers into fuels. Burner inside diameters were 0.75, 1.53, 3.02, 4.56, and 10.1 mm. Oxygen mole fractions were 0.21, 0.3, 0.4, and 1, and the balance was nitrogen. Fuels were methane, ethylene, and propane. The flames were observed in a weak co-flow of fuel inside a glass chimney. The flames were ignited at relatively high oxidizer flow rates, after which the oxidizer flow was reduced until extinction. The typical heat release rate of quenching inverse flame ranged from 1 - 2 W, compared to a typical heat release rate of quenching normal hydrocarbon flames of 3 W. The quenching limits of inverse flames were generally independent of burner diameter, were proportional to the fuel quenching distance in premixed flames, and scaled with oxygen mole fraction to the power of -1.5. The results may help assess the hazards of firefighter respirator leaks in underventilated fires.