M. S.

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Typescript. / Thesis (Ph. D.)--University of Hawaii at Manoa, 1973. / Bibliography: leaves [293]-300. / xvi, 300 l illus., tables

The objective of this research was to demonstrate the application of ultrasound to field sites having problems with precipitation build up and corrosion. PTW s are passive reactive walls containing zero-valent iron metal for in-situ remediation of contaminated groundwater. However, loss of reactivity over time due to build up of corrosion and other precipitates on the iron surface is a major concern. Ultrasound energy has been established as an effective tool for revitalizing iron surface. This research applied ultrasound energy to a zero-valent iron wall constructed below the ground surface to remove precipitates and iron corrosion, increasing iron reactivity. Two field sites were selected for the ultrasound application research project. These sites have PTWs installed for the remediation of chlorinated compounds such as TCE and its daughter products. The first site is located at Launch Complex 34 (LC 34), Cape Canaveral Air Station, Florida. The second site is located at Denver Federal Center, Lakewood, Colorado. The ultrasound was applied to these sites by introducing an ultrasonic transducer in wells installed within the wall or just upstream of the wall and then applying ultrasonic energy to the entire depth of the wall. The apparatus used for ultrasound application was an omni-directional tubular resonator. Two such ultrasound units with frequencies of 25 kHz and 40 kHz were used for ultrasound treatment. Kinetic batch studies were performed on iron samples taken before and after ultrasound treatment. The degradation rate constants and half-life values for TCE were then calculated and con1pared for pre-ultrasound and post-ultrasound iron san1p les. Sin1ultaneously grounchvater \vas analyzed for di ffercnt VOCs. Soni cation period as brief as 30 n1inutes sho\vcd signi fie ant in1pact on the firstorder rate constants for TCE degradation. An increase in sonication period proved to be even n1ore effective. A sonication period of 90 n1inutes decreased TCE half-life by 30-40% for the 40-kHz resonator and 60-75~o for the 25-kHz resonator, for both the field sites. The 25-kHz resonator proved to be more effective than the 40-kHz resonator. For both field sites, ultrasound treatn1ent significantly increased TCE degradation rates, indicating a ren1oval of corrosion products and precipitates from the iron surface due to ultrasound. This technology has shown a great potential in revitalizing iron reactivity, effectively increasing the PTW life expectancy.

Global simulations of atmospheric methyl chloride (CH3Cl) are conducted using the GEOS-Chem model in order to understand better its sources and sinks. Though CH3Cl is one of the most abundant organic chlorine species in the stratosphere, not much is known about its sources and the budget remains unbalanced. In addition to the known sources (1.5 Tg yr-1) from ocean, biomass burning, incineration/industry, salt marshes, and wetlands, a hypothetical aseasonal biogenic source of 2.9 Tg yr-1 is added in order to match needed emissions. Observations from 7 surface sites and 8 aircraft field experiments are used to evaluate the model simulations. The model results with a priori emissions and sinks reproduce CH3Cl observations at northern mid and high latitudes reasonably well. However, the seasonal variation of CH3Cl at southern mid and high latitudes is severely overestimated. Simulated vertical profiles show disagreements in the vicinities of major sources, principally reflecting the uncertainties in the estimated distributions of our added pseudo-biogenic and the biomass burning sources. Inverse modeling is applied to obtain optimal source distributions of CH3Cl on the basis of surface and aircraft observations and model results. We resolve the seasonal dependence of the biogenic and biomass burning sources for each hemisphere. The aircraft in situ measurements are found to provide better constraints on the emission sources than surface measurements. The a posteriori emissions result in better agreement with the observations particularly at southern high latitudes. The a posteriori biogenic and biomass burning source decrease by 13 and 11% to 2500 and 545 Gg yr-1, respectively, while the a posteriori net ocean source increases by about a factor of 2 to 761 Gg yr-1. The decrease in biomass burning emissions is largely due to the reduction in the emissions in seasons other than spring in the northern hemisphere. The inversion results indicate that the biogenic source has a clear winter minimum in both hemispheres, likely reflecting the decrease of biogenic activity during that season.

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Halogen chemistry plays an important role in spring time ozone and mercury depletion events (ODEs and MDEs) and may efficiently oxidize hydrocarbons such as the important greenhouse gas methane (CH4) in the polar marine boundary layer. This thesis presents a detailed study of bromine and chlorine chemistry in the Arctic boundary layer based on measurements of bromine and chlorine containing species using chemical ionization mass spectrometry (CIMS). The capability of CIMS to accurately measure bromine oxide (BrO) was demonstrated. The first direct measurements of hypobromous acid (HOBr) were achieved. Conditions that likely favor bromine activation (e.g. high wind speeds) was presented. To advance the understanding of bromine recycling, a time dependent model was built to simulate the bromine speciation. Unexpected high levels of molecular chlorine (Cl2) were observed at Barrow, AK, which had a large impact on methane oxidation and could contribute to ozone loss and mercury oxidation at Barrow, AK. Moreover, BrO levels observed at Summit, Greenland did not explain the under prediction of hydroxyl radical (OH). However, the enhanced OH was found to be coincident with elevated reactive gaseous mercury (RGM). In addition, airborne BrO measurements were found to be generally consistent with airborne observations of soluble bromide based on the response factors of mist chamber to the bromine species and bromine chemical mechanisms.