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Articles 61 - 66 of 66
Full-Text Articles in Environmental Indicators and Impact Assessment
Fage Determination Of Tropospheric Ho And Ho₂, Linda Acha George, Thomas M. Hard, Robert J. O'Brien
Fage Determination Of Tropospheric Ho And Ho₂, Linda Acha George, Thomas M. Hard, Robert J. O'Brien
Environmental Science and Management Faculty Publications and Presentations
FAGE (fluorescence assay with gas expansion) was developed as a sensitive technique for the detection of low-concentration free radicals in the atmosphere. The application of FAGE to tropospheric hydroxyl (H0) and hydroperoxyl (H0₂) radicals has yielded calibrated measurements of both species in both clean air and highly polluted urban air. For HO calibration, a continuously stirred tank reactor provides a uniform external HO concentration, which can be measured by gas chromatography of an HO-reactive hydrocarbon. The aerodynamics of the air-sampling process has been modeled computationally, with results that agree with empirical observations of the effects of nozzle diameter on HO …
Carbon Monoxide In The Antarctic Atmosphere: Observations Of Decreasing Concentrations, M. A. K. Khalil, R. A. Rasmussen
Carbon Monoxide In The Antarctic Atmosphere: Observations Of Decreasing Concentrations, M. A. K. Khalil, R. A. Rasmussen
Environmental Science and Management Faculty Publications and Presentations
Hydroxyl radicals remove hundreds, perhaps thousands, of organic gases from the atmosphere and are often regarded as am index of the oxidizing capacity of the Earth's atmosphere (Thompson 1992). In recent years, there have been growing concerns that, over the past century and now, human activities may be depleting hydroxyl concentrations by adding huge amounts of carbon monoxide and methane to the atmosphere. Reduction in the hydroxyl concentrations can then indirectly lead to more global warming, stratospheric ozone depletion, and other disturbances in atmospheric chemistry. Carbon monoxide (CO) is a key component in the determination of hydroxyl radical (OH) concentrations: …
Fage Measurements Of Tropospheric Ho With Measurements And Model Of Interferences, Thomas M. Hard, A. A. Mehrabzadeh, C. Y. Chan, Robert J. O'Brien
Fage Measurements Of Tropospheric Ho With Measurements And Model Of Interferences, Thomas M. Hard, A. A. Mehrabzadeh, C. Y. Chan, Robert J. O'Brien
Environmental Science and Management Faculty Publications and Presentations
Ambient HO measurements by low-pressure laser-excited fluorescence with chemical modulation, and supporting ozone and water-vapor data, are presented for periods in May and August 1987. The observed peak daytime ambient HO concentrations in Oregon are in the range (2.5 to 8) times106 molecules cm⁻³ and show small negative offsets due to photochemical interference. Direct measurements of the interference at fixed [O₃] give the dependence on ambient [H₂O] and on the modulating reagent [isobutane]. At ambient [O₃]=30 p.p.b. and 10 torr H₂O, with excitation and detection at a total pressure of 4 torr, the net interference is equal to [HO]=-1.3times10⁶ molecule …
Diurnal Ho₂ Cycles At Clean Air And Urban Sites In The Troposphere, Thomas M. Hard, C. Y. Chan, A. A. Mehrabzadeh, Robert J. O'Brien
Diurnal Ho₂ Cycles At Clean Air And Urban Sites In The Troposphere, Thomas M. Hard, C. Y. Chan, A. A. Mehrabzadeh, Robert J. O'Brien
Environmental Science and Management Faculty Publications and Presentations
HO₂ concentrations have been determined at two Oregon sites for continuous periods of 36 to 48 hours, using fluorescence assay with gas expansion. At the sea level coastal site (45degN 124degW), NNW winds prevailed during daytime, and a point measurement of very low total nonmethane hydrocarbon concentration indicated the presence of remote tropospheric air of oceanic origin. At the urban site, HO₂ was determined during moderately low ozone pollution levels. At both sites, maximum daily [HO₂] was in the range of 1-2times10⁸ cm⁻³ under clear-sky conditions, with an estimated overall uncertainty of 40%.
Third-Generation Fage Instrument For Tropospheric Hydroxyl Radical Measurement, C. Y. Chan, Thomas M. Hard, A. A. Mehrabzadeh, Robert J. O'Brien
Third-Generation Fage Instrument For Tropospheric Hydroxyl Radical Measurement, C. Y. Chan, Thomas M. Hard, A. A. Mehrabzadeh, Robert J. O'Brien
Environmental Science and Management Faculty Publications and Presentations
The authors have constructed a single-stage, frequency-doubled, copper vapor laser-pumped dye laser to be used in the measurement of atmospheric hydroxyl radical concentrations. This laser can be tuned to either the 1← 0 or 0← 0 vibrational transition in the electronic transition ²Sigmalarr²Pi. A new photon counting instrument is used for HO fluorescence detection. Theoretical and experimental studies of instrument performance show better sensitivities and reduced photolytic interferences than have been possible with previous systems based upon Nd:YAG pumping.
Interference Suppression In Ho Fluorescence Detection, Thomas M. Hard, Robert J. O'Brien, T. B. Cook, G. A. Tsongas
Interference Suppression In Ho Fluorescence Detection, Thomas M. Hard, Robert J. O'Brien, T. B. Cook, G. A. Tsongas
Environmental Science and Management Faculty Publications and Presentations
In this Letter we report preliminary results on a sampling method that greatly reduces the above interferences relative to hydroxyl fluorescence.