16 research outputs found
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Experimentally determined kinetic isotope effects in the reaction of CH4 with Cl: Implications for atmospheric CH4
We report experimental values for the carbon and hydrogen kinetic isotope effects (KIEs) in the reaction of CH4 with Cl at temperatures between 273 and 350 K. Isotope ratio mass spectrometry was utilized to measure 13CH4/12CH4 and CH3D/CH4 ratios on samples taken from a 5870- L reaction chamber. At 299 K, k(c12)/k(c13) = 1.0621±0.0001 (2σ) and k(h)/k(d) = 1.474±0.020 (2σ). For both KIEs, the ratio decreased with increasing temperature over the range studied. These results agree well with experimental studies using tunable diode laser absorption spectroscopy and FTIR absorption spectroscopy and with a recent theoretically-calculated set of values
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Experimentally determined kinetic isotope effects in the reaction of CH4 with Cl: Implications for atmospheric CH4
We report experimental values for the carbon and hydrogen kinetic isotope effects (KIEs) in the reaction of CH with Cl at temperatures between 273 and 350 K. Isotope ratio mass spectrometry was utilized to measure CH / CH and CH D/CH ratios on samples taken from a 5870- L reaction chamber. At 299 K, k(c12)/k(c13) = 1.0621±0.0001 (2σ) and k(h)/k(d) = 1.474±0.020 (2σ). For both KIEs, the ratio decreased with increasing temperature over the range studied. These results agree well with experimental studies using tunable diode laser absorption spectroscopy and FTIR absorption spectroscopy and with a recent theoretically-calculated set of values. 4 4 4 3 4 13 1
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Stable carbon isotopic composition of atmospheric methane: A comparison of surface level and free tropospheric air
We report CH4 mixing ratios and δ13C of CH4 values for remote air at two ground-based atmospheric sampling sites for the period December 1994 to August 1998 and similar data from aircraft sampling of air masses from near sea level to near tropopause in September and October of 1996 during the Global Tropospheric Experiment Pacific Exploratory Mission (PEM)-Tropics A. Surface values of δ13C-CH4 ranged from -47.02 to -47.52‰ at Niwot Ridge, Colorado (40° N, 105° W), and from -46.81 to -47.64‰ at Montaña de Oro, California (35° N, 121° W). Samples for isotopic analysis were taken from 2° to 27° S latitude and 81° to 158° W longitude and from sea level to 11.3 km in altitude during the PEM-Tropics A mission. They represent the first study of 13CH4 in the tropical free troposphere. At ∼11 km, δ13C-CH4 was ∼1‰ greater than surface level values. Methane was generally enriched in 13C as altitude increased and as latitude increased (toward the South Pole). Using criteria to filter out stratospheric subsidence and convective events on the basis of other trace gases present in the samples, we find evidence of a vertical gradient in δ13C-CH4 in the tropical troposphere. The magnitude of the isotopic shifts in atmospheric CH4 with altitude are examined with a two-dimensional tropospheric photochemical model and experimentally determined values for carbon kinetic isotope effects in chemical loss processes of CH4 Model-calculated values for δ13C-CH4 in both the troposphere and lower stratosphere significantly underpredict the enrichment in 13CH4 with altitude observed in our measurement data and data of other research groups. Copyright 1999 by the American Geophysical Union
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Stable carbon isotopic composition of atmospheric methane: A comparison of surface level and free tropospheric air
We report CH4 mixing ratios and δ13C of CH4 values for remote air at two ground-based atmospheric sampling sites for the period December 1994 to August 1998 and similar data from aircraft sampling of air masses from near sea level to near tropopause in September and October of 1996 during the Global Tropospheric Experiment Pacific Exploratory Mission (PEM)-Tropics A. Surface values of δ13C-CH4 ranged from -47.02 to -47.52‰ at Niwot Ridge, Colorado (40° N, 105° W), and from -46.81 to -47.64‰ at Montaña de Oro, California (35° N, 121° W). Samples for isotopic analysis were taken from 2° to 27° S latitude and 81° to 158° W longitude and from sea level to 11.3 km in altitude during the PEM-Tropics A mission. They represent the first study of 13CH4 in the tropical free troposphere. At ∼11 km, δ13C-CH4 was ∼1‰ greater than surface level values. Methane was generally enriched in 13C as altitude increased and as latitude increased (toward the South Pole). Using criteria to filter out stratospheric subsidence and convective events on the basis of other trace gases present in the samples, we find evidence of a vertical gradient in δ13C-CH4 in the tropical troposphere. The magnitude of the isotopic shifts in atmospheric CH4 with altitude are examined with a two-dimensional tropospheric photochemical model and experimentally determined values for carbon kinetic isotope effects in chemical loss processes of CH4 Model-calculated values for δ13C-CH4 in both the troposphere and lower stratosphere significantly underpredict the enrichment in 13CH4 with altitude observed in our measurement data and data of other research groups. Copyright 1999 by the American Geophysical Union