CH4 Profiles Research Articles

05/02677 Numerical study on NO formation in CH 4-O 2-N 2 diffusion flame diluted with CO 2

To optimize the good working of the common thermal oxidizers, a better understanding of the high temperature oxidation kinetic of volatile organic compounds (VOCs) in flame conditions is needed. So the experimental study of Ethyl Acetate (EA) oxidation in CH4/EA/O2/N2 low pressure flames has been investigated. Molecular species concentration profiles of CH4, O2, CO, CO2, H2O, C2H2, C2H4, C2H6, C3H6, C3H8, EA, CH3OH, C2H5OH, CH3CHO, C2H5CHO, CH3COCH3, CH3OCH3, CH3COOCHCH2 and CH3COOH have been obtained by coupling microprobe sampling with gas chromatography–mass spectrometry (GC/MS) analysis. A detailed kinetic mechanism has been developed to model the EA oxidation in these conditions. The kinetic scheme includes 23 oxygenated species involved in 142 reversible reactions. It takes into account the first steps of the EA oxidation and the oxidation processes of all the measured oxygenated intermediate compounds. The proposed mechanism globally well predicts the experimental results obtained in the methane/air flames even if some discrepancies are pointed out. Sensitivity analysis allows the determination of the main reactional pathways involved in the thermal degradation of EA.

ACE‐FTS measurements across the edge of the winter 2004 Arctic vortex

Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE‐FTS) solar occultation measurements have been used to create volume mixing ratio (VMR) profiles of H2O, CH4, and N2O inside and outside the 2004 Arctic vortex. Using derived meteorological quantities such as potential vorticity, we have classified 450 occultations (from February and March 2004 spanning 0 to 79.8°N) as vortex, vortex edge, or extravortex. We plot [CH4] versus [N2O] correlations to display the distinct patterns observed for measurements of different air masses and use these correlations to further classify the extravortex occultations as tropical, subtropical, or midlatitude. Using comparisons between high latitude profiles of [N2O], [CH4], and [H2O] inside and outside the Arctic vortex, we estimate upper stratospheric and lower mesospheric descent rates and find that descent in the winter 2004 Arctic vortex was rapid, with evidence of descent at higher altitudes than in past years.

Subtropical trace gas profiles determined by ground-based FTIR spectroscopy at Izaña (28° N, 16° W): Five-year record, error analysis, and comparison with 3-D CTMs

Abstract. Within the framework of the NDSC (Network for the Detection of Stratospheric Change) ground-based FTIR solar absorption spectra have been routinely recorded at Izaña Observatory (28° N, 16° W) on Tenerife Island since March 1999. By analyzing the shape of the absorption lines, and their different temperature sensitivities, the vertical distribution of the absorbers can be retrieved. Unique time series of subtropical profiles of O3, HCl, HF, N2O, and CH4 are presented. The effects of both dynamical and chemical annually varying trace gas cycles can be seen in the retrieved profiles. These include enhanced upwelling and photochemistry in summer and a more disturbed atmosphere in winter, which are typical of the subtropical stratosphere. A detailed error analysis has been performed for each profile. The output from two different three-dimensional (3-D) chemical transport models (CTMs), which are forced by ECMWF analyses, are compared to the measured profiles. Both models agree well with the measurements in tracking abrupt variations in the atmospheric structure, e.g. due to tropical streamers, in particular for the lower stratosphere. Simulated and measured profiles also reflect similar dynamical and chemical annual cycles. However, the differences between their mixing ratios clearly exceed the error bars estimated for the measured profiles. Possible reasons for this are discussed.

MANTRA ‐ A Balloon Mission to Study the Odd‐Nitrogen Budget of the Stratosphere

The Middle Atmosphere Nitrogen TRend Assessment (MANTRA) series of high‐altitude balloon flights is being undertaken to investigate changes in the concentrations of northern hemisphere mid‐latitude stratospheric ozone, and of nitrogen and chlorine compounds that play a role in ozone chemistry. Four campaigns have been carried out to date, all from Vanscoy, Saskatchewan, Canada (52°01'N, 107°02'W, 511.0 m). The first MANTRA mission took place in August 1998, with the balloon flight on 24 August 1998 being the first Canadian launch of a large high‐altitude balloon in about fifteen years. The balloon carried a payload of instruments to measure atmospheric composition, and made measurements from a float altitude of 32–38 km for one day. Three of these instruments had been flown on the Stratoprobe flights of the Atmospheric Environment Service (now the Meteorological Service of Canada) in the 1970s and early 1980s, providing a link to historical data predating the onset of mid‐latitude ozone loss. The primary measurements obtained from the balloon‐borne instruments were vertical profiles of ozone, NO2, HNO3, HCl, CFC‐11, CFC‐12, N2O, CH4, temperature, and aerosol backscatter. Total column measurements of ozone, NO2, SO2, and aerosol optical depth were made by three ground‐based spectrometers deployed during the campaign. Regular ozonesonde and radiosonde launches were also conducted during the two weeks prior to the main launch in order to characterize the local atmospheric conditions (winds, pressure, temperature, humidity) in the vicinity of the primary balloon flight. The data have been compared with the Model for Evaluating oZONe Trends (MEZON) chemical transport model, the University of California at Irvine photochemical box model, and the Canadian Middle Atmosphere Model (CMAM) to test our current understanding of model photochemistry and mid‐latitude species correlations. This paper provides an overview of the MANTRA 1998 mission, and serves as an introduction to the accompanying papers in this issue of Atmosphere‐Ocean that describe specific aspects and results of this campaign.

Lightweight diode laser spectrometer CHILD (Compact High-altitude In-situ Laser Diode) for balloonborne measurements of water vapor and methane

A new lightweight near-infrared tunable diode laser spectrometer CHILD (Compact High-altitude In-situ Laser Diode spectrometer) was developed for flights to the stratosphere as an additional in situ sensor on existing balloonborne payloads. Free-air absorption measurements in the near infrared are made with an open-path Herriott cell with new design features. It offers two individual absorption path lengths optimized for CH4 with 74 m (136 pass) and H2O with 36 m (66 pass). New electronic features include a real-time gain control loop that provides an autocalibration function. In flight-ready configuration the instrument mass is approximately 20 kg, including batteries. It successfully measured stratospheric CH4 and H2O profiles on high-altitude balloons on four balloon campaigns (Environmental Satellite validation) between October 2001 and June 2003. On these first flights, in situ spectra were recorded from ground level to 32,000-m altitude with a sensitivity of 0.1 ppm [(parts per million), ground] to 0.4 ppm (32,000 m) for methane and 0.15-0.5 ppm for water.

Atmospheric methane during the last four glacial‐interglacial cycles: Rapid changes and their link with Antarctic temperature

Atmospheric methane (CH4) recorded in Antarctic ice cores represents the closest ice proxy available for Greenland temperature changes beyond times when Greenland climate records are available. The record over four climatic cycles from the Vostok ice core offers the opportunity to study the phase relationship between Greenland and Antarctic climate changes through detailed CH4 profiles. Combining American and French analytical efforts, we have improved the time resolution of the existing CH4 record from Petit et al. [1999]. Spectral analyses reveal high‐ and low‐frequency variability (including a strong precessional component). The phase relationship between CH4 and the Antarctic temperature proxy (δD) shows a systematic lag of CH4 versus temperature by 1100 ± 200 years, on long timescales (50–400 kyr) and a more complex behavior over shorter timescales (i.e., ≤25 kyr), suggesting that Dansgaard/Oeschger‐type of climatic variability and associated interhemispheric linkage are robust features of late Quaternary climate.

Halogen Occultation Experiment (HALOE) and balloon-borne in situ measurements of methane in stratosphere and their relation to the quasi-biennial oscillation (QBO)

Abstract. Measurements of methane have been made from various observational platforms in the atmosphere. In this article, we have compared four high precision balloon-borne measurements from Hyderabad (17.5°N), India in the period of 1987 and 1998 with a part of HALOE/UARS global observations available since 1991. All the balloon measurements correspond to boreal spring (March and April) but belong to different years. A comparison shows fairly good agreement with each other. The gradient in CH4 profiles in the troposphere is controlled by the variation in vertical transport. The strongest effect of dynamical influence on methane vertical profiles is shown to be resulting from the dynamical quasi-biennial oscillation in the stratosphere, and that has been consistently derived from both the measurement techniques and chemistry-transport model simulations. It is observed that the QBO signal in CH4 anomaly exhibits interhemispheric asymmetry caused by the tropics to midlatitude circulation in the stratosphere. A mechanism is suggested to explain how and to what extent the methane vertical profiles over Hyderabad and higher latitudes could be modulated by the prevailing QBO winds in the tropics. We have also discussed how the same mechanism would affect ozone distribution in the stratosphere quite differently.

Trace gas measurements in nascent, aged, and cloud‐processed smoke from African savanna fires by airborne Fourier transform infrared spectroscopy (AFTIR)

We measured stable and reactive trace gases with an airborne Fourier transform infrared spectrometer (AFTIR) on the University of Washington Convair‐580 research aircraft in August/September 2000 during the SAFARI 2000 dry season campaign in Southern Africa. The measurements included vertical profiles of CO2, CO, H2O, and CH4 up to 5.5 km on six occasions above instrumented ground sites and below the TERRA satellite and ER‐2 high‐flying research aircraft. We also measured the trace gas emissions from 10 African savanna fires. Five of these fires featured extensive ground‐based fuel characterization, and two were in the humid savanna ecosystem that accounts for most African biomass burning. The major constituents that we detected in nascent smoke were (in order of excess molar abundance) H2O, CO2, CO, CH4, NO2, NO, C2H4, CH3COOH, HCHO, CH3OH, HCN, NH3, HCOOH, and C2H2. These are the first quantitative measurements of the initial emissions of oxygenated volatile organic compounds (OVOC), NH3, and HCN from African savanna fires. On average, we measured 5.3 g/kg of OVOC and 3.6 g/kg of hydrocarbons (including CH4) in the initial emissions from the fires. Thus, the OVOC will have profound, largely unexplored effects on tropical tropospheric chemistry. The HCN emission factor was only weakly dependent on fire type; the average value (0.53 g/kg) is about 20 times that of a previous recommendation. HCN may be useful as a tracer for savanna fires. ΔO3/ΔCO and ΔCH3COOH/ΔCO increased to as much as 9% in <1 h of photochemical processing downwind of fires. Direct measurements showed that cloud processing of smoke greatly reduced CH3OH, NH3, CH3COOH, SO2, and NO2 levels, but significantly increased HCHO and NO.

Methane in an acidic bog lake: The influence of peat in the catchment on the biogeochemistry of methane

Methanogenesis, an anaerobic microbial process in sediments, was investigated in a naturally acidic bog lake, Grosse Fuchskuhle, in northeastern Germany. The lake was artificially divided into four sub-basins: two western basins receiving humic substances from an adjacent Sphagnum bog and two eastern basins isolated from this acidic inflow with installed curtains. Within the northeast (NE) and southwest (SW) basin compartments, one of each group were studied. In the peat influenced western basins the pH-values in the water columns were kept low and increased in the others. The stratification period increased in all four compartments from some weeks to some months, from April to October, with the development of anoxic hypolimnia in all compartments after the installation of the curtains. The layer with the most active methane production moved from a sediment depth of more than 20 cm before separation to close to the surface sediments in all compartments. Methano genic microorganisms were found in the whole sediment core – from surface sediments to a depth of 25 cm. The proportion of methanogens was approximately 15% of total microbial cell numbers, which were approximately 2 × 106 cells/ml sediment. Oligonucleotide probes targeting nearly all families of the phylum Euryarchaeota were tested with fluorescence in-situ hybridization (FISH). In both basins, with and without the influence from peat, oligonucleotide probe (MSMX) targeting Methanosarcinaceae could be detected only as methanogens. The finding of only acetate-using methanogens by FISH indicated acetate as a major methanogenic substrate. Concentration profiles of CH4 as a function of sediment depth were characterized in both basins by high concentrations in the top layers. Methane bubbles were released in the late summer in the eastern sub-basin only, likely from oversaturation in the surface sediments.

Sequential assimilation of stratospheric chemical observations in a three‐dimensional model

We describe a technique to assimilate chemical observations in a three‐dimensional (3‐D) chemical transport model (CTM). The method uses the established sequential technique of Khattatov et al. [2000], but here, it is applied simultaneously to many observed species. Following the assimilation, care is taken to preserve compact correlations between all modeled long‐lived tracers and the total abundance of reactive families (e.g., inorganic chlorine). This way, the observations of long‐lived tracers and family members constrain many other species in the model. In this paper, we apply the technique to the assimilation of O3, CH4, H2O, and HCl from the Halogen Occultation Experiment (HALOE) in 1992. Despite the poor coverage of HALOE, the assimilation of species with long photochemical lifetimes is a useful global constraint on the model. Results of the assimilation model have been tested by comparison with Atmospheric Trace Molecule Spectroscopy Experiment (ATMOS) profiles of O3, CH4, H2O, HCl, and N2O. Direct comparison of the assimilated species shows that the assimilation model performs better in reproducing the independent observations. Comparison of the nonassimilated species (N2O) shows that assimilation has generally improved the comparison, especially in the midlatitude lower stratosphere.

Correlation relationships of stratospheric molecular constituents from high spectral resolution, ground‐based infrared solar absorption spectra

Comparisons of chemically active species with chemically inert tracers are useful to quantify transport and mixing and assess the accuracy of model predictions. We report measurements of chemically active species and chemically inert tracers in the stratosphere derived from the analysis of infrared solar absorption spectra recorded with a ground‐based Fourier transform spectrometer operated typically at 0.005‐ to 0.01‐cm−1 spectral resolution. The measurements were recorded from Kitt Peak in southern Arizona (latitude 31.9°N, 111.6°W, 2.09 km altitude). Time series of N2O, CH4, O3, and HNO3 vertical profiles have been retrieved from measurements in microwindows. From these results, correlations between N2O and CH4 stratospheric mixing ratios and between O3 and HNO3 lower stratospheric mixing ratios have been derived. The measured correlations between N2O versus CH4 mixing ratios are compact and show little variability with respect to season in quantitative agreement with Atmospheric Trace Molecule Spectroscopy Experiment (ATMOS) spring and autumn measurements recorded near the same latitude. Lower stratospheric O3 versus HNO3 mixing ratios measured during low to moderate aerosol loading time periods also show a compact relations though the HNO3/O3 slope is a factor of 2 lower than obtained from November 1994 ATMOS measurements near the same latitude. We also compare Kitt Peak and ATMOS N2O versus CH4 and O3 versus HNO3 relations obtained by averaging the measurements over two broad stratospheric layers. This comparison avoids bias from the a priori profiles and the limited vertical resolution of the ground‐based observations.

Decomposition of ethoxyethane in the cold plasma environment

A radio frequency (RF) plasma system was used to decompose the ethoxyethane (EOE) contained gas. The reactants and final products were analyzed by using an FTIR (Fourier Transform Infrared) spectrometer. The effects of plasma operational parameters, including input power wattage (W), equivalence ratios (Φ), feeding concentration (C) of EOE and total gas flow rate (Q) for EOE decomposition were evaluated. In addition, the possible reaction pathways for EOE decomposition and the formation of final products were built up and are discussed in this paper. The mole fraction profiles of C2H5OC2H5, CH3CHO, CH4, C2H6, C2H4, C2H2, CO2 and CO were detected and are also presented in this paper. At lower input power wattages, the creation of glow discharge is strongly dependent on the plasma production index (PPI). When input power wattages are smaller than 30 W, the minimum values of PPI to create glow discharge ranged between 18.2 and 19.0. The results of this study revealed that, in the RF plasma reactor, the decomposition fraction of EOE could reach 100% under most operational conditions. © 2000 Society of Chemical Industry

Atmospheric trace gas sounding with SCIAMACHY

Abstract SCIAMACHY (Scanning Imaging Absorption Spectrometer for Atmospheric CHartographY) is a contribution to the ENVISAT-1 satellite, which is to be launched in late 2000. The SCIAMACHY instrument is designed to measure sunlight transmitted, reflected and scattered by the Earth's atmosphere or surface. The instrument measures simultaneously from the UV to the NIR spectral spectral region (240 – 2380 nm). Observations are made in alternate nadir and limb viewing geometries and also for solar sunrise and lunar moonrise occultation. Inversion of the SCIAMACHY measurements will provide the following: the amount and distributions of some important trace gases O3, BrO, OClO, ClO, SO2, H2CO, NO2, CO, CO2, CH4, H2O, N2O, p, T, aerosol, and radiation flux profiles, cloud cover and cloud top height. Combination of the near simultaneous limb and nadir observations enables the tropospheric column amounts of O3, NO2, CO, CH4, H2O, N2O, SO2, and H2CO to be detected. SCIAMACHY will provide new insight into the global behaviour of the troposphere and the stratosphere.

CH4 emission from a hollow-ridge complex in a raised bog: The role of CH4 production and oxidation

The aim of this study was to correlate magnitude andcontrols of CH4 fluxes with the microtopographyand the vegetation in a hollow-ridge complex of araised bog. High CH4 emission rates were measuredfrom hollows and mud-bottom hollows, while hummocksconsumed atmospheric CH4 at a low rate. Thehighest emissions were measured from plots with Eriophorum vaginatum and Scheuchzeriapalustris. CH4 emission ceased after Scheuchzeria had been clipped below the water table,indicating the importance of this aerenchymatic plantas a conduit for CH4. Peat in the upper catotelm of hollows was younger andless decomposed than in hummocks. Potential CH4production in vitro was higher and themethanogenic association was better adapted to highertemperatures in hollow than in hummock peat. Highertemperatures in hollows resulted in a strongerCH4 source in hollows than in hummocks. Negativefluxes from hummocks indicated that even in wetlandsmethanotrophic bacteria exist that are able to oxidizeCH4 at atmospheric mixing ratios, and thatoxidation controls CH4 emission completely. TheCH4 mixing ratio was low in the acrotelm, but itincreased within the catotelm. Comparing fluxesmeasured in static chambers with fluxes calculatedfrom the porewater CH4 profiles it was deducedthat the zone of methane oxidation was located closeto the water table. In hollows, CH4 production at in situtemperature was far higher than emission into theatmosphere, corresponding to an oxidation rate ofnearly 99%. The CH4 flux between the catotelmand the acrotelm of hollows was also higher than theemission, indicating the importance of CH4oxidation in the aerobic acrotelm, too. CH4microprofiles showed that CH4 oxidation inmud-bottom hollows was confined to the topmost 2 mm,and that in Sphagnum-covered hollows CH4oxidation occurred at the lower edge of green Sphagnum-parts.

112 ILASによるオゾン層科学への貢献

The Improved Limb Atmospheric Spectrometer (ILAS) was a satellite sensor developed by Global Environment Division, the Environment Agency of Japan, for monitoring the ozone layer. ILAS was onboard the ADEOS (Advanced Earth Observing Satellite) MIDORI satellite which was launched on August 17, 1996 by the National Space Development Agency of Japan (NASDA). ILAS measured about 6700 vertical profiles of O3, HNO3, NO2, N2O, CH4, and H2O in the stratosphere both in the Northern and the Southern Hemispheres from November 1996 to June 1997. It provided valuable observational data for investigating the mechanism of ozone loss in high latitude stratosphere. Substantial ozone loss was found from the ILAS O3 data, which were taken in early spring in 1997 in the northern high latitude region. Simultaneously, frequent occurrence of Polar Stratospheric Clouds (PSCs) was observed.

Continuous measurements of CO and H2 deposition velocities onto an andisol: uptake control by soil moisture

Carbon monoxide (CO) and hydrogen (H2) net deposition velocities from the atmosphere onto soil and carbon dioxide (CO2) effluxes to the atmosphere have been measured in an andisol field in Tsukuba, Japan by the open-flow chamber method. The deposition velocities of CO and H2 were closely correlated (R = 0.87), with a ratio of 1.55, which was attributed to the difference in molecular diffusivities. However, the deposition velocities did not exhibit a direct relationship with the CO2 efflux. Deposition velocities of CO and H2 ranged from 0.00 to 0.06 cm s-1 and from 0.00 to 0.10 cm s-1, respectively, and were closely related to the level of the surface soil moisture (0–5 cm) and were higher in plowed plots than in compacted plots. CO deposition velocity was slightly lower in the daytime due to higher production rates affected by the soil temperature. These findings indicate that microbial CO and H2 consumption was limited by transport resistance in the soil and that the in situ CO and H2 uptake rates may be limited by a higher soil moisture level. CO and H2 deposition was estimated to be restricted to the surface soil (possibly only the top 2–3 cm). CH4 and CO2 gas profiles were also related to the variation of the soil moisture level.

Balloon‐borne in situ measurements of stratospheric H2O, CH4 and H2 at midlatitudes

Using a new Lyman α hygrometer, balloon‐borne measurements of H2O were performed on September 20, 1993 at midlatitudes (43° N). A cryogenic whole air sampler to measure CH4 and H2 amongst other long‐lived trace gases was operated on the same payload. The profile of the water vapor mixing ratio showed a well‐pronounced hygropause of 4 ppmv at θ = 420 K increasing to 6.0 ppmv at θ = 920 K. For θ > 400 K, total hydrogen ΣH2 = H2O + 2·CH4 + H2 appeared to be constant at a mean value of 7.72 ± 0.13 ppmv; the overall accuracy of this value is 0.65 ppmv. From the correlation functions of CH4 with H2 and H2O the water vapor yield from methane oxidation is determined with high precision to be 1.975 ± 0.030. The globally averaged water vapor mixing ratio entering the stratosphere from the troposphere was found to be 3.9 ± 1.0 ppmv. Intercomparison with H2O and CH4 profiles obtained nearby from the Halogen Occultation Experiment (HALOE) on board the UARS satellite showed good agreement within the experimental errors.

The Oxidation of 1,3-Butadiene: Experimental Results and Kinetic Modeling

The oxidation of 1,3-butadiene has been investigated in a jet-stirred reactor at high temperature (˜750-1250 K), variable pressure (1 and 10 atm) and variable equivalence ratio (0.25≤φ≤2). Molecular species concentration profiles of O2, H2,CO,CO2,CH2O,CH4,C2H2,C2H4, C2H6, C3H4 (allene and propyne), C3H6, acrolein, 1-C4H8, 2-C4H8 (cis and trans), 1,3-C4H6, vinyla-cetylene, cyclopentadiene, and benzene were obtained by probe sampling and GC analysis. The oxidation of 1,3-butadiene in these conditions was modeled using a detailed kinetic reaction mechanism (91 species and 666 reactions, most of them reversible). The proposed mechanism, also validated for the oxidation of simpler hydrocarbons and natural gas blends in the same conditions, is able to reasonably well-predict the experimental results obtained in this study. Sensitivity analyses and reaction path analyses, based on species net rate of reaction, are used to interpret the present results. The routes to benzene formation have been delineated: At low fuel conversion and low temperature, benzene is mostly formed through the addition of vinyl radical to 1,3-butadiene, yielding 1,3-cyclohexadiene, followed by two channels: (a) elimination of molecular hydrogen to yield benzene and (b) decomposition of 1,3-cyclohexadiene yielding cyclohexadienyl followed by its decomposition into benzene and H atom; at high fuel conversion and higher temperature, (c) the recombination of propargyl radicals and (d) the addition of vinyl to vinylacetylene increasingly yield to benzene formation.

Effect of gas bubbles on the diffusive flux of methane in anoxic paddy soil

The emission of CH4 from paddy soil is driven by CH4 concentration gradients in the submerged soil. CH4 concentration gradients between anoxic methanogenic and oxic methanotrophic soil layers were measured in paddy soil microcosms by using gas diffusion probes with a spatial resolution of 1 mm. The CH4 emission rate was measured by placing a microcosm into a chamber containing an atmosphere of either synthetic air (80% N2, 20% 02) or N2. The CH4 flux was 1.6 ± 5.4 nmol cm−2 d−1 under synthetic air and 288 ± 10 nmol cm−2 d−1 under N2. The difference between the oxic and the anoxic CH4 fluxes was due to CH4 oxidation. The vertical CH4 concentration gradients indicated CH4 oxidation at 2–3 mm depth. Below this depth CH4 concentrations increased steadily to about 10 mm depth, below which accumulation of gas bubbles was observed. The diffusive flux calculated by Fick's first law from the linear part of the gradient was 166 ± 14 nmol cm−2 d−1. Obviously, the flux calculated from molecular diffusion was smaller than the flux that was actually measured under N2. An important condition for the use of Fick's law is that the slope of the gradient used for the calculation is taken in the direction where the slope is steepest. This direction is not necessarily identical with the vertical direction if CH4 concentrations also change in horizontal direction. Measurement of horizontal and vertical CH4 profiles demonstrated that the gradients had a three‐dimensional structure. The reason for this structure was that the isopleths of identical CH4 concentrations followed the uneven surface of the gas bubble layer as the main direct source for the CH4 diffusion gradients. We conclude that gas bubbles do not only directly cause a CH4 flux by ebullition but also indirectly affect the diffusional flux of CH4 in soil or sediment. When the diffusive CH4 flux was calculated from the concentration gradient at 6–8 mm depth, it was larger (224 ± 70 nmol cm−2 d−1) than from that at 3–5 mm depth (125 ± 86 nmol cm−2 d−1). Thus, a transport process in addition to molecular diffusion seemed to be active in the upper soil layers, possibly bioirrigation.

Isobutene Oxidation and Ignition: Experimental and Detailed Kinetic Modeling Study

The oxidation of isobutene has been investigated for the first time in a jet-stirred reactor at high temperature (∼800-l230K) and at l,5and 10 atm. Molecular species concentration profiles of O2, H2, CO, CO2, CH2O, CH4, C2H2, C2H4, C2H6, C3H4 (allene and propyne), C3H6, acetone, acrolein, methacrolein, I-C4H8, i-C4H8, 1,3-C4H6, l-butyne, isoprene, 2-methyl-l-butene, 2-methyl-2-butene, and benzene were obtained by probe sampling and GC analysis. The oxidation of isobutene in these conditions and the ignition of isobutene-oxygen-argon mixtures in a shock-tube were modeled using a detailed kinetic reaction mechanism (l 10 species and 743 reactions, most of them reversible). The proposed mechanism, also validated for the oxidation of CH4, C2H2, C2H4, C2H4, C3H6, acetaldehyde, ethylene oxide, and natural gas blends in the same conditions, is able to predict experimental results obtained in our high-pressure jet stirred reactor and the ignition delays measured behind a reflected shock wave. Two routes to benzene formation have been delineated: (a) the addition of propargyl radicals to allene, and (b) the recombination of propargyl radicals. Sensitivity analyses and reaction path analyses are used to interpret the present results.