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DOE OSTI2020

The Global Methane Budget 2000–2017

Understanding and quantifying the global methane (CH 4 ) budget is important for assessing realistic pathways to mitigate climate change. Atmospheric emissions and concentrations of CH 4 are continuing to increase, making CH 4 the second most important human-influenced greenhouse gas in terms of climate forcing, after carbon dioxide (CO 2 ). Assessing the relative importance of CH 4 in comparison to CO 2 is complicated by its shorter atmospheric lifetime, stronger warming potential, and atmospheric growth rate variations over the past decade, the causes of which are still debated. Two major difficulties in reducing uncertainties arise from the variety of geographically overlapping CH 4 sources and from the destruction of CH 4 by short-lived hydroxyl radicals (OH). To address these difficulties, we have established a consortium of multi-disciplinary scientists under the umbrella of the Global Carbon Project to synthesize and stimulate new research aimed at improving and regularly updating the global methane budget. Following Saunois et al. (2016), we present here the second version of the living review paper dedicated to the decadal methane budget, integrating results of top-down studies (atmospheric observations within an atmospheric inverse-modelling framework) and bottom-up estimates (including process-based models for estimating land surface emissions and atmospheric chemistry, inventories of anthropogenic emissions, and data-driven extrapolations).

32 ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATI↗

NASA NTRS1973

Latitude measures of Jupiter in the 0.89-micron methane band.

Jupiter has been photographed by the Lunar and Planetary Laboratory in the 0.89-micron methane band since October 1968. A photometric evaluation of these photographs has not yet been carried out, but a visual study of this collection and a comparison with the color records has been made. This comparison, together with diameter and latitude measures of the methane records, shows that the albedos and latitudes of most features shown at 0.89 micron vary with time and that there is no simple correlation between the visual color and/or intensity of a feature and its intensity in the methane band. The latitudes of the Red Spot and South Tropical Zone have remained unchanged, while those of the Equatorial Zone, North Tropical Zone, and South Polar Hood have changed.

Minton, R. B.↗

NASA NTRS1974

Abundance and rotational temperature of telluric methane as determined from the 2 nu 3 band

The rotational temperature of the 2 nu 3 methane absorptions in the solar/telluric spectrum and the methane abundance in the earth's atmosphere have been measured and are found to be 243.4 plus or minus 5.3 K and 1.137 plus or minus 0.117 cm-atm (STP) in a vertical column, respectively. The data were reduced using the results of laboratory measurements with proper allowance for the fine-structure of the methane absorption manifold and the effects of vibration-rotation interaction (which causes an enhancement of the R-branch of this band).

Margolis, J. S.↗

NASA NTRS1974

Kinetics of oxygen atom formation during the oxidation of methane behind shock waves

An experimental and analytical study of the formation of oxygen atoms during the oxidation of methane and methane-hydrogen mixtures behind incident shock waves was carried out over the temperature range 1790-2584 K at reaction pressures between 1.2 and 1.7 atm. Oxygen atom levels were determined indirectly by measurement of emission from reaction of O with CO. On the basis of these data and ignition-delay data reported in the literature, a kinetic scheme for methane oxidation was assembled. The proposed kinetic mechanism, in general, predicts higher peak oxygen atom levels than the current oxidation mechanisms proposed by Bowman and Seery and by Skinner and his co-workers.

Jachimowski, C. J.↗

NASA NTRS1975

Analytical study of mechanisms for nitric oxide formation during combustion of methane in a jet-stirred combustor

The role of chemical kinetics in the formation of nitric oxide during the combustion of methane was examined analytically by means of a detailed chemical mechanism for the oxidation of methane, for the reaction between hydrocarbon fragments, and for the formation of nitric oxide. By comparing predicted nitric oxide levels with values reported in the literature from jet-stirred combuster experiments, it was determined that the nitric oxide levels observed in fuel-rich flames cannot be described by a mechanism in which the rate of nitric oxide formation is controlled solely by the kinetics of oxygen atom formation. A proposed mechanism for the formation of nitric oxide in methane-rich flames reproduces the observed levels. The oxidation of hydrogen cyanide appears to be an important factor in nitric oxide formation.

Jachimowski, C. J.↗

NASA NTRS1976

Pluto - Evidence for methane frost

Results are presented for infrared photometry of Pluto in the wavelength range from 1.2 to 2.2 microns, which includes the diagnostic absorption bands of water and methane frosts (designated as H1 and H2, respectively). Based on Pluto's observed J-H color and H1/H2 reflectance ratio as well as restrictions imposed by other observational and theoretical studies, it is concluded that methane frost is probably the dominant reflecting material on the planet's surface. It is suggested that this frost may be mixed with other materials, and some variation of the frost cover is indicated. Two plausible sources for the methane on Pluto are discussed, the average geometric albedo of the planet is assumed to be 0.4, and its diameter is estimated to be 3300 km.

Cruikshank, D. P.↗

NASA NTRS1982

Methane and nonmethane hydrocarbon concentrations in the Atlantic marine boundary layer

The reactions of reduced tropospheric trace gases with hydroxyl radicals are thought to play a significant role in global tropospheric photochemistry. Since tropospheric methane and nonmethane hydrocarbon (NMHC) concentrations will affect the tropospheric hydroxyl radical concentrations, accurate measurements of tropospheric CH4 and NMHC concentrations are necessary. Such measurements were, therefore, included in the first leg (from Hamburg, West Germany to Montevideo, Uruguay) of the 1980-1981 Antarctic Expedition. On the basis of the obtained data, a pronounced gradient between northern and southern hemispheric methane concentrations was observed in the marine boundary layer in the region of the intertropical convergence zone. Methane concentrations gradually decreased from about 1.70 ppmv at 40 deg N latitude to about 1.68 ppmv at 14 deg N latitude, then dropped sharply between 14 deg N and 10 deg N latitude to about 1.61 ppmv. NMHC data are also discussed.

Cofer, W. R., III↗

NASA NTRS1984

Detecting Methane Leaks

Remote sensor uses laser radiation backscattered from natural targets. He/Ne Laser System for remote scanning of Methane leaks employs topographic target to scatter light to receiver near laser transmitter. Apparatus powered by 1.5kW generator transported to field sites and pointed at suspected methane leaks. Used for remote detection of natural-gas leaks and locating methane emissions in landfill sites.

Grant, W. B.↗

NASA NTRS1983

Spatially resolved methane band photometry of Saturn. II - Cloud structure models at four latitudes

Saturn's cloud vertical structures in the Equatorial Zone, South Equatorial Belt, and North and South Temperate Regions near + or - 30 deg latitudes are determined by means of an analysis of spatially resolved reflectivity measurements in the 6190, 7250, and 8996 A methane bands. Radiative transfer models are computed for a structure whose parameters are the methane column abundance in an aerosol-free layer at the top of the atmosphere, and the specific abundance of methane in a semiinfinite homogeneous gas-and-cloud mixture deep in the atmosphere. The structure for the South Equatorial Belt resembles that for the North Temperate Region. The level where unit cloud optical depth occurs in the South Temperate Region is deeper than the corresponding level at other latitudes. The differences between model parameters derived by means of different absorption bands are discussed.

West, R. A.↗

NASA NTRS1983

An experimental investigation of the internal methane pressure in hydrogen attack

An experimental investigation of the internal methane pressure that is the driving force for bubble growth in hydrogen attack (HA) was done on pure iron (204 ppm C) and on two low carbon steels of slightly different compositions. The methane content N (c.c gas/g. material) in attacked specimens was measured by a vacuum extraction technique. The total void volume V (c.c) was determined from density measurements before and after HA exposure. The two values, N and V, were then used in an equation of state to calculate an average methane pressure P for the attack stages beyond a density loss (d.l.) greater than 0.05 pct. It was determined that N and P depend on hydrogen exposure conditions and the presence of traces of strong carbide forming alloying elements (in steel). They are independent of specimen size and grain size over a limited range. P varies as the bubble volume increases, showing a generally decreasing trend which brings it to values lower than calculated equilibrium pressures, although well within the same order of magnitude. Possible reasons for this behavior are discussed.

Natan, M.↗

NASA NTRS1983

Methane on Triton and Pluto - New CCD spectra

Spectra of Triton and of Pluto from 4553-9558 A at 25 A resolution are presented. Both spectra show the methane absorption near 8900 A; its equivalent width was 16 times greater on Pluto than on Triton at the time of the observations. This is the first published observation of the 8900 A feature in Triton's spectrum. The previously reported 27 m-amagat abundance of the Pluto atmosphere ignored contributions due to methane ice and should therefore be regarded as an upper limit. The observations of the Pluto spectrum discussed here show sharp structure in the 8900 and 8600 A bands; the case for an atmosphere on Pluto may turn on whether new laboratory measurements show that such structure is present in methane ice.

Apt, J.↗

NASA NTRS1984

Voyager 1 imaging and IRIS observations of Jovian methane absorption and thermal emission: Implications for cloud structure

Images from three filters of the Voyager 1 wide angle camera are used to measure the continuum reflectivity and spectral gradient near 6000 A and the 6190 A band methane/continuum ratio for a variety of cloud features in Jupiter's atmosphere. The dark barge features in the North Equatorial Belt have anomalously strong positive continuum spectral gradients suggesting unique composition. Methane absorption is shown at unprecedented spatial scales for the Great Red Spot and its immediate environment, for a dark barge feature in the North Equatorial Belt, and for two hot spot and plume regions in the North Equatorial Belt. Methane absorption and five micrometer emission are correlated in the vicinity of the Great Red Spot but are anticorrelated in one of the plume hot spot regions. Methane absorption and simultaneous maps of five micrometer brightness temperature is quantitatively compared to realistic cloud structure models which include multiple scattering at five micrometer as well as in the visible. Variability in H2 quadrupole lines are also investigated.

West, R. A.↗

NASA NTRS1984

Symmetry-dependent broadening parameters for methane

Experimental results for the pressure-broadened linewidths gamma of methane are presented and compared with theoretical results. A solar Fourier transform spectrometer with spectral resolving power of about one million was used together with a 6-m base length multitraversal absorption cell. Pressures were determined to an accuracy of + or - 0.2 Torr by means of a capacitance manometer. The pressuring gases were N2, H2, and He. Results are presented for numerical averages of all linewidths corresponding to a given tetrahedral symmetry for J = 0-6. For all cases studied, gamma(E) is found to be substantially less than gamma(A), and gamma(A) is found to be near to, but significantly smaller than, gamma(F). Relative inelastic cross sections calculated for argon scattering from methane are compared with relative pressure-broadened linewidths measured in several experiments involving methane and other gases.

Fox, K.↗

NASA NTRS1984

Portable remote laser sensor for methane leak detection

A portable laser system for remote detection of methane gas leaks and concentrations is disclosed. The system transmitter includes first and second lasers, tuned respectively to a wavelength coincident with a strong absorption line of methane and a reference wavelength which is weakly absorbed by methane gas. The system receiver includes a spherical mirror for collecting the reflected laser radiation and focusing the collected radiation through a narrowband optical filter onto an optial detector. The filter is tuned to the wavelength of the two lasers, and rejects background noise. The output of the optical detector is processed by a lock-in detector synchronized to the chopper, and which measures the difference between the first wavelength signal and the reference wavelength signal.

Grant, W. B.↗

NASA NTRS1985

Voyager 1 imaging and IRIS observations of Jovian methane absorption and thermal emission - Implications for cloud structure

Images from three filters of the Voyager 1 wide angle camera are used to measure the continuum reflectivity and spectral gradient near 6000 A and the 6190 A band methane/continuum ratio for a variety of cloud features in Jupiter's atmosphere. The dark barge features in the North Equatorial Belt have anomalously strong positive continuum spectral gradients suggesting unique composition. Methane absorption is shown at unprecedented spatial scales for the Great Red Spot and its immediate environment, for a dark barge feature in the North Equatorial Belt, and for two hot spot and plume regions in the North Equatorial Belt. Methane absorption and five micrometer emission are correlated in the vicinity of the Great Red Spot but are anticorrelated in one of the plume hot spot regions. Methane absorption and simultaneous maps of five micrometer brightness temperature are quantitatively compared to realistic cloud structure models which include multiple scattering at five micrometer as well as in the visible. Variability in H2 quadrupole lines are also investigated.

West, R. A.↗

NASA NTRS1986

Occultation determination of Neptune's oblateness and stratospheric methane mixing ratio

The occultation of a star by Neptune on August 20, 1985 was observed at 2.2 micron wavelength with telescopes at the European Southern Observatory (ESO) and the Cerro Tololo Inter-American Observatory (CTIO). The detection of a 'central flash' midway between immersion and emersion has allowed the determination of Neptune's oblateness and the atmospheric extinction at 2.2 microns, which is related to the stratospheric methane mixing ratio. An oblateness of (2.08 +0.19 or -0.18) x 10 to the 2nd is found and, assuming a stratospheric temperature of 120 K, a value of 0.6 percent is inferred (with an uncertainty of a factor of 10) for the methane mixing ratio (CH4/H2) at 0.3 mbar. The latter value may indicate supersaturation of methane in Neptune's stratosphere.

Lellouch, E.↗

NASA NTRS1987

Methane absorption variations in the spectrum of Pluto

The lightcurve phases of 0.18, 0.35, 0.49, and 0.98 covered by 5600-10,500 A absolute spectrophotometry of Pluto during four nights include minimum (0.98) light and one near-maximum (0.49) light. The spectra are noted to exhibit significant methane band absorption depth variations at 6200, 7200, 7900, 8400, 8600, 8900, and 10,000 A, with the minimum absorption occurring at minimum light and thereby indicating a 30-percent change in the methane column abundance in the course of three days. An attempt is made to model this absorption strength variation with rotational phase terms of an isotropic surface distribution of methane frost and a clear layer of CH4 gas.

Buie, Marc W.↗

NASA NTRS1988

The role of molecular hydrogen and methane oxidation in the water vapour budget of the stratosphere

The detailed photochemistry of methane oxidation has been studied in a coupled chemical/dynamical model of the middle atmosphere. The photochemistry of formaldehyde plays an important role in determining the production of water vapor from methane oxidation. At high latitudes, the production and transport of molecular hydrogen is particularly important in determining the water vapor distribution. It is shown that the ratio of the methane vertical gradient to the water vapor vertical gradient at any particular latitude should not be expected to be precisely 2, due both to photochemical and dynamical effects. Modeled H2O profiles are compared with measurements from the Limb Infrared Monitor of the Stratosphere (LIMS) experiment at various latitudes. Molecular hydrogen is shown to be responsible for the formation of a secondary maximum displayed by the model water vapor profiles in high latitude summer, a feature also found in the LIMS data.

Le Texier, H.↗