Loss Of CH Research Articles

Performance of polydimethylsiloxane membrane contactor process for selective hydrogen sulfide removal from biogas

H2S in biogas affects the co-generation performance adversely by corroding some critical components within the engine and it has to be removed in order to improve the biogas quality. This work presents the use of polydimethylsiloxane (PDMS) membrane contactor for selective removal of H2S from the biogas. Experiments were carried out to evaluate the effects of different pH of absorption liquid, biogas flowrate and temperature on the absorption performances. The results revealed that at the lowest loading rate (91mg H2S/m2·h) more than 98% H2S and 59% CO2 absorption efficiencies were achieved. The CH4 content in the treated gas increased from 60 to 80% with nearly 5% CH4 loss. Increasing the pH (7–10) and loading rate (91–355mg H2S/m2·h) enhanced the H2S absorption capacity, and the maximum H2S/CO2 and H2S/CH4 selectivity factors were 2.5 and 58, respectively. Temperature played a key role in the process and lower temperature was beneficial for intensifying H2S absorption performance. The highest H2S fluxes at pH 10 and 7 were 3.4g/m2·d and 1.8g/m2·d with overall mass transfer coefficients of 6.91×10−6 and 4.99×10−6m/s, respectively. The results showed that moderately high H2S fluxes with low CH4 loss may be achieved by using a robust and cost-effective membrane based absorption process for desulfurization of biogas. A tubular PDMS membrane contactor was tested for the first time to remove H2S from biogas under slightly alkaline conditions and the suggested process could be a promising for real scale applications.

Floating Aquatic Macrophytes Decrease the Methane Concentration in the Water Column of a Tropical Coastal Lagoon: Implications for Methane Oxidation and Emission

ABSTRACT In wetlands, the knowledge accumulated on the role of aquatic plants in the methane cycle focused on emergent macrophytes, to the detriment of other typologies. Herein, we evaluated whether the free-floating macrophyte Salvinia auriculata Aubl. and the floating-leaved macrophyte Eichhornia azurea (Sw.) Kunth. decrease the water column methane concentrations compared to a plant-free surface. We prepared microcosms by inserting an individual of S. auriculata or of E. azurea into chambers filled with lagoon water previously bubbled with CH4. Another set of chambers was incubated only with the prepared water, representing the plant-free surface. Half of the chambers were kept in the dark and half in sunlight to simulate a diel cycle. We observed greater loss of CH4, higher O2 uptake and lower CO2 outflow in the plants treatments. The decrease in methane concentrations in the E. azurea treatments was 93.5% in the light and 77.2% in the dark. In the S. auriculata treatments, the decreases were 74.2% and 67.4% in the light and in the dark, respectively. In plant-free surface the decrease was 58.7% in the light and 36.3% in the dark. These results indicate a role of floating aquatic macrophytes in the methane cycle in the water column. Moreover, our results suggest a diel variation of methane oxidation and methane emission, according to the differences observed in O2 uptake and CO2 outflow between dark and light conditions. Thus, future predictions of global methane budget should include the role played by floating aquatic macrophytes.

Methane dynamics downstream of a temperate run‐of‐the‐river reservoir

AbstractDams impact a significant portion of the world's rivers, and rivers downstream of the reservoirs created by large dams often emit significant amounts of methane (CH4). River CH4 emissions downstream of run‐of‐the‐river (ROR) dams are less well known. We investigated spatial and seasonal CH4 dynamics along a stretch of the Aare River (Switzerland) downstream from a bubbling ROR reservoir. We found that the CH4 horizontally accumulated in this vertically non‐stratifying ROR reservoir was consistently transported to the downstream river, but half was lost near a confluence with a tributary. We estimated CH4 diffusion using gas exchange coefficient (k) models that incorporate specific river characteristics and found CH4 emissions were threefold higher upstream of the confluence than downstream. We discuss the use of CO2‐derived k models in estimating k for CH4, and investigated it directly using a drifting chamber experiment. While chamber emissions only partly agreed with the k models, the models were robust enough to use in a CH4 mass balance along the river that indicated an overall CH4 loss from the study area. The loss predominantly occurred at the confluence and > 92% of it could not be accounted for by modelled or measured CH4 emissions. Ultimately, a ROR reservoir that does not form an anoxic, CH4‐rich hypolimnion enhanced downstream river CH4 emissions, but to a lesser extent than other systems (∼9% of total reservoir‐associated emissions). Regardless, small ROR dams and river features such as confluences should be considered when measuring, estimating or upscaling river CH4 emissions.

Microbial contributions to subterranean methane sinks.

Sources and sinks of methane (CH4 ) are critical for understanding global biogeochemical cycles and their role in climate change. A growing number of studies have reported that CH4 concentrations in cave ecosystems are depleted, leading to the notion that these subterranean environments may act as sinks for atmospheric CH4 . Recently, it was hypothesized that this CH4 depletion may be caused by radiolysis, an abiotic process whereby CH4 is oxidized via interactions with ionizing radiation derived from radioactive decay. An alternate explanation is that the depletion of CH4 concentrations in caves could be due to biological processes, specifically oxidation by methanotrophic bacteria. We theoretically explored the radiolysis hypothesis and conclude that it is a kinetically constrained process that is unlikely to lead to the rapid loss of CH4 in subterranean environments. We present results from a controlled laboratory experiment to support this claim. We then tested the microbial oxidation hypothesis with a set of mesocosm experiments that were conducted in two Vietnamese caves. Our results reveal that methanotrophic bacteria associated with cave rocks consume CH4 at a rate of 1.3-2.7mgCH4 ·m-2 ·d-1 . These CH4 oxidation rates equal or exceed what has been reported in other habitats, including agricultural systems, grasslands, deciduous forests, and Arctic tundra. Together, our results suggest that depleted concentrations of CH4 in caves are most likely due to microbial activity, not radiolysis as has been recently claimed. Microbial methanotrophy has the potential to oxidize CH4 not only in caves, but also in smaller-size open subterranean spaces, such as cracks, fissures, and other pores that are connected to and rapidly exchange with the atmosphere. Future studies are needed to understand how subterranean CH4 oxidation scales up to affect local, regional, and global CH4 cycling.

Gas Phase Reactions of Ions Derived from Anionic Uranyl Formate and Uranyl Acetate Complexes.

The speciation and reactivity of uranium are topics of sustained interest because of their importance to the development of nuclear fuel processing methods, and a more complete understanding of the factors that govern the mobility and fate of the element in the environment. Tandem mass spectrometry can be used to examine the intrinsic reactivity (i.e., free from influence of solvent and other condensed phase effects) of a wide range of metal ion complexes in a species-specific fashion. Here, electrospray ionization, collision-induced dissociation, and gas-phase ion-molecule reactions were used to create and characterize ions derived from precursors composed of uranyl cation (UVIO22+) coordinated by formate or acetate ligands. Anionic complexes containing UVIO22+ and formate ligands fragment by decarboxylation and elimination of CH2=O, ultimately to produce an oxo-hydride species [UVIO2(O)(H)]-. Cationic species ultimately dissociate to make [UVIO2(OH)]+. Anionic complexes containing acetate ligands exhibit an initial loss of acetyloxyl radical, CH3CO2•, with associated reduction of uranyl to UVO2+. Subsequent CID steps cause elimination of CO2 and CH4, ultimately to produce [UVO2(O)]-. Loss of CH4 occurs by an intra-complex H+ transfer process that leaves UVO2+ coordinated by acetate and acetate enolate ligands. A subsequent dissociation step causes elimination of CH2=C=O to leave [UVO2(O)]-. Elimination of CH4 is also observed as a result of hydrolysis caused by ion-molecule reaction with H2O. The reactions of other anionic species with gas-phase H2O create hydroxyl products, presumably through the elimination of H2. Graphical Abstract ᅟ.

Greenhouse gas fluxes from a grazed grassland soil after slurry injections and mineral fertilizer applications under the Atlantic climatic conditions of NW Spain

The number of studies that investigate how agricultural practices on dairy farms in the North West (NW) of Spain affect greenhouse gas (GHG) fluxes from soils is limited. Thus, the objective of this study was to quantify the effects of the application of mineral fertilizers and cattle slurry injections on GHG fluxes from a grassland soil with grazing dairy cattle, in Galicia (NW Spain). We also aimed to identify the type of fertilizer associated with high grass production and low GHG fluxes. To achieve this, fluxes of nitrous oxide (N2O), methane (CH4) and carbon dioxide (CO2), grass yields and soil mineral nitrogen (N) contents were monitored after three applications (in spring, summer and autumn) of surface broadcasted mineral fertilizer (MN) and injected cattle slurry (CS) and compared with no fertilization (zero N). Dry soil conditions (<60% water-filled pore space (WFPS)) were observed during the spring and summer, contrasting with the higher soil WFPS (>60%) in autumn due to the more frequent rainfall. Overall, total cumulative N2O fluxes from CS were similar than from MN (P>0.05), indicating that denitrification in this C-rich soil was not stimulated by slurry-carbon applications. Large losses of CH4 and CO2 were related to CS, but overall only total cumulative CH4 fluxes were larger with respect to MN (P<0.05). Dry weather conditions would have stimulated organic matter mineralization in this soil, which resulted in the low efficiency of both fertilizers to increase yields. As we obtained similar total CO2 equivalents to produce same yields with both types of fertilization (P>0.05), this study did not show a clear type of fertilization related to low GHG fluxes and high yields. We believe that longer-term studies are required to provide more robust estimations and conclusions about the effect of fertilizer applications on GHG fluxes from grassland soils in NW Spain.

Characterization of N-methylated amino acids by GC-MS after ethyl chloroformate derivatization.

Methylation is an essential metabolic process in the biological systems, and it is significant for several biological reactions in living organisms. Methylated compounds are known to be involved in most of the bodily functions, and some of them serve as biomarkers. Theoretically, all α-amino acids can be methylated, and it is possible to encounter them in most animal/plant samples. But the analytical data, especially the mass spectral data, are available only for a few of the methylated amino acids. Thus, it is essential to generate mass spectral data and to develop mass spectrometry methods for the identification of all possible methylated amino acids for future metabolomic studies. In this study, all N-methyl and N,N-dimethyl amino acids were synthesized by the methylation of α-amino acids and characterized by a GC-MS method. The methylated amino acids were derivatized with ethyl chloroformate and analyzed by GC-MS under EI and methane/CI conditions. The EI mass spectra of ethyl chloroformate derivatives of N-methyl (1-18) and N,N-dimethyl amino acids (19-35) showed abundant [M-COOC2 H5 ]+ ions. The fragment ions due to loss of C2 H4 , CO2 , (CO2 + C2 H4 ) from [M-COOC2 H5 ]+ were of structure indicative for 1-18. The EI spectra of 19-35 showed less number of fragment ions when compared with those of 1-18. The side chain group (R) caused specific fragment ions characteristic to its structure. The methane/CI spectra of the studied compounds showed [M + H]+ ions to substantiate their molecular weights. The detected EI fragment ions were characteristic of the structure that made easy identification of the studied compounds, including isomeric/isobaric compounds. Fragmentation patterns of the studied compounds (1-35) were confirmed by high-resolution mass spectra data and further substantiated by the data obtained from 13 C2 -labeled glycines and N-ethoxycarbonyl methoxy esters. The method was applied to human plasma samples for the identification of amino acids and methylated amino acids. Copyright © 2016 John Wiley & Sons, Ltd.

Effect of biochar amendment on greenhouse gas emission and bio-availability of heavy metals during sewage sludge co-composting

In the present study, we evaluated the feasibility of biochar amended with lime (B + L) to reduce the loss of ammonia, greenhouse gas (GHG) emissions and the bio-availability of heavy metals (HMs) during composting of dewatered fresh sewage sludge (DFSS) and to improve the end product quality. Biochar mixed with a low dosage of lime was supplemented at a 1:1 ratio into DFSS and wheat straw (dry weight basis), and compared with a lime only amendment and a control without any amendment. The CO2 emission profile clearly indicated that the B + L addition was effectively buffered a pH ∼8.0, and the B + L amendment experienced an enhanced the rate of decomposition compared to control and lime amended treatments. In aerobic co-composting, the B + L amendment effectively reduced the loss of ammonia, CH4, and N2O emission. B + L amended DFSS compost also showed significantly higher concentrations of humic-acid (17.23%) and fulvic-acid (3.79%) compounds, effectively reduced the bio-availability of HMs (34.81% Cu, 56.74% Zn, 87.96% Pb and 86.65% Ni) and improved compost maturity compared to control and lime amendments. Furthermore, the B + L amendment increased the adsorption of ammonium ions by reducing the ammonia loss and N2O emission, resulting in compost with higher nutrient concentrations. These results can be used to formulate the initial feedstock for industrial scale composting processes.

THE ROLE OF NITROGEN IN TITAN’S UPPER ATMOSPHERIC HYDROCARBON CHEMISTRY OVER THE SOLAR CYCLE

ABSTRACT Titan’s thermospheric photochemistry is primarily driven by solar radiation. Similarly to other planetary atmospheres, such as Mars’, Titan’s atmospheric structure is also directly affected by variations in the solar extreme-UV/UV output in response to the 11-year-long solar cycle. Here, we investigate the influence of nitrogen on the vertical production, loss, and abundance profiles of hydrocarbons as a function of the solar cycle. Our results show that changes in the atmospheric nitrogen atomic density (primarily in its ground state N(4S)) as a result of photon flux variations have important implications for the production of several minor hydrocarbons. The solar minimum enhancement of CH3, C2H6, and C3H8, despite the lower CH4 photodissociation rates compared with solar maximum conditions, is explained by the role of N(4S). N(4S) indirectly controls the altitude of termolecular versus bimolecular chemical regimes through its relationship with CH3. When in higher abundance during solar maximum at lower altitudes, N(4S) increases the importance of bimolecular CH3 + N(4S) reactions producing HCN and H2CN. The subsequent remarkable CH3 loss and decrease in the CH3 abundance at lower altitudes during solar maximum affects the overall hydrocarbon chemistry.

The ecology of methane in streams and rivers: patterns, controls, and global significance

AbstractStreams and rivers can substantially modify organic carbon (OC) inputs from terrestrial landscapes, and much of this processing is the result of microbial respiration. While carbon dioxide (CO2) is the major end‐product of ecosystem respiration, methane (CH4) is also present in many fluvial environments even though methanogenesis typically requires anoxic conditions that may be scarce in these systems. Given recent recognition of the pervasiveness of this greenhouse gas in streams and rivers, we synthesized existing research and data to identify patterns and drivers of CH4, knowledge gaps, and research opportunities. This included examining the history of lotic CH4 research, creating a database of concentrations and fluxes (MethDB) to generate a global‐scale estimate of fluvial CH4 efflux, and developing a conceptual framework and using this framework to consider how human activities may modify fluvial CH4 dynamics. Current understanding of CH4 in streams and rivers has been strongly influenced by goals of understanding OC processing and quantifying the contribution of CH4 to ecosystem C fluxes. Less effort has been directed towards investigating processes that dictate in situ CH4 production and loss. CH4 makes a meager contribution to watershed or landscape C budgets, but streams and rivers are often significant CH4 sources to the atmosphere across these same spatial extents. Most fluvial systems are supersaturated with CH4 and we estimate an annual global emission of 26.8 Tg CH4, equivalent to ~15‐40% of wetland and lake effluxes, respectively. Less clear is the role of CH4 oxidation, methanogenesis, and total anaerobic respiration to whole ecosystem production and respiration. Controls on CH4 generation and persistence can be viewed in terms of proximate controls that influence methanogenesis (organic matter, temperature, alternative electron acceptors, nutrients) and distal geomorphic and hydrologic drivers. Multiple controls combined with its extreme redox status and low solubility result in high spatial and temporal variance of CH4 in fluvial environments, which presents a substantial challenge for understanding its larger‐scale dynamics. Further understanding of CH4 production and consumption, anaerobic metabolism, and ecosystem energetics in streams and rivers can be achieved through more directed studies and comparison with knowledge from terrestrial, wetland, and aquatic disciplines.

Identification of Alkaloids in Stephania hainanensis by Liquid Chromatography Coupled with Quadrupole Time-of-flight Mass Spectrometry.

Plants in the genus Stephania can produce diverse bioactive alkaloids. Stephania hainanensis is a medicinal plant that contains effective alkaloids. However, only 10 alkaloids have been reported in this species. To characterise the alkaloids in Stephania hainanensis using liquid chromatography coupled with quadrupole time-of-flight tandem mass spectrometry (LC-QTOF-MS/MS). An LC-QTOF-MS/MS method was developed for structural characterisation of the alkaloids in Stephania hainanensis. The chromatographic separation was performed on a phenyl column with gradient elution, and the tandem mass spectra were obtained by using an electrospray ionisation (ESI) interface in positive ionisation mode. Compound identification was based on the exact masses, fragmentation pathways, retention behaviours and related botanical biogenesis. A total of 37 tetrahydroprotoberberine-, quaternary protoberberine-, aporphine-, proaporphine-, benzylisoquinoline- or bisbenzylisoquinoline-type alkaloids were identified or tentatively identified in a single LC run. Twenty-seven of these alkaloids, including the benzylisoquinoline-type of alkaloids, have not been previously reported in Stephania hainanensis. The possible fragmentation pathways of different types of alkaloids were proposed. Besides the general fragmentations, the characteristic losses of CH3 N = CH2 were observed for the benzylisoquinoline and aporphine alkaloids with two methyl groups on the nitrogen. The LC-QTOF-MS/MS method enabled profiling and rational, but tentative, identification of diverse alkaloids in Stephania hainanensis. The results obtained may be helpful for understanding the bioactivity of S. hainanensis and evaluating the quality of this plant. Copyright © 2016 John Wiley & Sons, Ltd.

Pluto's interaction with the solar wind

AbstractThis study provides the first observations of Plutogenic ions and their unique interaction with the solar wind. We find ~20% solar wind slowing that maps to a point only ~4.5 RP upstream of Pluto and a bow shock most likely produced by comet‐like mass loading. The Pluto obstacle is a region of dense heavy ions bounded by a “Plutopause” where the solar wind is largely excluded and which extends back &gt;100 RP into a heavy ion tail. The upstream standoff distance is at only ~2.5 RP. The heavy ion tail contains considerable structure, may still be partially threaded by the interplanetary magnetic field (IMF), and is surrounded by a light ion sheath. The heavy ions (presumably CH4+) have average speed, density, and temperature of ~90 km s−1, ~0.009 cm−3, and ~7 × 105 K, with significant variability, slightly increasing speed/temperature with distance, and are N‐S asymmetric. Density and temperature are roughly anticorrelated yielding a pressure ~2 × 10−2 pPa, roughly in balance with the interstellar pickup ions at ~33 AU. We set an upper bound of &lt;30 nT surface field at Pluto and argue that the obstacle is largely produced by atmospheric thermal pressure like Venus and Mars; we also show that the loss rate down the tail (~5 × 1023 s−1) is only ~1% of the expected total CH4 loss rate from Pluto. Finally, we observe a burst of heavy ions upstream from the bow shock as they are becoming picked up and tentatively identify an IMF outward sector at the time of the NH flyby.

Hydrothermal dewatering of a Chinese lignite and properties of the solid products

A lignite from Shengli mine in Chinese Inner Mongolia was upgraded by hydrothermal dewatering (HTD). The effects of the HTD process on the solid product yield and the removal rate of elements, organics and inorganics in this coal were determined. The moisture re-adsorption capacities and spontaneous combustion behaviors of coal products from the HTD process were studied, using the saturated salt solution and wire basket reactor methods respectively. FT-IR, XRD, CO2 adsorption, mercury intrusion porosimetry (MIP) and helium pycnometry were carried out to characterize the physicochemical properties of samples. The results show that during the HTD process, organic matter was removed due to the loss of CH3, CH2, and O-containing functional groups, while the inorganic matter was mainly removed in the forms of Ca- and Fe-containing minerals. The amounts of both organic and inorganic matter removed increased with temperature. The equilibrium moisture contents of the hydrothermally dewatered coals at 30°C in the relative humidity range 11–97% were lower than those of the raw coal. They decreased as the HTD temperature rose, probably due to the increase in the loss of hydrophilic O-containing functional groups and decrease in surface area with the increasing of HTD temperature. Moreover, the critical ignition temperature (Tcr) of all the hydrothermally dewatered coals, as measured by wire basket spontaneous combustion experiments, was lower than that of raw coal.

Potentialities of a dense skin hollow fiber membrane contactor for biogas purification by pressurized water absorption

Gas purification technologies are a key step in the technological chain using energy sources such as natural gas, coal gas or biogas, and represent a significant part of production costs. Numerous separation technologies exist to achieve required purity specifications depending on the particular application (absorption, adsorption, cryogenic, membranes). The objective of this work is to present an evaluation of the potentialities of using a membrane contactor based on a dense skin Poly (Phenylene Oxide) (PPO) hollow fiber module for CO2 absorption from biogas by pressurized water through simulations and experiments. In this system, the liquid flowing on the shell-side is in direct contact with the dense skin, thus enabling pressurization and avoiding membrane wetting in order to maintain stable absorption performances.The overall mass transfer coefficient Kov, CO2 removal efficiency, and CH4 loss are evaluated for a range of liquid and gas flow rates. A one-dimensional (1D) model based on resistance in series provides very good predictions of the experimental results when the Graetz approach is used for shell-side mass transfer calculations.Membrane mass transfer resistance is shown to be negligible compared to the liquid resistance. Consequently, the overall mass transfer performances of the dense skin contactor are close to typical values of packed columns (10−5–10−4m/s). Thanks to the increased gas-liquid interfacial area offered by the membrane contactor, a significant volume reduction of the absorption unit (up to 15 process intensification factor) is potentially achievable. Moreover, a selective dense skin could minimize methane losses.

Atmospheric methane evolution the last 40 years

Abstract. Observations at surface sites show an increase in global mean surface methane (CH4) of about 180 parts per billion (ppb) (above 10 %) over the period 1984–2012. Over this period there are large fluctuations in the annual growth rate. In this work, we investigate the atmospheric CH4 evolution over the period 1970–2012 with the Oslo CTM3 global chemical transport model (CTM) in a bottom-up approach. We thoroughly assess data from surface measurement sites in international networks and select a subset suited for comparisons with the output from the CTM. We compare model results and observations to understand causes for both long-term trends and short-term variations. Employing Oslo CTM3 we are able to reproduce the seasonal and year-to-year variations and shifts between years with consecutive growth and stagnation, both at global and regional scales. The overall CH4 trend over the period is reproduced, but for some periods the model fails to reproduce the strength of the growth. The model overestimates the observed growth after 2006 in all regions. This seems to be explained by an overly strong increase in anthropogenic emissions in Asia, having global impact. Our findings confirm other studies questioning the timing or strength of the emission changes in Asia in the EDGAR v4.2 emission inventory over recent decades. The evolution of CH4 is not only controlled by changes in sources, but also by changes in the chemical loss in the atmosphere and soil uptake. The atmospheric CH4 lifetime is an indicator of the CH4 loss. In our simulations, the atmospheric CH4 lifetime decreases by more than 8 % from 1970 to 2012, a significant reduction of the residence time of this important greenhouse gas. Changes in CO and NOx emissions, specific humidity, and ozone column drive most of this, and we provide simple prognostic equations for the relations between those and the CH4 lifetime. The reduced lifetime results in substantial growth in the chemical CH4 loss (relative to its burden) and dampens the CH4 growth.

Ion chemistries in triethylborane

The absolute total and partial cross sections of electron ionization on triethylborane (TEB) are measured in an electron energy range of 6.6–200eV. The total cross section reaches a maximum of 2.2×10−15cm2 in 65–85eV energy region. Product ions from the electron ionization include C2H2–5+, BCH4+, BC2H2,4,6+, BC3H4,6+, BC4H8,10+ and BC6H15+, among which BC6H15+, BC4H10+ and BC2H6+ are the most abundant ions. Gas-phase reactions between TEB and certain major ions from the electron ionization of TEB are studied. Common products from the ion-molecule reactions are BC4H10+ and BC2H6+, as well as association products for certain heavy-ion reactions. The mechanisms of forming BC4H10+ and BC2H6+ in ion-molecule reactions are believed to be simple ethide transfer and ethide transfer followed by dissociation (loss of C2H4), respectively.

Soil incubations reproduce field methane dynamics in a subarctic wetland

A major challenge in peatland carbon cycle modeling is the estimation of subsurface methane (CH4) and carbon dioxide (CO2) production and consumption rates and pathways. The most common methods for modeling these processes are soil incubations and stable isotope modeling, both of which may involve departures from field conditions. To explore the impacts of these departures, we measured CH4/CO2 concentration ratios and 13C fractionation factors (αC, indicating CH4 production pathways) in field pore water from a thawing subarctic peatland, and compared these values to those observed in incubations of corresponding peat samples. Incubation CH4/CO2 production ratios were significantly and positively correlated with observed field CH4/CO2 concentration ratios, though observed field ratios were ~20 % of those in incubations due to CH4’s lower solubility in pore water. After correcting the field ratios for CH4 loss with an isotope mass balance model, the incubation CH4/CO2 ratios and αC were both significantly positively correlated with field ratios and αC (respectively), both with slopes indistinguishable from 1. Although CH4/CO2 ratios and αC were slightly higher in the incubations, these shifts were consistent along the thaw progression, indicating that ex situ incubations can replicate trends in in situ CH4 production.

Electrospray production and collisional dissociation of lanthanide/methylsulfonyl anion complexes: Sulfur dioxide anion as a ligand

Gas-phase lanthanide-SO2 complexes, Ln(CH3SO2)3(SO2)−, were produced by collision induced dissociation (CID) of Ln(CH3SO2)4− precursors prepared by electrospray ionization. For all lanthanides except Eu, CID of Ln(CH3SO2)4− resulted in CH3 loss to form Ln(CH3SO2)3(SO2)−, which spontaneously react with O2 to form Ln(CH3SO2)3(O2)−. CID of Eu(CH3SO2)4− produced only Eu(CH3SO2)3−, with reduction from Eu(III) to Eu(II). For Ln=Yb and Sm, the Ln(CH3SO2)4− underwent neutral ligand loss to form Ln(CH3SO2)3−, which reacted with O2 to yield Ln(CH3SO2)3(O2)−, recovering the Ln(III) oxidation state. The CID results show parallels to condensed-phase Ln3+/Ln2+ redox chemistry. Density functional theory (DFT) calculations on Ln(CH3SO2)3(SO2)− for Ln=La, Yb and Lu reveal that SO2 acts as a bidentate oxygen bound ligand for doublet ground-state La(CH3SO2)3(SO2)− and Lu(CH3SO2)3(SO2)−, while the ground state for Yb(CH3SO2)3(SO2)− is an open-shell singlet with a monodentate SO2 ligand. Loss of CH3 is computed to be much more favorable than neutral ligand loss for La(CH3SO2)4− and Lu(CH3SO2)4−, whereas both channels are comparable in energy for Yb(CH3SO2)4−, in accord with the experiments. DFT results for fragmentation of Cu(CH3SO2)2− reveal that formation of the organometallic complex, Cu(CH3SO2)(CH3)−, is energetically most favorable, in agreement with contrasting fragmentation pathways of copper and lanthanide complexes.

Vibrational Spectroscopy Reveals Varying Structural Motifs in Cu(+)(CH4)(n) and Ag(+)(CH4)(n) (n = 1-6).

Vibrational spectra are measured for Cu(+)(CH4)(Ar)2, Cu(+)(CH4)2(Ar), Cu(+)(CH4)n (n = 3-6), and Ag(+)(CH4)n (n = 1-6) in the C-H stretching region (2500-3100 cm(-1)) using photofragment spectroscopy. Spectra are obtained by monitoring loss of Ar or CH4. Interaction with the metal ion produces substantial red shifts in the C-H stretches of proximate hydrogens. The magnitude of the shift reflects the metal-methane distance and the coordination to the metal ion of the methane hydrogens (η(2) or η(3)). The structures of the complexes are determined by comparing the measured spectra with spectra calculated for candidate geometries using the B3LYP and CAM-B3LYP density functionals with 6-311++G(3df,3pd) and aug-cc-pVTZ-PP basis sets. Because of the d(10) electronic configuration of the metal ions, the complexes are expected to adopt symmetric structures, which is confirmed by the experiments. All of the complexes have η(2) hydrogen coordination in the first shell, in accord with theoretical predictions; second-shell ligands sometimes show η(3) hydrogen coordination. The vibrational spectrum of Cu(+)(CH4)(Ar)2 shows extensive structure due to Fermi resonance between the lowest-frequency C-H stretch and overtones of the H-C-H bends. The Cu(+)(CH4) cluster has a smaller red shift in the lowest-frequency C-H stretch than M(+)(CH4), M(+) = Co(+) (d(8)) and Ni(+) (d(9)). Although all three ions have similar binding energies, the metal-ligand electrostatic interaction is largest for Cu(+), while the contribution from covalent interactions is largest for Co(+). The larger ionic radius of Ag(+) leads to a larger metal-ligand distance and weaker interaction, resulting in substantially smaller red shifts than in the Cu(+) complexes. The Cu(+)(CH4)2 and Ag(+)(CH4)2 clusters have symmetrical structures, with the methanes on opposite sides of the metal, while Cu(+)(CH4)3 and Ag(+)(CH4)3 adopt symmetrical, trigonal planar structures with all M-C distances equal. For Cu(+)(CH4)4, the tetrahedral structure dominates the observed spectrum, although a trigonal pyramidal structure may contribute; however, only the tetrahedral structure is observed for Ag(+)(CH4)4. The structures of Cu(+)(CH4)n and Ag(+)(CH4)n differ for clusters with n > 4. For copper complexes, these are primarily formed by adding outer-shell methane ligand(s) to the tetrahedral n = 4 core. The observed spectra of the larger Ag(+) clusters are dominated by symmetrical structures in which all of the Ag-C distances are similar: Ag(+)(CH4)5 has a trigonal bipyramidal geometry and Ag(+)(CH4)6 is octahedral.

Methane Emissions from United States Natural Gas Gathering and Processing.

New facility-level methane (CH4) emissions measurements obtained from 114 natural gas gathering facilities and 16 processing plants in 13 U.S. states were combined with facility counts obtained from state and national databases in a Monte Carlo simulation to estimate CH4 emissions from U.S. natural gas gathering and processing operations. Total annual CH4 emissions of 2421 (+245/-237) Gg were estimated for all U.S. gathering and processing operations, which represents a CH4 loss rate of 0.47% (±0.05%) when normalized by 2012 CH4 production. Over 90% of those emissions were attributed to normal operation of gathering facilities (1697 +189/-185 Gg) and processing plants (506 +55/-52 Gg), with the balance attributed to gathering pipelines and processing plant routine maintenance and upsets. The median CH4 emissions estimate for processing plants is a factor of 1.7 lower than the 2012 EPA Greenhouse Gas Inventory (GHGI) estimate, with the difference due largely to fewer reciprocating compressors, and a factor of 3.0 higher than that reported under the EPA Greenhouse Gas Reporting Program. Since gathering operations are currently embedded within the production segment of the EPA GHGI, direct comparison to our results is complicated. However, the study results suggest that CH4 emissions from gathering are substantially higher than the current EPA GHGI estimate and are equivalent to 30% of the total net CH4 emissions in the natural gas systems GHGI. Because CH4 emissions from most gathering facilities are not reported under the current rule and not all source categories are reported for processing plants, the total CH4 emissions from gathering and processing reported under the EPA GHGRP (180 Gg) represents only 14% of that tabulated in the EPA GHGI and 7% of that predicted from this study.