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California methane (CH 4 ) emissions are quantified for three years from two tower networks and one aircraft campaign. We used backward trajectory simulations and a mesoscale Bayesian inverse model, initialized by three inventories, to achieve the emission quantification. Results show total statewide CH 4 emissions of 2.05 ± 0.26 (at 95% confidence) Tg/yr, which is 1.14 to 1.47 times greater than the anthropogenic emission estimates by California Air Resource Board (CARB). Some of differences could be biogenic emissions, superemitter point sources, and other episodic emissions which may not be completely included in the CARB inventory. San Joaquin Valley (SJV) has the largest CH 4 emissions (0.94 ± 0.18 Tg/yr), followed by the South Coast Air Basin, the Sacramento Valley, and the San Francisco Bay Area at 0.39 ± 0.18, 0.21 ± 0.04, and 0.16 ± 0.05 Tg/yr, respectively. The dairy and oil/gas production sources in the SJV contribute 0.44 ± 0.36 and 0.22 ± 0.23 Tg CH 4 /yr, respectively. This study has important policy implications for regulatory programs, as it provides a thorough multiyear evaluation of the emissions inventory using independent atmospheric measurements and investigates the utility of a complementary multiplatform approach in understanding the spatial and temporal patterns of CH 4 emissions in the state and identifies opportunities for the expansion and applications of the monitoring network.

In this study, we explore observational, experimental, methodological, and practical aspects of the flux quantification of greenhouse gases from local point sources by using in situ airborne observations, and suggest a series of conceptual changes to improve flux estimates. We address the major sources of uncertainty reported in previous studies by modifying (1) the shape of the typical flight path, (2) the modeling of covariance and anisotropy, and (3) the type of interpolation tools used. We show that a cylindrical flight profile offers considerable advantages compared to traditional profiles collected as curtains, although this new approach brings with it the need for a more comprehensive subsequent analysis. The proposed flight pattern design does not require prior knowledge of wind direction and allows for the derivation of an ad hoc empirical correction factor to partially alleviate errors resulting from interpolation and measurement inaccuracies. The modified approach is applied to a use-case for quantifying CH4 emission from an oil field south of San Ardo, CA, and compared to a bottom-up CH4 emission estimate.

The development of laser spectroscopy has made it possible to measure minute changes in the concentrations of trace gases and their isotopic analogs. These single or even multiply substituted species occur at ratios from percent to below parts per million and contain important information concerning trace gas sources and transformations. Due to their low abundance, minimizing spectral interference from other gases in a mixture is essential. Options including traps and membranes are available to remove many specific impurities. Methods for removing CH4 , however, are extremely limited as methane has low reactivity and adsorbs poorly to most materials. Here we demonstrate a novel method for CH4 removal via chlorine-initiated oxidation. Our motivation in developing the technique was to overcome methane interference in measurements of N2O isotopic analogs when using a cavity ring-down spectrometer. We describe the design and validation of a proof-of-concept device and a kinetic model to predict the dependence of the methane removal efficiency on the methane concentration [ CH4 ], chlorine photolysis rate JCl2 , chlorine concentration [ Cl2 ] and residence time tR. The model was validated by comparison to experimental data and then used to predict the possible formation of troublesome side products and by-products including CCl4 and HCl. The removal of methane could be maintained with a peak removal efficiency >98 % for ambient levels of methane at a flow rate of 7.5 mL min -1 with [ Cl2 ] at 50 ppm. These tests show that our method is a viable option for continuous methane scrubbing. Additional measures may be needed to avoid complications due to the introduction of Cl2 and formation of HCl. Note that the method will also oxidize most other common volatile organic compounds. The system was tested in combination with a cavity ring-down methane spectrometer, and the developed method was shown to be successful at removing methane interference. [ABSTRACT FROM AUTHOR]

Inland aquatic systems, such as reservoirs, contribute substantially to global methane (CH4) emissions; yet are among the most uncertain components of the total CH4 budget. Reservoirs have received recent attention as they may generate high CH4 fluxes. Improved quantification of these CH4 fluxes, particularly their spatiotemporal distribution, is key to realistically incorporating them in CH4 modeling and budget studies. Here we report on a new global, gridded (0.25° lat × 0.25° lon) study of reservoir CH4 emissions, accounting for new knowledge regarding reservoir areal extent and distribution, and spatiotemporal emission patterns influenced by diurnal variability, temperature‐dependent seasonality, satellite‐derived freeze‐thaw dynamics, and eco‐climatic zone. The results of this new data set comprise daily CH4 emissions throughout the full annual cycle and show that reservoirs cover 297 × 103 km2 globally and emit 10.1 Tg CH4 yr−1 (1σ uncertainty range of 7.2–12.9 Tg CH4 yr−1) from diffusive (1.2 Tg CH4 yr−1) and ebullitive (8.9 Tg CH4 yr−1) emission pathways. This analysis of reservoir CH4 emission addresses multiple gaps and uncertainties in previous studies and represents an important contribution to studies of the global CH4 budget. The new data sets and methodologies from this study provide a framework to better understand and model the current and future role of reservoirs in the global CH4 budget and to guide efforts to mitigate reservoir‐related CH4 emissions. Plain Language Summary: Methane (CH4) is a greenhouse gas which contributes significantly to global warming and has atmospheric concentrations which have increased considerably in the last few decades, primarily due to human‐induced emissions. Natural sources such as wetlands and inland aquatic systems (i.e., reservoirs, lakes, and rivers) contribute substantially to global emissions but these natural systems comprise the most uncertain components of the CH4 budget. This study addresses multiple gaps and uncertainties associated with global CH4 emissions from reservoirs and undertakes a spatial and temporal assessment of global reservoir emissions. The results from this study suggest that reservoirs occupy a global area about 300,000 km2 (comparable to the size of the country of Italy) and emit 10.1 Tg CH4 yr−1. We identify data and methodological elements of previous estimates that indicate they may overestimate these emissions. This work provides a suite of global data sets, gridded at 0.25° × 0.25°, considering reservoir surface area, spatial distribution, eco‐climatic system type, and the full annual cycle of daily CH4 emissions. Key Points: This study reports on a new global, gridded data set of spatiotemporal CH4 emission from reservoirsGlobal reservoirs occupy an area of 297 × 103 km2 and emit 10.1 Tg CH4 yr−1 via diffusion (1.2 Tg CH4 yr−1) and ebullition (8.9 Tg CH4 yr−1)This study addresses several key gaps and uncertainties existing in estimates of global reservoir CH4 emission [ABSTRACT FROM AUTHOR]

Methane (CH4) is emitted from lakes by several processes: bubbles released from bottom sediments that reach the atmosphere (ebullition); spring release of CH4 trapped in bubbles in and under the ice during fall freeze (bubble release), and diffusion of CH4 from sediments to the surface. Each of these emission routes is highly variable over space and time, and episodic in the extreme, making reliable measurements difficult to carry out. However, lakes are receiving increasing interest for their important contribution to global CH4 emissions. Their area, distribution and emissions respond to interannual and longer-term climate fluctuations and close to half the world's lake area is in high northern latitudes that are experiencing rapidly-warming temperatures and lengthening thaw periods. We report on a new spatially-explicit data set of lakes > 50°N, classified with methane-relevant criteria. The seasonality of daily CH4 fluxes is driven with satellite observations of thaw timing and duration. We found that observed thaw seasons are 10–30% shorter than those assumed in previous studies. The area of lakes is 1,095 × 103 km2 and total CH4 emission is 13.8–17.7 Tg CH4 year−1: 11.2–14.4 Tg via diffusion and ebullition and 2.6–3.3 Tg from spring release of CH4 stored in bubbles in winter lake ice. This novel suite of data and methodologies provides a unique framework to model CH4 emission from lakes under current, past and future climates. [ABSTRACT FROM AUTHOR]

This study analyzes source apportioned methane (CH 4 ) emissions and atmospheric mixing ratios in northern California during the Discover-AQ-CA field campaign using airborne measurement data and model simulations. Source apportioned CH 4 emissions from the Emissions Database for Global Atmospheric Research (EDGAR) version 4.2 were applied in the 3-D chemical transport model GEOS-Chem and analyzed using airborne measurements taken as part of the Alpha Jet Atmospheric eXperiment over the San Francisco Bay Area (SFBA) and northern San Joaquin Valley (SJV). During the time period of the Discover-AQ-CA field campaign EDGAR inventory CH 4 emissions were ∼5.30 Gg day −1 (Gg = 1.0 × 10 9 g) (equating to ∼1.90 × 10 3 Gg yr −1 ) for all of California. According to EDGAR, the SFBA and northern SJV region contributes ∼30% of total CH 4 emissions from California. Source apportionment analysis during this study shows that CH 4 mixing ratios over this area of northern California are largely influenced by global emissions from wetlands and local/global emissions from gas and oil production and distribution, waste treatment processes, and livestock management. Model simulations, using EDGAR emissions, suggest that the model under-estimates CH 4 mixing ratios in northern California (average normalized mean bias (NMB) = −5.2% and linear regression slope = 0.20). The largest negative biases in the model were calculated on days when large amounts of CH 4 were measured over local emission sources and atmospheric CH 4 mixing ratios reached values >2.5 parts per million. Sensitivity emission studies conducted during this research suggest that local emissions of CH 4 from livestock management processes are likely the primary source of the negative model bias. These results indicate that a variety, and larger quantity, of measurement data needs to be obtained and additional research is necessary to better quantify source apportioned CH 4 emissions in California. [ABSTRACT FROM AUTHOR]

The kinetic isotope effects in the reaction of methane (CH4) with Cl atoms are studied in a relative rate experiment at 298 ± 2 K and 1013 ± 10 mbar. The reaction rates of 13CH4, 12CH3D, 12CH2D2, 12CHD3, and 12CD4 with Cl radicals are measured relative to 12CH4 in a smog chamber using long path FTIR detection. The experimental data are analyzed with a nonlinear least squares spectral fitting method using measured high-resolution spectra as well as cross sections from the HITRAN database. The relative reaction rates of 12CH4, 13CH4, 12CH3D, 12CH2D2, 12CHD3, and 12CD4 with Cl are determined as kCl+12CH4/kCl+13CH4 = 1.06 ± 0.01, kCl+12CH4/kCl+12CH3D = 1.47 ± 0.03, kCl+12CH4/kCl+12CH2D2 = 2.45 ± 0.05, kCl+12CH4/kCl+12CHD3 = 4.7 ± 0.1, kCl+12CH4/kCl+12CD4 = 14.7 ± 0.3. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 37: 110–118, 2005 [ABSTRACT FROM AUTHOR]

A series of experiments were carried out to determine the clumped ( 13 CH 3 D) methane kinetic isotope effects during oxidation of methane by OH and Cl radicals, the major sink reactions for atmospheric methane. Experiments were performed in a 100 L quartz photochemical reactor, in which OH was produced from the reaction of O( 1 D) (from O 3 photolysis) with H 2 O, and Cl was from photolysis of Cl 2 . Samples were taken from the reaction cell and analyzed for methane ( 12 CH 4 , 12 CH 3 D, 13 CH 4 , 13 CH 3 D) isotopologue ratios using tunable infrared laser direct absorption spectroscopy. Measured kinetic isotope effects for singly substituted species were consistent with previous experimental studies. For doubly substituted methane, 13 CH 3 D, the observed kinetic isotope effects closely follow the product of the kinetic isotope effects for the 13 C and deuterium substituted species (i.e., 13,2 KIE = 13 KIE × 2 KIE). The deviation from this relationship is 0.3‰ ± 1.2‰ and 3.5‰ ± 0.7‰ for OH and Cl oxidation, respectively. This is consistent with model calculations performed using quantum chemistry and transition state theory. The OH and Cl reactions enrich the residual methane in the clumped isotopologue in open system reactions. In a closed system, however, this effect is overtaken by the large D/H isotope effect, which causes the residual methane to become anti-clumped relative to the initial methane. Based on these results, we demonstrate that oxidation of methane by OH, the predominant oxidant for tropospheric methane, will only have a minor (∼0.3‰) impact on the clumped isotope signature (Δ 13 CH 3 D, measured as a deviation from a stochastic distribution of isotopes) of tropospheric methane. This paper shows that Δ 13 CH 3 D will provide constraints on methane source strengths, and predicts that Δ 12 CH 2 D 2 can provide information on methane sink strengths. [ABSTRACT FROM AUTHOR]