Natural gas (NG) is a potential “bridge fuel” during transition to a decarbonized energy system: It emits less carbon dioxide during combustion than other fossil fuels and can be used in many industries. However, because of the high global warming potential of methane (CH4, the major component of NG), climate benefits from NG use depend on system leakage rates. Some recent estimates of leakage have challenged the benefits of switching from coal to NG, a large near-term greenhouse gas (GHG) reduction opportunity ( 1 – 3 ). Also, global atmospheric CH4 concentrations are on the rise, with the causes still poorly understood ( 4 ).

https://cedmcenter.org/wp-content/uploads/2014/04/Novim_Overview_CMU_03312014.pdf

Methane leaks from North American natural gas systems

Previous studies on ice core analyses and recent in situ measurements have shown that CH 4 has increased from about 075–173 μmol/mol during the past 150 years Here, we review sources and sink estimates and we present global 3D model calculations, showing that the main features of the global CH 4 distribution are well represented The model has been used to derive the total CH 4 emission source, being about 600 Tg yr -1  Based on published results of isotope measurements the total contribution of fossil fuel related CH 4 emissions has been estimated to be about 110 Tg yr -1  However, the individual coal, natural gas and oil associated CH 4 emissions can not be accurately quantified In particular natural gas and oil associated emissions remain speculative Since the total anthropogenic CH 4 source is about 410 Tg yr -1 (∼70% of the total source) and the mean recent atmospheric CH 4 increase is ∼20 Tg yr -1 an anthropogenic source reduction of 5% could stabilize the atmospheric CH 4 level We have calculated the indirect chemical effects of increasing CH 4 on climate forcing on the basis of global 3D chemistry-transport and radiative transfer calculations These indicate an enhancement of the direct radiative effect by about 30%, in agreement with previous work The contribution of CH 4 (direct and indirect effects) to climate forcing during the past 150 years is 057W m −2 (direct 044W m −2 , indirect 013 W m −2 ) This is about 35% of the climate forcing by CO 2 (16W m −2 ) and about 22% of the forcing by all long-lived greenhouse gases (26 W m −2 ) Scenario calculations (IPCC-IS92a) indicate that the CH 4 lifetime in the atmosphere increased by about 25–30%during the past 150 years to a current value of 79 years Future lifetime changes are expected to be much smaller, about 6%, mostly due to the expected increase of tropospheric O 3 (→OH) in the tropics The global mean concentration of CH 4 may increase to about 255 μmol/mol, its lifetime is expected to increase to 84 years in the year 2050 Further, we have calculated a CH 4 global warming potential (GWP) of 21 (kgCH 4 /kgCO 2 ) over a time horizon of 100 years, in agreement with IPCC (1996) Scenario calculations indicate that the importance of the climate forcing by CH 4 (including indirect effects) relative to that of CO 2 will decrease in future; currently this is about 35%, while this is expected to decrease to about 15% in the year 2050 DOI: 101034/j1600-08891998t01-1-00002x

Changing concentration, lifetime and climate forcing of atmospheric methane

Engineering estimates of methane emissions from natural gas production have led to varied projections of national emissions. This work reports direct measurements of methane emissions at 190 onshore natural gas sites in the United States (150 production sites, 27 well completion flowbacks, 9 well unloadings, and 4 workovers). For well completion flowbacks, which clear fractured wells of liquid to allow gas production, methane emissions ranged from 0.01 Mg to 17 Mg (mean = 1.7 Mg; 95% confidence bounds of 0.67–3.3 Mg), compared with an average of 81 Mg per event in the 2011 EPA national emission inventory from April 2013. Emission factors for pneumatic pumps and controllers as well as equipment leaks were both comparable to and higher than estimates in the national inventory. Overall, if emission factors from this work for completion flowbacks, equipment leaks, and pneumatic pumps and controllers are assumed to be representative of national populations and are used to estimate national emissions, total annual emissions from these source categories are calculated to be 957 Gg of methane (with sampling and measurement uncertainties estimated at ±200 Gg). The estimate for comparable source categories in the EPA national inventory is ∼1,200 Gg. Additional measurements of unloadings and workovers are needed to produce national emission estimates for these source categories. The 957 Gg in emissions for completion flowbacks, pneumatics, and equipment leaks, coupled with EPA national inventory estimates for other categories, leads to an estimated 2,300 Gg of methane emissions from natural gas production (0.42% of gross gas production).

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Measurements of methane emissions at natural gas production sites in the United States

Methane mass balance at three landfill sites: what is the efficiency of capture by gas collection systems?

Methane emissions from natural gas delivery and end use must be quantified to evaluate the environmental impacts of natural gas and to develop and assess the efficacy of emission reduction strategies. We report natural gas emission rates for 1 y in the urban region of Boston, using a comprehensive atmospheric measurement and modeling framework. Continuous methane observations from four stations are combined with a high-resolution transport model to quantify the regional average emission flux, 18.5 ± 3.7 (95% confidence interval) g CH4⋅m−2⋅y−1. Simultaneous observations of atmospheric ethane, compared with the ethane-to-methane ratio in the pipeline gas delivered to the region, demonstrate that natural gas accounted for ∼60–100% of methane emissions, depending on season. Using government statistics and geospatial data on natural gas use, we find the average fractional loss rate to the atmosphere from all downstream components of the natural gas system, including transmission, distribution, and end use, was 2.7 ± 0.6% in the Boston urban region, with little seasonal variability. This fraction is notably higher than the 1.1% implied by the most closely comparable emission inventory.

https://www.pnas.org/content/pnas/112/7/1941.full.pdf

Methane emissions from natural gas infrastructure and use in the urban region of Boston, Massachusetts

Environ. Sci. Techno/. 1995, 29, 1468-1 479 Introduction Tracer -Methods To Measure Downloaded by UNIV OF CALIFORNIA IRVINE on August 26, 2015 | http://pubs.acs.org Publication Date: June 1, 1995 | doi: 10.1021/es00006a007 Gas Faciliies and U h n Areas BRIAN K. LAMB,*,’ J. BARRY MCMANUS,* JOANNE H. SHORTER,* CHARLES E . KOLB,* BYARD MOSHER,$ R O B E R T C . HARRISS,§ EUGENE ALLWINE,’ DENISE BLAHA,$ T O U C H E H O W A R D , ” A L E X GUENTHER,l ROBERT A. LOTT,A ROBERT SIVERSON,’ HAL WESTBERG,’ AND PAT ZIMMERMAN- Laboratory for Atmospheric Research, Department of Civil & Environmental Engineering, Washington State University, Pullman, WA 99164-2910, Center for Chemical and Environmental Physics, Aerodyne Research, Inc., Billerica, Massachusetts 01821, Institute for the Study of Earth, Oceans and Space, University of New Hampshire, Durham, New Hampshire 03824, Indaco Air Quality Services, Inc., Pullman, Washington 99163, National Center for Atmospheric Research, Boulder, Colorado 80303, and Gas Research Institute, Chicago, Illinois 60631 -3562 A new, integrated methodologyto locate and measure methane emissions from natural gas systems has been developed. Atmospheric methane sources are identified by elevated ambient CH4 concentrations measured with a mobile laser-based methane analyzer. The total methane emission rate from a source is obtained by simulating the source with a sulfur hexafluoride (SFS) tracer gas release and by measuring methane and tracer concentrations along downwind sampling paths using mobile, real-time analyzers. Combustion sources of methane are dis- tinguished from noncombustion sources by concur- rent ambient carbon dioxide measurements. Three variations on the tracer ratio method are described for application to (1) small underground vaults, (2) above- ground natural gas facilities, and (3) diffuse methane emissions from an entire town. Results from controlled releases and from replicate tests demonstrate thatthe tracer ratio approach can yield total emission rates to within approximately &15%. The estimated accuracy of emission estimates for urban areas with a variety of diffuse emissions is &50%. Methane (CH4) has been a contributor to the increasing burden of greenhouse gases in the earth’s atmosphere for more than a century (1). Faced with significant risks identified in scenarios of increasing greenhouse gas con- centrations, many countries are developing plans to reduce emissions. However, uncertainties in specific source emission rates for CH4 and other non-COz greenhouse gases currently limit the quantitative risk-benefit analysis needed to answer key policy questions related to the socioeconomic impacts of large-scale mitigation actions (2, 3 ) . Initial attempts to estimate CH4 losses to the atmosphere from natural gas production and use assumed that emis- sions could be approximated by industry reports of “unac- counted for” gas (e.g., ref 4). Unaccounted for gas, defined as the difference between the amount of natural gas metered into a system and the amount of gas metered out of a system, does not account for gas losses from wells to the processing plant, gas used as fuel in facilities, theft of gas, meter inaccuracies, and differences in accounting procedures between companies (4,5). Thus, the unaccounted for gas estimates cannot unambiguously be considered an upper or lower bound on emissions (5). Extrapolation of engi- neering estimates or data obtained from component by component sniffing methods also leads to large uncertain- ties in estimated emissions. In the United Kingdom, the British Gas Company estimates CH4 emissions from gas distribution system components to be less than 1% of throughput, while others estimate losses as high as 11% of gas throughput (6). A recent estimate of CH4 leakage from the natural gas system in the former Soviet Union, which was characterized as “tentative and highly conditional” suggested a range of total losses from 3.3% to 7% of gas production ( 7 ) . The U.S. Environmental Protection Agency (EPA) and the Gas Research Institute (GRI) have recently sponsored an integrated field measurement and analysis program to better define methane emissions from the U.S. natural gas system. Drawing on initial measurements using some of the techniques reported here as weil as engineering estimates, GRI has developed a preliminary estimate of methane emissions from the gas industry that equals approximately 1.5 i 0.5% of annual throughput (8). In the case of CH4 emissions due to the use of natural gas, there is an added motivation for correctly prescribing the methane source strength. Since natural gas typically produces 32-45% less COz per unit of thermal output compared to coal and 30% less compared to fuel oil, switching from coal and fuel oil to natural gas has the potential to reduce carbon dioxide emissions and reduce global warming (5). However, CH4 is a more potent greenhouse gas than CO2 on a molecule for molecule basis (9- 1 I). As a result, increasing the usage of natural gas may * To whom correspondence should be addressed: e-mail address: blamb@wsu.edu. + Washington State University. Aerodyne Research, Inc. 5 University of New Hampshire. I’ Indaco Air Quality Services, Inc. National Center for Atmospheric Research. A Gas Research Institute. 1468 ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 6, 1995 0013-936)(/95/0929-1468$09.00/0 @ 1995 American Chemical Society

/pdf/development-of-atmospheric-tracer-methods-to-measure-methane-16fjb0bb5b.pdf

Development of atmospheric tracer methods to measure methane emissions from natural gas facilities and urban areas.

Natural and anthropogenic methane sources in New England

We have compiled a unique high-resolution ambient-air methane data set consisting of approximately 125,000 independently measured data points for the years 1991-1995 that have been collected at a site in the northeastern United States. The annual median mixing ratio of methane for all measurements was 1808 ppbv in 1992, increasing at a variable rate to 1837 ppbv in 1995. The lower 10-30% of the data from each month were defined as representative of background air and were compared with the global Climate Monitoring and Diagnostics Laboratory (CMDL) data set. The background data exhibit a variable upward trend of 5.5 + 2 ppbv/yr during the 4-year time period, with most of the increase observed during 1993 and 1994. The seasonal cycle for the back- ground data set is similar to what is observed by CMDL stations and varies from 24 to 35 ppbv. The amplitude of the seasonal cycle for the full data set was larger, ranging from 35 to 44 ppbv. Differ- ences between the full and background mixing ratios vary on a seasonal basis and are largest in the winter and smallest in the summer. These differences appear to be controlled by changes in atmo- spheric stability and changes in emissions from local and regional sources throughout the year. Diurnal cycles exist in the data, witlthe magnitude and timing of maximum and minimum values being controlled by inputs from local sources and atmospheric stability. Nearby weftands contribute to an ovemight buildup of methane in tle late spring and summer. The magnitude of the daily cycle is largest in July and August (--23 ppbv), when inputs from weftands are large and wind speed is generally low. In April, the daily cycle is smallest (--6 ppbv), when inputs from local sources are low and more vigorous atmospheric mixing limits pollution buildup. In 1993 the United States released a climate action plan propos- ing to return U.S. greenhouse gas emissions to the 1990 level by the year 2000. The largest contributor to the potential of global warming is carbon dioxide (CO2), followed by methane (CH4). Stabilizing the atmospheric burden of CO 2 will require a reduction in emissions of 60-80%. This is unlikely in the near future given industrial societies' strong dependence on fuels coupled with the 200-year lifetime of CO 2. In comparison, CH 4 emissions need to be reduced only 10-15% to stabilize its global concentration, and the 11-year lifetin-re of CH 4 means the effects of mitigation strategies may be observed within several decades. A critical factor in determining if mitigation strategies are working is the paucity of quantitative measurements of CH 4 at local to regional scales in which changes in emissions over long time periods may be verified. To address this problem, automated high-frequency (8-11 min) CH 4 measurements have been made by Patrick M. Crill from the University of New Hampshire at the Harvard Forest (HF) research site since 1992. The proximity of

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/98JD00106

Atmospheric methane measurements in central New England: An analysis of the long‐term trend and the seasonal and diurnal cycles

The Harvard Forest research site located in central New England is influenced by numerous anthropogenic methane sources on a year-round basis. Methane is strongly correlated to other chemical species that have an anthropogenic component, including acetylene, propane, ethane, hexane, and additional short-lived nonmethane hydrocarbons. The correlation between methane and acetylene is due to the colocation of landfills and cities. The correlation between methane and other short-lived species implies that emissions from local and regional rather than distant sources are the primary cause of elevated events. Wind roses of chemical species are examined for annual and seasonal time periods with enhancements in anthropogenic species corresponding to the location of large cities and landfills. The southwest quadrant is subjected to the most severe pollution events and is impacted by outflow from nearby cities in that sector, including Northampton and Springfield, Massachusetts. Emissions from cities in other quadrants, including Boston and Worcester, Massachusetts, Providence, Rhode Island, and the close-by town of Petersham, Massachusetts, also affect the site, but to a lesser degree. Case studies are used to identify atmospheric conditions that lead to high concentrations of methane and other species. The co-occurrence of a persistent wind direction, light wind speed, and stable atmospheric conditions is the ideal scenario in which emissions from nearby cities and landfills are advected to the site. Emissions from local and regional, rather than distant sources, are the primary cause of elevated events.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/98JD00967

Denise Blaha

Papers

Development of atmospheric tracer methods to measure methane emissions from natural gas facilities and urban areas.

Natural and anthropogenic methane sources in New England

Atmospheric methane measurements in central New England: An analysis of the long‐term trend and the seasonal and diurnal cycles

Methane measurements in central New England: An assessment of regional transport from surrounding sources