Attributing Atmospheric Methane to Anthropogenic Emission\nSources

Abstract

Methane\nis a greenhouse gas, and increases in atmospheric methane\nconcentration over the past 250 years have driven increased radiative\nforcing of the atmosphere. Increases in atmospheric methane concentration\nsince 1750 account for approximately 17% of increases in radiative\nforcing of the atmosphere, and that percentage increases by approximately\na factor of 2 if the effects of the greenhouse gases produced by the\natmospheric reactions of methane are included in the assessment. Because\nof the role of methane emissions in radiative forcing of the atmosphere,\nthe identification and quantification of sources of methane emissions\nis receiving increased scientific attention. Methane emission sources\ninclude biogenic, geogenic, and anthropogenic sources; the largest\nanthropogenic sources are natural gas and petroleum systems, enteric\nfermentation (livestock), landfills, coal mining, and manure management.\nWhile these source categories are well-known, there is significant\nuncertainty in the relative magnitudes of methane emissions from the\nvarious source categories. Further, the overall magnitude of methane\nemissions from all anthropogenic sources is actively debated, with\nestimates based on source sampling extrapolated to regional or national\nscale (“bottom-up analyses”) differing from estimates\nthat infer emissions based on ambient data (“top-down analyses”)\nby 50% or more.To address the important problem of attribution\nof methane to specific\nsources, a variety of new analytical methods are being employed, including\nhigh time resolution and highly sensitive measurements of methane,\nmethane isotopes, and other chemical species frequently associated\nwith methane emissions, such as ethane. This Account describes the\nuse of some of these emerging measurements, in both top-down and bottom-up\nmethane emission studies. In addition, this Account describes how\ndata from these new analytical methods can be used in conjunction\nwith chemical mass balance (CMB) methods for source attribution. CMB\nmethods have been developed over the past several decades to quantify\nsources of volatile organic compound (VOC) emissions and atmospheric\nparticulate matter. These emerging capabilities for making measurements\nof methane and species coemitted with methane, rapidly, precisely,\nand at relatively low cost, used together with CMB methods of source\nattribution can lead to a better understanding of methane emission\nsources. Application of the CMB approach to source attribution in\nthe Barnett Shale oil and gas production region in Texas demonstrates\nboth the importance of extensive and simultaneous source testing in\nthe region being analyzed and the potential of CMB method to quantify\nthe relative strengths of methane emission sources.