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Published September 4, 2012 | public
Journal Article

On the Sources of Methane to the Los Angeles Atmosphere


We use historical and new atmospheric trace gas observations to refine the estimated source of methane (CH_4) emitted into California's South Coast Air Basin (the larger Los Angeles metropolitan region). Referenced to the California Air Resources Board (CARB) CO emissions inventory, total CH4 emissions are 0.44 ± 0.15 Tg each year. To investigate the possible contribution of fossil fuel emissions, we use ambient air observations of methane (CH_4), ethane (C_(2)H_(6)), and carbon monoxide (CO), together with measured C_(2)H_6 to CH_4 enhancement ratios in the Los Angeles natural gas supply. The observed atmospheric C_(2)H_6 to CH_4 ratio during the ARCTAS (2008) and CalNex (2010) aircraft campaigns is similar to the ratio of these gases in the natural gas supplied to the basin during both these campaigns. Thus, at the upper limit (assuming that the only major source of atmospheric C_(2)H_6 is fugitive emissions from the natural gas infrastructure) these data are consistent with the attribution of most (0.39 ± 0.15 Tg yr^(–1)) of the excess CH_4 in the basin to uncombusted losses from the natural gas system (approximately 2.5–6% of natural gas delivered to basin customers). However, there are other sources of C_(2)H_6 in the region. In particular, emissions of C_(2)H_6 (and CH_4) from natural gas seeps as well as those associated with petroleum production, both of which are poorly known, will reduce the inferred contribution of the natural gas infrastructure to the total CH_4 emissions, potentially significantly. This study highlights both the value and challenges associated with the use of ethane as a tracer for fugitive emissions from the natural gas production and distribution system.

Additional Information

© 2012 American Chemical Society. Received: March 23, 2012; Revised: July 19, 2012; Accepted: August 1, 2012; Published: August 1, 2012. Data used in this analysis were obtained with support of NASA, NOAA, and the California Air Resources Board. We thank Stephanie A. Vay for her efforts to obtain the CO2 data during ARCTAS. We thank the Southern California Gas Company for their interest and support in this study. The analysis was supported by the California Institute of Technology. Support for the analysis of the remote sensing data was provided by NASA's Terrestrial Ecology Program. W.M. acknowledges support from a NSF Graduate Research Fellowship. This work was funded in part by the W. M. Keck Institute for Space Studies. G.S. acknowledges support from NSF and EPA STAR graduate fellowships. We thank Joseph Fischer, Larry Hunsaker, Webster Tassat, Marc Vayssières, and Ying-Kang Hsu for sharing advice and data. This work was supported by the California Energy Commission's Public Interest Environmental Research (CEC-PIER) program, the California Air Resources Board, and the US Dept. of Energy through the LBNL Laboratory Directed Research and Development, under contract No. DE-AC02-05CH11231.

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August 19, 2023
October 20, 2023