Spatial patterns and source attribution of urban methane in the Los Angeles Basin - eScholarship

PUBLICATIONS Journal of Geophysical Research: Atmospheres RESEARCH ARTICLE 10.1002/2015JD024429 Key Points: • Atmospheric methane levels are highly variable across Los Angeles • The majority of Los Angeles methane emissions are from fossil sources • Mobile laboratory approach can identify and apportion methane emissions regionally Supporting Information: • Supporting Information S1 • Map S1 • Data Set S1 Correspondence to: F. M. Hopkins, fhopkins@uci.edu Citation: Hopkins, F. M., E. A. Kort, S. E. Bush, J. R. Ehleringer, C.-T. Lai, D. R. Blake, and J. T. Randerson (2016), Spatial patterns and source attribution of urban methane in the Los Angeles Basin, J. Geophys. Res. Atmos., 121, doi:10.1002/ 2015JD024429. Received 3 NOV 2015 Accepted 17 FEB 2016 Accepted article online 20 FEB 2016 Spatial patterns and source attribution of urban methane in the Los Angeles Basin Francesca M. Hopkins 1,2 , Eric A. Kort 3 , Susan E. Bush 4 , James R. Ehleringer 4,5 , Chun-Ta Lai 6 , Donald R. Blake 7 , and James T. Randerson 1 Department of Earth System Science, University of California, Irvine, California, USA, 2 Now at Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA, 3 Department of Atmospheric, Ocean, and Space Sciences, University of Michigan, Ann Arbor, Michigan, USA, 4 Department of Biology, University of Utah, Salt Lake City, Utah, USA, 5 Global Change and Sustainability Center, University of Utah, Salt Lake City, Utah, USA, 6 Department of Biology, San Diego State University, San Diego, California, USA, 7 Department of Chemistry, University of California, Irvine, California, USA Abstract Urban areas are increasingly recognized as a globally important source of methane to the atmosphere; however, the location of methane sources and relative contributions of source sectors are not well known. Recent atmospheric measurements in Los Angeles, California, USA, show that more than a third of the city’s methane emissions are unaccounted for in inventories and suggest that fugitive fossil emissions are the unknown source. We made on-road measurements to quantify fine-scale structure of methane and a suite of complementary trace gases across the Los Angeles Basin in June 2013. Enhanced methane levels were observed across the basin but were unevenly distributed in space. We identified 213 methane hot spots from unknown emission sources. We made direct measurements of ethane to methane (C 2 H 6 /CH 4 ) ratios of known methane emission sources in the region, including cattle, geologic seeps, landfills, and compressed natural gas fueling stations, and used these ratios to determine the contribution of biogenic and fossil methane sources to unknown hot spots and to local urban background air. We found that 75% of hot spots were of fossil origin, 20% were biogenic, and 5% of indeterminate source. In regionally integrated air, we observed a wider range of C 2 H 6 /CH 4 values than observed previously. Fossil fuel sources accounted for 58–65% of methane emissions, with the range depending on the assumed C 2 H 6 /CH 4 ratio of source end-members and model structure. These surveys demonstrated the prevalence of fugitive methane emissions across the Los Angeles urban landscape and suggested that uninventoried methane sources were widely distributed and primarily of fossil origin. 1. Introduction Methane (CH 4 ) is an important atmospheric pollutant: the second largest contributor to global warming and a key constituent regulating CO and O 3 [Cicerone and Oremland, 1988]. Most sources of CH 4 to the atmo- sphere have been identified; however, their relative importance to the global budget is uncertain [Kirschke et al., 2013]. CH 4 source budgets are even more uncertain at continental and regional scales [e.g., Kort et al., 2008; Miller et al., 2013]. The majority (50–65%) of CH 4 emissions globally come from anthropogenic sources, with a flux of approximately 330 Tg CH 4 per year [Kirschke et al., 2013]. Reduction of CH 4 emissions has been suggested to be an effective short-term strategy to reduce global warming because of CH 4 ’s high radiative forcing relative to CO 2 , around 28 times on a mass basis over a 100 year time horizon [Shindell et al., 2012; Myhre et al., 2013]. However, mitigation of anthropogenic CH 4 emissions requires an accurate CH 4 budget, including knowledge of location and sectoral contributions of different CH 4 emitters, particularly at scales where mitigation policies may be enacted [Hsu et al., 2010; Jeong et al., 2013]. ©2016. American Geophysical Union. All Rights Reserved. HOPKINS ET AL. Observations of elevated CH 4 levels in cities demonstrate that significant emissions of anthropogenic CH 4 are derived from urban areas [Blake et al., 1984; Wunch et al., 2009]. According to inventory estimates, 35% of the anthropogenic CH 4 in North America is emitted from urban regions [Marcotullio et al., 2013]. However, recent atmospheric studies at the state and city levels in California suggest a 30–80% underestimation of CH 4 emissions in the state greenhouse gas inventory, using stationary and airborne trace gas measurements [Wunch et al., 2009; Hsu et al., 2010; Wennberg et al., 2012; Jeong et al., 2013; Peischl et al., 2013; Wong et al., 2015]. Uncounted fugitive emissions, such as leaks from natural gas pipelines, are hypothesized to account for this mismatch between bottom-up inventories and top-down measurements [Brandt et al., 2014]. On-road surveys in major cities such as Boston and Washington, DC have revealed large fugitive leaks from natural gas distribution MOBILE SURVEY OF LA METHANE