“…For example, emission factors for oil and gas operations are difficult to estimate because a small number of emitters often account for a large fraction of emissions (e.g., Allen, 2014;Allen et al, 2015;Brantley et al, 2014;Lan et al, 2015;Mitchell et al, 2015) and because there are so many points along the natural gas production, processing, transmission, and distribution cycle that leak methane (e.g., Kang et al, 2014;Allen et al, 2015;McKain et al, 2015;Subramanian et al, 2015;Zimmerle et al, 2015).…”