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. Other Test Method 33A (OTM 33A) is a near-source flux measurement methoddeveloped by the Environmental Protection Agency (EPA) primarily used tolocate and estimate emission fluxes of methane from oil and gas (O&amp;amp;G)production facilities without requiring site access. A recent nationalestimate of methane emissions from O&amp;amp;G production included a large number offlux measurements of upstream O&amp;amp;G facilities made using OTM 33A andconcluded the EPA National Emission Inventory underestimates this sector by afactor of ∼2.1 (Alvarez et al., 2018). The study presented hereinvestigates the accuracy of OTM 33A through a series of test releasesperformed at the Methane Emissions Technology Evaluation Center (METEC), afacility designed to allow quantified amounts of natural gas to be releasedfrom decommissioned O&amp;amp;G equipment to simulate emissions from real facilities(Fig. 1). This study includes test releases from single andmultiple points, from equipment locations at different heights, and spannedmethane release rates ranging from 0.16 to 2.15  kg h−1. Approximately95 % of individual measurements (N=45) fell within ±70 % ofthe known release rate. A simple linear regression of OTM 33A versus knownrelease rates at the METEC site gives an average slope of 0.96 with 95 %CI (0.66,1.28), suggesting that an ensemble of OTM 33A measurements mayhave a small but statistically insignificant low bias.

Constraining the accuracy of flux estimates using OTM 33A