Abstract. Other Test Method 33A (OTM 33A) is a near-source flux measurement method
developed by the Environmental Protection Agency (EPA) primarily used to
locate and estimate emission fluxes of methane from oil and gas (O&G)
production facilities without requiring site access. A recent national
estimate of methane emissions from O&G production included a large number of
flux measurements of upstream O&G facilities made using OTM 33A and
concluded the EPA National Emission Inventory underestimates this sector by a
factor of ∼2.1 (Alvarez et al., 2018). The study presented here
investigates the accuracy of OTM 33A through a series of test releases
performed at the Methane Emissions Technology Evaluation Center (METEC), a
facility designed to allow quantified amounts of natural gas to be released
from decommissioned O&G equipment to simulate emissions from real facilities
(Fig. 1). This study includes test releases from single and
multiple points, from equipment locations at different heights, and spanned
methane release rates ranging from 0.16 to 2.15 kg h−1. Approximately
95 % of individual measurements (N=45) fell within ±70 % of
the known release rate. A simple linear regression of OTM 33A versus known
release 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 may
have a small but statistically insignificant low bias.
Constraining the accuracy of flux estimates using OTM 33A
