Identifying under-characterized atmospheric methane emission sources in Western Maryland

Abstract Methane (CH4), as the second largest component of greenhouse gases in the atmosphere, can be formed through biogenic or thermogenic processes. A 2015 aircraft survey conducted by the National Oceanic and Atmospheric Administration (NOAA) observed a 2.7 ppm methane plume (40% increase relative to background) at an altitude of 500 m above ground in Western Maryland. Specific sources were not attributed, however, due to rough terrain and a variety of possible sources including coal mines, landfills, and natural gas infrastructure (production wells, storage wells, abandoned wells, pipelines and compressor stations). National Energy Technology Laboratory (NETL) conducted a stationary ambient air monitoring campaign during 2014 Spring/Summer (May to August) and Fall/Winter (November to February) and a 5-day mobile sampling survey in September and October 2018 to identify the potential source(s). The ambient air monitoring laboratory was located in Western Maryland in an area containing production, abandoned, and storage wells. By using cavity ring-down spectroscopy, we observed CH4 concentrations up to 7.4 ppm. The plume events had a thermogenic δ13C signature of −35.2 ± 0.6‰. Mobile survey routes were optimized based on locations of previously identified CH4 sources. We observed a 10-min long CH4 plume event while stopping/driving in a township downwind of a compressor station and repeated 10 ppm plumes near the landfill in the northeast of Western Maryland. CH4 emissions from the landfill had a δ13C value of −54.0 ± 0.4‰, indicative of biogenic origins. We calculated the CH4 emission flux from the landfill to be 1575 ± 1173 tons/year by using inverse dispersion modeling. The flux estimation agreed with Maryland Department of the Environment inventory if assuming no day to day variation in the emission rate. CH4 sources in Western Maryland include natural gas infrastructure and the landfill. The 2.7 ppm plume observed by the aircraft was most likely from the landfill near Frostburg in the northeast of the study area.