Activity data: Data on the magnitude of a human activity resulting in emissions or removals taking place during a given time period (e.g., head of cattle per year or number of gas processing facilities) (IPCC, 2006).
Atmospheric inverse models: As opposed to a forward model, which uses a prescribed forcing such as surface emissions, to calculate atmospheric concentration, an inverse model uses information about atmospheric concentration to estimate the forcing. In practice, this involves using a first guess of surface emissions (prior emissions) to simulate the atmospheric responses and then systematically comparing simulated and observed concentrations to produce revised estimates of surface forcing that result in optimal agreement with observations. Numerical estimation techniques used in atmospheric inversions to obtain surface emissions are similar to those used in numerical weather prediction and include variational and ensemble approaches. Resulting estimated (posterior) emissions are a weighted combination of information coming from observations and prior flux estimates. The weight of prior versus observational constraints in the estimated emissions is determined by the input uncertainties of observational and prior emissions where the uncertainty of observational information reflects measurement uncertainty as well as the ability to accurately simulate atmospheric concentration at measurement sites.
Atmospheric transport models: A mathematical model that solves the mass continuity equation for atmospheric trace species, resulting in simulated atmospheric concentrations. The mass continuity equation accounts for sources and sinks of trace species such as surface emissions and chemical loss processes taking place throughout the atmospheric column. It also accounts for transport by atmospheric motions. Transport due to small-scale processes that cannot be resolved because of limited model spatiotemporal resolution such as turbulent diffusion and convection are parameterized. Models can range in complexity from global average models to highly resolved global three-dimensional models. Although some models generate their own atmospheric motions, many models used for trace gas studies use atmospheric state variables such as winds and temperatures from atmospheric analyses or reanalyses from parent general circulation or weather models.
“Beyond the meter”: Natural gas infrastructure (i.e., pipes, fittings, valves, and other components) after distribution service lines and customer meters, but before use (e.g., combustion) of the natural gas.
Blowdowns: Natural gas released due to maintenance and/or depressurization operations including compressor depressurization, pipeline depressurization, and emergency shutdown system testing (EPA, 2016b).
Bottom-up method: Based on measurements from a single facility or source; these source or facility-level measurements can then be extrapolated to larger scales (regional, national, global) in a bottom-up estimate.
Carbon budget: Quantification of the amount of carbon (often in the form of either methane or carbon dioxide) contained in different environments—including the atmosphere, ocean, and terrestrial biosphere—and the movement of this carbon among these sources and sinks.
Emission factor: A coefficient that quantifies the emissions or removals of a greenhouse gas per unit of activity. Emission factors are often based on a sample of measurement data, averaged to develop a representative rate of emission for a given activity level, under a given set of operating conditions (IPCC, 2006).
Eructation: Release of methane gas from the digestive tract (mainly the rumen) by ruminant animals (i.e., burping or belching).
Expiration: The phase in breathing when air leaves the lungs.
Flatulence: Release of methane gas produced in the intestines of ruminant animals through the rectum.
Fugitive emissions: Intentional or unintentional release of greenhouse gases that may occur during the extraction, processing, and delivery of fossil fuels to the point of final use (IPCC, 2006).
Greenhouse gas inventories: an estimation of the amount of greenhouse gas emissions discharged into the atmosphere over a specific spatial and temporal scale.
Greenhouse gas sinks: any process, activity, or mechanism that removes a GHG from the atmosphere. There are also biological sinks: methane is oxidized in soils and sediments by indigenous microorganisms under both aerobic and anaerobic conditions (IPCC, 2014).
Greenhouse Gas Reporting Program: As the GHGRP is codified in 40 CFR Part 98. The GHGRP requires reporting of greenhouse gas (GHG) data and other relevant informa-
tion from large GHG emission sources, fuel and industrial gas suppliers, and CO2 injection sites in the United States—generally, facilities emitting ≥25,000 metric tonnes carbon dioxide equivalent (CO2eq) per year.
Greenhouse gas sources: Any identified process or activity (natural or human-caused) that produces or releases a GHG to the atmosphere. Sources are typically characterized by sector, category, or components.
Gridded inventory: Presentation of anthropogenic greenhouse gas emission data at smaller spatial (e.g., 0.1°× 0.1°) and temporal (e.g., monthly) scales than are available at the national level. Development of a gridded inventory may include the use of the U.S. Environmental Protection Agency Greenhouse Gas Inventory data or other datasets and disaggregation of this inventory using state-, county-, local-, and point-source-level databases to allocate emissions to individual source types (e.g., Maasakkers et al., 2016).
High-emitting sources: Also known as fat tails or super-emitters, high-emitting sources are a small number of sources that contribute to a disproportionately high fraction of the cumulative total emissions recorded. They are typically found in the natural gas production and distribution sectors. These sources may be a result of episodic events, malfunctions, or their size and operating/design limitations.
Landfill gas/landfill biogas: A mixture of gases consisting primarily of methane and carbon dioxide that is produced by methanogenesis in landfills. Landfill gas (as recovered by engineered systems) can also contain oxygen and nitrogen resulting from air intrusion and a large number of trace components due to volatilization and internal landfill reactions.
Liquid unloadings: Gas well liquid unloading is a procedure, implemented periodically, where liquids that have accumulated in a gas well are removed to surface equipment. The liquid accumulation can include oil, condensate, and water, and may be due to a variety of causes, including decreases in gas velocity in the well, decreases in reservoir pressure, or changing gas-to-liquid ratios. As liquids accumulate, well production can decline and an operator may choose to unload the liquids from the well to restore gas production.1
Longwall mining: a type of underground mining where coal is extracted mechanically in long (up to 3,650 m) rectangular blocks, after which the mined area collapses, to be differentiated from older, room-and-pillar mining where “pillars” are left in place with only incidental collapses after mining is completed (Kentucky Foundation, 2007).
Methane oxidation: The process by which methane is aerobically (presence of oxygen) converted to carbon dioxide and water vapor by indigenous methanotrophic microorganisms in soils and sediments. Methane can also be anaerobically (absence of oxygen) oxidized in soils and sediments via multiple pathways.
Methanogenesis: The process by which methanogenic microorganisms biologically produce methane in the absence of air (anaerobic conditions). This process occurs in water-saturated soils and sediments in marine and terrestrial environments (e.g., wetlands). Normally, aerobic soils will also produce methane during periods of saturation. Methane produced during methanogenesis is considered to be anthropogenic when linked to human activities (landfills, wastewater, rice production, manure management systems, and rearing livestock for milk or meat).
Overburden: In mining it defines the material located above the economically valuable coal seam. Overburden can consist of hard rock, soft sediment, and soil.
Pneumatic controllers: Devices used in petroleum and natural gas systems to regulate liquid levels, valves, and gas pressure. When open, controllers powered by natural gas pressure release methane (EPA, 2016c).
Posterior emissions: Estimated emissions produced by atmospheric inversions. These are emissions that have been revised from prior emissions to agree optimally with observations.
Prior emissions: Emissions used as a first guess in atmospheric inversions. Emission inventories and results from process models of emissions are common choices for prior emissions.
Rumen: The largest segment of the complex stomach of ruminant animals, in which methanogenic archaea generate methane (predominantly) from hydrogen and carbon dioxide.
Top-down method: Approaches based on atmospheric measurements that are directed toward estimating emissions from regions that could include multiple facilities (Heath et al., 2015).