Airborne Quantification of Methane Emissions over the Four Corners Region

A Gridded Inventory of Annual 2012-2018 U.S. Anthropogenic Methane Emissions

Nationally reported greenhouse gas inventories are a core component of the Paris Agreement's transparency framework. Comparisons with emission estimates derived from atmospheric observations help identify improvements to reduce uncertainties and increase the confidence in reported values. To facilitate comparisons over the contiguous United States, we present a 0.1° × 0.1° gridded inventory of annual 2012-2018 anthropogenic methane emissions, allocated to 26 individual source categories, with scale-dependent error estimates. Our inventory is consistent with the U.S. Environmental Protection Agency (EPA) Inventory of U.S. Greenhouse Gas Emissions and Sinks (GHGI), submitted to the United Nations in 2020. Total emissions and patterns (spatial/temporal) reflect the activity and emission factor data underlying the GHGI, including many updates relative to a previous gridded version of the GHGI that has been extensively compared with observations. These underlying data are not generally available in global gridded inventories, and comparison to EDGAR version 6 shows large spatial differences, particularly for the oil and gas sectors. We also find strong regional variability across all sources in annual 2012-2018 spatial trends, highlighting the importance of understanding regional- and facility-level activities. Our inventory represents the first time series of gridded GHGI methane emissions and enables robust comparisons of emissions and their trends with atmospheric observations.

Enhancements in Ammonia and Methane from Agricultural Sources in the Northeastern Colorado Front Range Using Observations from a Small Research Aircraft

Quantifying ammonia (NH₃) to methane (CH₄) enhancement ratios from agricultural sources is important for understanding air pollution and nitrogen deposition. The northeastern Colorado Front Range is home to concentrated animal feeding operations (CAFOs) that produce large emissions of NH₃ and CH₄. Isolating enhancements of NH₃ and CH₄ in this region due to agriculture is complicated because CAFOs are often located within regions of oil and natural gas (O&NG) extraction that are a major source of CH₄ and other alkanes. Here, we utilize a small research aircraft to collect in situ 1 Hz measurements of gas-phase NH₃, CH₄, and ethane (C₂H₆) downwind of CAFOs during three flights conducted in November 2019. Enhancements in NH₃ and CH₄ are distinguishable up to 10 km downwind of CAFOs with the most concentrated portions of the plumes typically below 0.25 km AGL. We demonstrate that NH₃ and C₂H₆ can be jointly used to separate near-source enhancements in CH₄ from agriculture and O&NG. Molar enhancement ratios of NH₃ to CH₄ are quantified for individual CAFOs in this region, and they range from 0.8 to 2.7 ppbv ppbv^-1. A multivariate regression model produces enhancement ratios and quantitative regional source contributions that are consistent with prior studies.

Why are methane emissions from China's oil & natural gas systems still unclear? A review of current bottom-up inventories

There is growing awareness and concern on methane (CH₄) emissions from China's oil and natural gas (ONG) systems owing to the carbon neutral target. This paper aims to present a comprehensive review on the bottom-up inventories of the CH₄ emissions from the perspective of the ONG systems in China. The trend and magnitude of total emissions in the last four decades were revealed and limitations of current estimations were explored. Previous studies showed that the average CH₄ emissions from China's ONG systems have almost tripled from 1980 (760 Gg) to 2015 (2180 Gg) with a trend of steady increase. However, the estimated values varied by an order-of-magnitude with the largest discrepancy of 2700 Gg. This discrepancy was unlikely caused mainly by the incompleteness of estimation, since dominant emission sources were all covered by representative studies. Moreover, the differences of activity-level data were within ±10%, which ruled out the possibility that it was the main contributor to the large discrepancies. The emissions estimate has huge variation in large part because of differences in assumed emission factors (EFs) that vary by an order of magnitude. The difficulty was to determine which of the EFs were accurate due to measurement-based data availability. Thus, the large discrepancies stem from the scarcity of publicly available data, which enlarged the impact from various methods adopted by previous studies. For better understanding of CH₄ emissions from the ONG systems in China, the measurements of facility-level emissions and statistics on the ONG infrastructure are required urgently. Due to the high cost and experience-oriented measurement work, international cooperation and communications are critical prerequisites for future CH₄ emission estimates and effective mitigation strategies.

Joint Use of in-Scene Background Radiance Estimation and Optimal Estimation Methods for Quantifying Methane Emissions Using PRISMA Hyperspectral Satellite Data: Application to the Korpezhe Industrial Site

Methane (CH4) is one of the most contributing anthropogenic greenhouse gases (GHGs) in terms of global warming. Industry is one of the largest anthropogenic sources of methane, which are currently only roughly estimated. New satellite hyperspectral imagers, such as PRISMA, open up daily temporal monitoring of industrial methane sources at a spatial resolution of 30 m. Here, we developed the Characterization of Effluents Leakages in Industrial Environment (CELINE) code to inverse images of the Korpezhe industrial site. In this code, the in-Scene Background Radiance (ISBR) method was combined with a standard Optimal Estimation (OE) approach. The ISBR-OE method avoids the use of a complete and time-consuming radiative transfer model. The ISBR-OEM developed here overcomes the underestimation issues of the linear method (LM) used in the literature for high concentration plumes and controls a posteriori uncertainty. For the Korpezhe site, using the ISBR-OEM instead of the LM -retrieved CH4 concentration map led to a bias correction on CH4 mass from 4 to 16% depending on the source strength. The most important CH4 source has an estimated flow rate ranging from 0.36 ± 0.3 kg·s−1 to 4 ± 1.76 kg·s−1 on nine dates. These local and variable sources contribute to the CH4 budget and can better constrain climate change models.

Quantifying Time-Averaged Methane Emissions from Individual Coal Mine Vents with GHGSat-D Satellite Observations

Satellite observations of atmospheric methane plumes offer a means for global mapping of methane point sources. Here we use the GHGSat-D satellite instrument with 50 m effective spatial resolution and 9-18% single-pass column precision to quantify mean source rates for three coal mine vents (San Juan, United States; Appin, Australia; and Bulianta, China) over a two-year period (2016-2018). This involves averaging wind-rotated observations from 14 to 24 overpasses to achieve satisfactory signal-to-noise. Our wind rotation method optimizes the wind direction information for individual plumes to account for error in meteorological databases. We derive source rates from the time-averaged plumes using integrated mass enhancement (IME) and cross-sectional flux (CSF) methods calibrated with large eddy simulations. We find time-averaged source rates ranging from 2320 to 5850 kg h^-1 for the three coal mine vents, with 40-45% precision (1σ), and generally consistent with previous estimates. The IME and CSF methods agree within 15%. Our results demonstrate the potential of space-based monitoring for annual reporting of methane emissions from point sources and suggest that future satellite instruments with similar pixel resolution but better precision should be able to constrain a wide range of point sources.

Airborne Assessment of Methane Emissions from Offshore Platforms in the U.S. Gulf of Mexico

Methane (CH₄) emissions from oil and gas activities are large and poorly quantified, with onshore studies showing systematic inventory underestimates. We present aircraft measurements of CH₄ emissions from offshore oil and gas platforms collected over the U.S. Gulf of Mexico in January 2018. Flights sampled individual facilities as well as regions of 5-70 facilities. We combine facility-level samples, production data, and inventory estimates to generate an aerial measurement-based inventory of CH₄ emissions for the U.S. Gulf of Mexico. We compare our inventory and the Environmental Protection Agency Greenhouse Gas Inventory (GHGI) with regional airborne estimates. The new inventory and regional airborne estimates are consistent with the GHGI in deep water but appear higher for shallow water. For the full U.S. Gulf of Mexico our inventory estimates total emissions of 0.53 Tg CH₄/yr [0.40-0.71 Tg CH₄/yr, 95% CI] and corresponds to a loss rate of 2.9% [2.2-3.8%] of natural gas production. Our estimate is a factor of 2 higher than the GHGI updated with 2018 platform counts. We attribute this disagreement to incomplete platform counts and emission factors that both underestimate emissions for shallow water platforms and do not account for disproportionately high emissions from large shallow water facilities.

Quantifying methane emissions from the largest oil-producing basin in the United States from space

Using new satellite observations and atmospheric inverse modeling, we report methane emissions from the Permian Basin, which is among the world's most prolific oil-producing regions and accounts for >30% of total U.S. oil production. Based on satellite measurements from May 2018 to March 2019, Permian methane emissions from oil and natural gas production are estimated to be 2.7 ± 0.5 Tg a^-1, representing the largest methane flux ever reported from a U.S. oil/gas-producing region and are more than two times higher than bottom-up inventory-based estimates. This magnitude of emissions is 3.7% of the gross gas extracted in the Permian, i.e., ~60% higher than the national average leakage rate. The high methane leakage rate is likely contributed by extensive venting and flaring, resulting from insufficient infrastructure to process and transport natural gas. This work demonstrates a high-resolution satellite data-based atmospheric inversion framework, providing a robust top-down analytical tool for quantifying and evaluating subregional methane emissions.

Beyond-the-Meter: Unaccounted Sources of Methane Emissions in the Natural Gas Distribution Sector

The United States Environmental Protection Agency maintains an inventory of greenhouse gas emissions in accordance with the Intergovernmental Panel on Climate Change. Methane (CH₄), a potent gas with a global warming potential 86-125× that of carbon dioxide (CO₂) over a twenty-year period, is the main component of natural gas (NG). As NG becomes an increasingly larger percentage of the energy resources used in the United States, it is ever more important to evaluate the CH₄ emissions inventory. However, the inventory also does not account for all possible sources of CH₄ leaks, contributing to uncertainty in the national CH₄ inventory. Discrepancies between top-down and bottom-up inventories of CH₄ emissions imply that there are significant unaccounted-for sources of CH₄ leaks, especially over cities. Diffuse CH₄ plumes above cities that are not attributable to distribution pipelines or other NG infrastructure suggest many small beyond-the-meter leaks together contribute to large emissions. Here, we evaluate the distribution sector of the CH₄ emissions inventory and make suggestions to improve the inventory by analyzing end-user emissions. Preliminary research into beyond-the-meter emissions suggests that while individually small, the appliances and buildings that make up the residential sector could contribute significantly to national scale emissions. Furnaces are the most leak-prone of appliances, contributing to 0.14% of total CH₄ emissions from the NG sector in the United States. Combining measurements from whole house emissions and steady-state operation of appliances, we estimate that residential homes and appliances could release 9.1 Gg CH₄ yearly in the United States, totaling over 2% of the CH₄ released from the NG sector. While factors such as appliance age and usage, climate, and residential setting could influence the emissions profile of individual appliances, these preliminary estimates justify further exploration of beyond-the-meter emissions.

Top-Down Constraints on Methane Point Source Emissions From Animal Agriculture and Waste Based on New Airborne Measurements in the U.S. Upper Midwest

Agriculture and waste are thought to account for half or more of the U.S. anthropogenic methane source. However, current bottom-up inventories contain inherent uncertainties from extrapolating limited in situ measurements to larger scales. Here, we employ new airborne methane measurements over the U.S. Corn Belt and Upper Midwest, among the most intensive agricultural regions in the world, to quantify emissions from an array of key agriculture and waste point sources. Nine of the largest concentrated animal feeding operations in the region and two sugar processing plants were measured, with multiple revisits during summer (August 2017), winter (January 2018), and spring (May-June 2018). We compare the top-down fluxes with state-of-science bottom-up estimates informed by U.S. Environmental Protection Agency methodology and site-level animal population and management practices. Top-down point source emissions are consistent with bottom-up estimates for beef concentrated animal feeding operations but moderately lower for dairies (by 37% on average) and significantly lower for sugar plants (by 80% on average). Swine facility results are more variable. The assumed bottom-up seasonality for manure methane emissions is not apparent in the aircraft measurements, which may be due to on-site management factors that are difficult to capture accurately in national-scale inventories. If not properly accounted for, such seasonal disparities could lead to source misattribution in top-down assessments of methane fluxes.

Barometric-pumping controls fugitive gas emissions from a vadose zone natural gas release

Subsurface natural gas release from leaking oil and gas wells is a major environmental concern. Gas migration can cause aquifer contamination, explosive conditions in soil gas, and greenhouse gas emissions. Gas migration is controlled by complex interacting processes, thus constraining the distribution and magnitude of “fugitive gas” emissions remains a challenge. We simulated wellbore leakage in the vadose zone through a controlled release experiment and demonstrate that fugitive gas emissions can be directly influenced by barometric pressure changes. Decreases in barometric-pressure led to surface gas breakthroughs (>20-fold increase in <24 hours), even in the presence of low-permeability surficial soils. Current monitoring strategies do not consider the effect of barometric pressure changes on gas migration and may not provide adequate estimates of fugitive gas emissions. Frequent or continuous monitoring is needed to accurately detect and quantify fugitive gas emissions at oil and gas sites with a deep water table.

Large Fugitive Methane Emissions From Urban Centers Along the U.S. East Coast

Urban emissions remain an underexamined part of the methane budget. Here we present and interpret aircraft observations of six old and leak-prone major cities along the East Coast of the United States. We use direct observations of methane (CH4), carbon dioxide (CO2), carbon monoxide (CO), ethane (C2H6), and their correlations to quantify CH4 emissions and attribute to natural gas. We find the five largest cities emit 0.85 (0.63, 1.12) Tg CH4/year, of which 0.75 (0.49, 1.10) Tg CH4/year is attributed to natural gas. Our estimates, which include all thermogenic methane sources including end use, are more than twice that reported in the most recent gridded EPA inventory, which does not include end-use emissions. These results highlight that current urban inventory estimates of natural gas emissions are substantially low, either due to underestimates of leakage, lack of inclusion of end-use emissions, or some combination thereof.

Characterizing Regional Methane Emissions from Natural Gas Liquid Unloading

A "bottom-up" probabilistic model was developed using engineering first-principles to quantify annualized throughput normalized methane emissions (TNME) from natural gas liquid unloading activities for 18 basins in the United States in 2016. For each basin, six discrete liquid-unloading scenarios are considered, consisting of combinations of well types (conventional and unconventional) and liquid-unloading systems (nonplunger, manual plunger lift, and automatic plunger lift). Analysis reveals that methane emissions from liquids unloading are highly variable, with mean TNMEs ranging from 0.0093% to 0.38% across basins. Automatic plunger-lift systems are found to have significantly higher per-well methane emissions rates relative to manual plunger-lift or non-plunger systems and on average constitute 28% of annual methane emissions from liquids unloading over all basins despite representing only ∼0.43% of total natural gas well count. While previous work has advocated that operational malfunctions and abnormal process conditions explain the existence of super-emitters in the natural gas supply chain, this work finds that super-emitters can arise naturally due to variability in underlying component processes. Additionally, average cumulative methane emissions from liquids unloading, attributed to the natural gas supply chain, across all basins are ∼4.8 times higher than those inferred from the 2016 Greenhouse Gas Reporting Program (GHGRP). Our new model highlights the importance of technological disaggregation, uncertainty quantification, and regionalization in estimating episodic methane emissions from liquids unloading. These insights can help reconcile discrepancies between "top-down" (regional or atmospheric studies) and "bottom-up" (component or facility-level) studies.

Identification, spatial extent and distribution of fugitive gas migration on the well pad scale

Global methane (CH₄) emissions are becoming increasingly important due to the contribution of CH₄ to global warming. Leaking oil and gas wells can lead to subsurface CH₄ gas migration (GM), which can cause both aquifer contamination and atmospheric emissions. Despite the need to identify and quantify GM at oil and gas well pads, effective and reliable monitoring techniques are lacking. In this field study, we used CH₄ and carbon dioxide (CO₂) efflux measurements together with soil gas stable carbon isotopic signatures to identify the occurrence and to characterize the spatio-temporal migration of fugitive gas across 17 selected well pads in Northeastern British Columbia, Canada. At 13 of these sites, operators had previously reported the occurrence of GM; however, subsequent inspections based on visual, olfactory or auditory evidence only identified GM at two of these sites. Using the soil gas efflux method, evidence for GM was found at 15 of the 17 well pads with CH₄ and CO₂ effluxes ranging from 0.017 to 180μmolm^-2s^-1(0.024 to 250gCH₄m^-2d^-1) and 0.50 to 32μmolm^-2s^-1 (1.9 to 122gCO₂m^-2d^-1), respectively. Stable carbon isotopic composition was assessed at 10 of the 17 well pads with 9 well pads showing evidence of GM. The isotopic values indicated that CH₄ in soil gas was from the same origin as CH₄ in the surface casing vent flow gas. There was no correlation between CH₄ effluxes and the distance from the well head; an equal portion of elevated effluxes were detected >10m from the well head as were detected <5m from the well head. In addition, CH₄ effluxes varied temporally with values changing by up to an order of magnitude over 2h. Although the study was carried out in Northeastern British Columbia, the results are applicable on a global scale, suggesting that inspections mostly based on visual evidence (e.g. bubbling at the well head) are not reliable for the identification of GM and, that infrequent survey measurements at predefined locations close to the well head may overestimate, underestimate or even miss CH₄ effluxes. Repetitive and relatively densely spaced gas efflux measurements using a dynamic closed chamber method proved to be a useful tool for detecting GM.

Methane Emissions from Natural Gas Production Sites in the United States: Data Synthesis and National Estimate

We used site-level methane (CH₄) emissions data from over 1000 natural gas (NG) production sites in eight basins, including 92 new site-level CH₄ measurements in the Uinta, northeastern Marcellus, and Denver-Julesburg basins, to investigate CH₄ emissions characteristics and develop a new national CH₄ emission estimate for the NG production sector. The distribution of site-level emissions is highly skewed, with the top 5% of sites accounting for 50% of cumulative emissions. High emitting sites are predominantly also high producing (>10 Mcfd). However, low NG production sites emit a larger fraction of their CH₄ production. When combined with activity data, we predict that this creates substantial variability in the basin-level CH₄ emissions which, as a fraction of basin-level CH₄ production, range from 0.90% for the Appalachian and Greater Green River to >4.5% in the San Juan and San Joaquin. This suggests that much of the basin-level differences in production-normalized CH₄ emissions reported by aircraft studies can be explained by differences in site size and distribution of site-level production rates. We estimate that NG production sites emit total CH₄ emissions of 830 Mg/h (95% CI: 530-1200), 63% of which come from the sites producing <100 Mcfd that account for only 10% of total NG production. Our total CH₄ emissions estimate is 2.3 times higher than the U.S. Environmental Protection Agency's estimate and likely attributable to the disproportionate influence of high emitting sites.

Assessment of methane emissions from the U.S. oil and gas supply chain

Methane emissions from the U.S. oil and natural gas supply chain were estimated by using ground-based, facility-scale measurements and validated with aircraft observations in areas accounting for ~30% of U.S. gas production. When scaled up nationally, our facility-based estimate of 2015 supply chain emissions is 13 ± 2 teragrams per year, equivalent to 2.3% of gross U.S. gas production. This value is ~60% higher than the U.S. Environmental Protection Agency inventory estimate, likely because existing inventory methods miss emissions released during abnormal operating conditions. Methane emissions of this magnitude, per unit of natural gas consumed, produce radiative forcing over a 20-year time horizon comparable to the CO₂ from natural gas combustion. Substantial emission reductions are feasible through rapid detection of the root causes of high emissions and deployment of less failure-prone systems.