Methane (CH4) is a potent greenhouse gas and ozone precursor. Quantifying methane emissions is critical for projecting and mitigating changes to climate and air quality. Here we present CH4 ...observations made from space combined with Earth‐based remote sensing column measurements. Results indicate the largest anomalous CH4 levels viewable from space over the conterminous U.S. are located at the Four Corners region in the Southwest U.S. Emissions exceeding inventory estimates, totaling 0.59 Tg CH4/yr 0.50–0.67; 2σ, are necessary to bring high‐resolution simulations and observations into agreement. This underestimated source approaches 10% of the EPA estimate of total U.S. CH4 emissions from natural gas. The persistence of this CH4 signal from 2003 onward indicates that the source is likely from established gas, coal, and coalbed methane mining and processing. This work demonstrates that space‐based observations can identify anomalous CH4 emission source regions and quantify their emissions with the use of a transport model.
Key Points
Four Corners exhibits largest CH4 anomaly seen from spaceEmissions of >0.5 Tg CH4/yr have persisted since 2003Space‐ and ground‐based CH4 identify missing emissions from fossil fuel extraction
Atmospheric observations of greenhouse gases provide essential information on sources and sinks of these key atmospheric constituents. To quantify fluxes from atmospheric observations, representation ...of transport—especially vertical mixing—is a necessity and often a source of error. We report on remotely sensed profiles of vertical aerosol distribution taken over a 2 year period in Pasadena, California. Using an automated analysis system, we estimate daytime mixing layer depth, achieving high confidence in the afternoon maximum on 51% of days with profiles from a Sigma Space Mini Micropulse LiDAR (MiniMPL) and on 36% of days with a Vaisala CL51 ceilometer. We note that considering ceilometer data on a logarithmic scale, a standard method, introduces, an offset in mixing height retrievals. The mean afternoon maximum mixing height is 770 m Above Ground Level in summer and 670 m in winter, with significant day‐to‐day variance (within season σ = 220m≈30%). Taking advantage of the MiniMPL's portability, we demonstrate the feasibility of measuring the detailed horizontal structure of the mixing layer by automobile. We compare our observations to planetary boundary layer (PBL) heights from sonde launches, North American regional reanalysis (NARR), and a custom Weather Research and Forecasting (WRF) model developed for greenhouse gas (GHG) monitoring in Los Angeles. NARR and WRF PBL heights at Pasadena are both systematically higher than measured, NARR by 2.5 times; these biases will cause proportional errors in GHG flux estimates using modeled transport. We discuss how sustained lidar observations can be used to reduce flux inversion error by selecting suitable analysis periods, calibrating models, or characterizing bias for correction in post processing.
Key Points
Aerosol lidar maps LA mixing depth in space (pilot mobile study) and time (2 years data)
Automatic mixing depth retrieval system finds daily variability far exceeds seasonal difference
PBL heights in models used for GHG monitoring show biases that will carry over to flux estimates
Flaring is widely used by the fossil fuel industry to dispose of natural gas. Industry and governments generally assume that flares remain lit and destroy methane, the predominant component of ...natural gas, with 98% efficiency. Neither assumption, however, is based on real-world observations. We calculate flare efficiency using airborne sampling across three basins responsible for >80% of US flaring and combine these observations with unlit flare prevalence surveys. We find that both unlit flares and inefficient combustion contribute comparably to ineffective methane destruction, with flares effectively destroying only 91.1% (90.2, 91.8; 95% confidence interval) of methane. This represents a fivefold increase in methane emissions above present assumptions and constitutes 4 to 10% of total US oil and gas methane emissions, highlighting a previously underappreciated methane source and mitigation opportunity.
Fueling global warming
Flaring, the process of burning natural gas escaping from oil and gas wells, is primarily intended to combust the powerful greenhouse gas methane to minimize its emission. But is flaring as effective as is claimed? Plant
et al
. used airborne sampling to measure flare efficiency in three major gas production regions in the United States and found that methane emissions are five times higher than currently thought (see the Perspective by Duren and Gordon). Therefore, flaring is often not as efficient as presumed—or methane plumes simply are not combusted at all. —HJS
Natural gas flaring destroys much less methane than thought.
Anthropogenic methane emissions from urban centers are important and addressable, yet remain poorly characterized, and the representativeness of studies from individual cities is unknown. A ...satellite-based approach provides a pathway to tackle this challenge on a national or global scale. Here we present a space-based method that uses the simultaneous, daily observation of methane and carbon monoxide from the TROPOMI satellite to estimate urban methane emissions. We assess and validate the method and demonstrate that using these simultaneous observations enables robust assessment of methane emissions from urban centers without relying on atmospheric transport models. Initial assessments with this approach in eight United States cities suggest emission inventory underestimates previously discovered in older East Coast cities are more broadly representative, with aggregated emissions totaling 1.47 (0.56, 3.19, 95% confidence interval) Tg CH4/year, compared to the Environmental Protection Agency estimate of 0.52 Tg CH4/year. We show this data driven approach provides a pathway to study urban methane emissions across the globe and track how emissions respond if urban mitigation measures are implemented by investigating three additional megacities outside the United States.
•We present a new data-driven method to quantify urban CH4 emissions from space.•Daily and simultaneous observations of CH4 and CO from TROPOMI are analyzed.•The approach is evaluated with synthetic and aircraft data.•U.S. results suggest that inventory underestimates of urban CH4 may be widespread.•This approach enables future study of global urban CH4 emissions.
The concentration of carbon dioxide (CO2) in Earth's atmosphere is increasing due to human activities and the resulting effects on the global climate system have initiated several policy‐driven ...approaches to reduce emissions of this greenhouse gas. Quantifying the effectiveness of such policies requires both bottom‐up and top‐down approaches to estimate CO2 emissions. This work investigates, for the first time, the potential of using Snapshot Area Map observations from NASA's OCO‐3 instrument to disaggregate sector‐specific emissions from instrument observations. Optimized sector‐specific timeseries were produced using Bayesian inversion techniques and compared to proxy activity data from transportation, commercial maritime, and industrial sectors in the Los Angeles Basin. Results demonstrate that dense space‐based observations of atmospheric CO2 are capable of disentangling sector‐specific CO2 fluxes, paving the way for accurate monitoring of the effects of carbon‐reduction policies and operational carbon monitoring systems.
Plain Language Summary
Carbon dioxide (CO2) is a key greenhouse gas and several local‐to‐international policies are in place to reduce the amount being emitted by human activities. This work investigates the amount of CO2 emitted within the Los Angeles Basin during the period between January 2020 and December 2021 using NASA's Orbiting Carbon Observatory‐3. The observed CO2 is used to assess contributions from specific sectors (on‐road transportation, industrial sources, commercial maritime activity, etc.). The results of this work demonstrate that urban CO2 emissions observed from space‐based instrumentation can be disaggregated to several socioeconomic sectors to study trends that may be present in each one. Notable detected features include the sudden reduction of on‐road CO2 emissions due to the COVID‐19 lockdown period and the steady increase in off‐shore emissions due to ship idling and delays. The effectiveness of current and future policies regarding sector‐specific reductions have the potential to be observed over time using the framework presented here.
Key Points
NASA's OCO‐3 instrument provides the densest spatial coverage of urban XCO2 from space, which includes information on spatially variant surface fluxes. We show this spatial coverage makes it possible to disaggregate sectoral emissions information from observations
Using OCO‐3 and the Los Angeles Basin as a case study, three emission sectors from an emission inventory are optimized to include effects from COVID‐19 lockdowns. In two contributing sectors, On‐road Transportation and Industry, optimized CO2 flux decreased considerably around the time COVID‐19 lockdowns were implemented in the Los Angeles area. In the third sector, Maritime Transportation, optimized CO2 flux steadily increased over time
The timeseries of optimized fluxes followed sector‐specific proxy data
Over the past decade, 1000s of cities have pledged reductions in carbon dioxide emissions. However, tracking progress toward these pledges has largely relied exclusively on activity-based, ...self-reported emissions inventories, which often underestimate emissions due to incomplete accounting. Furthermore, the lack of a consistent framework that may be deployed broadly, across political boundaries, hampers understanding of changes in both city-scale emissions and the global summation of urban emissions mitigation actions, with insight being particularly limited for cities within the global south. Given the pressing need for rapid decarbonization, development of a consistent framework that tracks progress toward city-scale emissions reduction targets, while providing actionable information for policy makers, will be critical. Here, we combine satellite-based observations of atmospheric carbon dioxide and an atmospheric model to present an atmospherically-based framework for monitoring changes in urban emissions and related intensity metrics. Application of this framework to 77 cities captures ∼16% of global carbon dioxide emissions, similar in magnitude to the total direct emissions of the United States or Europe, and demonstrates the framework’s ability to track changes in emissions via satellite-observation. COVID-19 lockdowns correspond to an average ∼21% reduction in emissions across urban systems over March–May of 2020 relative to non-lockdown years. Urban scaling analyses suggest that per capita energy savings drive decreases in emissions per capita as population density increases, while local affluence and economic development correspond to increasing emissions. Results highlight the potential for a global atmospherically-based monitoring framework to complement activity-based inventories and provide actionable information regarding interactions between city-scale emissions and local policy actions.
Limiting emissions of climate-warming methane from oil and gas (O&G) is a major opportunity for short-term climate benefits. We deploy a basin-wide airborne survey of O&G extraction and ...transportation activities in the New Mexico Permian Basin, spanning 35 923 km2, 26 292 active wells, and over 15 000 km of natural gas pipelines using an independently validated hyperspectral methane point source detection and quantification system. The airborne survey repeatedly visited over 90% of the active wells in the survey region throughout October 2018 to January 2020, totaling approximately 98 000 well site visits. We estimate total O&G methane emissions in this area at 194 (+72/–68, 95% CI) metric tonnes per hour (t/h), or 9.4% (+3.5%/–3.3%) of gross gas production. 50% of observed emissions come from large emission sources with persistence-averaged emission rates over 308 kg/h. The fact that a large sample size is required to characterize the heavy tail of the distribution emphasizes the importance of capturing low-probability, high-consequence events through basin-wide surveys when estimating regional O&G methane emissions.
Urban regions are responsible for emitting significant amounts of fossil fuel carbon dioxide (FFCO2), and emissions at the finer, city scales are more uncertain than those aggregated at the global ...scale. Carbon-observing satellites may provide independent top-down emission evaluations and compensate for the sparseness of surface CO2 observing networks in urban areas. Although some previous studies have attempted to derive urban CO2 signals from satellite column-averaged CO2 data (XCO2) using simple statistical measures, less work has been carried out to link upwind emission sources to downwind atmospheric columns using atmospheric models. In addition to Eulerian atmospheric models that have been customized for emission estimates over specific cities, the Lagrangian modeling approach – in particular, the Lagrangian particle dispersion model (LPDM) approach – has the potential to efficiently determine the sensitivity of downwind concentration changes to upwind sources. However, when applying LPDMs to interpret satellite XCO2, several issues have yet to be addressed, including quantifying uncertainties in urban XCO2 signals due to receptor configurations and errors in atmospheric transport and backgroundXCO2.In this study, we present a modified version of the Stochastic Time-Inverted Lagrangian Transport (STILT) model, “X-STILT”, for extracting urbanXCO2 signals from NASA's Orbiting Carbon Observatory 2 (OCO-2)XCO2 data. X-STILT incorporates satellite profiles and provides comprehensive uncertainty estimates of urban XCO2 enhancements on a per sounding basis. Several methods to initialize receptor/particle setups and determine background XCO2 are presented and discussed via sensitivity analyses and comparisons. To illustrate X-STILT's utilities and applications, we examined five OCO-2 overpasses over Riyadh, Saudi Arabia, during a 2-year time period and performed a simple scaling factor-based inverse analysis. As a result, the model is able to reproduce most observedXCO2 enhancements. Error estimates show that the 68 % confidence limit of XCO2 uncertainties due to transport (horizontal wind plus vertical mixing) and emission uncertainties contribute to ∼33 % and ∼20 % of the mean latitudinally integrated urban signals, respectively, over the five overpasses, using meteorological fields from the Global Data Assimilation System (GDAS). In addition, a sizeable mean difference of -0.55 ppm in background derived from a previous study employing simple statistics (regional daily median) leads to a ∼39 % higher mean observed urban signal and a larger posterior scaling factor. Based on our signal estimates and associated error impacts, we foresee X-STILT serving as a tool for interpreting column measurements, estimating urban enhancement signals, and carrying out inverse modeling to improve quantification of urban emissions.
We present high time resolution airborne ethane (C2H6) and methane (CH4) measurements made in March and October 2013 as part of the Barnett Coordinated Campaign over the Barnett Shale formation in ...Texas. Ethane fluxes are quantified using a downwind flight strategy, a first demonstration of this approach for C2H6. Additionally, ethane-to-methane emissions ratios (C2H6:CH4) of point sources were observationally determined from simultaneous airborne C2H6 and CH4 measurements during a survey flight over the source region. Distinct C2H6:CH4 × 100% molar ratios of 0.0%, 1.8%, and 9.6%, indicative of microbial, low-C2H6 fossil, and high-C2H6 fossil sources, respectively, emerged in observations over the emissions source region of the Barnett Shale. Ethane-to-methane correlations were used in conjunction with C2H6 and CH4 fluxes to quantify the fraction of CH4 emissions derived from fossil and microbial sources. On the basis of two analyses, we find 71–85% of the observed methane emissions quantified in the Barnett Shale are derived from fossil sources. The average ethane flux observed from the studied region of the Barnett Shale was 6.6 ± 0.2 × 103 kg hr–1 and consistent across six days in spring and fall of 2013.
Natural gas flaring is a common practice employed in many United States (U.S.) oil and gas regions to dispose of gas associated with oil production. Combustion of predominantly hydrocarbon gas ...results in the production of nitrogen oxides (NOx). Here, we present a large field data set of in situ sampling of real world flares, quantifying flaring NOx production in major U.S. oil production regions: the Bakken, Eagle Ford, and Permian. We find that a single emission factor does not capture the range of the observed NOx emission factors within these regions. For all three regions, the median emission factors fall within the range of four emission factors used by the Texas Commission for Environmental Quality. In the Bakken and Permian, the distribution of emission factors exhibits a heavy tail such that basin-average emission factors are 2–3 times larger than the value employed by the U.S. Environmental Protection Agency. Extrapolation to basin scale emissions using auxiliary satellite assessments of flare volumes indicates that NOx emissions from flares are skewed, with 20%–30% of the flares responsible for 80% of basin-wide flaring NOx emissions. Efforts to reduce flaring volume through alternative gas capture methods would have a larger impact on the NOx oil and gas budget than current inventories indicate.
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