Long-term decline of global atmospheric ethane concentrations and implications for methane

Elevated airborne radioactivity downwind of a Colorado oil refinery

Airborne radioactivity from fossil fuel production systems is poorly characterized, but a recent study showed elevated ambient levels with proximity to oil and gas production wells. Here, we report year-long, high temporal resolution monitoring results of airborne alpha radioactivity from both radon gas and radon progeny attached to particulates immediately northeast of an oil refinery in Commerce City, Colorado, USA, in an environmental justice community of concern. Gas and particle-associated radioactivity contributed nearly evenly to the total alpha radioactivity. Total radioactivity levels of 30-40 Bq m-3 were 2-3 times higher than background levels (~10-15 Bq m-3) when winds were light and southwesterly, suggesting the refinery as the geographic origin. Furthermore, elevated airborne radioactivity tracked most closely with the light hydrocarbon and natural gas tracer ethane. Thus, the data imply natural gas as the radon emission carrier. Our findings are unique and suggest a need for further investigations of radon emissions from oil and gas infrastructure such as natural gas processing plants, compressor stations, petrochemical plants, and oil refineries that process oil and natural gas from unconventional production.Implications: Regulatory agencies currently do not mandate or conduct monitoring of radioactivity releases and public exposure from petroleum industry air emissions. This study reports elevated radioactivity from radon gas and nonvolatile radon decay products attached to particulate matter, at about 2-3 times above background levels in proximity to Colorado's largest oil refinery. Observations were within an environmental justice community of concern that experiences well above-average exposure to many other harmful atmospheric pollutants, suggesting potential adverse health effects from this cumulative exposure. Our findings offer actionable insights for policymakers, industry stakeholders, and affected communities alike.

Near-Infrared Dual-Range Methane Sensor Using the OAIC-HC Mode Based on VMD-SG-Assisted Optical Noise Suppression

Based on tunable diode laser absorption spectroscopy and off-axis integrated cavity output spectroscopy, a dual-range methane hybrid sensor was constructed utilizing the overtone absorption band of CH4 gas molecules at 1653.7 nm. By simultaneously utilizing an off-axis integrated cavity and Herriott cell with an effective absorption path of 11 and 405 m, respectively, the two received photoelectric signals are decomposed into different frequency components by VMD and then reconstructed after SG filtering. Applying the global optimization algorithm DA-ELM to CH4 concentration inversion, the correlation coefficient R2 is as high as 0.9995. Through long-term stability verification, the instrument's standard deviation at 1 ppm is 27 ppb. After Allan-Werle deviation analysis, the sensor's limit of detection is 2.298 ppb at an integration time of 138 s. Using the self-developed sensor, the detection of dual-range trace CH4 gas is achieved, enabling a dynamic detection range of trace CH4 gas ranging from 400 ppb to 1000 ppm. The sensor realizes dual-range methane trace detection and actively controls methane emissions to improve environmental quality while taking into account the safety benefits of reducing production accidents.

Space-based observations of tropospheric ethane map emissions from fossil fuel extraction

Ethane is the most abundant non-methane hydrocarbon in the troposphere, where it impacts ozone and reactive nitrogen and is a key tracer used for partitioning emitted methane between anthropogenic and natural sources. However, quantification has been challenged by sparse observations. Here, we present a satellite-based measurement of tropospheric ethane and demonstrate its utility for fossil-fuel source quantification. An ethane spectral signal is detectable from space in Cross-track Infrared Sounder (CrIS) radiances, revealing ethane signatures associated with fires and fossil fuel production. We use machine-learning to convert these signals to ethane abundances and validate the results against surface observations (R2 = 0.66, mean CrIS/surface ratio: 0.65). The CrIS data show that the Permian Basin in Texas and New Mexico exhibits the largest persistent ethane enhancements on the planet, with regional emissions underestimated by seven-fold. Correcting this underestimate reveals Permian ethane emissions that represent at least 4-7% of the global fossil-fuel ethane source.

U.S. Ethane Emissions and Trends Estimated from Atmospheric Observations

Oil and natural gas (O&G) production and processing activities have changed markedly across the U.S. over the past several years. However, the impacts of these changes on air pollution and greenhouse gas emissions are not clear. In this study, we examine U.S. ethane (C2H6) emissions, which are primarily from O&G activities, during years 2015-2020. We use C2H6 observations made by the NOAA Global Monitoring Laboratory and partner organizations from towers and aircraft and estimate emissions from these observations by using an inverse model. We find that U.S. C2H6 emissions (4.43 ± 0.2 Tg·yr-1) are approximately three times those estimated by the EPA's 2017 National Emissions Inventory (NEI) platform (1.54 Tg·yr-1) and exhibit a very different seasonal cycle. We also find that changes in U.S. C2H6 emissions are decoupled from reported changes in production; emissions increased 6.3 ± 7.6% (0.25 ± 0.31 Tg) between 2015 and 2020 while reported C2H6 production increased by a much larger amount (78%). Our results also suggest an apparent correlation between C2H6 emissions and C2H6 spot prices, where prices could be a proxy for pressure on the infrastructure across the supply chain. Overall, these results provide insight into how U.S. C2H6 emissions are changing over time.

Revising VOC emissions speciation improves the simulation of global background ethane and propane

Non-Methane Volatile Organic Compounds (NMVOCs) generate ozone (O3) when they are oxidized in the presence of oxides of nitrogen, modulate the oxidative capacity of the atmosphere and can lead to the formation of aerosol. Here, we assess the capability of a chemical transport model (GEOS-Chem) to simulate NMVOC concentrations by comparing ethane, propane and higher alkane observations in remote regions from the NOAA Flask Network and the World Meteorological Organization's Global Atmosphere Watch (GAW) network. Using the Community Emissions Data System (CEDS) inventory we find a significant underestimate in the simulated concentration of both ethane (35%) and propane (64%), consistent with previous studies. We run a new simulation where the total mass of anthropogenic NMVOC emitted in a grid box is the same as that used in CEDS, but with the NMVOC speciation derived from regional inventories. For US emissions we use the National Emissions Inventory (NEI), for Europe we use the UK National Atmospheric Emissions Inventory (NAEI), and for China, the Multi-resolution Emission Inventory for China (MEIC). These changes lead to a large increase in the modelled concentrations of ethane, improving the mean model bias from -35% to -4%. Simulated propane also improves (from -64% to -48% mean model bias), but there remains a substantial model underestimate. There were relatively minor changes to other NMVOCs. The low bias in simulated global ethane concentration is essentially removed, resolving one long-term issue in global simulations. Propane concentrations are improved but remain significantly underestimated, suggesting the potential for a missing global propane source. The change in the NMVOC emission speciation results in only minor changes in tropospheric O3 and OH concentrations.

Identifying emission sources of CH4 in East Asia based on in-situ observations of atmospheric δ13C-CH4 and C2H6

Methane (CH4) is the second most important greenhouse gas influenced by human activity. The increase in atmospheric CH4 concentrations contributed ~23 % to the anthropogenic radiative forcing (Saunois et al., 2020). The current anthropogenic CH4 emissions trajectory implies that large emissions reductions are needed to meet the target of the Paris Agreement (Nisbet et al., 2019). For effective regulation of CH4, it is important to identify spatiotemporal emission sources, in particular those from East Asia - one of the largest CH4 emitters. In this study, we present in-situ observations of atmospheric CH4 concentrations (i.e., dry air mole fractions in part per billion (ppb)) and carbon isotopic compositions of CH4 made during 2017-2020 at the Gosan station (GSN, 33.3°N, 126.2°E, 72 m a.s.l) which is representative of regional background conditions in East Asia. The annual growth rate of the observed CH4 baseline concentrations was 11 ± 1 ppb yr-1. The enhanced pollution concentrations of CH4 showed seasonally distinctive correlations with the corresponding δ13C-CH4. The CH4 source isotopic signature for winter derived based on both the Keeling and Miller-Tans approaches was -40.7 ± 3.4 ‰, suggesting dominant thermogenic sources (e.g., coal and/or gas combustion), whereas the source signature for summer was estimated as -54.1 ± 1.2 ‰, which seemed to represent both microbial sources (e.g., rice paddies) and fossil fuel sources of CH4 emissions. Based on the δ13C-CH4 source signatures, we were able to infer that the proportional contribution of microbial sources to CH4 summer emissions was ranges from 45 to 79 %. The finding indicates that microbial sources account for a substantial portion of CH4 summer emissions, consistent with estimates of 74-80 % derived from the observed correlation between CH4 and C2H6, which serves as a complementary tracer for fossil fuel sources.

CH4, C2H6, and CO2 Multi-Gas Sensing Based on Portable Mid-Infrared Spectroscopy and PCA-BP Algorithm

A multi-gas sensing system was developed based on the detection principle of the non-dispersive infrared (NDIR) method, which used a broad-spectra light source, a tunable Fabry-Pérot (FP) filter detector, and a flexible low-loss infrared waveguide as an absorption cell. CH₄, C₂H₆, and CO₂ gases were detected by the system. The concentration of CO₂ could be detected directly, and the concentrations of CH₄ and C₂H₆ were detected using a PCA-BP neural network algorithm because of the interference of CH₄ and C₂H₆. The detection limits were achieved to be 2.59 ppm, 926 ppb, and 114 ppb for CH₄, C₂H₆, and CO₂ with an averaging time of 429 s, 462 s, and 297 s, respectively. The root mean square error of prediction (RMSEP) of CH₄ and C₂H₆ were 10.97 ppm and 2.00 ppm, respectively. The proposed system and method take full advantage of the multi-component gas measurement capability of the mid-infrared broadband source and achieve a compromise between performance and system cost.

Hydrocarbon Tracers Suggest Methane Emissions from Fossil Sources Occur Predominately Before Gas Processing and That Petroleum Plays Are a Significant Source

We use global airborne observations of propane (C₃H₈) and ethane (C₂H₆) from the Atmospheric Tomography (ATom) and HIAPER Pole-to-Pole Observations (HIPPO), as well as U.S.-based aircraft and tower observations by NOAA and from the NCAR FRAPPE campaign as tracers for emissions from oil and gas operations. To simulate global mole fraction fields for these gases, we update the default emissions' configuration of C₃H₈ used by the global chemical transport model, GEOS-Chem v13.0.0, using a scaled C₂H₆ spatial proxy. With the updated emissions, simulations of both C₃H₈ and C₂H₆ using GEOS-Chem are in reasonable agreement with ATom and HIPPO observations, though the updated emission fields underestimate C₃H₈ accumulation in the arctic wintertime, pointing to additional sources of this gas in the high latitudes (e.g., Europe). Using a Bayesian hierarchical model, we estimate global emissions of C₂H₆ and C₃H₈ from fossil fuel production in 2016-2018 to be 13.3 ± 0.7 (95% CI) and 14.7 ± 0.8 (95% CI) Tg/year, respectively. We calculate bottom-up hydrocarbon emission ratios using basin composition measurements weighted by gas production and find their magnitude is higher than expected and is similar to ratios informed by our revised alkane emissions. This suggests that emissions are dominated by pre-processing activities in oil-producing basins.

Vehicle-Deployed Off-Axis Integrated Cavity Output Spectroscopic CH4/C2H6 Sensor System for Mobile Inspection of Natural Gas Leakage

A vehicle-deployed parts-per-billion in volume (ppbv)-level off-axis integrated cavity output spectroscopic (OA-ICOS) CH₄/C₂H₆ sensor system was experimentally presented for mobile inspection of natural gas leakage in urban areas. For the time-division-multiplexing-based dual-gas sensor system, an antivibration 35-cm-long optical cavity with an effective path length of ∼2510 m was fabricated with a high-stability temperature and pressure control design. An Allan deviation analysis yielded a minimum detection limit of 0.2 ppbv for CH₄ detection and 10 ppbv for C₂H₆ detection for a 1 s averaging time. A natural gas leakage source location algorithm was proposed using an improved hybrid Nelder-Mead simplex search method and a particle swarm optimization (NM-PSO) algorithm. For field industrial application, the accuracy of the sensor system and leakage source location algorithm was confirmed through a CH₄/C₂H₆ cylinder leakage experiment on the campus. Furthermore, through natural gas pipeline network inspection measurements in urban areas, three types of leakage sources, including natural gas, biogas, and possible leakage source were respectively located and confirmed using the global positioning system and wind speed and direction measurement system, verifying the reliability and potential application of the vehicle-deployed inspection system for future natural gas pipeline leakage monitoring.

Quantifying Methane and Ozone Precursor Emissions from Oil and Gas Production Regions across the Contiguous US

We present an updated fuel-based oil and gas (FOG) inventory with estimates of nitrogen oxide (NO_x) emissions from oil and natural gas production in the contiguous US (CONUS). We compare the FOG inventory with aircraft-derived ("top-down") emissions for NO_x over footprints that account for ∼25% of US oil and natural gas production. Across CONUS, we find that the bottom-up FOG inventory combined with other anthropogenic emissions is on average within ∼10% of top-down aircraft-derived NO_x emissions. We also find good agreement in the trends of NO_x from drilling- and production-phase activities, as inferred by satellites and in the bottom-up inventory. Leveraging tracer-tracer relationships derived from aircraft observations, methane (CH₄) and non-methane volatile organic compound (NMVOC) emissions have been added to the inventory. Our total CONUS emission estimates for 2015 of oil and natural gas are 0.45 ± 0.14 Tg NO_x/yr, 15.2 ± 3.0 Tg CH₄/yr, and 5.7 ± 1.7 Tg NMVOC/yr. Compared to the US National Emissions Inventory and Greenhouse Gas Inventory, FOG NO_x emissions are ∼40% lower, while inferred CH₄ and NMVOC emissions are up to a factor of ∼2 higher. This suggests that NMVOC/NO_x emissions from oil and gas basins are ∼3 times higher than current estimates and will likely affect how air quality models represent ozone formation downwind of oil and gas fields.

Mapping Urban Methane Sources in Paris, France

Megacities, with their large and complex infrastructures, are significant sources of methane emissions. To develop a simple, low-cost methodology to quantify these globally important methane sources, this study focuses on mobile measurements of methane (CH₄) and its isotopic composition in Paris. Data collected between September 2018 to March 2019 resulted in 17 days of measurements, which provided spatial distribution of street-level methane mixing ratios, source type identification, and emission quantification. Consequently, 90 potential leaks were detected in Paris sorted into three leak categories: natural gas distribution network emissions (63%), sewage network emissions (33%), and emissions from heating furnaces of buildings (4%). The latter category has not previously been reported in urban methane studies. Accounting for the detectable emissions from the ground, the total estimated CH₄ emission rate of Paris was 5000 L/min (190 t/yr), with the largest contribution from gas leaks (56%). This ranks Paris as a city with medium CH₄ emissions. Two areas of clusters were found, where 22% and 56% of the total potential emissions of Paris were observed. Our findings suggest that the natural gas distribution network, the sewage system, and furnaces of buildings are ideal targets for street-level CH₄ emission reduction efforts for Paris.

Intramolecular isotopic evidence for bacterial oxidation of propane in subsurface natural gas reservoirs

Microbial anaerobic oxidation of hydrocarbons is a key process potentially involved in a myriad of geological and biochemical environments yet has remained notoriously difficult to identify and quantify in natural environments. We performed position-specific carbon isotope analysis of propane from cracking and incubation experiments. Anaerobic bacterial oxidation of propane leads to a pronounced and previously unidentified ¹³C enrichment in the central position of propane, which contrasts with the isotope signature associated with the thermogenic process. This distinctive signature allows the detection and quantification of anaerobic oxidation of hydrocarbons in diverse natural gas reservoirs and suggests that this process may be more widespread than previously thought. Position-specific isotope analysis can elucidate the fate of natural gas hydrocarbons and provide insight into a major but previously cryptic process controlling the biogeochemical cycling of globally significant greenhouse gases.

Interpreting contemporary trends in atmospheric methane

Atmospheric methane plays a major role in controlling climate, yet contemporary methane trends (1982-2017) have defied explanation with numerous, often conflicting, hypotheses proposed in the literature. Specifically, atmospheric observations of methane from 1982 to 2017 have exhibited periods of both increasing concentrations (from 1982 to 2000 and from 2007 to 2017) and stabilization (from 2000 to 2007). Explanations for the increases and stabilization have invoked changes in tropical wetlands, livestock, fossil fuels, biomass burning, and the methane sink. Contradictions in these hypotheses arise because our current observational network cannot unambiguously link recent methane variations to specific sources. This raises some fundamental questions: (i) What do we know about sources, sinks, and underlying processes driving observed trends in atmospheric methane? (ii) How will global methane respond to changes in anthropogenic emissions? And (iii), What future observations could help resolve changes in the methane budget? To address these questions, we discuss potential drivers of atmospheric methane abundances over the last four decades in light of various observational constraints as well as process-based knowledge. While uncertainties in the methane budget exist, they should not detract from the potential of methane emissions mitigation strategies. We show that net-zero cost emission reductions can lead to a declining atmospheric burden, but can take three decades to stabilize. Moving forward, we make recommendations for observations to better constrain contemporary trends in atmospheric methane and to provide mitigation support.

Atmospheric implications of large C2-C5 alkane emissions from the U.S. oil and gas industry

Emissions of C₂-C₅ alkanes from the U.S. oil and gas sector have changed rapidly over the last decade. We use a nested GEOS-Chem simulation driven by updated 2011NEI emissions with aircraft, surface and column observations to 1) examine spatial patterns in the emissions and observed atmospheric abundances of C₂-C₅ alkanes over the U.S., and 2) estimate the contribution of emissions from the U.S. oil and gas industry to these patterns. The oil and gas sector in the updated 2011NEI contributes over 80% of the total U.S. emissions of ethane (C₂H₆) and propane (C₃H₈), and emissions of these species are largest in the central U.S. Observed mixing ratios of C₂-C₅ alkanes show enhancements over the central U.S. below 2 km. A nested GEOS-Chem simulation underpredicts observed C₃H₈ mixing ratios in the boundary layer over several U.S. regions and the relative underprediction is not consistent, suggesting C₃H₈ emissions should receive more attention moving forward. Our decision to consider only C₄-C₅ alkane emissions as a single lumped species produces a geographic distribution similar to observations. Due to the increasing importance of oil and gas emissions in the U.S., we recommend continued support of existing long-term measurements of C₂-C₅ alkanes. We suggest additional monitoring of C₂-C₅ alkanes downwind of northeastern Colorado, Wyoming and western North Dakota to capture changes in these regions. The atmospheric chemistry modeling community should also evaluate whether chemical mechanisms that lump larger alkanes are sufficient to understand air quality issues in regions with large emissions of these species.

Large changes in biomass burning over the last millennium inferred from paleoatmospheric ethane in polar ice cores

Biomass burning drives changes in greenhouse gases, climate-forcing aerosols, and global atmospheric chemistry. There is controversy about the magnitude and timing of changes in biomass burning emissions on millennial time scales from preindustrial to present and about the relative importance of climate change and human activities as the underlying cause. Biomass burning is one of two notable sources of ethane in the preindustrial atmosphere. Here, we present ice core ethane measurements from Antarctica and Greenland that contain information about changes in biomass burning emissions since 1000 CE (Common Era). The biomass burning emissions of ethane during the Medieval Period (1000-1500 CE) were higher than present day and declined sharply to a minimum during the cooler Little Ice Age (1600-1800 CE). Assuming that preindustrial atmospheric reactivity and transport were the same as in the modern atmosphere, we estimate that biomass burning emissions decreased by 30 to 45% from the Medieval Period to the Little Ice Age. The timing and magnitude of this decline in biomass burning emissions is consistent with that inferred from ice core methane stable carbon isotope ratios but inconsistent with histories based on sedimentary charcoal and ice core carbon monoxide measurements. This study demonstrates that biomass burning emissions have exceeded modern levels in the past and may be highly sensitive to changes in climate.

Street-level emissions of methane and nitrous oxide from the wastewater collection system in Cincinnati, Ohio

Recent studies have indicated that urban streets can be hotspots for emissions of methane (CH₄) from leaky natural gas lines, particularly in cities with older natural gas distribution systems. The objective of the current study was to determine whether leaking sewer pipes could also be a source of street-level CH₄ as well as nitrous oxide (N₂O) in Cincinnati, Ohio, a city with a relatively new gas pipeline network. To do this, we measured the carbon (δ¹³C) and hydrogen (δ²H) stable isotopic composition of CH₄ to distinguish between biogenic CH₄ from sewer gas and thermogenic CH₄ from leaking natural gas pipelines and measured CH₄ and N₂O flux rates and concentrations at sites from a previous study of street-level CH₄ enhancements (77 out of 104 sites) as well as additional sites found through surveying sewer grates and utility manholes (27 out of 104 sites). The average isotopic signatures for δ¹³C-CH₄ and δ²H-CH₄ were -48.5‰ ± 6.0‰ and -302‰ ± 142‰. The measured flux rates ranged from 0.0 to 282.5 mg CH₄ day^-1 and 0.0-14.1 mg N₂O day^-1 (n = 43). The average CH₄ and N₂O concentrations measured in our study were 4.0 ± 7.6 ppm and 392 ± 158 ppb, respectively (n = 104). 72% of sites where fluxes were measured were a source of biogenic CH₄. Overall, 47% of the sampled sites had biogenic CH₄, while only 13% of our sites had solely thermogenic CH₄. The other sites were either a source of both biogenic and thermogenic CH₄ (13%), and a relatively large portion of sites had an unresolved source (29%). Overall, this survey of emissions across a large urban area indicates that production and emission of biogenic CH₄ and N₂O is considerable, although CH₄ fluxes are lower than those reported for cities with leaky natural gas distribution systems.

Non-methane hydrocarbons in a controlled ecological life support system

Non-methane hydrocarbons (NMHCs) are vital to people's health and plants' growth, especially inside a controlled ecological life support system (CELSS) built for long-term space explorations. In this study, we measured 54 kinds of NMHCs to study their changing trends in concentration levels during a 4-person-180-day integrated experiment inside a CELSS with four cabins for plants growing and other two cabins for human daily activities and resources management. During the experiment, the total mixing ratio of measured NMHCs was 423 ± 283 ppbv at the first day and it approached 2961 ± 323 ppbv ultimately. Ethane and propane were the most abundant alkanes and their mixing ratios kept growing from 27.5 ± 19.4 and 31.0 ± 33.6 ppbv to 2423 ± 449 ppbv and 290 ± 10 ppbv in the end. For alkenes, ethylene and isoprene presented continuously fluctuating states during the experimental period with average mixing ratios of 30.4 ± 19.3 ppbv, 7.4 ± 5.8 ppbv. For aromatic hydrocarbons, the total mixing ratios of benzene, toluene, ethylbenzene and xylenes declined from 48.0 ± 44 ppbv initially to 3.8 ± 1.1 ppbv ultimately. Biomass burning, sewage treatment, construction materials and plants all contributed to NMHCs inside CELSS. In conclusion, the results demonstrate the changing trends of NMHCs in a long-term closed ecological environment's atmosphere which provides valuable information for both the atmosphere management of CELSS and the exploration of interactions between humans and the total environment.

Methane, Black Carbon, and Ethane Emissions from Natural Gas Flares in the Bakken Shale, North Dakota

Incomplete combustion during flaring can lead to production of black carbon (BC) and loss of methane and other pollutants to the atmosphere, impacting climate and air quality. However, few studies have measured flare efficiency in a real-world setting. We use airborne data of plume samples from 37 unique flares in the Bakken region of North Dakota in May 2014 to calculate emission factors for BC, methane, ethane, and combustion efficiency for methane and ethane. We find no clear relationship between emission factors and aircraft-level wind speed or between methane and BC emission factors. Observed median combustion efficiencies for methane and ethane are close to expected values for typical flares according to the US EPA (98%). However, we find that the efficiency distribution is skewed, exhibiting log-normal behavior. This suggests incomplete combustion from flares contributes almost 1/5 of the total field emissions of methane and ethane measured in the Bakken shale, more than double the expected value if 98% efficiency was representative. BC emission factors also have a skewed distribution, but we find lower emission values than previous studies. The direct observation for the first time of a heavy-tail emissions distribution from flares suggests the need to consider skewed distributions when assessing flare impacts globally.

Ambiguity in the causes for decadal trends in atmospheric methane and hydroxyl

Methane is the second strongest anthropogenic greenhouse gas and its atmospheric burden has more than doubled since 1850. Methane concentrations stabilized in the early 2000s and began increasing again in 2007. Neither the stabilization nor the recent growth are well understood, as evidenced by multiple competing hypotheses in recent literature. Here we use a multispecies two-box model inversion to jointly constrain 36 y of methane sources and sinks, using ground-based measurements of methane, methyl chloroform, and the C¹³/C¹² ratio in atmospheric methane (δ¹³CH₄) from 1983 through 2015. We find that the problem, as currently formulated, is underdetermined and solutions obtained in previous work are strongly dependent on prior assumptions. Based on our analysis, the mathematically most likely explanation for the renewed growth in atmospheric methane, counterintuitively, involves a 25-Tg/y decrease in methane emissions from 2003 to 2016 that is offset by a 7% decrease in global mean hydroxyl (OH) concentrations, the primary sink for atmospheric methane, over the same period. However, we are still able to fit the observations if we assume that OH concentrations are time invariant (as much of the previous work has assumed) and we then find solutions that are largely consistent with other proposed hypotheses for the renewed growth of atmospheric methane since 2007. We conclude that the current surface observing system does not allow unambiguous attribution of the decadal trends in methane without robust constraints on OH variability, which currently rely purely on methyl chloroform data and its uncertain emissions estimates.

Atmospheric oxygen regulation at low Proterozoic levels by incomplete oxidative weathering of sedimentary organic carbon

It is unclear why atmospheric oxygen remained trapped at low levels for more than 1.5 billion years following the Paleoproterozoic Great Oxidation Event. Here, we use models for erosion, weathering and biogeochemical cycling to show that this can be explained by the tectonic recycling of previously accumulated sedimentary organic carbon, combined with the oxygen sensitivity of oxidative weathering. Our results indicate a strong negative feedback regime when atmospheric oxygen concentration is of order pO2∼0.1 PAL (present atmospheric level), but that stability is lost at pO2<0.01 PAL. Within these limits, the carbonate carbon isotope (δ13C) record becomes insensitive to changes in organic carbon burial rate, due to counterbalancing changes in the weathering of isotopically light organic carbon. This can explain the lack of secular trend in the Precambrian δ13C record, and reopens the possibility that increased biological productivity and resultant organic carbon burial drove the Great Oxidation Event.

Formaldehyde column density measurements as a suitable pathway to estimate near-surface ozone tendencies from space

In support of future satellite missions that aim to address the current shortcomings in measuring air quality from space, NASA's Deriving Information on Surface Conditions from Column and Vertically Resolved Observations Relevant to Air Quality (DISCOVER-AQ) field campaign was designed to enable exploration of relationships between column measurements of trace species relevant to air quality at high spatial and temporal resolution. In the DISCOVER-AQ data set, a modest correlation (r 2 = 0.45) between ozone (O3) and formaldehyde (CH2O) column densities was observed. Further analysis revealed regional variability in the O3-CH2O relationship, with Maryland having a strong relationship when data were viewed temporally and Houston having a strong relationship when data were viewed spatially. These differences in regional behavior are attributed to differences in volatile organic compound (VOC) emissions. In Maryland, biogenic VOCs were responsible for ~28% of CH2O formation within the boundary layer column, causing CH2O to, in general, increase monotonically throughout the day. In Houston, persistent anthropogenic emissions dominated the local hydrocarbon environment, and no discernable diurnal trend in CH2O was observed. Box model simulations suggested that ambient CH2O mixing ratios have a weak diurnal trend (±20% throughout the day) due to photochemical effects, and that larger diurnal trends are associated with changes in hydrocarbon precursors. Finally, mathematical relationships were developed from first principles and were able to replicate the different behaviors seen in Maryland and Houston. While studies would be necessary to validate these results and determine the regional applicability of the O3-CH2O relationship, the results presented here provide compelling insight into the ability of future satellite missions to aid in monitoring near-surface air quality.

Atmospheric methane isotopic record favors fossil sources flat in 1980s and 1990s with recent increase

Observations of atmospheric methane (CH4) since the late 1970s and measurements of CH4 trapped in ice and snow reveal a meteoric rise in concentration during much of the twentieth century. Since 1750, levels of atmospheric CH4 have more than doubled to current globally averaged concentration near 1,800 ppb. During the late 1980s and 1990s, the CH4 growth rate slowed substantially and was near or at zero between 1999 and 2006. There is no scientific consensus on the drivers of this slowdown. Here, we report measurements of the stable isotopic composition of atmospheric CH4 ((13)C/(12)C and D/H) from a rare air archive dating from 1977 to 1998. Together with more modern records of isotopic atmospheric CH4, we performed a time-dependent retrieval of methane fluxes spanning 25 y (1984-2009) using a 3D chemical transport model. This inversion results in a 24 [18, 27] Tg y(-1) CH4 increase in fugitive fossil fuel emissions since 1984 with most of this growth occurring after year 2000. This result is consistent with some bottom-up emissions inventories but not with recent estimates based on atmospheric ethane. In fact, when forced with decreasing emissions from fossil fuel sources our inversion estimates unreasonably high emissions in other sources. Further, the inversion estimates a decrease in biomass-burning emissions that could explain falling ethane abundance. A range of sensitivity tests suggests that these results are robust.

A 21st-century shift from fossil-fuel to biogenic methane emissions indicated by ¹³CH₄

Between 1999 and 2006, a plateau interrupted the otherwise continuous increase of atmospheric methane concentration [CH4] since preindustrial times. Causes could be sink variability or a temporary reduction in industrial or climate-sensitive sources. We reconstructed the global history of [CH4] and its stable carbon isotopes from ice cores, archived air, and a global network of monitoring stations. A box-model analysis suggests that diminishing thermogenic emissions, probably from the fossil-fuel industry, and/or variations in the hydroxyl CH4 sink caused the [CH4] plateau. Thermogenic emissions did not resume to cause the renewed [CH4] rise after 2006, which contradicts emission inventories. Post-2006 source increases are predominantly biogenic, outside the Arctic, and arguably more consistent with agriculture than wetlands. If so, mitigating CH4 emissions must be balanced with the need for food production.

Airborne Ethane Observations in the Barnett Shale: Quantification of Ethane Flux and Attribution of Methane 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 × 10(3) kg hr(-1) and consistent across six days in spring and fall of 2013.

The contribution of trees to ecosystem methane emissions in a temperate forested wetland

Wetland-adapted trees are known to transport soil-produced methane (CH₄ ), an important greenhouse gas to the atmosphere, yet seasonal variations and controls on the magnitude of tree-mediated CH₄ emissions remain unknown for mature forests. We examined the spatial and temporal variability in stem CH₄ emissions in situ and their controls in two wetland-adapted tree species (Alnus glutinosa and Betula pubescens) located in a temperate forested wetland. Soil and herbaceous plant-mediated CH₄ emissions from hollows and hummocks also were measured, thus enabling an estimate of contributions from each pathway to total ecosystem flux. Stem CH₄ emissions varied significantly between the two tree species, with Alnus glutinosa displaying minimal seasonal variations, while substantial seasonal variations were observed in Betula pubescens. Trees from each species emitted similar quantities of CH₄ from their stems regardless of whether they were situated in hollows or hummocks. Soil temperature and pore-water CH₄ concentrations best explained annual variability in stem emissions, while wood-specific density and pore-water CH₄ concentrations best accounted for between-species variations in stem CH₄ emission. Our study demonstrates that tree-mediated CH₄ emissions contribute up to 27% of seasonal ecosystem CH₄ flux in temperate forested wetland, with the largest relative contributions occurring in spring and winter. Tree-mediated CH₄ emissions currently are not included in trace gas budgets of forested wetland. Further work is required to quantify and integrate this transport pathway into CH₄ inventories and process-based models.

Emissions of methane from offshore oil and gas platforms in Southeast Asia

Methane is a substantial contributor to climate change. It also contributes to maintaining the background levels of tropospheric ozone. Among a variety of CH₄ sources, current estimates suggest that CH₄ emissions from oil and gas processes account for approximately 20% of worldwide anthropogenic emissions. Here, we report on observational evidence of CH₄ emissions from offshore oil and gas platforms in Southeast Asia, detected by a highly time-resolved spectroscopic monitoring technique deployed onboard cargo ships of opportunity. We often encountered CH₄ plumes originating from operational flaring/venting and fugitive emissions off the coast of the Malay Peninsula and Borneo. Using night-light imagery from satellites, we discovered more offshore platforms in this region than are accounted for in the emission inventory. Our results demonstrate that current knowledge regarding CH₄ emissions from offshore platforms in Southeast Asia has considerable uncertainty and therefore, emission inventories used for modeling and assessment need to be re-examined.

Demonstration of an ethane spectrometer for methane source identification

Methane is an important greenhouse gas and tropospheric ozone precursor. Simultaneous observation of ethane with methane can help identify specific methane source types. Aerodyne Ethane-Mini spectrometers, employing recently available mid-infrared distributed feedback tunable diode lasers (DFB-TDL), provide 1 s ethane measurements with sub-ppb precision. In this work, an Ethane-Mini spectrometer has been integrated into two mobile sampling platforms, a ground vehicle and a small airplane, and used to measure ethane/methane enhancement ratios downwind of methane sources. Methane emissions with precisely known sources are shown to have ethane/methane enhancement ratios that differ greatly depending on the source type. Large differences between biogenic and thermogenic sources are observed. Variation within thermogenic sources are detected and tabulated. Methane emitters are classified by their expected ethane content. Categories include the following: biogenic (<0.2%), dry gas (1-6%), wet gas (>6%), pipeline grade natural gas (<15%), and processed natural gas liquids (>30%). Regional scale observations in the Dallas/Fort Worth area of Texas show two distinct ethane/methane enhancement ratios bridged by a transitional region. These results demonstrate the usefulness of continuous and fast ethane measurements in experimental studies of methane emissions, particularly in the oil and natural gas sector.

Natural gas fugitive emissions rates constrained by global atmospheric methane and ethane

The amount of methane emissions released by the natural gas (NG) industry is a critical and uncertain value for various industry and policy decisions, such as for determining the climate implications of using NG over coal. Previous studies have estimated fugitive emissions rates (FER)--the fraction of produced NG (mainly methane and ethane) escaped to the atmosphere--between 1 and 9%. Most of these studies rely on few and outdated measurements, and some may represent only temporal/regional NG industry snapshots. This study estimates NG industry representative FER using global atmospheric methane and ethane measurements over three decades, and literature ranges of (i) tracer gas atmospheric lifetimes, (ii) non-NG source estimates, and (iii) fossil fuel fugitive gas hydrocarbon compositions. The modeling suggests an upper bound global average FER of 5% during 2006-2011, and a most likely FER of 2-4% since 2000, trending downward. These results do not account for highly uncertain natural hydrocarbon seepage, which could lower the FER. Further emissions reductions by the NG industry may be needed to ensure climate benefits over coal during the next few decades.

Air quality in Mecca and surrounding holy places in Saudi Arabia during Hajj: initial survey

The Arabian Peninsula experiences severe air pollution, the extent and sources of which are poorly documented. Each year in Saudi Arabia this situation is intensified during Hajj, the Holy Pilgrimage of Islam that draws millions of pilgrims to Mecca. An initial study of air quality in Mecca and surrounding holy sites during the 2012 Hajj (October 24-27) revealed strongly elevated levels of the combustion tracer carbon monoxide (CO, up to 57 ppmv) and volatile organic compounds (VOCs) along the pilgrimage route-especially in the tunnels of Mecca-that are a concern for human health. The most abundant VOC was the gasoline evaporation tracer i-pentane, which exceeded 1200 ppbv in the tunnels. Even though VOC concentrations were generally lower during a follow-up non-Hajj sampling period (April 2013), many were still comparable to other large cities suffering from poor air quality. Major VOC sources during the 2012 Hajj study included vehicular exhaust, gasoline evaporation, liquefied petroleum gas, and air conditioners. Of the measured compounds, reactive alkenes and CO showed the strongest potential to form ground-level ozone. Because the number of pilgrims is expected to increase in the future, we present emission reduction strategies to target both combustive and evaporative fossil fuel sources.

Validation and application of cavity-enhanced, near-infrared tunable diode laser absorption spectrometry for measurements of methane carbon isotopes at ambient concentrations

Methane is an effective greenhouse gas but has a short residence time in the atmosphere, and therefore, reductions in emissions can alleviate its greenhouse gas warming effect within a decadal time frame. Continuous and high temporal resolution measurements of methane concentrations and carbon isotopic ratios (δ(13)CH4) can inform on mechanisms of formation, provide constraints on emissions sources, and guide future mitigation efforts. We describe the development, validation, and deployment of a cavity-enhanced, near-infrared tunable diode laser absorption spectrometry system capable of quantifying δ(13)CH4 at ambient methane concentrations. Laboratory validation and testing show that the instrument is capable of operating over a wide dynamic range of methane concentration and provides a measurement precision for δ(13)CH4 of better than ± 0.5 ‰ (1σ) over 1000 s of data averaging at ambient methane concentrations. The analyzer is accurate to better than ± 0.5 ‰, as demonstrated by measurements of characterized methane/air samples with minimal dependence (<1 ‰) of measured carbon isotope ratio on methane concentration. Deployment of the instrument at a marsh over multiple days demonstrated how methane fluxes varied by an order of magnitude over 2 day deployment periods, and showed a 17 ‰ variability in δ(13)CH4 of the emitted methane during the growing season.