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Methane Emissions from Offshore Oil and Gas Platforms in the Gulf of Mexico

  • Tara I. Yacovitch*
    Tara I. Yacovitch
    Aerodyne Research, Inc., 45 Manning Road, Billerica, Massachusetts 01821, United States
    *E-mail: [email protected]
    More by Tara I. Yacovitch
    Orcidhttp://orcid.org/0000-0002-9604-116X
  • , 
  • Conner Daube
    Conner Daube
    Aerodyne Research, Inc., 45 Manning Road, Billerica, Massachusetts 01821, United States
    More by Conner Daube
  • , and 
  • Scott C. Herndon
    Scott C. Herndon
    Aerodyne Research, Inc., 45 Manning Road, Billerica, Massachusetts 01821, United States
    More by Scott C. Herndon
Cite this: Environ. Sci. Technol. 2020, 54, 6, 3530–3538
Publication Date (Web):March 9, 2020
https://doi.org/10.1021/acs.est.9b07148

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Abstract

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Shipboard measurements of offshore oil and gas facilities were conducted in the Gulf of Mexico in February 2018. Species measured at 1 s include methane, ethane, carbon-13 (13C) and deuterium (D) isotopes of methane, and several combustion tracers. Significant variability in the emission composition is observed between individual sites, with typical ethane/methane ratios around 5.3% and 13C and D methane isotopic compositions around −40 and −240‰, respectively. Offshore plumes were spatially narrower than expectations of the plume width based on terrestrial atmospheric stability classes; a modified Gaussian dispersion methodology using empirically measured horizontal plume widths was used to estimate the emission rates. A total of 103 sites were studied, including shallow and deepwater offshore platforms and drillships. Methane emission rates range from 0 to 190 kg/h with 95% confidence limits estimated at a factor of 10. The observed distribution is skewed with the top two emitters accounting for 20% of the total methane emissions of all sampled sites. Despite the greater throughput of the deepwater facilities, they had moderate emission rates compared to shallow-water sites. Analysis of background ethane enhancements also suggests a source region in shallow waters. A complete 1 s measurement database is published for use in future studies of offshore dispersion.

Introduction

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Offshore oil and gas methane emissions have not received the same attention from the measurement community as on-shore assets. The U.S. Energy Information Administration has projected that oil production in the Gulf of Mexico (federal areas) would account for approximately 16% of the total U.S. production in 2018 and 2019. (1) Globally, offshore oil production accounted for around 30% of the overall production in 2015 and is increasing. (2)
Offshore production platforms are structures used to drill and service wells on the ocean floor. They may have compressors and some treatment equipment like separators, and often have equipment for use in workovers. A large variety of platform types are in operation, (3) and equipment is also placed underwater. (4,5) This includes wells marked by valve manifolds and flowlines (pipelines) connecting multiple wells to gathering manifolds. Umbilicals from surface platforms are needed to power, monitor, and control subsea equipment, and can include fluid lines for chemical injections. Underwater gathering flowlines ultimately converge at a riser, a section of pipeline that brings the produced fluids up to a surface facility. Offshore platforms are one example of such a surface facility, but extracted hydrocarbons (or “product”) can also be piped directly to the shore if wells are close enough, or up a riser to a floating production storage and offloading vessel (FPSO), which acts as a portable processing and storage plant. The resulting separated streams of product are then reinjected into subsea transmission pipelines or transported to the shore via a tanker.
Offshore platforms are thus not analogous to well pads on land. Their purpose and function are extremely varied, and they potentially occupy multiple spots in the offshore oil and gas supply chain. Historically, shallow-water reserves were easier to exploit, and these areas tend to have numerous fixed platforms and less equipment on the seabed. Deep-water plays are much more difficult and expensive to develop. They will leverage modern equipment and techniques, maximize the use of subsea equipment, and minimize the number of floating and semisubmersible platforms required. In 2015, deepwater production, at depths ≥125 m, accounted for 36% of the total US offshore production. (6)
A limited number of measurement campaigns have focused on the offshore sector. Two recent ship-based campaigns investigated North Sea platforms. Hensen et al. visited over 50 platforms and included tracer release experiments at a subset of sites. (7) Riddick et al. performed a dispersion analysis of eight platforms. (8) Measurements along shipping routes in the offshore regions of Southeast Asia yielded methane emission rates for 14 methane plumes. (9) Aircraft measurements in the Norwegian Sea have quantified emissions of NOx and aerosols, (10) and flights over the North Sea observed a broad methane plume, partially attributed to offshore activity. (11) Other emission estimates have used component-based measurements (12) or inventory analysis. (13) A large number of studies have examined hydrocarbon emissions from offshore emergency events: the Deepwater Horizon oil spill in the Gulf of Mexico in 2010 (14−17) and the Elgin platform gas leak in the UK North Sea in 2012. (18) The Deepwater Horizon spill did not release significant amounts of methane into the atmosphere due to dissolution in the ocean. The Elgin gas leak, on the other hand, occurred at the platform itself, with substantial methane emission rates.
This paper presents shipboard measurements downwind of offshore oil and gas platforms in the Gulf of Mexico. Measurements were conducted off the coasts of Texas and Louisiana between February 12, 2018 and February 22, 2018. In situ measurements of methane, ethane, the isotopes of methane (δ13C and δD), and several other combustion tracers were made. Gaussian inversion methods were used to estimate the methane emission rates at 103 offshore sites.

Materials and Methods

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Equipment

A suite of meteorological and analytical equipment was installed aboard the Research Vessel Trident (RV Trident), owned and operated by the Texas A&M University at Galveston. Shipboard measurements were conducted during a two-week period between February 12, 2018 and February 22, 2018. The RV Trident was staffed 24 h a day by captains, deckhands, and scientists, and was capable of outings lasting as long as 36 h. The vessel was harbored in Galveston, Texas, and in Grand Isle, Louisiana, during the campaign. Measurements were conducted when weather conditions were forecast to be favorable for both sampling (winds >2 m/s) and sailing (<6 feet seas, at the captain’s discretion). All told, approximately 120 h were spent at sea during the 11-day and night measurement period, a duty cycle close to 50%.
Three tunable infrared laser direct absorption spectroscopy (TILDAS) trace-gas monitors from Aerodyne Research, Inc. (19) were used for performing gas phase measurements of ambient air. A dual-laser TILDAS instrument equipped with a custom 400 m multipass absorption cell measured methane (CH4) and its carbon and deuterium isotopes. Two single-laser mini-TILDAS instruments measured: (1) nitrous oxide, carbon monoxide, and water (N2O, CO, and H2O) and (2) ethane (C2H6) (20) and redundant CH4 and H2O. The TILDAS instruments were operated with cell pressures between 30 and 50 Torr. A LI-COR 7000 analyzer was used to measure carbon dioxide (CO2) and an Aerodyne cavity-attenuated phase-shift spectrometer (CAPS-NO2) for measuring nitrogen dioxide (NO2).
A 10 m-long 1/2″ Teflon inlet line was connected to the gas phase instrumentation in the vessel’s laboratory room and mounted partway up the vessel’s mast. The final height of the inlet tip was 10 m above the water. 7.8 standard liters per minute of air was drawn down the inlet through a particle filter and through the instruments with a scroll pump (Agilent TriScroll TM 600) for continuous sampling of outdoor air at ambient humidity levels. Every 15 min, clean air (ultra-zero air, hydrocarbon-free) was delivered in excess of the intake flow. These gas additions served to spectroscopically background the TILDAS instruments and to check zero values for the other instruments.
Meteorological conditions and global positioning system (GPS)-based positions were measured near the base of the mast. A Hemisphere (V103) GPS Compass provided the GPS position, vessel heading, bearing, pitch, yaw, and roll. An RM Young 3D Anemometer (model 81000RE) was used to measure the wind at a 10 Hz data rate. The raw measured wind was corrected for vessel speed and orientation (including pitch, roll, and yaw) in near-real time using data from the GPS compass. The vessel itself was also equipped with navigational equipment, including access to NOAA navigational maps and radar. An ARISense monitor was mounted onto a railing near the base of the mast, which measured particulate matter size; insolation; and CO, NO, NO2, and ozone mixing ratios every 10 s.
Real-time data were logged and displayed on an analysis computer in the RV Trident’s laboratory space, and could be accessed remotely on a laptop from the vessel’s bridge. This live data access from the bridge was crucial in allowing collaboration between scientists and the ship’s captain on the downwind sampling strategy and on decisions to measure sites opportunistically along a pre-planned route. Notes were recorded on this same analysis computer, with particular effort taken to note the GPS coordinates of the measured sites and to flag periods of data showing enhancements above background (plumes). The research vessel exhaust was identified based on the wind direction and high concentrations of combustion species (CO2, CO, and NO2).
A 1 s measurement dataset is available for download. Data are presented in a comma-separated format, and are accompanied by a readme file (see Section S1 of the Supporting Information and 1 s Dataset supplemental file).
Calibrations for the mixing ratios of CH4, C2H6, N2O, and CO were accomplished by precision blending ppm-level standards with a diluent, ultra-zero air. These calibrations were repeated two to three times over the course of the campaign (see Section S11). The instrument performance was assessed for CO2 and NO2 before and after the campaign. The methane isotope instrument was operated on an automated calibration schedule (7 min every 3 h) using dynamic dilutions of four high-concentration isotope standards mixed with ambient air. Details of this isotope calibration strategy are presented in Section S12. Keeling analysis (21,22) was used to separate the signature of the source from that of the ambient background (see Section S13).

Campaign Design

Offshore production data from well locations were analyzed prior to the measurement campaign in an effort to select sites representative of the offshore sector in the Gulf of Mexico (Section S3). Wellbore production data between July and November 2017 was used. (23) Examination of these data shows a boundary between relatively low-producing sites closer to the shore and high-producing sites in deepwaters (Figure 1, dotted line): 19% of wells are south of the divide, but account for 67% of gas and 88% of liquid production (Table S2). Platform location data available from the Bureau of Ocean Energy Management (BOEM, see Figure S2) (24) reveal that only 3% of platforms are south of the divide.

Figure 1

Figure 1. Vessel path (solid black line) over the course of the campaign, showing measured sites (squares) colored by site choice type. Offshore well daily gas production (circles) for the period June–February 2017 (23) is plotted in thousand cubic feet per day (Mcfd). A dividing line between lower and higher production magnitudes is shown (dashed line). The shoreline map was obtained from NOAA. (25)

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A set of waypoints were chosen (Figure S3) to optimize the spatial coverage across the Gulf and across the two shallow/deep zones (dashed line in Figure 1). The locations of offshore platforms differ from the well locations (Figure 1, circle markers), and the density of platforms can be much lower, particularly in deeper waters. Platforms sampled were chosen based on their proximity to the waypoints and are shown in green in Figure 1. Additional fortuitous sites (blue) were added during the voyage based on their proximity to the transit route. A handful more sites were identified during the post-campaign analysis (red, generally along transit routes). The decision to add a site did not depend on whether or not emissions were observed.
Logistical considerations dictated deviations from the original sampling plan. Inclement weather (high seas, storms) prevented certain sorties and required that the return trip from Grand Isle to Galveston be close to shore for safety. Rarely, mechanical problems or insufficient wind speeds for sampling cut a sortie short.
The preferred sampling strategy (Section S4) at planned sites was to navigate a zig-zag path downwind of the chosen site, intercepting the emission plume at a range of distances (typically 1–10 km). In cases where potential interfering sites were present on maps and/or radar, an upwind pass of the selected site was also done. Congested areas with many nearby platforms, the presence of other nearby vessels, and large closest point of approach restrictions for deepwater drillships required simplified sampling schemes. Most of the fortuitous sites and all of the post-analysis sites were measured with a single pass, and occasionally repeated on different days (sites 114, 127, and 128; 2 days each).

Emission Estimates

Methane emissions from offshore sites were estimated as in previous studies (20,26) using the standard Gaussian dispersion equation (27) (Section S5). Horizontal (σy) and vertical (σz) dispersion parameters describe the extent of spread a plume undergoes as it is transported downwind. These parameters are typically looked up based on the Pasquill–Gifford stability classes. Traditionally, meteorological conditions such as wind speed, cloud cover, ceiling height, and insolation are used to define the stability class; the corresponding σy and σz are then determined. The standard deviation of the wind can also be used to look up the stability class, as in the OTM-33A dispersion method. (28) In the dataset shown here, both of these methodologies yield stability classes that are far too unstable (A–D) to match the width of the measured plumes. Indeed, studies of offshore dispersion find that very stable conditions are common. (29,30) Hanna et al. supplemented the standard set of Pasquill–Gifford stability classes (A–F) with an additional extra-stable class, G, in their published offshore model. (30) However, Hanna’s calculation of this extra-stable stability class requires knowledge of meteorological conditions that are not known for this campaign, such as the vertical air temperature gradient in the lowest 100 m. The published model itself was also designed with stationary hourly shore-based measurements in mind.
In order to account for the stable conditions encountered at sea, an empirical approach to estimate the stability class and dispersion parameters was developed. The research vessel transects the emission plume, directly measuring the degree of horizontal dispersion, σy. The inlet is at a fixed vertical height, and so the vertical dispersion σz is not measured. However, with a known source location, the downwind distance y is known. This allows for a reverse lookup of the stability class that comes closest to reproducing the observed horizontal σy width. This empirical best-guess stability class is then used to determine the vertical dispersion, σz. In the final simulated result, a combination of the best-guess σz and the measured σy is used. Section S6 demonstrates Gaussian simulations using this methodology. Previous aircraft campaigns have also used the measured dispersion parameters for offshore dispersion calculations, though with the ability to measure both σz and σy. (18)
The average wind bearing measurement during a transect did not always accurately describe the observed direction of transport of the plume. This discrepancy is likely due to the breakdown of the constant wind direction assumption: plume meandering results in a difference between the average wind direction measured during the 2–3 min transect compared to the true transport direction in the 2–20 min timeframe between the period when emissions leave the platform and are intercepted by the vessel. In these cases, the wind bearing is allowed to float in order to reproduce the observed direction of plume transport.
Gaussian dispersion calculations were run on 290 individual time periods or “plumes” in order to produce the emission results presented here. The analysis strategy relies on quality metrics like the plume width and the R2 between the simulated and measured time traces, as well as secondary tracers like ethane, CO, and CO2. It is outlined in the flow chart in Section S7. Uncertainties in site assignment are discussed in the Results and Discussion section.
There is a high degree of uncertainty in these simulated emission magnitudes, stemming from a variety of factors. Land-based tracer-release studies of the Gaussian dispersion methodology (without using a measured σy) applied to mobile data found that the method itself has 95% confidence intervals of [0.33x, 3.34x] for an emission of magnitude x, (20) that is, approximately a factor of 3.17. Sensitivity analysis studies suggest that even larger errors are possible. (31) This factor of 3.17 errors does not include any uncertainty in either the release height or the release location, both of which are present in this dataset. Furthermore, no field testing of the measured σy methodology used here has been done at sea or otherwise. Clearly, field studies aimed at collecting tracer-release data for method validation are warranted. For this reason, factor-of-10 errors at 95% confidence (i.e., [0.1x, 10x]) are asserted on all dispersion results. For those sites with multiple downwind transects, the standard deviation of Gaussian quantifications is also reported (Section S2 and Site Averages supplemental file), as a means to assess any changes in site activity. The small standard deviations typically found in this study mean that uncertainty from plume-to-plume variability is unimportant when compared to the potentially large errors/biases stemming from the methodology itself.
In cases where no downwind methane enhancements were observed, the site was assigned a nominal emission magnitude of 0 kg/h and marked as “null” in the dataset. Since factor-of-10 errors on 0 are still zero, the error on these null sites is determined by running a Gaussian dispersion simulation on the baseline methane trace. The stability class is manually selected based on the results from previous or subsequent sites (usually, class F or E). The error in these cases is a function of the variability of the methane baseline downwind of the facility.

Results and Discussion

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Offshore Platforms

During this two-week campaign, 103 sites were measured. All except for 5 sites consist of a single platform structure or, rarely, a drillship. Five sites included multiple platform structures. The 103 sites include 62 sites that were targeted based on the original measurement plan and 41 fortuitous or opportunistic sites, when platforms were located on the way to the next waypoint. Thirty-seven sites had three or more replicate plume measurements and 46 had only one. Of the 103 sites, 75 had observed CH4 enhancements above background accompanied by C2H6; the remaining 28 had no such observed enhancements, and are termed “null” in the dataset. One of these “null” sites had observed C2H6 enhancements with no associated CH4.
There is a varying degree of uncertainty in the assignment of CH4 emissions (or lack thereof) to a given site location. The dataset flags 41 sites with location uncertainty. Location uncertainty is discussed in greater detail in subsequent paragraphs.
A dataset of site-average-estimated CH4 emission rates; site locations; and plume characteristics like C2H6/CH4 ratios, CO/CO2 ratios, and isotopic signatures for δ13C-CH4 and δD-CH4 is available (Section S2 and Site Averages supplemental file). Ethane/methane ratios in these measurements span 0.16–17%, with the distribution peaking around 3.8% (Figure S21). The measured δ13C-CH4 isotope signatures span −72 to −15‰ and the distribution peaks around −40‰ (see Figure S18), as expected for fossil fuel sources with a typical composition of −56 to −26‰. (32,33) Only sparse data for δD-CH4 are available due to the difficulty in measuring this isotope and the sparsity of plumes with sufficient enhancements (>600 ppb CH4) to yield good results; δD data in this study span −314 to −186‰ with an average of −238‰. Typical fossil fuel sources are in the −250 to −150‰ (32,33) range for δD.
Several interesting sites pop out when examining these plume characteristics. One site, ID 32, has a curiously low ethane/methane ratio of 0.16 (0.01)% (the value in parentheses corresponds to a 1σ error); a typical value for transmission-grade natural gas with low ethane content would be 1.5–2%. (30) The associated isotope signatures are suggestive of microbial sources: δ13C = −71 (16)‰ and δD = −186 (115)‰. This site was in relatively shallow waters, off the coast of Louisiana, and had a large number of plume encounters (n = 20). The observed isotopic signatures are puzzling, since nearby sites had C2H6/CH4 ratios greater than 5% [ID 43 at 5.3 (0.5)%; ID 33 at 5.2 (0.2)%] and less negative isotope signatures. We also observed CO and CO2 concomitant with methane enhancements. Analysis of correlated CO and CO2 allows an estimate of the modified combustion efficiency (0.954). (34) If we assume the combustion is due to flaring, the CO/CO2 data suggest that the flare was operating ∼4.5 times less efficiently than the typical specification (0.99). One hypothesis is that emissions from this site are dominated by flaring emissions, and that ethane is selectively depleted through combustion. Another hypothesis is that this platform is accessing gas from a different geologic source than its neighbors with an unusual chemical signature, as is seen in certain unconventional reserves. (35)
On the other hand, several sites had ethane/methane ratios that skewed higher: up to 17%. Such ethane/methane ratios are not unusual, and have been observed on land in regions producing larger amounts of oil and natural gas liquids, resulting in emissions richer in higher hydrocarbons. In general, ethane/methane ratios are higher in deeper waters and toward the east (Figure S22), though still with a fair degree of variation.
Though the precision in isotopic measurements is not as good as for laboratory mass-based methods typically used with extractive canister or bag samples, the spectroscopic measurements reported here have several unique advantages. First, isotopic quantification is done on the 1 s data stream, eliminating the need to collect and properly time the canister samples; all the collected data are candidates for isotopic analysis. Second, canister samples yield only a single data point, so that numerous samples are required to perform a Keeling analysis. Keeling plots (δ vs 1/CH4, see Section S13, for several examples) are used to separate the signature of the source from that of the ambient background. (21,22) Shipboard measurements also provide more time in the plume than analogous aircraft measurements would, which is fully leveraged in the Keeling analysis. Practically, canister sampling studies often lump together samples from many sites in order to accumulate enough data to identify an isotopic signature for a given source type. Here, each individual plume encounter yields a true Keeling plot, and assuming a sufficient dynamic range of CH4 concentrations, an isotopic signature unique to that specific plume. As a result, variations in the isotopic signatures from site to site are resolved.
Figure 2 plots C2H6/CH4 (top) and δD (bottom) versus δ13C-CH4 for offshore sites. These figures show data that largely fall within the expected regions for thermogenic sources. (33,36) The δ13C, δD, and C2H6/CH4 source signatures are often used to interrogate the geologic formations at the source of emissions. For example, the C2H6/CH4 ratio of natural gas tends to decrease and the δ13C isotopic signature becomes less negative (13C-enriched) with increasing thermal maturity of the reservoir. (36) Furthermore, no ethane will be present in bacterially formed methane, (35,37) and the δD isotopic signatures provide an additional way to distinguish bacterial from thermogenic methane and to separate specific methanogenic pathways (carbonate reduction vs methyl-type fermentation). Though the measurements here span regions with rocks of varying geologic ages (generally younger closer to the shore and older further asea), (38,39) correlations are not readily visible in Figure 2 plots. However, when comparing this dataset to published gas compositions (33) in this region, we find good agreement for δ13C and C2H6/CH4, in both range and variability. Discrepancies arise primarily due to differences in sampled areas, with this dataset measuring more sites in deep offshore waters south of Louisiana, and fewer sites far from shore south of the Texas/Louisiana border (Figures S19 and S22). Previously published data in this region are lacking for δD. (33)

Figure 2

Figure 2. Relationship between the isotopic source signatures of methane (δ13C and δD) and ethane/methane ratios. Each data point represents the average composition of a site’s emissions. 1σ error bars are shown.

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The uncertainty in the source isotopic signatures shown in Figure 2 is determined from propagating the error in the standard deviation of the Keeling intercept for each replicate plume measurement. This Keeling intercept uncertainty (typical value of 8‰ for δ13C, for example) decreases with the CH4 dynamic range during a plume, and in this dataset, it is usually much greater than the instrumental performance on the 1 s data stream (typical 1 s 1σ value for δ13C of 0.5‰ stationary or 2‰ while in motion, see Figure S12). δD measurements are more challenging than δ13C measurements due to the spectroscopy and abundance of CH3D in the sampled air (typical Keeling intercept uncertainty of 50‰; 1 s 1σ performance of 18‰ stationary and 62‰ in motion, see Figure S12). For this reason, higher plume concentration enhancements were required for δD quantification (see Sections S12 and S13), and fewer data points are reported.
The estimated CH4 emission magnitudes, in kg/h, are shown in Figure 3. Null sites are assigned a nominal emission magnitude of 0, and are shown in pale pink or blue. The median emission magnitude is 5.3 kg/h with an average of 17 kg/h and a maximum of 185 kg/h. This distribution is highly skewed: the top 2% of sites account for 20% of the total emissions. Plotting the distribution of emitters on a logarithmic scale better shows the variability in emission magnitudes (Figure 3, bottom). This distribution spans over three orders of magnitude, peaking at 13 kg/h. The distribution is similar if restricted to only sites with three or more replicates (Section S8).

Figure 3

Figure 3. Estimated methane emission rates in kg/h for offshore sites. Emission histograms are shown on a linear scale (top, 10 kg/h bins) and a logarithmic scale (bottom). Sites are classified based on water depths <1000 feet (red) or ≥1000 feet (blue). Sites with no detected emissions are shown in pale pink or pale blue.

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Two recent measurement campaigns, both in the North Sea, have significantly expanded the available emission data for offshore platforms. Riddick et al. reported median emissions of 24.5 kg/h from five platforms in the UK, with a maximum emission of 80 kg/h. (8) Hensen et al. reported emissions from 37 platforms on the Netherlands side, with median emissions of 6.12 kg/h and a maximum value of 151 kg/h. (7) Hensen et al. also performed tracer-release experiments on a subset of sites for validation purposes. Both of these datasets fall within the range of emissions reported here (median: 5.3 kg/h and maximum: 185 kg/h). In the study by Hensen et al., there were enough sites to construct a distribution of emitters. Their distribution was slightly broader, and peaks were observed at lower emission rates (3 kg/h vs 17 kg/h here, see Figure S23). They measured just one site with emissions <0.1 kg/h, whereas we found more than 25 such sites.
One other study looks at methane emissions from platforms in the Gulf of Mexico. Bylin et al. reported methane emissions of 224 kg/h per platform, as calculated from equipment counts and activity data. (13) This is substantially higher than the median emission magnitude estimated in this study (median: 5.3 kg/h) and exceeds the maximum emission of 185 kg/h. However, there are potential differences in the platform populations investigated. Bylin et al. measured 15 operational platforms chosen to represent the size of platforms producing in deepwater areas of the Gulf. Ten of these had wet seal centrifugal compressors, the largest of the sources of emissions tabulated. In contrast, our study measured 103 sites, only 75 of which had detected methane enhancements downwind. Our sample contains numerous shallow-water sites, and included sites that were no longer producing. While we have no tabulated list of equipment, anecdotally there was a large diversity in the platform size and associated equipment.
In order to put emission magnitudes into context, it would be useful to know the amount of product handled by a given offshore platform. Offshore operators are responsible for reporting production data to BOEM (Section S17). However, at the time of writing, production data were reported only for wells, and the databases did not provide any generalized way to associate a given well to a platform. Adding to the complexity of this issue, platforms may serve a great variety of functions in the offshore landscape, and it is possible that some of the throughput they manage is associated with gases or liquids that never come to the surface. We note that a dataset hosted on the website of the department of the interior was found via a web search, and lists gas and oil production data associated with individual platforms; (40) however, this database is not accessible via any website link, and has no associated description or metadata. Collaboration and data sharing with database managers and industry stakeholders will thus be beneficial to future studies.
Other types of assets in the offshore industry are not considered here and warrant future study. Notably, no FPSOs were measured. These vessels act as portable processing and storage plants, and can even perform some subsea maintenance. They can be used as an alternative to a permanent platform in some areas. Tankers and coastal transfer stations also warrant study, particularly the transfer locations where product finally enters the land supply chain.

Uncertainties in Emission Estimates

Source height is a particularly uncertain parameter in this dataset—and of critical importance for Gaussian dispersion methodology. At only 26% of sites were we able to find site lease planning documents showing a platform height above the water for use in simulations (main deck heights are between 7 and 30 m, with the average of 15.24 m used for all other sites). Furthermore, the true height of an emission is likely different from the platform height, depending on its exact source vector.
Underestimating heights of the simulated sources will lead to underestimation of their emissions. For representative meteorological conditions, assuming a 15 m source height, emissions could be overestimated by 9% if the source was actually at 7 m, or underestimated by 37% if the source was actually at 30 m (Section S9 and Figure S9). In comparison, the underestimation in emissions due to plume rise would be 7% if the source were a normally operating flare (see Figure S9).
The simple Gaussian model does not include the impact of emissions being reflected beneath a low marine boundary layer. For this dataset, we estimate boundary layer heights around 500–800 m based on an examination of vertical structure data from the 2006 TexAQS campaign. (41) At these boundary layer heights, the impact of including the reflection terms is negligible (Figure S10). Those terms become important only for extremely low boundary layer heights: emissions will be overestimated by a factor of 2.5 for a 20 m boundary layer at a typical site measured at a 10 km distance. Another important factor to consider is the possibility of emissions being emitted above a very low marine boundary layer. If emissions are released above this layer, they are unlikely to mix down and be observed by the measurement vessel; these sites will have undetected or underestimated emissions. Measurements were aborted when winds ceased, and so this is unlikely to be a significant problem in this dataset.
Another compounding source of error in this dataset is the uncertainty in the distance between the vessel and the site (i.e., the exact site location because the vessel’s position is continuously logged). This location uncertainty includes inaccuracies or data gaps in the reported positions of sites in the BOEM platform database (24) or NOAA navigational charts. More importantly though, it includes errors in identifying which site of multiple candidates was responsible for a given plume. Measurements at most planned sites (Figure 1) were assigned without much ambiguity. Ambiguities or uncertainties in site assignment were more common in congested areas, where the vessel’s sampling path was restricted, and for fortuitous or post-analysis sites (Figure 1). In all cases, the analysis (Section S7) leveraged information like other trace gases, plume widths, and wind data to identify reasonable site locations, discarding data that were too ambiguous. The dataset of site-average results (Section S2 and GulfSite Averages file) includes a “location uncertainty” column, indicating whether site assignment was certain or ambiguous.
Ultimately, the factor-of-10 error bars asserted here for Gaussian dispersion results are meant to encompass the uncertainties in input parameters like height (factor-of-0.5 for the above example) and location, as well as more fundamental uncertainties related to Gaussian dispersion (factor-of-3.17, see the Materials and Methods section) and to the empirical methodology used to account for the extra stable conditions encountered in this study. In many cases, replicate downwind transects were done for a site. These replicate measurements showed variability in the Gaussian plume emission determination smaller or much smaller than a factor of 10 or even 3. This further highlights the fact that the error in these estimates is dominated by systematic or methodological errors, and not variability in the underlying measured data.

Background Concentrations

While previous sections have focused on plume enhancements originating from offshore facilities, significant variations in offshore ethane background values were also observed. From a typical background level of 2 ppb, background ethane mixing ratios varied by factors ranging from a half to a seven-fold increase. In order to understand whether these observed enhancements in ethane were due to either a continental outflow, or a flow from shallow or deepwater offshore regions, surface-level sampling footprints were calculated using Hysplit. (42−45) The details of the calculation parameters used are reported in Section S16. The model was used to create a sampling contour arriving at the research vessel for each 10 min period. The background signals of ethane and methane (all plumes removed) were apportioned onto the sampling footprint, and geographically averaged for the whole measurement campaign.
Figure 4 depicts the result of this footprint analysis. The average campaign footprint (white/cyan contour lines, Figure 4) indicates that the project overwhelmingly sampled air that, in the prior 12 h, was present in the marine boundary layer of the Gulf waters. The background ethane signal mapped onto this footprint (pink color map, Figure 4) reveals several periods of elevated concentrations. An analogous figure showing methane background footprints is shown in Figure S25. High mixing ratios of ethane were measured during the brief period when the RV Trident sampled continental air from the Texas/Louisiana border. This is not surprising, as this part of the country is home to the Haynesville oil and gas play.

Figure 4

Figure 4. Results of Hysplit back-trajectory analysis of the ethane background. The measured ethane mixing ratio during each 10 min period is drawn onto the associated footprint and averaged for the entire campaign (pink color map). The average sampling footprint over the course of the campaign is shown as blue contour lines. The shoreline map was obtained from NOAA. (25)

High Resolution Image
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More interestingly, the waters south-west of Grand Isle also exhibit elevated mixing ratios of ethane, suggesting a distributed source in the shallow Gulf waters. Offshore natural seeps could be one such distributed source. Hu et al. sailed to deepwater areas in the Gulf of Mexico, and suggested that the impact of such emissions on atmospheric methane is not significant. (46) Still, their light hydrocarbon measurements in the atmosphere and water column suggest at least some contribution from deepwater hydrocarbon seeps. Pisso et al. reported aircraft measurements west of Svalbard, an area with known methane clathrates. (47) They found that these areas did not significantly influence atmospheric methane concentrations, and calculated an upper limit on methane fluxes from the region. Bacterial sources of methane in shallow shelf areas have been shown to have an impact on atmospheric methane. (48) However, neither clathrates nor bacterial sources would be associated with ethane. Thus, while it is possible that a natural seep is present in the shallow region with higher levels of ethane, it is also possible that the numerous offshore oil and gas facilities in shallow waters are effectively acting as a distributed source through their emissions.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.est.9b07148.

  • Dataset descriptions, additional methods (site selection, sampling and analysis strategy, and instrument calibration and performance), isotopic and ethane/methane ratio results, comparisons with other studies, and a description of offshore datasets used (PDF)

  • Site Averages: Comma-separated value file and associated readme file for summary emission data for the measured offshore sites (TXT) (TXT)

  • 1 s Dataset: Comma-separated value file and the associated readme file for the dataset of 1 s measurements, including trace gas measurements, wind and geospatial information (TXT) (TXT)

  • Hysplit simulation files (ZIP)

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Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html.

Author Information

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  • Corresponding Author
  • Authors
    • Conner Daube - Aerodyne Research, Inc., 45 Manning Road, Billerica, Massachusetts 01821, United States
    • Scott C. Herndon - Aerodyne Research, Inc., 45 Manning Road, Billerica, Massachusetts 01821, United States
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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We gratefully acknowledge Captain Jason Brunson, First Mate Jon Wilson, and the crew of the RV Trident (Lucy Flipse, Emily Brzozowske, and Harrison Hines) who ensured a safe and successful field mission. We thank Stefan Schwietzke and Owen Sherwood for allowing us to use their global gas composition database and for helpful discussions during manuscript preparation. This work was funded under the Climate and Clean Air Coalition (CCAC) Oil and Gas Methane Science Studies. The studies were managed by United Nations Environment Programme in collaboration with the Chief Scientist, Steven Hamburg of the Environmental Defense Fund. Funding was provided by the Environmental Defense Fund, OGCI Companies (Shell, BP, ENI, Petrobras, Repsol, Total, Equinor, CNPC, Saudi Aramco, and Pemex), CCAC, and the European Commission.

References

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    Mobile Laboratory Observations of Methane Emissions in the Barnett Shale Region
    Yacovitch, Tara I.; Herndon, Scott C.; Petron, Gabrielle; Kofler, Jonathan; Lyon, David; Zahniser, Mark S.; Kolb, Charles E.
    Environmental Science & Technology (2015), 49 (13), 7889-7895CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    Results of mobile ground-based atm. measurements conducted during the Barnett Shale Coordinated Campaign in spring and fall of 2013 are presented. Methane and ethane are continuously measured downwind of facilities such as natural gas processing plants, compressor stations, and prodn. well pads. Gaussian dispersion simulations of these methane plumes, using an iterative forward plume dispersion algorithm, are used to est. both the source location and the emission magnitude. The distribution of emitters is peaked in the 0-5 kg/h range, with a significant tail. The ethane/methane molar enhancement ratio for this same distribution is investigated, showing a peak at ∼1.5% and a broad distribution between ∼4% and ∼17%. The regional distributions of source emissions and ethane/methane enhancement ratios are examd.: the largest methane emissions appear between Fort Worth and Dallas, while the highest ethane/methane enhancement ratios occur for plumes obsd. in the northwestern potion of the region. Individual facilities, focusing on large emitters, are further analyzed by constraining the source location.
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    Keeling, C. D. The concentration and isotopic abundances of atmospheric carbon dioxide in rural areas. Geochim. Cosmochim. Acta 1958, 13, 322– 334,  DOI: 10.1016/0016-7037(58)90033-4
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    The concentration and isotopic abundances of atmospheric carbon dioxide in rural areas
    Keeling, Charles D.
    Geochimica et Cosmochimica Acta (1958), 13 (), 322-34CODEN: GCACAK; ISSN:0016-7037.
    cf. Craig, C.A. 51, 17599i. Fifty samples of air collected near the Pacific coast of Washington and California were analyzed for CO2 and isotopic abundance of C13 and O18. The meteorological data for the samples analyzed are tabulated. Min. concns. of CO2 in the air were noted in the afternoons and max. concns. in the evening or early morning hours. The pronounced regularity with which the C isotope ratio follows changes in CO2 concn. is due to the CO2 added to or subtracted from the air, diurnally, by plants and their decay products. It is assumed that the air initially contains 0.031 vol. % CO2, C13/C12 ratio -0.7%, to which is added CO2 of plant origin with a ratio of approx. -2.3%. The uniform min. concn. and C isotope ratio of afternoon air samples, regardless of location, must, on the other hand, be the result of ground-level air mixing with air from above or beyond the zone of vegetative influence. The variation in concn. is only 0.0307-0.0316%; in C13/C12 ratio, -0.67 to -0.74%; it is thought to represent Pacific maritime air. O isotope ratios are about the same as for CO2 in equil. with av. ocean water, at 25°, -0.1%. Samples assocd. with min. concns. range from +1.3 to -0.2%; forest and grassland samples from +2.9 to -1.9%. The variations are apparently not correlated with any measured meteorological or chem. factor. One case was noted where a definite change in O isotope ratio reflected change in barometric pressure, whereas the C isotope ratio and CO2 concn. remained const. It could have been due to partial mixing of air from different air masses that were equilibrated with H2O of different O-isotope compn. or at different temps.
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    Keeling, C. D. The concentration and isotopic abundances of carbon dioxide in rural and marine air. Geochim. Cosmochim. Acta 1961, 24, 277– 298,  DOI: 10.1016/0016-7037(61)90023-0
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    Concentration and isotopic abundances of carbon dioxide in rural and marine air
    Keeling, Charles D.
    Geochimica et Cosmochimica Acta (1961), 24 (), 277-98CODEN: GCACAK; ISSN:0016-7037.
    cf. CA 52, 12477a. Addnl. analyses are reported of 106 samples of rural air in forest, grassland, and desert and of 13 samples of air over tropical waters of the eastern Pacific Ocean. The data are correlated with meteorological conditions. In areas away from urban effects and terrestrial plant growth, the concn. and C13 abundance of CO2 in air are nearly const., but O18 abundance of CO2 shows systematic variation with air and ocean water temps. or season. The C13::C12 ratio ranges from -6.7 to -7.4 per mil and O18:O16 from +0.8 to -0.6 per mil. Wickman's hypothesis (CA 46, 9167c) on C-isotope enrichment of terrestrial plants is corroborated. The previous correlation between C13 abundance and concn. of CO2 in forest air is noted again. The C13:C12 ratio of CO2 from forest plants was computed and is -21 to -26 per mil. The O18 abundance of CO2 in forest air is variable and shows no obvious correlation with other measurable properties.
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    Yacovitch, T. I.; Neininger, B.; Herndon, S. C.; van der Gon, H. D.; Jonkers, S.; Hulskotte, J.; Roscioli, J. R.; Zavala-Araiza, D. Methane emissions in the Netherlands: The Groningen field. Elem. Sci. Anth. 2018, 6, 57,  DOI: 10.1525/elementa.308
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    Hanna, S. R.; Schulman, L. L.; Paine, R. J.; Pleim, J. E.; Baer, M. Development and Evaluation of the Offshore and Coastal Dispersion Model. Journal of the Air Pollution Control Association 1985, 35, 1039– 1047,  DOI: 10.1080/00022470.1985.10466003
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    Development and evaluation of the offshore and coastal dispersion model
    Hanna, Steven R.; Schulman, Lloyd L.; Paine, Robert J.; Pleim, Jonathan E.; Baer, Mitchell
    Journal of the Air Pollution Control Association (1985), 35 (10), 1039-47CODEN: JPCAAC; ISSN:0002-2470.
    The Offshore and Coastal Dispersion (OCD) model for detg. the impact of offshore and onshore emissions from point sources on the air quality of coastal regions was constructed on the framework of the EPA guideline model MPTER and incorporates overwater plume transport and dispersion as well as changes that occur as the plume crosses the shoreline. Hourly meteorol. data are needed from both offshore and onshore locations, including wind direction and speed, mixing height, overwater air temp. and relative humidity, and the sea surface temp. Obsd. turbulence intensities are preferred but are not mandatory. Building downwash and plume rise are incorporated into the OCD model and partial plume penetration into elevated inversions is treated using Briggs' model. Dispersion coeffs. are proportional to turbulence intensities. A virtual source technique is used to change the rate of plume growth as the overwater plume intercepts the overland internal boundary layer. The continuous shoreline fumigation case is treated using an approach suggested by J.W. Deardorff and G.E. Willis (1982). Calcn. of plume reflection from elevated terrain follows the Rough Terrain Dispersion Model. The OCD model and the modified EPA model used as an interim model for overwater applications by the Minerals Management Service (MMS) were tested with measurements from 3 offshore tracer expts. The OCD model was an improvement over the EPA model and was officially approved by the MMS in Mar. 1985.
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    Abdel-Rahman, A. A. On the Atmospheric Dispersion and Gaussian Plume Model. In 2nd International Conference on Waste Management, Water Pollution, Air Pollution, Indoor Climate, Corfu, Greece, October 26–28, 2008; Mastorakis, N. E., Ed.; WSEAS Press, 2008; pp 31– 39.
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    Rigby, M.; Manning, A. J.; Prinn, R. G. The value of high-frequency, high-precision methane isotopologue measurements for source and sink estimation. J. Geophys. Res.: Atmos. 2012, 117, D12312,  DOI: 10.1029/2011JD017384
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    The value of high-frequency, high-precision methane isotopologue measurements for source and sink estimation
    Rigby, M.; Manning, A. J.; Prinn, R. G.
    Journal of Geophysical Research: Atmospheres (2012), 117 (June), D12312/1-D12312/14CODEN: JGRDE3 ISSN:. (American Geophysical Union)
    We present an observing system simulation expt. examg. the potential benefits of new methane isotopologues measurements for global- and national-scale source and sink inversions. New measurements are expected in the coming years, using quantum cascade laser spectroscopy with sample preconcn., that will allow observations of δ13C - CH4 and δD - CH4 at approx. hourly intervals and higher precision than previously possible. Using model-generated "pseudo-data", we predict the variability that these new systems should encounter in the atm., and est. the addnl. uncertainty redn. that should result from their use in source and sink inversions. We find that much of the δ-value variability from seasonal to daily timescales should be resolvable at the target precision of the new observations. For global source estn., we find addnl. uncertainty redns. of between 3-9 Tg/yr for four major source categories (microbial, biomass burning, landfill and fossil fuel), compared to mole fraction-only inversions, if the higher end of the anticipated isotopologue-measurement precisions can be achieved. On national scales, we obtain av. uncertainty redns. of ∼10% of the source strength for countries close to high-frequency monitoring sites, although the degree of uncertainty redn. on such small scales varies significantly (from close to 0% to almost 50%) for different sources and countries.
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    Sherwood, O. A.; Schwietzke, S.; Arling, V. A.; Etiope, G. Global Inventory of Gas Geochemistry Data from Fossil Fuel, Microbial and Burning Sources, version 2017. Earth Syst. Sci. Data 2017, 9, 639– 656,  DOI: 10.5194/essd-9-639-2017
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    Briggs, N. L.; Jaffe, D. A.; Gao, H.; Hee, J. R.; Baylon, P. M.; Zhang, Q.; Zhou, S.; Collier, S. C.; Sampson, P. D.; Cary, R. A. Particulate Matter, Ozone, and Nitrogen Species in Aged Wildfire Plumes Observed at the Mount Bachelor Observatory. Aerosol and Air Quality Research 2016, 16, 3075– 3087,  DOI: 10.4209/aaqr.2016.03.0120
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    Particulate matter, ozone, and nitrogen species in aged wildfire plumes observed at the mount bachelor observatory
    Briggs, Nicole L.; Jaffe, Daniel A.; Gao, Honglian; Hee, Jonathan R.; Baylon, Pao M.; Zhang, Qi; Zhou, Shan; Collier, Sonya C.; Sampson, Paul D.; Cary, Robert A.
    Aerosol and Air Quality Research (2016), 16 (12), 3075-3087CODEN: AAQRAV; ISSN:1680-8584. (Taiwan Association for Aerosol Research)
    During the summer of 2012 and 2013, we measured carbon monoxide (CO), carbon dioxide (CO2), ozone (O3), nitrogen oxides (NOx), reactive nitrogen (NOy), peroxyacetyl nitrate (PAN), aerosol scattering (ssp) and absorption, elemental and org. carbon (EC and OC), and aerosol chem. at the Mount Bachelor Observatory (2.8 km above sea level, Oregon, US). Here we analyze 23 of the individual plumes from regional wildfires to better understand prodn. and loss of aerosols and gaseous species. We also developed a new method to calc. enhancement ratios and Modified Combustion Efficiency (MCE), which takes into account possible changes in background concns. during transport. We compared this new method to existing methods for calcg. enhancement ratios. The MCE values ranged from 0.79- 0.98, ΔO3/ ΔCO ranged from 0.01-0.07 ppbv ppbv1, Δσsp/δ CO ranged from 0.23-1.32 Mm1 (at STP) ppbv-1, ΔNOy/ΔCO ranged from 2.89-12.82 pptv ppbv1, and δ PAN/δ CO ranged from 1.46-6.25 pptv ppbv1. A comparison of three different methods to calc. enhancement ratios (ER) showed that the methods generally resulted in similar Δσsp/δ CO, ΔNOy/ΔCO, and ΔPAN/ΔCO; however, there was a significant bias between the methods when calcg. δO3/δCO due to the small abs. enhancement of O3 in the plumes. The ΔO3/ΔCO ERs calcd. using two common methods were biased low (~ 20-30%) when compared to the new proposed method. Two pieces of evidence suggest moderate secondary particulate formation in many of the plumes studied: 1 mean obsd. δOC/δ CO2 was 0.028 g particulate-C gC1 (as CO2)-27% higher than the midpoint of the biomass burning emission ratio range reported by a recent review-and 2 single scattering albedo (ω) was relatively const. at all MCE values, in contrast with results for fresh plumes. The obsd. NOx, PAN, and aerosol nitrate represented 6-48%, 25-57%, and 20-69% of the obsd. NOy in the aged plumes, resp., and other species represented on av. 11% of the obsd. NOy.
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    Roscioli, J. R.; Herndon, S. C.; Yacovitch, T. I.; Knighton, W. B.; Zavala-Araiza, D.; Johnson, M. R.; Tyner, D. R. Characterization of methane emissions from five cold heavy oil production with sands (CHOPS) facilities. J. Air Waste Manage. Assoc. 2018, 68, 671– 684,  DOI: 10.1080/10962247.2018.1436096
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    Characterization of methane emissions from five cold heavy oil production with sands facilities
    Roscioli, Joseph R.; Herndon, Scott C.; Yacovitch, Tara I.; Knighton, W. Berk; Zavala-Araiza, Daniel; Johnson, Matthew R.; Tyner, David R.
    Journal of the Air & Waste Management Association (2018), 68 (7), 671-684CODEN: JAWAFC; ISSN:1096-2247. (Taylor & Francis Ltd.)
    Cold heavy oil prodn. with sands CHOPS is a common oil extn. method in the Canadian provinces of Alberta and Saskatchewan that can result in significant methane emissions due to annular venting. Little is known about the magnitude of these emissions, nor their contributions to the regional methane budget. Here the authors present the results of field measurements of methane emissions from CHOPS wells and compare them with self-reported venting rates. The tracer ratio method was used not only to analyze total site emissions but at one site it was also used to locate primary emission sources and quantify their contributions to the facility-wide emission rate, revealing the annular vent to be a dominant source. Emissions measured from five different CHOPS sites in Alberta showed large discrepancies between the measured and reported rates, with emissions being mainly underreported. These methane emission rates are placed in the context of current reporting procedures and the role that gas-oil ratio measurements play in vented vol. ests. In addn. to methane, emissions of higher hydrocarbons were also measured; a chem. "fingerprint" assocd. with CHOPS wells in this region reveals very low emission ratios of ethane, propane, and aroms. vs. methane. The results of this study may inform future studies of CHOPS sites and aid in developing policy to mitigate regional methane emissions. Implications: Methane measurements from cold heavy oil prodn. with sand CHOPS sites identify annular venting to be a potentially major source of emissions at these facilities. The measured emission rates are generally larger than reported by operators, with uncertainty in the gas-oil ratio possibly playing a large role in this discrepancy. These results have potential policy implications for reducing methane emissions in Alberta in order to achieve the Canadian government's goal of reducing methane emissions by 40-45% below 2012 levels within 8 yr.
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    Yacovitch, T. I.; Herndon, S. C.; Roscioli, J. R.; Floerchinger, C.; McGovern, R. M.; Agnese, M.; Pétron, G.; Kofler, J.; Sweeney, C.; Karion, A.; Conley, S. A.; Kort, E. A.; Nähle, L.; Fischer, M.; Hildebrandt, L.; Koeth, J.; McManus, J. B.; Nelson, D. D.; Zahniser, M. S.; Kolb, C. E. Demonstration of an Ethane Spectrometer for Methane Source Identification. Environ. Sci. Technol. 2014, 48, 8028– 8034,  DOI: 10.1021/es501475q
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    Demonstration of an Ethane Spectrometer for Methane Source Identification
    Yacovitch, Tara I.; Herndon, Scott C.; Roscioli, Joseph R.; Floerchinger, Cody; McGovern, Ryan M.; Agnese, Michael; Petron, Gabrielle; Kofler, Jonathan; Sweeney, Colm; Karion, Anna; Conley, Stephen A.; Kort, Eric A.; Nahle, Lars; Fischer, Marc; Hildebrandt, Lars; Koeth, Johannes; McManus, J. Barry; Nelson, David D.; Zahniser, Mark S.; Kolb, Charles E.
    Environmental Science & Technology (2014), 48 (14), 8028-8034CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    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-IR 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 obsd. 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 liqs. (>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 exptl. studies of methane emissions, particularly in the oil and natural gas sector.
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    Warwick, P. D. Geologic Assessment Of Undiscovered Conventional Oil and Gas Resources in the Lower Paleogene Midway and Wilcox Groups, and the Carrizo Sand of the Claiborne Group, of the Northern Gulf Coast Region; Open-File Report 2017–1111; U.S. Geological Survey: Reston, Virginia, 2017; p 67.
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    Hu, L.; Yvon-Lewis, S. A.; Kessler, J. D.; MacDonald, I. R. Methane fluxes to the atmosphere from deepwater hydrocarbon seeps in the northern Gulf of Mexico. J. Geophys. Res.: Oceans 2012, 117, C01009,  DOI: 10.1029/2011JC007208
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    Methane fluxes to the atmosphere from deepwater hydrocarbon seeps in the northern Gulf of Mexico
    Hu, Lei; Yvon-Lewis, Shari A.; Kessler, John D.; MacDonald, Ian R.
    Journal of Geophysical Research: Oceans (2012), 117 (Jan.), C01009/1-C01009/13CODEN: JGRCEY ISSN:. (American Geophysical Union)
    Three deepwater hydrocarbon seep sites in the northern Gulf of Mexico that feature near-seafloor gas hydrates, MC118 (depth = 900 m), GC600 (depth = 1250 m) and GC185 (depth = 550 m), were investigated during the Remote Sensing and Sea-Truth Measurements of Methane Flux to the Atm. (HYFLUX) study in July 2009. Continuous measurements of air and sea surface concns. of methane were made to obtain high spatial and temporal resoln. of the diffusive net sea-to-air fluxes. The atm. methane fluctuated between 1.70 and 2.40 ppm (ppm) during the entire cruise except for high concns. (up to 4.01 ppm) sampled during the end of the occupation of GC600 and the transit between GC600 and GC185. In conjunction with air-mass back trajectory anal., these high concns. are likely from a localized methane source to the atm. Methane concns. in surface seawater and methane net sea-to-air fluxes show high temporal and spatial variability within and between sites. The presence of ethane and propane in the surface seawater indicates a thermogenic source in the plume areas, suggesting the surface methane could be at least partly attributable to transport from the deepwater hydrocarbon seeps. Results from interpolations within the survey areas show the daily methane fluxes to the atm. at the three sites range from 0.744 to 300 mol d-1. Extrapolating the highest daily sea-to-air flux of methane to other deepwater seeps in the northern Gulf of Mexico suggests that the net diffusive sea-to-air flux from deepwater hydrocarbon seeps in this region is an insignificant source to the atm. methane.
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    Pisso, I.; Myhre, C. L.; Platt, S. M.; Eckhardt, S.; Hermansen, O.; Schmidbauer, N.; Mienert, J.; Vadakkepuliyambatta, S.; Bauguitte, S.; Pitt, J.; Allen, G.; Bower, K. N.; O’Shea, S.; Gallagher, M. W.; Percival, C. J.; Pyle, J.; Cain, M.; Stohl, A. Constraints on oceanic methane emissions west of Svalbard from atmospheric in situ measurements and Lagrangian transport modeling. J. Geophys. Res.: Atmos. 2016, 121, 14188– 14200,  DOI: 10.1002/2016JD025590
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    Constraints on oceanic methane emissions west of Svalbard from atmospheric in situ measurements and Lagrangian transport modeling
    Pisso I; Myhre C Lund; Platt S M; Eckhardt S; Hermansen O; Schmidbauer N; Stohl A; Mienert J; Vadakkepuliyambatta S; Bauguitte S; Pitt J; Allen G; Bower K N; O'Shea S; Percival C J; Gallagher M W; Pyle J; Cain M
    Journal of geophysical research. Atmospheres : JGR (2016), 121 (23), 14188-14200 ISSN:2169-897X.
    Methane stored in seabed reservoirs such as methane hydrates can reach the atmosphere in the form of bubbles or dissolved in water. Hydrates could destabilize with rising temperature further increasing greenhouse gas emissions in a warming climate. To assess the impact of oceanic emissions from the area west of Svalbard, where methane hydrates are abundant, we used measurements collected with a research aircraft (Facility for Airborne Atmospheric Measurements) and a ship (Helmer Hansen) during the Summer 2014 and for Zeppelin Observatory for the full year. We present a model-supported analysis of the atmospheric CH4 mixing ratios measured by the different platforms. To address uncertainty about where CH4 emissions actually occur, we explored three scenarios: areas with known seeps, a hydrate stability model, and an ocean depth criterion. We then used a budget analysis and a Lagrangian particle dispersion model to compare measurements taken upwind and downwind of the potential CH4 emission areas. We found small differences between the CH4 mixing ratios measured upwind and downwind of the potential emission areas during the campaign. By taking into account measurement and sampling uncertainties and by determining the sensitivity of the measured mixing ratios to potential oceanic emissions, we provide upper limits for the CH4 fluxes. The CH4 flux during the campaign was small, with an upper limit of 2.5 nmol m(-2) s(-1) in the stability model scenario. The Zeppelin Observatory data for 2014 suggest CH4 fluxes from the Svalbard continental platform below 0.2 Tg yr(-1). All estimates are in the lower range of values previously reported.
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  48. 48
    Bange, H. W.; Bartell, U. H.; Rapsomanikis, S.; Andreae, M. O. Methane in the Baltic and North Seas and a reassessment of the marine emissions of methane. Global Biogeochem. Cycles 1994, 8, 465– 480,  DOI: 10.1029/94GB02181
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    48
    Methane in the Baltic and North seas and a reassessment of the marine emissions of methane
    Bange, H. W.; Bartell, U. H.; Rapsonmanikis, S.; Andreae, M. O.
    Global Biogeochemical Cycles (1994), 8 (4), 465-80CODEN: GBCYEP; ISSN:0886-6236. (American Geophysical Union)
    During three measurement campaigns on the Baltic and North seas, atm. and dissolved methane was detd. with an automated gas chromatog. system. Area-weighted mean satn. values in the sea surface waters were 113 ± 5% and 395 ± 82% (Baltic Sea, Feb. and July 1992) and 126 ± 8% (south central North Sea, Sept. 1992). On the bases of our data and a compilation of literature data the global oceanic emissions of methane were reassessed by introducing a concept for regional gas transfer coeffs. Our ests. computed with two different air-sea exchange models lie in the range of 11-18 Tg CH4 yr-1. Despite the fact that shelf areas and estuaries only represent a small part of the world's ocean they contribute about 75% to the global oceanic emissions. We applied a simple, coupled, three-layer model to evaluate the time dependent variation of the oceanic flux to the atm. The model calcns. indicate that even with increasing tropospheric methane concn., the ocean will remain a source of atm. methane.
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  • Abstract

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    Figure 1

    Figure 1. Vessel path (solid black line) over the course of the campaign, showing measured sites (squares) colored by site choice type. Offshore well daily gas production (circles) for the period June–February 2017 (23) is plotted in thousand cubic feet per day (Mcfd). A dividing line between lower and higher production magnitudes is shown (dashed line). The shoreline map was obtained from NOAA. (25)

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    Figure 2

    Figure 2. Relationship between the isotopic source signatures of methane (δ13C and δD) and ethane/methane ratios. Each data point represents the average composition of a site’s emissions. 1σ error bars are shown.

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    Figure 3

    Figure 3. Estimated methane emission rates in kg/h for offshore sites. Emission histograms are shown on a linear scale (top, 10 kg/h bins) and a logarithmic scale (bottom). Sites are classified based on water depths <1000 feet (red) or ≥1000 feet (blue). Sites with no detected emissions are shown in pale pink or pale blue.

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    Figure 4

    Figure 4. Results of Hysplit back-trajectory analysis of the ethane background. The measured ethane mixing ratio during each 10 min period is drawn onto the associated footprint and averaged for the entire campaign (pink color map). The average sampling footprint over the course of the campaign is shown as blue contour lines. The shoreline map was obtained from NOAA. (25)

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  • References

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    This article references 48 other publications.

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      Riddick, S. N.; Mauzerall, D. L.; Celia, M.; Harris, N. R. P.; Allen, G.; Pitt, J.; Staunton-Sykes, J.; Forster, G. L.; Kang, M.; Lowry, D.; Nisbet, E. G.; Manning, A. J. Methane emissions from oil and gas platforms in the North Sea. Atmos. Chem. Phys. 2019, 19, 9787– 9796,  DOI: 10.5194/acp-19-9787-2019
      8
      Methane emissions from oil and gas platforms in the North Sea
      Riddick, Stuart N.; Mauzerall, Denise L.; Celia, Michael; Harris, Neil R. P.; Allen, Grant; Pitt, Joseph; Staunton-Sykes, John; Forster, Grant L.; Kang, Mary; Lowry, David; Nisbet, Euan G.; Manning, Alistair J.
      Atmospheric Chemistry and Physics (2019), 19 (15), 9787-9796CODEN: ACPTCE; ISSN:1680-7324. (Copernicus Publications)
      Since 1850 the concn. of atm. methane (CH4), a potent greenhouse gas, has more than doubled. Recent studies suggest that emission inventories may be missing sources and underestimating emissions. To investigate whether offshore oil and gas platforms leak CH4 during normal operation, we measured CH4 mole fractions around eight oil and gas prodn. platforms in the North Sea which were neither flaring gas nor offloading oil. We use the measurements from summer 2017, along with meteorol. data, in a Gaussian plume model to est. CH4 emissions from each platform. We find CH4 mole fractions of between 11 and 370 ppb above background concn. downwind of the platforms measured, corresponding to a median CH4 emission of 6.8 gCH4 s-1 for each platform, with a range of 2.9 to 22.3 gCH4 s-1. When matched to prodn. records, during our measurements individual platforms lost between 0.04% and 1.4% of gas produced with a median loss of 0.23%. When the measured platforms are considered collectively (i.e. the sum of platforms' emission fluxes weighted by the sum of the platforms' prodn.), we ests. the CH4 loss to be 0.19% of gas prodn. These ests. are substantially higher than the emissions most recently reported to the National Atm. Emission Inventory (NAEI) for total CH4 loss from United Kingdom platforms in the North Sea. The NAEI reports CH4 losses from the offshore oil and gas platforms we measured to be 0.13% of gas prodn., with most of their emissions coming from gas flaring and offshore oil loading, neither of which was taking place at the time of our measurements. All oil and gas platforms we obsd. were found to leak CH4 during normal operation, and much of this leakage has not been included in UK emission inventories. Further research is required to accurately det. total CH4 leakage from all offshore oil and gas operations and to properly include the leakage in national and international emission inventories.
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    9. 9
      Nara, H.; Tanimoto, H.; Tohjima, Y.; Mukai, H.; Nojiri, Y.; Machida, T. Emissions of methane from offshore oil and gas platforms in Southeast Asia. Sci. Rep. 2014, 4, 6503,  DOI: 10.1038/srep06503
      9
      Emissions of methane from offshore oil and gas platforms in Southeast Asia
      Nara, Hideki; Tanimoto, Hiroshi; Tohjima, Yasunori; Mukai, Hitoshi; Nojiri, Yukihiro; Machida, Toshinobu
      Scientific Reports (2014), 4 (), 6503CODEN: SRCEC3; ISSN:2045-2322. (Nature Publishing Group)
      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 ests. suggest that CH4 emissions from oil and gas processes account for approx. 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-examd.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXktlOnu7o%253D&md5=92737c6cba0c54b30fe4d781231f6065
    10. 10
      Tuccella, P.; Thomas, J. L.; Law, K. S.; Raut, J.-C.; Marelle, L.; Roiger, A.; Weinzierl, B.; Denier van der Gon, H. A. C.; Schlager, H.; Onishi, T. Air pollution impacts due to petroleum extraction in the Norwegian Sea during the ACCESS aircraft campaign. Elem. Sci. Anth. 2017, 5, 25,  DOI: 10.1525/elementa.124
      There is no corresponding record for this reference.
    11. 11
      Cain, M.; Warwick, N. J.; Fisher, R. E.; Lowry, D.; Lanoisellé, M.; Nisbet, E. G.; France, J.; Pitt, J.; O’Shea, S.; Bower, K. N.; Allen, G.; Illingworth, S.; Manning, A. J.; Bauguitte, S.; Pisso, I.; Pyle, J. A. A cautionary tale: A study of a methane enhancement over the North Sea. J. Geophys. Res.: Atmos. 2017, 122, 7630– 7645,  DOI: 10.1002/2017JD026626
      11
      A cautionary tale: A study of a methane enhancement over the North Sea
      Cain, M.; Warwick, N. J.; Fisher, R. E.; Lowry, D.; Lanoiselle, M.; Nisbet, E. G.; France, J.; Pitt, J.; O'Shea, S.; Bower, K. N.; Allen, G.; Illingworth, S.; Manning, A. J.; Bauguitte, S.; Pisso, I.; Pyle, J. A.
      Journal of Geophysical Research: Atmospheres (2017), 122 (14), 7630-7645CODEN: JGRDE3; ISSN:2169-8996. (Wiley-Blackwell)
      Airborne measurements of a methane (CH4) plume over the North Sea from August 2013 are analyzed. The plume was only obsd. downwind of circumnavigated gas fields, and three methods are used to det. its source. First, a mass balance calcn. assuming a gas field source gives a CH4 emission rate between 2.5 ± 0.8×104 and 4.6 ± 1.5×104 kg h-1. This would be greater than the industry's reported 0.5% leak rate if it were emitting for more than half the time. Second, annual av. UK CH4 emissions are combined with an atm. dispersion model to create pseudo-observations. Clean air from the North Atlantic passed over mainland UK, picking up anthropogenic emissions. To best explain the obsd. plume using pseudo-observations, an addnl. North Sea source from the gas rigs area is added. Third, the δ13C-CH4 from the plume is shown to be -53‰, which is lighter than fossil gas but heavier than the UK av. emission. We conclude that either an addnl. small-area mainland source is needed, combined with temporal variability in emission or transport in small-scale meteorol. features. Alternatively, a combination of addnl. sources that are at least 75% from the mainland (-58‰) and up to 25% from the North Sea gas rigs area (-32‰) would explain the measurements. Had the isotopic anal. not been performed, the likely conclusion would have been of a gas field source of CH4. This demonstrates the limitation of analyzing mole fractions alone, as the simplest explanation is rejected based on anal. of isotopic data.
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    12. 12
      Countess, R. J.; Browne, D. Fugitive Hydrocarbon Emissions from Pacific Offshore Oil Platforms: Models, Emission Factors, and Platform Emissions. J. Air Waste Manage. Assoc. 1993, 43, 1455– 1460,  DOI: 10.1080/1073161X.1993.10467218
      There is no corresponding record for this reference.
    13. 13
      Bylin, C.; Schaffer, Z.; Goel, V.; Robinson, D. R.; Campos, A. d. N.; Borensztein, F. Designing the Ideal Offshore Platform Methane Mitigation Strategy. SPE International Conference on Health, Safety and Environment in Oil and Gas Exploration and Production, Rio de Janeiro, Brazil; Society of Petroleum Engineers, 2010; p 27.
      There is no corresponding record for this reference.
    14. 14
      Camilli, R.; Reddy, C. M.; Yoerger, D. R.; Van Mooy, B. A. S.; Jakuba, M. V.; Kinsey, J. C.; McIntyre, C. P.; Sylva, S. P.; Maloney, J. V. Tracking Hydrocarbon Plume Transport and Biodegradation at Deepwater Horizon. Science 2010, 330, 201– 204,  DOI: 10.1126/science.1195223
      14
      Tracking Hydrocarbon Plume Transport and Biodegradation at Deepwater Horizon
      Camilli, Richard; Reddy, Christopher M.; Yoerger, Dana R.; Van Mooy, Benjamin A. S.; Jakuba, Michael V.; Kinsey, James C.; McIntyre, Cameron P.; Sylva, Sean P.; Maloney, James V.
      Science (Washington, DC, United States) (2010), 330 (6001), 201-204CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      The Deepwater Horizon blow-out is the largest offshore oil spill in history. Results of a subsurface hydrocarbon survey using an autonomous underwater vehicle and a ship-cabled sampler are reported. Results indicated the presence of a continuous oil plume, >35 km long, at ∼1100 m depth which persisted for months without substantial biodegrdn. Samples collected within the plume showed monoarom. petroleum hydrocarbon concns. >50 μg/L. These data indicated that monoarom. input to this plume was at least 5500 kg/day, more than double the total source rate of all natural seeps of monoarom. petroleum hydrocarbons in the northern Gulf of Mexico. Dissolved O2 concns. suggested microbial respiration rates within the plume were not appreciably >1 μM O2/day.
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    15. 15
      Reddy, C. M.; Arey, J. S.; Seewald, J. S.; Sylva, S. P.; Lemkau, K. L.; Nelson, R. K.; Carmichael, C. A.; McIntyre, C. P.; Fenwick, J.; Ventura, G. T.; Van Mooy, B. A. S.; Camilli, R. Composition and fate of gas and oil released to the water column during the Deepwater Horizon oil spill. Proc. Natl. Acad. Sci. U.S.A. 2012, 109, 20229– 20234,  DOI: 10.1073/pnas.1101242108
      15
      Composition and fate of gas and oil released to the water column during the Deepwater Horizon oil spill
      Reddy, Christopher M.; Arey, J. Samuel; Seewald, Jeffrey S.; Sylva, Sean P.; Lemkau, Karin L.; Nelson, Robert K.; Carmichael, Catherine A.; McIntyre, Cameron P.; Fenwick, Judith; Ventura, G. Todd; Van Mooy, Benjamin A. S.; Camilli, Richard
      Proceedings of the National Academy of Sciences of the United States of America (2012), 109 (50), 20229-20234, S20229/1-S20229/10CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
      Quant. information regarding the endmember compn. of the gas and oil that flowed from the Macondo well during the Deepwater Horizon oil spill is essential for detg. the oil flow rate, total oil vol. released, and trajectories and fates of hydrocarbon components in the marine environment. Using isobaric gas-tight samplers, we collected discrete samples directly above the Macondo well on June 21, 2010, and analyzed the gas and oil. We found that the fluids flowing from the Macondo well had a gas-to-oil ratio of 1600 std. ft3/petroleum barrel. Based on the measured endmember gas-to-oil ratio and the Federally estd. net liq. oil release of 4.1 million barrels, the total amt. of C1-C5 hydrocarbons released to the water column was 1.7 × 1011 g. The endmember gas and oil compns. then enabled us to study the fractionation of petroleum hydrocarbons in discrete water samples collected in June 2010 within a southwest trending hydrocarbon-enriched plume of neutrally buoyant water at a water depth of 1100 m. The most abundant petroleum hydrocarbons larger than C1-C5 were benzene, toluene, ethylbenzene, and total xylenes at concns. ≤78 μg/L. Comparison of the endmember gas and oil compn. with the compn. of water column samples showed that the plume was preferentially enriched with water-sol. components, indicating that aq. dissoln. played a major role in plume formation, whereas the fates of relatively insol. petroleum components were initially controlled by other processes.
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    16. 16
      Yvon-Lewis, S. A.; Hu, L.; Kessler, J. Methane flux to the atmosphere from the Deepwater Horizon oil disaster. Geophys. Res. Lett. 2011, 38, L01602,  DOI: 10.1029/2010GL045928
      There is no corresponding record for this reference.
    17. 17
      Ryerson, T. B.; Aikin, K. C.; Angevine, W. M.; Atlas, E. L.; Blake, D. R.; Brock, C. A.; Fehsenfeld, F. C.; Gao, R.-S.; de Gouw, J. A.; Fahey, D. W.; Holloway, J. S.; Lack, D. A.; Lueb, R. A.; Meinardi, S.; Middlebrook, A. M.; Murphy, D. M.; Neuman, J. A.; Nowak, J. B.; Parrish, D. D.; Peischl, J.; Perring, A. E.; Pollack, I. B.; Ravishankara, A. R.; Roberts, J. M.; Schwarz, J. P.; Spackman, J. R.; Stark, H.; Warneke, C.; Watts, L. A. Atmospheric emissions from the Deepwater Horizon spill constrain air-water partitioning, hydrocarbon fate, and leak rate. Geophys. Res. Lett. 2011, 38, L07803,  DOI: 10.1029/2011GL046726
      17
      Atmospheric emissions from the Deepwater Horizon spill constrain air-water partitioning, hydrocarbon fate, and leak rate
      Ryerson, T. B.; Aikin, K. C.; Angevine, W. M.; Atlas, E. L.; Blake, D. R.; Brock, C. A.; Fehsenfeld, F. C.; Gao, R.-S.; de Gouw, J. A.; Fahey, D. W.; Holloway, J. S.; Lack, D. A.; Lueb, R. A.; Meinardi, S.; Middlebrook, A. M.; Murhy, D. M.; Neuman, J. A.; Nowak, J. B.; Parrish, D. D.; Peischl, J.; Perring, A. E.; Pollack, I. B.; Ravishankara, A. R.; Roberts, J. M.; Schwarz, J. P.; Spackman, J. R.; Stark, H.; Warneke, C.; Watts, L. A.
      Geophysical Research Letters (2011), 38 (April), L07803/1-L07803/6CODEN: GPRLAJ; ISSN:1944-8007. (American Geophysical Union)
      The fate of deepwater releases of gas and oil mixts. is initially detd. by soly. and volatility of individual hydrocarbon species; these attributes det. partitioning between air and water. Quantifying this partitioning is necessary to constrain simulations of gas and oil transport, to predict marine bioavailability of different fractions of the gas-oil mixt., and to develop a comprehensive picture of the fate of leaked hydrocarbons in the marine environment. Anal. of airborne atm. data shows massive amts. (∼258,000 kg/day) of hydrocarbons evapg. promptly from the Deepwater Horizon spill; these data collected during two research flights constrain air-water partitioning, thus bioavailability and fate, of the leaked fluid. This anal. quantifies the fraction of surfacing hydrocarbons that dissolves in the water column (∼33% by mass), the fraction that does not dissolve, and the fraction that evaps. promptly after surfacing (∼14% by mass). We do not quantify the leaked fraction lacking a surface expression; therefore, calcn. of atm. mass fluxes provides a lower limit to the total hydrocarbon leak rate of 32,600 to 47,700 barrels of fluid per day, depending on reservoir fluid compn. information. This study demonstrates a new approach for rapid-response airborne assessment of future oil spills.
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    18. 18
      Lee, J. D.; Mobbs, S. D.; Wellpott, A.; Allen, G.; Bauguitte, S. J.-B.; Burton, R. R.; Camilli, R.; Coe, H.; Fisher, R. E.; France, J. L.; Gallagher, M.; Hopkins, J. R.; Lanoiselle, M.; Lewis, A. C.; Lowry, D.; Nisbet, E. G.; Purvis, R. M.; O’Shea, S.; Pyle, J. A.; Ryerson, T. B. Flow rate and source reservoir identification from airborne chemical sampling of the uncontrolled Elgin platform gas release. Atmos. Meas. Tech. 2018, 11, 1725– 1739,  DOI: 10.5194/amt-11-1725-2018
      18
      Flow rate and source reservoir identification from airborne chemical sampling of the uncontrolled Elgin platform gas release
      Lee, James D.; Mobbs, Stephen D.; Wellpott, Axel; Allen, Grant; Bauguitte, Stephane J.-B.; Burton, Ralph R.; Camilli, Richard; Coe, Hugh; Fisher, Rebecca E.; France, James L.; Gallagher, Martin; Hopkins, James R.; Lanoiselle, Mathias; Lewis, Alastair C.; Lowry, David; Nisbet, Euan G.; Purvis, Ruth M.; O'Shea, Sebastian; Pyle, John A.; Ryerson, Thomas B.
      Atmospheric Measurement Techniques (2018), 11 (3), 1725-1739CODEN: AMTTC2; ISSN:1867-8548. (Copernicus Publications)
      An uncontrolled gas leak from 25 March to 16 May 2012 led to evacuation of the Total Elgin wellhead and neighboring drilling and prodn. platforms in the UK North Sea. Initially the atm. flow rate of leaking gas and condensate was very poorly known, hampering environmental assessment and well control efforts. Six flights by the UK FAAM chem. instrumented BAe-146 research aircraft were used to quantify the flow rate. The flow rate was calcd. by assuming the plume may be modelled by a Gaussian distribution with two different soln. methods: Gaussian fitting in the vertical and fitting with a fully mixed layer. When both soln. methods were used they compared within 6% of each other, which was within combined errors. Data from the first flight on 30 March 2012 showed the flow rate to be 1.3±0.2 kgCH4 s-1, decreasing to less than half that by the second flight on 17 Apr. 2012. δ13CCH4 in the gas was found to be -43 ‰, implying that the gas source was unlikely to be from the main high pressure, high temp. Elgin gas field at 5.5 km depth, but more probably from the overlying Hod Formation at 4.2 km depth. This was deemed to be smaller and more manageable than the high pressure Elgin field and hence the response strategy was considerably simpler. The first flight was conducted within 5 days of the blowout and allowed a flow rate est. within 48 h of sampling, with δ13CCH4 characterization soon thereafter, demonstrating the potential for a rapid-response capability that is widely applicable to future atm. emissions of environmental concern. Knowledge of the Elgin flow rate helped inform subsequent decision making. This study shows that leak assessment using appropriately designed airborne plume sampling strategies is well suited for circumstances where direct access is difficult or potentially dangerous. Measurements such as this also permit unbiased regulatory assessment of potential impact, independent of the emitting party, on timescales that can inform industry decision makers and assist rapid-response planning by government.
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    19. 19
      McManus, J. B.; Zahniser, M. S.; Nelson, D. D.; Shorter, J. H.; Herndon, S. C.; Jervis, D.; Agnese, M.; McGovern, R.; Yacovitch, T. I.; Roscioli, J. R. Recent progress in laser-based trace gas instruments: performance and noise analysis. Appl. Phys. B 2015, 119, 203– 218,  DOI: 10.1007/s00340-015-6033-0
      19
      Recent progress in laser-based trace gas instruments: performance and noise analysis
      McManus, J. B.; Zahniser, M. S.; Nelson, D. D.; Shorter, J. H.; Herndon, S. C.; Jervis, D.; Agnese, M.; McGovern, R.; Yacovitch, T. I.; Roscioli, J. R.
      Applied Physics B: Lasers and Optics (2015), 119 (1), 203-218CODEN: APBOEM; ISSN:0946-2171. (Springer)
      We review our recent results in development of high-precision laser spectroscopic instrumentation using mid-IR quantum cascade lasers, interband cascade lasers and antimonide diode lasers. These instruments are primarily for high-precision and high-sensitivity measurements of atm. trace gases, as required for atm. research. The instruments are based on direct absorption spectroscopy with rapid sweeps, integration and precision fitting, under the control of high-capability software. By operating in the mid-IR with long absorption path lengths at reduced pressure, we achieve excellent sensitivity. Some instruments have demonstrated a fractional precision of 10-4 for atm. trace gases at ambient concn., allowing real-time isotopologue measurements of CO2, CO, CH4, N2O and H2O. Trace gas detection in ambient air at the low part-per-trillion levels is feasible. We also describe signal processing methods to identify and reduce measurement noise. Anal. of spectral information is largely based on loading spectra into arrays and then applying block operations such as filters, Fourier anal., multivariate fitting and principal component anal. We present math. expressions for averaged spectra in arrays and note different ways frequency aliasing can occur. We present an extended example of anal. of instrument noise and find an electronic signal mixing with an interference fringe.
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    20. 20
      Yacovitch, T. I.; Herndon, S. C.; Pétron, G.; Kofler, J.; Lyon, D.; Zahniser, M. S.; Kolb, C. E. Mobile Laboratory Observations of Methane Emissions in the Barnett Shale Region. Environ. Sci. Technol. 2015, 49, 7889– 7895,  DOI: 10.1021/es506352j
      20
      Mobile Laboratory Observations of Methane Emissions in the Barnett Shale Region
      Yacovitch, Tara I.; Herndon, Scott C.; Petron, Gabrielle; Kofler, Jonathan; Lyon, David; Zahniser, Mark S.; Kolb, Charles E.
      Environmental Science & Technology (2015), 49 (13), 7889-7895CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
      Results of mobile ground-based atm. measurements conducted during the Barnett Shale Coordinated Campaign in spring and fall of 2013 are presented. Methane and ethane are continuously measured downwind of facilities such as natural gas processing plants, compressor stations, and prodn. well pads. Gaussian dispersion simulations of these methane plumes, using an iterative forward plume dispersion algorithm, are used to est. both the source location and the emission magnitude. The distribution of emitters is peaked in the 0-5 kg/h range, with a significant tail. The ethane/methane molar enhancement ratio for this same distribution is investigated, showing a peak at ∼1.5% and a broad distribution between ∼4% and ∼17%. The regional distributions of source emissions and ethane/methane enhancement ratios are examd.: the largest methane emissions appear between Fort Worth and Dallas, while the highest ethane/methane enhancement ratios occur for plumes obsd. in the northwestern potion of the region. Individual facilities, focusing on large emitters, are further analyzed by constraining the source location.
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    21. 21
      Keeling, C. D. The concentration and isotopic abundances of atmospheric carbon dioxide in rural areas. Geochim. Cosmochim. Acta 1958, 13, 322– 334,  DOI: 10.1016/0016-7037(58)90033-4
      21
      The concentration and isotopic abundances of atmospheric carbon dioxide in rural areas
      Keeling, Charles D.
      Geochimica et Cosmochimica Acta (1958), 13 (), 322-34CODEN: GCACAK; ISSN:0016-7037.
      cf. Craig, C.A. 51, 17599i. Fifty samples of air collected near the Pacific coast of Washington and California were analyzed for CO2 and isotopic abundance of C13 and O18. The meteorological data for the samples analyzed are tabulated. Min. concns. of CO2 in the air were noted in the afternoons and max. concns. in the evening or early morning hours. The pronounced regularity with which the C isotope ratio follows changes in CO2 concn. is due to the CO2 added to or subtracted from the air, diurnally, by plants and their decay products. It is assumed that the air initially contains 0.031 vol. % CO2, C13/C12 ratio -0.7%, to which is added CO2 of plant origin with a ratio of approx. -2.3%. The uniform min. concn. and C isotope ratio of afternoon air samples, regardless of location, must, on the other hand, be the result of ground-level air mixing with air from above or beyond the zone of vegetative influence. The variation in concn. is only 0.0307-0.0316%; in C13/C12 ratio, -0.67 to -0.74%; it is thought to represent Pacific maritime air. O isotope ratios are about the same as for CO2 in equil. with av. ocean water, at 25°, -0.1%. Samples assocd. with min. concns. range from +1.3 to -0.2%; forest and grassland samples from +2.9 to -1.9%. The variations are apparently not correlated with any measured meteorological or chem. factor. One case was noted where a definite change in O isotope ratio reflected change in barometric pressure, whereas the C isotope ratio and CO2 concn. remained const. It could have been due to partial mixing of air from different air masses that were equilibrated with H2O of different O-isotope compn. or at different temps.
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    22. 22
      Keeling, C. D. The concentration and isotopic abundances of carbon dioxide in rural and marine air. Geochim. Cosmochim. Acta 1961, 24, 277– 298,  DOI: 10.1016/0016-7037(61)90023-0
      22
      Concentration and isotopic abundances of carbon dioxide in rural and marine air
      Keeling, Charles D.
      Geochimica et Cosmochimica Acta (1961), 24 (), 277-98CODEN: GCACAK; ISSN:0016-7037.
      cf. CA 52, 12477a. Addnl. analyses are reported of 106 samples of rural air in forest, grassland, and desert and of 13 samples of air over tropical waters of the eastern Pacific Ocean. The data are correlated with meteorological conditions. In areas away from urban effects and terrestrial plant growth, the concn. and C13 abundance of CO2 in air are nearly const., but O18 abundance of CO2 shows systematic variation with air and ocean water temps. or season. The C13::C12 ratio ranges from -6.7 to -7.4 per mil and O18:O16 from +0.8 to -0.6 per mil. Wickman's hypothesis (CA 46, 9167c) on C-isotope enrichment of terrestrial plants is corroborated. The previous correlation between C13 abundance and concn. of CO2 in forest air is noted again. The C13:C12 ratio of CO2 from forest plants was computed and is -21 to -26 per mil. The O18 abundance of CO2 in forest air is variable and shows no obvious correlation with other measurable properties.
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    23. 23
      DI Desktop 2017 Drillinginfo; Drillinginfo—An International Oil & Gas Intelligence Company: Austin, TX, 2017.
      There is no corresponding record for this reference.
    24. 24
      BOEM. Platforms; Bureau of Ocean Energy Management: New Orleans, LA, 2019, https://www.data.boem.gov/Main/Mapping.aspx (accessed Feb 15, 2019).
      There is no corresponding record for this reference.
    25. 25
      NOAA. Global Self-Consistent, Hierarchical, High-Resolution Geography Database (GSHHG) , version 2.2.0., 2013, http://www.ngdc.noaa.gov/mgg/shorelines/gshhs.html.
      There is no corresponding record for this reference.
    26. 26
      Yacovitch, T. I.; Neininger, B.; Herndon, S. C.; van der Gon, H. D.; Jonkers, S.; Hulskotte, J.; Roscioli, J. R.; Zavala-Araiza, D. Methane emissions in the Netherlands: The Groningen field. Elem. Sci. Anth. 2018, 6, 57,  DOI: 10.1525/elementa.308
      There is no corresponding record for this reference.
    27. 27
      Turner, B. D. Workbook of Atmospheric Dispersion Estimates: An Introduction to Dispersion Modeling, 2nd ed.; CRC Press, Inc.: Boca Raton, Florida, 1994.
      There is no corresponding record for this reference.
    28. 28
      Thoma, E.; Squier, B. OTM 33 Geospatial Measurement of Air Pollution, Remote Emissions Quantification (GMAP-REQ) and OTM33A Geospatial Measurement of Air Pollution-Remote Emissions Quantification-Direct Assessment (GMAP-REQ-DA) , 2014. https://cfpub.epa.gov/si/si_public_record_report.cfm?dirEntryId=309632.
      There is no corresponding record for this reference.
    29. 29
      Erbrink, H. J.; Scholten, R. D. A. Atmospheric Turbulence above Coastal Waters: Determination of Stability Class and a Simple Model for Offshore Flow Including Advection and Dissipation. J. Appl. Meteorol. 1995, 34, 2278– 2293,  DOI: 10.1175/1520-0450(1995)034<2278:atacwd>2.0.co;2
      There is no corresponding record for this reference.
    30. 30
      Hanna, S. R.; Schulman, L. L.; Paine, R. J.; Pleim, J. E.; Baer, M. Development and Evaluation of the Offshore and Coastal Dispersion Model. Journal of the Air Pollution Control Association 1985, 35, 1039– 1047,  DOI: 10.1080/00022470.1985.10466003
      30
      Development and evaluation of the offshore and coastal dispersion model
      Hanna, Steven R.; Schulman, Lloyd L.; Paine, Robert J.; Pleim, Jonathan E.; Baer, Mitchell
      Journal of the Air Pollution Control Association (1985), 35 (10), 1039-47CODEN: JPCAAC; ISSN:0002-2470.
      The Offshore and Coastal Dispersion (OCD) model for detg. the impact of offshore and onshore emissions from point sources on the air quality of coastal regions was constructed on the framework of the EPA guideline model MPTER and incorporates overwater plume transport and dispersion as well as changes that occur as the plume crosses the shoreline. Hourly meteorol. data are needed from both offshore and onshore locations, including wind direction and speed, mixing height, overwater air temp. and relative humidity, and the sea surface temp. Obsd. turbulence intensities are preferred but are not mandatory. Building downwash and plume rise are incorporated into the OCD model and partial plume penetration into elevated inversions is treated using Briggs' model. Dispersion coeffs. are proportional to turbulence intensities. A virtual source technique is used to change the rate of plume growth as the overwater plume intercepts the overland internal boundary layer. The continuous shoreline fumigation case is treated using an approach suggested by J.W. Deardorff and G.E. Willis (1982). Calcn. of plume reflection from elevated terrain follows the Rough Terrain Dispersion Model. The OCD model and the modified EPA model used as an interim model for overwater applications by the Minerals Management Service (MMS) were tested with measurements from 3 offshore tracer expts. The OCD model was an improvement over the EPA model and was officially approved by the MMS in Mar. 1985.
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    31. 31
      Abdel-Rahman, A. A. On the Atmospheric Dispersion and Gaussian Plume Model. In 2nd International Conference on Waste Management, Water Pollution, Air Pollution, Indoor Climate, Corfu, Greece, October 26–28, 2008; Mastorakis, N. E., Ed.; WSEAS Press, 2008; pp 31– 39.
      There is no corresponding record for this reference.
    32. 32
      Rigby, M.; Manning, A. J.; Prinn, R. G. The value of high-frequency, high-precision methane isotopologue measurements for source and sink estimation. J. Geophys. Res.: Atmos. 2012, 117, D12312,  DOI: 10.1029/2011JD017384
      32
      The value of high-frequency, high-precision methane isotopologue measurements for source and sink estimation
      Rigby, M.; Manning, A. J.; Prinn, R. G.
      Journal of Geophysical Research: Atmospheres (2012), 117 (June), D12312/1-D12312/14CODEN: JGRDE3 ISSN:. (American Geophysical Union)
      We present an observing system simulation expt. examg. the potential benefits of new methane isotopologues measurements for global- and national-scale source and sink inversions. New measurements are expected in the coming years, using quantum cascade laser spectroscopy with sample preconcn., that will allow observations of δ13C - CH4 and δD - CH4 at approx. hourly intervals and higher precision than previously possible. Using model-generated "pseudo-data", we predict the variability that these new systems should encounter in the atm., and est. the addnl. uncertainty redn. that should result from their use in source and sink inversions. We find that much of the δ-value variability from seasonal to daily timescales should be resolvable at the target precision of the new observations. For global source estn., we find addnl. uncertainty redns. of between 3-9 Tg/yr for four major source categories (microbial, biomass burning, landfill and fossil fuel), compared to mole fraction-only inversions, if the higher end of the anticipated isotopologue-measurement precisions can be achieved. On national scales, we obtain av. uncertainty redns. of ∼10% of the source strength for countries close to high-frequency monitoring sites, although the degree of uncertainty redn. on such small scales varies significantly (from close to 0% to almost 50%) for different sources and countries.
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    33. 33
      Sherwood, O. A.; Schwietzke, S.; Arling, V. A.; Etiope, G. Global Inventory of Gas Geochemistry Data from Fossil Fuel, Microbial and Burning Sources, version 2017. Earth Syst. Sci. Data 2017, 9, 639– 656,  DOI: 10.5194/essd-9-639-2017
      There is no corresponding record for this reference.
    34. 34
      Briggs, N. L.; Jaffe, D. A.; Gao, H.; Hee, J. R.; Baylon, P. M.; Zhang, Q.; Zhou, S.; Collier, S. C.; Sampson, P. D.; Cary, R. A. Particulate Matter, Ozone, and Nitrogen Species in Aged Wildfire Plumes Observed at the Mount Bachelor Observatory. Aerosol and Air Quality Research 2016, 16, 3075– 3087,  DOI: 10.4209/aaqr.2016.03.0120
      34
      Particulate matter, ozone, and nitrogen species in aged wildfire plumes observed at the mount bachelor observatory
      Briggs, Nicole L.; Jaffe, Daniel A.; Gao, Honglian; Hee, Jonathan R.; Baylon, Pao M.; Zhang, Qi; Zhou, Shan; Collier, Sonya C.; Sampson, Paul D.; Cary, Robert A.
      Aerosol and Air Quality Research (2016), 16 (12), 3075-3087CODEN: AAQRAV; ISSN:1680-8584. (Taiwan Association for Aerosol Research)
      During the summer of 2012 and 2013, we measured carbon monoxide (CO), carbon dioxide (CO2), ozone (O3), nitrogen oxides (NOx), reactive nitrogen (NOy), peroxyacetyl nitrate (PAN), aerosol scattering (ssp) and absorption, elemental and org. carbon (EC and OC), and aerosol chem. at the Mount Bachelor Observatory (2.8 km above sea level, Oregon, US). Here we analyze 23 of the individual plumes from regional wildfires to better understand prodn. and loss of aerosols and gaseous species. We also developed a new method to calc. enhancement ratios and Modified Combustion Efficiency (MCE), which takes into account possible changes in background concns. during transport. We compared this new method to existing methods for calcg. enhancement ratios. The MCE values ranged from 0.79- 0.98, ΔO3/ ΔCO ranged from 0.01-0.07 ppbv ppbv1, Δσsp/δ CO ranged from 0.23-1.32 Mm1 (at STP) ppbv-1, ΔNOy/ΔCO ranged from 2.89-12.82 pptv ppbv1, and δ PAN/δ CO ranged from 1.46-6.25 pptv ppbv1. A comparison of three different methods to calc. enhancement ratios (ER) showed that the methods generally resulted in similar Δσsp/δ CO, ΔNOy/ΔCO, and ΔPAN/ΔCO; however, there was a significant bias between the methods when calcg. δO3/δCO due to the small abs. enhancement of O3 in the plumes. The ΔO3/ΔCO ERs calcd. using two common methods were biased low (~ 20-30%) when compared to the new proposed method. Two pieces of evidence suggest moderate secondary particulate formation in many of the plumes studied: 1 mean obsd. δOC/δ CO2 was 0.028 g particulate-C gC1 (as CO2)-27% higher than the midpoint of the biomass burning emission ratio range reported by a recent review-and 2 single scattering albedo (ω) was relatively const. at all MCE values, in contrast with results for fresh plumes. The obsd. NOx, PAN, and aerosol nitrate represented 6-48%, 25-57%, and 20-69% of the obsd. NOy in the aged plumes, resp., and other species represented on av. 11% of the obsd. NOy.
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    35. 35
      Roscioli, J. R.; Herndon, S. C.; Yacovitch, T. I.; Knighton, W. B.; Zavala-Araiza, D.; Johnson, M. R.; Tyner, D. R. Characterization of methane emissions from five cold heavy oil production with sands (CHOPS) facilities. J. Air Waste Manage. Assoc. 2018, 68, 671– 684,  DOI: 10.1080/10962247.2018.1436096
      35
      Characterization of methane emissions from five cold heavy oil production with sands facilities
      Roscioli, Joseph R.; Herndon, Scott C.; Yacovitch, Tara I.; Knighton, W. Berk; Zavala-Araiza, Daniel; Johnson, Matthew R.; Tyner, David R.
      Journal of the Air & Waste Management Association (2018), 68 (7), 671-684CODEN: JAWAFC; ISSN:1096-2247. (Taylor & Francis Ltd.)
      Cold heavy oil prodn. with sands CHOPS is a common oil extn. method in the Canadian provinces of Alberta and Saskatchewan that can result in significant methane emissions due to annular venting. Little is known about the magnitude of these emissions, nor their contributions to the regional methane budget. Here the authors present the results of field measurements of methane emissions from CHOPS wells and compare them with self-reported venting rates. The tracer ratio method was used not only to analyze total site emissions but at one site it was also used to locate primary emission sources and quantify their contributions to the facility-wide emission rate, revealing the annular vent to be a dominant source. Emissions measured from five different CHOPS sites in Alberta showed large discrepancies between the measured and reported rates, with emissions being mainly underreported. These methane emission rates are placed in the context of current reporting procedures and the role that gas-oil ratio measurements play in vented vol. ests. In addn. to methane, emissions of higher hydrocarbons were also measured; a chem. "fingerprint" assocd. with CHOPS wells in this region reveals very low emission ratios of ethane, propane, and aroms. vs. methane. The results of this study may inform future studies of CHOPS sites and aid in developing policy to mitigate regional methane emissions. Implications: Methane measurements from cold heavy oil prodn. with sand CHOPS sites identify annular venting to be a potentially major source of emissions at these facilities. The measured emission rates are generally larger than reported by operators, with uncertainty in the gas-oil ratio possibly playing a large role in this discrepancy. These results have potential policy implications for reducing methane emissions in Alberta in order to achieve the Canadian government's goal of reducing methane emissions by 40-45% below 2012 levels within 8 yr.
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    36. 36
      Whiticar, M. Correlation of Natural Gases with Their Sources. In The Petroleum System—From Source to Trap; Magoon, L. B., Dow, W. G., Eds.; AAPG, 1994; pp 261– 283.
      There is no corresponding record for this reference.
    37. 37
      Yacovitch, T. I.; Herndon, S. C.; Roscioli, J. R.; Floerchinger, C.; McGovern, R. M.; Agnese, M.; Pétron, G.; Kofler, J.; Sweeney, C.; Karion, A.; Conley, S. A.; Kort, E. A.; Nähle, L.; Fischer, M.; Hildebrandt, L.; Koeth, J.; McManus, J. B.; Nelson, D. D.; Zahniser, M. S.; Kolb, C. E. Demonstration of an Ethane Spectrometer for Methane Source Identification. Environ. Sci. Technol. 2014, 48, 8028– 8034,  DOI: 10.1021/es501475q
      37
      Demonstration of an Ethane Spectrometer for Methane Source Identification
      Yacovitch, Tara I.; Herndon, Scott C.; Roscioli, Joseph R.; Floerchinger, Cody; McGovern, Ryan M.; Agnese, Michael; Petron, Gabrielle; Kofler, Jonathan; Sweeney, Colm; Karion, Anna; Conley, Stephen A.; Kort, Eric A.; Nahle, Lars; Fischer, Marc; Hildebrandt, Lars; Koeth, Johannes; McManus, J. Barry; Nelson, David D.; Zahniser, Mark S.; Kolb, Charles E.
      Environmental Science & Technology (2014), 48 (14), 8028-8034CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
      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-IR 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 obsd. 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 liqs. (>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 exptl. studies of methane emissions, particularly in the oil and natural gas sector.
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    38. 38
      Warwick, P. D. Geologic Assessment Of Undiscovered Conventional Oil and Gas Resources in the Lower Paleogene Midway and Wilcox Groups, and the Carrizo Sand of the Claiborne Group, of the Northern Gulf Coast Region; Open-File Report 2017–1111; U.S. Geological Survey: Reston, Virginia, 2017; p 67.
      There is no corresponding record for this reference.
    39. 39
      Crawford, T. G.; Burgess, G. L.; Haley, S. M.; Harrison, P. F.; Kinler, C. J.; Klocek, G. D.; Shepard, N. K. Estimated Oil and Gas Reserves, Gulf of Mexico, December 31, 2006; OCS Report MMS 2009-064; Gulf of Mexico OCS Regional Office, 2009; p 58. https://www.boem.gov/BOEM-Newsroom/Library/Publications/2009/2009-064.aspx.
      There is no corresponding record for this reference.
    40. 40
      BOEM. Oil and gas production in 2015 for platforms in the Gulf of Mexico; Bureau of Ocean Energy Management: New Orleans, LA, 2016, https://www.doi.gov/sites/doi.gov/files/uploads/bsee_2016_data.xlsx (accessed Nov 29, 2018).
      There is no corresponding record for this reference.
    41. 41
      NOAA. TexAQS 2006 HRDL Lidar Data; NOAA Earth Systems Research Laboratory, Chemical Sciences Division, 2006, https://www.esrl.noaa.gov/csd/groups/csd3/measurements/texaqs06/hrdl/ (accessed Nov 14, 2019).
      There is no corresponding record for this reference.
    42. 42
      Stein, A. F.; Draxler, R. R.; Rolph, G. D.; Stunder, B. J. B.; Cohen, M. D.; Ngan, F. NOAA’s HYSPLIT Atmospheric Transport and Dispersion Modeling System. Bull. Am. Meteorol. Soc. 2015, 96, 2059– 2077,  DOI: 10.1175/bams-d-14-00110.1
      There is no corresponding record for this reference.
    43. 43
      Draxler, R. R.; Hess, G. D. An Overview of the HYSPLIT_4 modeling system of trajectories, dispersion, and deposition. Aust. Meteor. Mag. 1998, 47, 295– 308
      There is no corresponding record for this reference.
    44. 44
      Draxler, R. R.; Hess, G. D. Description of the HYSPLIT_4 modeling system. In NOAA Technical Memorandum; NOAA Air Resources Laboratory: Silver Spring, MD, 1997; p 24.
      There is no corresponding record for this reference.
    45. 45
      Draxler, R. R. HYSPLIT_4 User’s Guide. In NOAA Technical Memorandum; NOAA Air Resources Laboratory: Silver Spring, MD, 1999, https://www.arl.noaa.gov/documents/reports/arl-230.pdf (accessed Oct 06, 2017).
      There is no corresponding record for this reference.
    46. 46
      Hu, L.; Yvon-Lewis, S. A.; Kessler, J. D.; MacDonald, I. R. Methane fluxes to the atmosphere from deepwater hydrocarbon seeps in the northern Gulf of Mexico. J. Geophys. Res.: Oceans 2012, 117, C01009,  DOI: 10.1029/2011JC007208
      46
      Methane fluxes to the atmosphere from deepwater hydrocarbon seeps in the northern Gulf of Mexico
      Hu, Lei; Yvon-Lewis, Shari A.; Kessler, John D.; MacDonald, Ian R.
      Journal of Geophysical Research: Oceans (2012), 117 (Jan.), C01009/1-C01009/13CODEN: JGRCEY ISSN:. (American Geophysical Union)
      Three deepwater hydrocarbon seep sites in the northern Gulf of Mexico that feature near-seafloor gas hydrates, MC118 (depth = 900 m), GC600 (depth = 1250 m) and GC185 (depth = 550 m), were investigated during the Remote Sensing and Sea-Truth Measurements of Methane Flux to the Atm. (HYFLUX) study in July 2009. Continuous measurements of air and sea surface concns. of methane were made to obtain high spatial and temporal resoln. of the diffusive net sea-to-air fluxes. The atm. methane fluctuated between 1.70 and 2.40 ppm (ppm) during the entire cruise except for high concns. (up to 4.01 ppm) sampled during the end of the occupation of GC600 and the transit between GC600 and GC185. In conjunction with air-mass back trajectory anal., these high concns. are likely from a localized methane source to the atm. Methane concns. in surface seawater and methane net sea-to-air fluxes show high temporal and spatial variability within and between sites. The presence of ethane and propane in the surface seawater indicates a thermogenic source in the plume areas, suggesting the surface methane could be at least partly attributable to transport from the deepwater hydrocarbon seeps. Results from interpolations within the survey areas show the daily methane fluxes to the atm. at the three sites range from 0.744 to 300 mol d-1. Extrapolating the highest daily sea-to-air flux of methane to other deepwater seeps in the northern Gulf of Mexico suggests that the net diffusive sea-to-air flux from deepwater hydrocarbon seeps in this region is an insignificant source to the atm. methane.
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    47. 47
      Pisso, I.; Myhre, C. L.; Platt, S. M.; Eckhardt, S.; Hermansen, O.; Schmidbauer, N.; Mienert, J.; Vadakkepuliyambatta, S.; Bauguitte, S.; Pitt, J.; Allen, G.; Bower, K. N.; O’Shea, S.; Gallagher, M. W.; Percival, C. J.; Pyle, J.; Cain, M.; Stohl, A. Constraints on oceanic methane emissions west of Svalbard from atmospheric in situ measurements and Lagrangian transport modeling. J. Geophys. Res.: Atmos. 2016, 121, 14188– 14200,  DOI: 10.1002/2016JD025590
      47
      Constraints on oceanic methane emissions west of Svalbard from atmospheric in situ measurements and Lagrangian transport modeling
      Pisso I; Myhre C Lund; Platt S M; Eckhardt S; Hermansen O; Schmidbauer N; Stohl A; Mienert J; Vadakkepuliyambatta S; Bauguitte S; Pitt J; Allen G; Bower K N; O'Shea S; Percival C J; Gallagher M W; Pyle J; Cain M
      Journal of geophysical research. Atmospheres : JGR (2016), 121 (23), 14188-14200 ISSN:2169-897X.
      Methane stored in seabed reservoirs such as methane hydrates can reach the atmosphere in the form of bubbles or dissolved in water. Hydrates could destabilize with rising temperature further increasing greenhouse gas emissions in a warming climate. To assess the impact of oceanic emissions from the area west of Svalbard, where methane hydrates are abundant, we used measurements collected with a research aircraft (Facility for Airborne Atmospheric Measurements) and a ship (Helmer Hansen) during the Summer 2014 and for Zeppelin Observatory for the full year. We present a model-supported analysis of the atmospheric CH4 mixing ratios measured by the different platforms. To address uncertainty about where CH4 emissions actually occur, we explored three scenarios: areas with known seeps, a hydrate stability model, and an ocean depth criterion. We then used a budget analysis and a Lagrangian particle dispersion model to compare measurements taken upwind and downwind of the potential CH4 emission areas. We found small differences between the CH4 mixing ratios measured upwind and downwind of the potential emission areas during the campaign. By taking into account measurement and sampling uncertainties and by determining the sensitivity of the measured mixing ratios to potential oceanic emissions, we provide upper limits for the CH4 fluxes. The CH4 flux during the campaign was small, with an upper limit of 2.5 nmol m(-2) s(-1) in the stability model scenario. The Zeppelin Observatory data for 2014 suggest CH4 fluxes from the Svalbard continental platform below 0.2 Tg yr(-1). All estimates are in the lower range of values previously reported.
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    48. 48
      Bange, H. W.; Bartell, U. H.; Rapsomanikis, S.; Andreae, M. O. Methane in the Baltic and North Seas and a reassessment of the marine emissions of methane. Global Biogeochem. Cycles 1994, 8, 465– 480,  DOI: 10.1029/94GB02181
      48
      Methane in the Baltic and North seas and a reassessment of the marine emissions of methane
      Bange, H. W.; Bartell, U. H.; Rapsonmanikis, S.; Andreae, M. O.
      Global Biogeochemical Cycles (1994), 8 (4), 465-80CODEN: GBCYEP; ISSN:0886-6236. (American Geophysical Union)
      During three measurement campaigns on the Baltic and North seas, atm. and dissolved methane was detd. with an automated gas chromatog. system. Area-weighted mean satn. values in the sea surface waters were 113 ± 5% and 395 ± 82% (Baltic Sea, Feb. and July 1992) and 126 ± 8% (south central North Sea, Sept. 1992). On the bases of our data and a compilation of literature data the global oceanic emissions of methane were reassessed by introducing a concept for regional gas transfer coeffs. Our ests. computed with two different air-sea exchange models lie in the range of 11-18 Tg CH4 yr-1. Despite the fact that shelf areas and estuaries only represent a small part of the world's ocean they contribute about 75% to the global oceanic emissions. We applied a simple, coupled, three-layer model to evaluate the time dependent variation of the oceanic flux to the atm. The model calcns. indicate that even with increasing tropospheric methane concn., the ocean will remain a source of atm. methane.
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    • Dataset descriptions, additional methods (site selection, sampling and analysis strategy, and instrument calibration and performance), isotopic and ethane/methane ratio results, comparisons with other studies, and a description of offshore datasets used (PDF)

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